\r\n\t• As a consequence of the pandemic spread of obesity, NAFLD is one of the most important causes of liver disease worldwide in adults and children. The large volume of patients sets NAFLD apart from other liver diseases, meaning the major focus of clinical care is discerning those at the highest risk of progressive liver disease
\r\n\t• Because the overweighting in childhood and adolescence is associated with an increased risk of NAFLD later in life, the threshold of liver-related morbidity and/or mortality is reached at a younger age. Patients with NAFLD have a high risk of liver-related morbidity and mortality along with metabolic comorbidities and might place a growing strain on healthcare systems
\r\n\t• Whereas animal studies have demonstrated a potential causal role of gut microbiota in NAFLD, human studies have only just started to describe microbiome signatures in NAFLD.
\r\n\t• Proteobacteria are consistently enriched in steatosis and non-alcoholic steatohepatitis.
\r\n\t• Discrepant microbiome signatures across studies could be linked to the heterogeneity of geographical regions, ethnicity, population characteristics, microbiome sequencing tools, NAFLD diagnostic tools, disease spectrum, drug consumption, and circadian rhythm.
\r\n\t• The composition of the diet, in particular the types of lipids and carbohydrates, have an important role in the progression of NAFLD to NASH and fibrosis
\r\n\t• A complex interplay between the environment (especially diet), host genetics, and the gut microflora is crucial for the development and progression of NAFLD
\r\n\t• Activation of the innate immune system has an essential role in maintaining homeostasis and liver regeneration, as well as disease pathogenesis, acting in a cooperative rather than independent fashion
\r\n\t• Discoveries that characterized the importance of cell death in NAFLD progression triggered the development of novel therapeutic and diagnostic approaches for NAFLD
\r\n\t• Various types of cell death contribute to the development of NAFLD; extensive crosstalk and biochemical cooperation exists between these cell death pathways to drive disease progression
\r\n\t• Chronic hepatic inflammation represents the driving force in the evolution of nonalcoholic steatohepatitis (NASH) to liver fibrosis and/or cirrhosis.
\r\n\t• In both humans and rodents, NASH is characterized by B cell and T cell infiltration of the liver as well as by the presence of circulating antibodies targeting antigens originating from oxidative stress.
\r\n\t• Alterations in regulatory T cell and hepatic dendritic cell homeostasis have a role in triggering immune responses during the progression of NASH.
\r\n\t• While NAFLD progression to non-alcoholic steatohepatitis is becoming the leading cause of end-stage liver failure, the leading causes of death in patients with NAFLD are complications of cardiometabolic disease and a tight relationship exists between NAFLD, insulin resistance, and type II diabetes mellitus.
\r\n\t• A NAFLD has become the most common liver disease globally, yet there are currently no approved therapies and It is likely that developing therapeutics that target both NAFLD and cardiometabolic risk factors might be extremely beneficial.
\r\n\t
As a viscoelastic body, the cell exhibits both elastic and viscous characteristics (Kasza, 07). Although these mechanical properties have not been attributed wholly to a single element, such as the cytoskeletal network, the cytoplasm, the cell membrane, or the extracellular network (Janmey et al., 2007), it is agreed that they are determined predominantly by the cytoskeleton, a network of biopolymers in the form of actin filaments, microtubules, and intermediate filaments. The dynamic assembly and disassembly of these biopolymers give the cell the ability to move and to modulate its shape, elasticity, and mechanical strength in responses to mechanical and chemical stimuli from the external environment (Fletcher & Mullins, 2010). Among these cytoskeletal polymers, actin filaments are known to be primarily responsible for the rigidity of the cell. An increase in the concentration of actin filaments typically results in an increase in the rigidity of the cell, which can be characterized by Young’s modulus (Satcher Jr & Dewey Jr, 1996).
The cytoskeleton is also essential in regulation of cell signaling and trafficking (Janmey, 1998; Papakonstanti & Stournaras, 2008). In particular, the structure of the cytoskeleton plays an essential role in EGFR signaling and trafficking that is initiated by the binding of epidermal growth factor (EGF) to the EGF receptor (EGFR) (Ridley, 1994; Song et al., 2008). EGF is a protein molecule known to play a crucial role in the regulation of cell growth, proliferation, differentiation and motility. EGFR is a transmembrane receptor that consists of an extracellular ligand-binding domain, a transmembrane domain, an intracellular tyrosine kinase domain, and a C-terminal regulatory domain (Scaltriti & Baselga, 2006). Binding of EGF to the extracellular domain of EGFR leads to the dimerization of EGFR, which in turn stimulates tyrosine kinase activity of the receptors and triggers autophosphorylation of specific tyrosine residues within the cytoplasmic regulatory domain. The activation of tyrosine kinases initiates multiple downstream signaling pathways such as Ras/Raf-1/MAPK (Scaltriti & Baselga, 2006), PI3Kinase/Akt/mTOR (Ono & Kuwano, 2006), Src/NFKb (Lee C.-W. et al., 2007; Silva, 2004), catenin/cytoskeleton (Yasmeen et al., 2006) and PAK‐1 /Rac pathways (McManus et al., 2000).
It is known that EGFR signaling induces drastic morphological changes, such as rounding of cells, induction of membrane ruffling and extension of filopodia (Bretscher, 1989; Chinkers et al., 1981). These changes can be attributed to the remodeling of cytoskeletal structures (Rijken et al., 1991), which may also alter mechanical properties of the cells (Kasza et al., 2007; Stamenovic, 2005). Currently, the connection between cell signalling and alterations of the mechanical properties of cells is still not fully understood in general. Information concerning the effects of EGF stimulation on the mechanical properties of cells will certainly provide insights into this connection. In addition, since EGFR is highly expressed in a variety of human tumors (Dei Tos & Ellis, 2005) and mutations in EGFR can produce aberrant cell signaling that often leads to uncontrolled cell growth and a malignant phenotype, such information will also shed light on the link between cell mechanical properties and human diseases (Bao & Suresh, 2003).
Many highly sensitive techniques have been developed over the years to assess mechanical properties of cells (Addae-Mensah & Wikswo, 2008). These include atomic force microscopy (Smith et al., 2005), magnetic twisting cytometry (Wang et al., 1993), micropipette aspiration (Alexopoulos et al., 2003), optical tweezers (Svoboda & Block, 1994), Shear-flow methods (Usami et al., 1993), particle-tracking microrheology (Wirtz, 2009), cantilever beams (Galbraith & Sheetz, 1997), and others. Each technique probes a cell or cells in a different manner and does not necessarily measure the same aspects of a cell as another technique. Thus, the use of more than one technique to study the same object (i.e., cell) may prove useful. This chapter describes the application of two sensitive techniques, the atomic force microscopy and the quartz crystal microbalance with dissipation monitoring, to the study of the mechanical properties of cells in response to exposure to EGF.
Atomic force microscopy (AFM) is one of the most popular choices for probing the mechanical properties of cells, because individual cells can be probed in high sensitivity and resolution with a minimum of force (Radmacher Manfred, 2007). To measure the mechanical properties of the cell with AFM, the top surface of a live cell is indented with the sharp tip located at the end of a cantilever (a probe). The cantilever is mounted on a piezoelectric tube that moves the cantilever down and up in the vertical direction toward and away from the surface of the cell. The deflection in the cantilever is typically measured by a laser that tracks a spot on the tip of the cantilever. From the position of the cantilever and its deflection, force-displacement curves during the indentation of the cell by the probe are generated as shown in Figure 1 (Radmacher M., 1997).
Typical force-displacement curves generated for the approach and retraction of the AFM probe. Approach and retraction correspond to mechanical loading and unloading of the AFM probe on the top surface of the cell.
To indent epithelial cells that are being treated with biologically active molecules, the force applied on the cells by the AFM probe is often kept approximately 50 nN or slightly lower to minimize the adverse effects on the cells caused by the probe. With the force at this level, the probe can have the probing depth that is sufficiently deep (100 to 500 nm) to register the cytoskeleton remodeling (Schillers et al., 2010) but is still shallow enough to avoid influence from the nucleus and the solid substrate on which the cells rest (Melzak et al., 2011). In addition to a low magnitude of loading, the velocity of loading should be kept low enough so that the transient friction interactions between the probe tip and the cell surface are avoided (Alcaraz et al., 2003).
Force-displacement curves acquired with all of these precautions in place then can be used to estimate values for Young’s modulus and energy dissipation of the cell. The Young’s modulus of a cell can be extracted from a curve of unloading force displacement with the aid of the Herzian elastic contact model for a conically shaped tip indenting an elastic body (Touhami et al., 2003):
where
In the estimation of the Young\'s modulus, the cell is assumed to be an elastic body, i.e., to return all of the energy deposited during the loading portion of the indentation process. However, in reality, the cell is not perfectly elastic but exhibits some dissipative behaviour. This dissipative behavior is manifested as a loss (as heat to the surroundings) of some of the energy stored during loading, and can be seen in the indentation process as hysteresis in a cycle of force displacement (Figure 1). In a cell, energy dissipation is believed to be accomplished by internal friction and/or viscous damping mechanisms (Alcaraz et al., 2003; Smith et al., 2005). In AFM, the mechanical energy dissipated per cycle of indentation is given quantitatively by the area of the hysteresis loop enclosed by the approach and retraction curves (Alcaraz et al., 2003), as shown in Figure 1.
In contrast to AFM, the quartz crystal microbalance with dissipation monitoring (QCM-D) has not been widely used in characterization of cell mechanics. The QCM-D is an ultrasensitive piezoelectric device (Hook F. et al., 1998; Rodahl Michael et al., 1996; Rodahl M. & Kasemo, 1996) that is able to detect mass coupled (adsorbed or adhered) to the surface of the sensing element. The sensing element is a single piezoelectric quartz crystal in the form of a thin disc with a metal electrode deposited on its underside. This sensor crystal is set into free vibration in shear mode by means of a pulse of current. In air, the sensor crystal has a characteristic resonant frequency; this frequency is changed when any material, liquid or solid, is coupled or attached to it by adsorption or adhesion.
The mass of an ultrathin and elastic adsorbed layer, which exhibits negligible dissipation, is given by the Sauerbrey equation:
where ∆
The QCM has been used to assess the changes in mass and mechanical properties of a layer of biomolecules immobilized on the surface of the quartz crystal (Dixon, 2008). Specific examples include the use of the QCM to show protein adsorption (Hook F. et al., 1998), to indicate changes in an immobilized layer caused by ligand–receptor interactions (Janshoff et al., 1997; Lee H. et al., 2010), to detect nucleic acid hybridization (Furtado & Thompson, 1998), and to study immunoresponse (Aizawa et al., 2001).
Diagram of a layer of cells attached to the surface of a sensor crystal in the QCM-D technique. The vibrational wave originating from the piezoelectric sensor crystal penetrates the cells from the bottom and diminishes with distance above the sensor surface. This figure is adapted with permission from (
In the field of cell biology, the QCM technique has become particularly attractive for its capability to study cells in a label-free manner (Heitmann et al., 2007; Janshoff et al., 1996; Matsuda et al., 1992; Redepenning et al., 1993; Wegener et al., 1998). Importantly, the technique is non-invasive to mammalian cells when the amplitude of shear oscillation is kept under 1 nm (Heitmann & Wegener, 2007). It has been used for determining the kinetics of cell attachment and spreading (Fredriksson et al., 1998; Nimeri et al., 1998) and for monitoring the long term growth of cells (Otto et al., 1999; Reipa et al., 2006). More recently, the QCM has been applied to characterization of cell viscoelasticity (Alessandrini et al., 2006; Galli Marxer et al., 2003; Li et al., 2008; Marx et al., 2005; Pax et al., 2005; Voinova et al., 2004). When the instrument used has the capability for monitoring the change in dissipation factor as well as frequency, the technique is termed QCM-D (QCM with dissipation monitoring). Because the acoustic signal diminishes exponentially with distance above the surface of the quartz crystal oscillator on which the cells are deposited, the QCM probes primarily the basal area of the cell monolayer (Heitmann et al., 2007; Le Guillou-Buffello et al., 2011). The test configuration is shown in Figure 2. Thus, ∆
An example of the use of the AFM to assess change in the mechanical response of cells upon exposure to a biologically active molecule is illustrated in this section. Figure 3 shows the Young’s moduli, obtained at a probe speed of 5.8 µm/s and indentation depth of ~500 nm, for two hundred randomly selected A431 cells before and after the treatment with a 40-nM EGF solution (Yang et al., 2012). The focus is on the comparison, rather than on the modulus values themselves. The two samples show a statistically significant increase (p < 0.05) in average modulus as a result of the treatment: 11.2 ± 2.8 kPa for untreated cells and 18.7 ± 2.0 kPa for treated cells.
Histograms of the distributions of the Young’s modulus of two hundred randomly selected A431 cells before (A) and after (B) the treatment with a 40-nM EGF solution in buffer (
Figure 4 shows changes in amount of the dissipated energy of cells upon EGF stimulation. It is evident that energy dissipation of a cell, as indicated by the area of the hysteresis loop, increases after the treatment with 40 nM EGF (Figures 4A and 4B). Figure 4C summarize the distribution of such differences exhibited by one hundred randomly selected cells. A statistically significant difference (p < 0.05) in energy dissipation per cycle is shown: 3.09 ± 0.79 fJ before the treatment and 5.10 ± 0.71 fJ after the treatment.
Force-displacement curves for a single cell before (A) and after (B) the treatment with 40 nM EGF. Histograms (C) of energy dissipation for one hundred randomly selected cells before (blue) and after (red) the treatment with EGF.
The underlying meaning of a simultaneous increase in both stiffness (Young’s modulus) and energy dissipation of the cells upon the EGF treatment can be better understood with the soft glassy rheology (SGR) model (Fabry et al., 2001). In this model, the cell is considered as a soft glassy material that is structurally disordered and metastable (Sollich, 1998). The hysteresivity of the cell,
Although both stiffness and dissipation of the cells increase simultaneously, the hysteresivity of the cells, which is determined based on the ratio of area within the hysteresis loop to area under the approach curve (Figure 1) (Collinsworth et al., 2002; Fung, 1984; Smith et al., 2005), is not constant but increases as a function of time, shown in Figure 5. According to the SGR model, upon a non-thermal stimulation (e.g., ATP depletion, or cell relaxing agent), the cell can undergo a change in mechanical ordering state either toward the glass transition as hysteresivity decreases or away from the glass transition as hysteresivity increases. So the cell can modulate its mechanical state between a more solid-like state and a more liquid-like state (Smith et al., 2005). The increase in hysteresivity in Figure 5 implies that the mechanical state of the EGF-treated cells moves away from the glass transition and possibly takes on a more fluidic behavior. This interpretation is consistent with morphological changes observed in A431 cells responding to EGF stimulation, where the cells undergo cell rounding, membrane ruffling, and filopodia extension, all of which might be facilitated by a more fluid-like state of the cells (Chinkers et al., 1981).
Hysteresivity versus time for exposure of A431 cells to EGF at 0, 10 nM, 20 nM, and 40 nM (
An example of the use of QCM-D to assess change in the mechanical response of cells upon exposure to a biologically active molecule is shown in Figure 6. This figure shows ∆
Figure 7 shows the QCM-D measurement of ∆
Typical response of A431 cells to EGF. Both ∆
Real-time QCM-D measurements of the Δ
A likely cause of the reduction in ∆
It is desirable to connect the EGF-induced changes in ∆
The ΔD response of the cells pretreated with EGFR tyrosine kinase inhibitor, PD158780, showing suppression of 10 nM EGF‐induced response. This figure is adapted with permission from (
The change in mechanical properties of cells has been attributed to remodeling of the cytoskeleton (Kuznetsova et al., 2007), which can be induced by EGF treatment (Rijken et al., 1995; Rijken et al., 1998). Direct evidence of remodeling of the cytoskeleton can be obtained with fluorescence imaging. For this, the cells were first treated with a 10-nM EGF solution for 60 min under the same conditions used for both AFM and QCM-D measurements. The actin filaments of the cytoskeleton were then stained with fluorescently labeled phalloidin and imaged with an inverted fluorescence microscope.
Remodeling of actin filaments in a monolayer of A431 cells induced by the treatment of 10 nM of EGF at 37°C. (A) and (B) show fluorescence-stained actin cortex at the top-half of the cell layer before and after the 60-min treatment with EGF solution, respectively (
As shown in Figures 9A and B, the top portion of the membrane skeleton exhibited an increase in brightness after the 60-min EGF treatment, indicating an increase of the number and size of the cortical actin filaments. Considering that the cortical actin provides cells with a structural framework, the increase in cortical actin can be assumed responsible for the increased rigidity of the cells, which is manifested as the increase in Young’s modulus measured by AFM. In addition, because both energy dissipation and hysteresivity were derived from the same set of force-displacement curves used for determining Young’s modulus of the top region of the cells, it is reasonable to assume that the increases in dissipation and hysteresivity were also related to the increase in cortical actin filaments.
The basal area of the cell monolayer, i.e., the area probed by the QCM-D, was also examined by means of fluorescence imaging. In this case, the stress fibers, which are actin filaments that reside in the bottom portion of the cells and are involved in the formation of focal adhesions attaching the cell to the substrate, were studied. As shown in Figures 10A and 10B, the cells displayed a decrease in amount and size of stress fibers after 60 min of exposure to EGF. Thus the decrease in dissipation observed by means of the QCM-D technique can be reasonably assumed to be related to a reduction in number and size of the actin stress fibers in the basal region of the cells. In addition, this reduction in stress fibers suggests a loss of adhesion and/or contact between the cells and the solid substrate, which is consistent with the EGF-induced cell rounding and retracting that has been reported previously (Chinkers et al., 1979; 1981).
Remodeling of actin filaments in a monolayer of A431 cells induced by the treatment of 10 nM of EGF at 37°C. (A) and (B) show fluorescence-stained stress fibers at the basal area of the cell layer before and after the 60-min EGF treatment, respectively.
The two sensitive techniques, AFM and QCM-D, described in this chapter probe two different regions of cells in a monolayer on a solid surface. The nature of the AFM limits it to probing the top surface and immediately underlying volume of individual cells. The nature of the QCM‐D limits it to probing the basal areas of the cells in a monolayer. The AFM technique revealed an increase in Young’s modulus, energy dissipation, and hysteresivity in response to EGF, while the QCM-D technique revealed a decrease in energy dissipation factor. The fluorescence studies of the changes in actin in the top and bottom surfaces upon exposure to EGF corroborate the differences observed in mechanical properties.
Because both hysteresivity (
Interestingly, both hysteresivity (
In this chapter, the examples were provided to illustrate how to employ a combination of AFM and QCM-D to characterize the mechanical behavior of cells in response to exposure to EGF. This unique combination allowed the comparative assessment of the upper volume of the cell bodies as well as the basal areas of the cells. Results from both parts of the A431 cells reveal a regionally specific mechanical behavior of the cells, which can be attributed to the distinct cytoskeleton structures utilized by the cells to alter the local structure in response to EGF stimulation. The signaling pathways that mediate the remodeling of the cytoskeleton in the upper volume of the cell bodies are likely closely coupled to those in the basal areas of the cells. There is a clear correlation between the time-dependent mechanical response and the dynamic process of EGFR signaling.
Overall, the combination of AFM and QCM-D is able to provide a more complete and refined mechanical profile of the cells during the dynamic cell signaling process than either technique alone. The use of combined techniques to track real-time cell signaling based on the measurement of cellular mechanical response in a label-free manner is a powerful approach for investigating the role of EGFR in causing abnormal cell behavior. This combined, real-time approach may have the potential to be applied to the study of other types of receptor-mediated cell signaling and trafficking. This approach should contribute to the fundamental understanding of the correlation between cell function and cell mechanical properties.
This manuscript recapitulates some information from original works that have previously been published in
The total body potassium (K+) in an average 70-kg adult is approximately 3000–4000 mEq (50–55 mEq/kg) [1]. About 98% of this is intracellular, approximating to a concentration of 140 mEq/L and 2% in the extracellular compartment, which amounts to 4–5 mEq/L. Potassium’s two primary physiologic functions are cellular metabolism, protein, glycogen, deoxyribonucleic acid (DNA) synthesis, and resting potential membrane maintenance. Multiple physiologic processes have been identified in humans that maintain potassium homeostasis with a goal of appropriate tissue potassium distribution [1]. They can be classified into two groups based on the mechanisms involved: transcellular shifts and potassium excess excretion.
Transcellular mechanisms maintain the ICF (intracellular fluid): ECF (extracellular fluid) potassium ratio by acting immediately within the first few minutes to hours by regulating Na+/K+-ATPase (sodium/potassium adenosine triphosphatase pump), resulting in transcellular shifts. It is an electrogenic pump transporting sodium and potassium in the ratio of three sodium to two potassium. It is an integral membrane protein and serves as an ion channel, maintaining the electrochemical gradient across the cell membrane. The delayed mechanisms are slower to kick in but play a significant role in the excretion of the excess potassium from the body
Insulin is released after a meal when the plasma glucose concentration increases. Insulin plays a vital role in shifting dietary potassium into the cells and avoiding hyperkalemia before the kidneys can work on the excretion of the extra potassium [2, 3]. Insulin attaches to specific cell surface receptors and inserts GLUT4 (glucose transporter type-4) into the cell membranes, promoting glucose uptake in insulin-responsive tissues such as skeletal muscles, adipocytes, and cardiomyocytes. More than 80% of glucose is transported into the muscle cells. Also seen is an upregulation of the GLUT4-mediated glucose transport in elevated transport needs, like elevated blood glucose during a carbohydrate-rich meal or during increased metabolic demands by skeletal muscles during exercise [4]. The potassium uptake is also increased during this process by increasing the Na+/K+-ATPase activity. Insulin is also noted to have differential glucose and K uptake regulation, as noted in a metabolic syndrome where insulin-mediated glucose is compromised, but the cellular K+ uptake usually occurs [5, 6].
Catecholamines play a critical role in potassium homeostasis. They alter internal K+ distribution by acting through alpha and beta receptors. Beta-2 receptors enhance K+ uptake by cells by activating the Na+/K+-ATPase pump [7, 8]. This effect appears to be present at basal catecholamine levels [9, 10]. The beta-adrenergic activity also increases insulin secretion from the pancreas by directly stimulating and enhancing glycolysis, which increases blood glucose levels. A stress response results in epinephrine release that causes an acute drop in plasma K+ by approximately 0.5– 0.6 mEq/L. Alpha-1,2 adrenoreceptors mediate the initial hyperkalemia by activating hepatic calcium-dependent potassium channels [11, 12]. Beta-3 adrenoreceptor stimulation may also affect the plasma potassium [12].
Exercise causes an elevation in plasma K+ levels. The rise is proportional to the exercise intensity. There is an increase of 0.3–0.4 mEq/L with slow walking, 0.7–1.2 mEq/L with moderate exercise, and a max of 2.0 mEq/L with strenuous exercise leading to exhaustion [13, 14, 15]. These changes are transient for few minutes and then normalize. This transient increase is due to the release of potassium from the exercising cells. Skeletal muscle cells have ATP-dependent K+ channels. A reduction of ATP (adenosine triphosphate) levels causes the opening of more K+ channels. The release of potassium from the muscles causes a local increase in the plasma K+ concentration, which causes vasodilation, and hence increases blood supply to the muscles during exercise. This mechanism is attenuated in physical conditioning as there is an increase in both cellular K+ concentration and Na+/K+-ATPase activity.
The changes in acid-base balance cause alterations in K+ levels to maintain electroneutrality. Hence, such changes are seen only in the setting of acidosis caused by inorganic acids. Electroneutrality is maintained by the movement of K+ and Na+into the ECF, as chloride (Clˉ) movement into the cells is limited. The wide range of variation is possibly due to other mechanisms involved in regulating potassium homeostasis [16, 17].
Skeletal muscles also have another mechanism to regulate intracellular pH
Intracellular pH regulation by skeletal muscles.
The changes in plasma tonicity also affect K+ levels. Effective osmoles such as glucose, mannitol, and sucrose accumulate in the ECF, resulting in an osmotic gradient that causes water movement from ICF to ECF [18, 19, 20]. This effect leads to a decrease in cell volume. During this process, the intracellular K+ concentration increases and causes K+ efflux through K+ permeable channels. This process is shown in Figure 2.
Plasma tonicity.
The plasma K+ concentration itself influences the movement of K+ in and out of cells
Clinical conditions such as severe trauma, tumor lysis syndrome, acute tissue ischemia, and necrosis cause cell breakdown and release of intracellular K+ into the ECF [23]. The hyperkalemia severity is dependent on the ability of other cells to uptake the excess K+ and the capability of the kidney to excrete K+ quickly. Conversely, there is a movement of ECF K+ to ICF due to increased cellular metabolic needs like protein synthesis in situations with increase in rapid cell production. This effect is observed in cases of severe vitamin B 12 or folic acid deficiency. When such individuals are supplemented with vitamin B 12 or folic acid, there is a marked increase in erythropoiesis and red blood cell production, causing hypokalemia; hence, it is recommended to monitor labs and supplement potassium to replete the levels [24].
The kidney plays a significant role in the excretion of excess K+ and maintaining K+ balance. The colon plays a trivial role as small amounts of K+ are lost through feces each day. Insignificant amounts are lost through sweat.
Around 80% of the filtered potassium is reabsorbed in the proximal collecting tubule. The movement of K+ in the PCT is mainly passive, following sodium and water. The Na+ absorption increases across the PCT, by both active and passive processes causing net fluid reabsorption. This process drives K+ reabsorption by solvent drag. There is also a minimal change in the transepithelial voltage from negative to slightly positive as we move down the PCT, favoring K+ and calcium (Ca2+) reabsorption
Potassium handling in proximal convoluted tubule (PCT).
K+ reabsorption in this segment occurs by both paracellular and transcellular pathways. Na+/K+/Clˉ co-transporter plays a significant role in the reabsorption of potassium here. This channel is located on the luminal side. The low-intracellular Na+ concentration and high-transcellular Na+ gradient caused by the Na+/K+-ATPase pump activity in the basolateral membrane help activate the Na+/K+/Clˉ co-transporter. This electroneutral process causes an increase in intracellular K+, which then exits
Potassium handling in the thick ascending limb.
Less than 10% of the filtered load reaches the distal tubule. The lumen’s K+ concentration increases down the lumen and is secondary to voltage-dependent K+secretion mediated by the ROMK channel. Late DCT has an epithelial Na+ channel (ENaC) responsible for sodium absorption and creating lumen-negative electrochemical gradient and hence effusion of K+, similar to collecting duct [26].
Most potassium secretion is in the connecting segment and the collecting tubules (cortical, outer medullary, and inner medullary). This secretion varies according to the physiologic needs of the body. The connecting tubule has two major types of cells: principal cells and intercalated cells.
In principal cells, K+ movement out of the cell into the tubular lumen is due to the electrochemical gradient generated by the entry of Na+ into the cell
Potassium handling in principal cells.
Intercalated cells are of two types, type A and type B, involved in acid-base regulation. In type A cells, the H+/K+ATPase pump on the luminal side results in H+ secretion and K+ reabsorption (Figure 6) [28, 29]. The activity of this pump is increased in K+-depleted states and decreased in the setting of elevated K+ levels.
Potassium handling in intercalated type A cell.
In intercalated type B cells, the reabsorption of K+ is along with the Clˉ reabsorption
Potassium handling in intercalated type B cell.
Aldosterone is a steroid hormone. It enters the cell, binds with a cytosolic receptor and moves to the nucleus, increasing the synthesis of sgk mRNA (serum and glucocorticoid regulated kinase messenger ribonucleic acid), resulting in sgk protein [32]. sgk protein stimulates the ENaC activity sevenfold. An increase in serum K+ levels as little as 0.1–0.2 mEq/L can stimulate significant aldosterone levels [33].
Increasing the distal tubular flow rate potentiates K+ secretion in those segments. This effect is more prominent when a person consumes a high K+ diet as it also causes a simultaneous increase in aldosterone levels. Under normal circumstances, the fluid entering the distal tubule has a K+ concentration of <1 mEq/L as most of the K+ filtered is absorbed in the earlier parts of the nephron. Due to water reabsorption in the presence of ADH (antidiuretic hormone) and K+ secretion in the tube’s distal parts, the tubular K+ concentration increases. However, if the distal flow rate is increased, enhanced flow washes the secreted K+, keeping the K+ concentration in the tubular fluid relatively lower and creating a favorable concentration gradient for K+ secretion [34, 35].
Increased distal flow also increases Na+ delivery to the distal nephron, increasing Na entry into the cells and potentiating the changes to create a favorable electrochemical gradient for K+ secretion.
Two major K+ channels are found in the thick ascending loop, DCT, cortical, and medullary collecting ducts. ROMK, also known as the constitutive K+ secretary channel, is responsible for K+ secretion during normal tubular flow. Cortical ROMK expression is upregulated by aldosterone and increased plasma K+ concentration, whereas medullary ROMK expression is regulated by plasma K+ levels and not aldosterone.
BK channels are also known as big/large conductance channels or “Maxi K” channels opened by the high tubular flow, hence were earlier called a flow-dependent channel. High tubular flow increases Na+ delivery and eventually causes apical membrane depolarization. The depolarization leads to increased intracellular calcium levels and activates the BK channels. Recent studies suggest that K+ secretion by ROMK is also increased in the setting of increased tubular flow. Both these channels work together in maintaining potassium homeostasis and preventing hyperkalemia. Either of them is upregulated in the absence of the other channel [36].
The primary absorption site for K+ from the diet is the small intestine. The colon plays a minimal role in the absorption and secretion of potassium. Potassium secretion occurs primarily by passive mechanisms; however, in the rectum and sigmoid colon, K+ is secreted by an active process [37].
Animal studies have shown that active K+ secretion is mediated by apical K+ channels, which coordinate with the basolateral Na+/K+/Clˉ co-transporter. Intermediate conductance K+ (IK) and large-conductance K+ (BK) channels are present on the apical membrane of colonic epithelia. In patients with chronic renal insufficiency, especially when creatinine clearance is less than 10 mL/min, the net colonic K+ secretion increases compared to normal renal function due to increased expression and activity of the BK channel. Colonic K+ losses increase in the setting of diarrhea and by using cation resins (e.g., sodium polystyrene sulfonate) [38].
Feedforward control means the response to a specific signal is preset and happens irrespective of the changes in the environment, unlike in the feedback mechanism where the changes in the environment control the pathway.
An increase in dietary intake of K+ causes an increase in renal excretion of potassium, even though the K+ concentration is not sufficient to cause any changes in plasma K+ concentration or stimulate aldosterone. It was also noted that this mechanism acts independently and is not altered by changing the tubular flow rate of urine, urine pH, renal Na+ excretion, or an aldosterone antagonist. `The feedforward mechanism is wholly dissociated from the common pathways involved in the feedback mechanisms of K+ homeostasis [39].
The precise mechanism is still unknown, but it is hypothesized that there is a possible gastrointestinal-renal signaling pathway in humans responsible for this feedforward control of K+ homeostasis [40].
Potassium excretion is also influenced by the circadian rhythm irrespective of the activity levels or posture. The peak potassium excretion is observed in the middle of the day [41]. The presence of an oscillator system in DCT, connecting tubule and cortical collecting duct renal tubular cells, is responsible for the circadian variation of potassium excretion. The pathway includes cellular receptors for central nervous system signals, intracellular messenger’s, effectors, and renal tubule membrane transporters [42]. The oscillations result in circadian gene and transcriptional factor expression changes modifying the expression of vasopressin V2 receptor and multiple transcellular channels (Aquaporin 2, Aquaporin 4, alpha ENaC, and ROMK-1), maintaining the plasma sodium and potassium concentration [39, 43]. ROMK gene expression is higher during physical activity, whereas the H+/K+ATPase gene expression is higher at rest [44]. Circadian rhythm causes a variation in aldosterone secretion, thus impacting renal potassium excretion [45].
To maintain a normal potassium concentration, both the feedback and feedforward mechanisms work in tandem. After a potassium-rich diet, the increased plasma potassium levels gain entry into the cells
No external funding was received in preparation of this manuscript.
We declare no conflict of interest.
We thank the editor for allowing us to author this manuscript.
potassium milliequivalent/liter kilogram deoxyribonucleic acid intracellular fluid extracellular fluid sodium/potassium-adenosine triphosphatase sodium glucose transporter type 4 adenosine triphosphate chloride a scale used to specify the acidity or basicity of an aqueous solution hydrogen monocarbohydrate transporter sodium hydrogen exchanger 1 proximal convoluted tubule calcium sodium/potassium/chloride co-transporter renal outer medullary potassium channel distal-convoluted tubule epithelial sodium channel hydrogen/potassium adenosine triphosphatase chloride/bicarbonate exchanger messenger ribonucleic acid antidiuretic hormone big/large conductance channels intermediate conductance
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",metaTitle:"What Does It Cost?",metaDescription:"Open Access publishing helps remove barriers and allows everyone to access valuable information, but article and book processing charges also exclude talented authors and editors who can’t afford to pay. The goal of our Women in Science program is to charge zero APCs, so none of our authors or editors have to pay for publication.",metaKeywords:null,canonicalURL:null,contentRaw:'[{"type":"htmlEditorComponent","content":"We are currently in the process of collecting sponsorship. If you have any ideas or would like to help sponsor this ambitious program, we’d love to hear from you. Contact us at info@intechopen.com.
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\n\n\r\n\tPollution is caused by a wide variety of human activities and occurs in diverse forms, for example biological, chemical, et cetera. In recent years, significant efforts have been made to ensure that the environment is clean, that rigorous rules are implemented, and old laws are updated to reduce the risks towards humans and ecosystems. However, rapid industrialization and the need for more cultivable sources or habitable lands, for an increasing population, as well as fewer alternatives for waste disposal, make the pollution control tasks more challenging. Therefore, this topic will focus on assessing and managing environmental pollution. It will cover various subjects, including risk assessment due to the pollution of ecosystems, transport and fate of pollutants, restoration or remediation of polluted matrices, and efforts towards sustainable solutions to minimize environmental pollution.
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Dr. Rahman was also adjunctly attached with Kanazawa University, Japan (Visiting Research Professor, Dec 2014 to Mar 2015; JSPS Postdoctoral Research Fellow, Apr 2012 to Mar 2014), and Tokyo Institute of Technology, Japan (TokyoTech-UNESCO Research Fellow, Oct 2004–Sep 2005). \nHe received his Ph.D. degree in Environmental Analytical Chemistry from Kanazawa University, Japan (2011). He also achieved a Diploma in Environment from the Tokyo Institute of Technology, Japan (2005). Besides, he has an M.Sc. degree in Applied Chemistry and a B.Sc. degree in Chemistry, all from the University of Chittagong, Bangladesh. \nDr. Rahman’s research interest includes the study of the fate and behavior of environmental pollutants in the biosphere; design of low energy and low burden environmental improvement (remediation) technology; implementation of sustainable waste management practices for treatment, handling, reuse, and ultimate residual disposition of solid wastes; nature and type of interactions in organic liquid mixtures for process engineering design applications.",institutionString:null,institution:{name:"Fukushima University",institutionURL:null,country:{name:"Japan"}}},editorTwo:{id:"201020",title:"Dr.",name:"Zinnat Ara",middleName:null,surname:"Begum",slug:"zinnat-ara-begum",fullName:"Zinnat Ara Begum",profilePictureURL:"https://mts.intechopen.com/storage/users/201020/images/system/201020.jpeg",biography:"Zinnat A. 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