Electrophysiological and molecular characterization of the inward rectifier in juxtaglomerular cells from rat kidney

Role of renal vascular potassium channels in physiology and pathophysiology

The control of renal vascular tone is important for the regulation of salt and water balance, blood pressure and the protection against damaging elevated glomerular pressure. The K⁺ conductance is a major factor in the regulation of the membrane potential (V_m ) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by V_m via its effect on the opening probability of voltage-operated Ca²⁺ channels (VOCC) in VSMC. When K⁺ conductance increases V_m becomes more negative and vasodilation follows, while deactivation of K⁺ channels leads to depolarization and vasoconstriction. K⁺ channels in EC indirectly participate in the control of vascular tone by endothelium-derived vasodilation. Therefore, by regulating the tone of renal resistance vessels, K⁺ channels have a potential role in the control of fluid homoeostasis and blood pressure as well as in the protection of the renal parenchyma. The main classes of K⁺ channels (calcium activated (K_Ca ), inward rectifier (K_ir ), voltage activated (K_v ) and ATP sensitive (K_ATP )) have been found in the renal vessels. In this review, we summarize results available in the literature and our own studies in the field. We compare the ambiguous in vitro and in vivo results. We discuss the role of single types of K⁺ channels and the integrated function of several classes. We also deal with the possible role of renal vascular K⁺ channels in the pathophysiology of hypertension, diabetes mellitus and sepsis.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

Clinical features and long exercise test in Chinese patients with Andersen-Tawil syndrome

INTRODUCTION

Andersen-Tawil syndrome (ATS) is a rare multisystem channelopathy characterized by periodic paralysis, ventricular arrhythmias, and developmental dysmorphology. There are few reports concerning ATS in the Chinese population. We analyzed clinical features and evaluated the long exercise test as a tool for diagnosis of periodic paralysis in ATS.

METHODS

Direct sequencing of KCNJ2 was performed in 12 subjects from mainland China with suspected ATS. Clinical features, therapeutic responses, and long exercise tests (LET) were retrospectively analyzed.

RESULTS

Twelve patients were genetically confirmed to have ATS. A small mandible and clinodactyly were demonstrated in all patients. Premature ventricular contractions were the most prevalent form of cardiac arrhythmia. The LET revealed an early amplitude decrement.

CONCLUSIONS

Chinese ATS patients shared some common clinical features with reported subjects in other countries. An early amplitude decrement in LET may be useful for diagnosis of ATS. Muscle Nerve 54: 1059-1063, 2016.

The degradation of the inwardly rectifying potassium channel, Kir2.1, depends on the expression level: examination with fluorescent proteins

The expression of ion channels is regulated by their synthesis as well as degradation, and some ion channels are degraded in an expression level-dependent way. Recently, new techniques of fluorescent proteins have been developed and seem to be useful to study protein degradation. To examine the regulation of the degradation of strongly inwardly rectifying potassium channel (Kir2.1) and the usefulness of the fluorescent proteins, we constructed Kir2.1 fusion proteins with SNAP tag and fluorescent timer (FT). The SNAP tag, which covalently binds to a specific membrane-permeable fluorescent dye, enables a pulse-chase experiment with fluorescence. When the SNAP-Kir2.1 proteins were expressed in 293T cells by low and high expression plasmids, the half-life of the fusion protein expressed by a high-expression plasmid was shorter (18.2±1.9 h) than that expressed by a low-expression plasmid (35.1+2.3h). The addition of Ba(2+), a selective blocker of Kir2.1, slowed the degradation, suggesting a current-dependency of degradation. Consistently, patch-clamp recording showed that cultivation in the presence of Ba(2+) increased the whole cell conductance of SNAP-Kir2.1. Since the fluorescence of FT changes gradually changes from green to red, the green/red ratio should allow us to monitor the changes in the degradation rate of FT-Kir2.1. Using this method, we confirmed the slower degradation by Ba(2+). The results suggest a homeostatic regulation of the degradation of Kir2.1 in the 293T cells, and the usefulness of fluorescence-based methods for examining the degradation of ion channels.

Role of vascular potassium channels in the regulation of renal hemodynamics

K+ conductance is a major determinant of membrane potential ( Vm) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by Vm through the action of voltage-operated Ca2+ channels (VOCC) in VSMC. Increased K+ conductance leads to hyperpolarization and vasodilation, while inactivation of K+ channels causes depolarization and vasoconstriction. K+ channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K+ channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K+ channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K+ channels [calcium activated (KCa), inward rectifier (Kir), voltage activated (KV), and ATP sensitive (KATP)] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K+ channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K+ channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K+ channel function, the role of renal vascular K+ channels in the control of salt and water excretion deserves attention.

Mechanisms of K(+) induced renal vasodilation in normo- and hypertensive rats in vivo

AIM

We investigated the mechanisms behind K(+) -induced renal vasodilation in vivo in normotensive Sprague-Dawley (SD) rats and spontaneously hypertensive rats (SHR).

METHODS

Renal blood flow (RBF) was measured utilizing an ultrasonic Doppler flow probe. Renal vascular resistance (RVR) was calculated as the ratio of mean arterial pressure (MAP) and RBF (RVR = MAP/RBF). Test drugs were introduced directly into the renal artery. Inward rectifier K(+) (K(ir) ) channels and Na(+) ,K(+) -ATPase were blocked by Ba(2+) and ouabain (estimated plasma concentrations ∼20 and ∼7 μm) respectively.

RESULTS

Confocal immunofluorescence microscopy demonstrated K(ir) 2.1 channels in pre-glomerular vessels of SD and SHR. Ba(2+) caused a transient (6-13%) increase in baseline RVR in both SD and SHR. Ouabain had a similar effect. Elevated renal plasma [K(+) ] (∼12 mm) caused a small and sustained decrease (5-13%) in RVR in both strains. This decrease was significantly larger in SHR than in SD. The K(+) -induced vasodilation was attenuated by Ba(2+) in control SD and SHR and by ouabain in SD. Nitric oxide (NO) blockade using l-NAME treatment increased MAP and decreased RBF in both rat strains, but did not affect the K(+) -induced renal vasodilation.

CONCLUSION

K(+) -induced renal vasodilation is larger in SHR, mediated by K(ir) channels in SD and SHR, and in addition, by Na(+) ,K(+) -ATPase in SD. In addition, NO is not essential for K(+) -induced renal vasodilation.

The mammalian K(IR)2.x inward rectifier ion channel family: expression pattern and pathophysiology

Inward rectifier currents based on K(IR)2.x subunits are regarded as essential components for establishing a stable and negative resting membrane potential in many excitable cell types. Pharmacological inhibition, null mutation in mice and dominant positive and negative mutations in patients reveal some of the important functions of these channels in their native tissues. Here we review the complex mammalian expression pattern of K(IR)2.x subunits and relate these to the outcomes of functional inhibition of the resultant channels. Correlations between expression and function in muscle and bone tissue are observed, while we recognize a discrepancy between neuronal expression and function.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Inwardly rectifying potassium channels: their structure, function, and physiological roles

Inwardly rectifying K(+) (Kir) channels allow K(+) to move more easily into rather than out of the cell. They have diverse physiological functions depending on their type and their location. There are seven Kir channel subfamilies that can be classified into four functional groups: classical Kir channels (Kir2.x) are constitutively active, G protein-gated Kir channels (Kir3.x) are regulated by G protein-coupled receptors, ATP-sensitive K(+) channels (Kir6.x) are tightly linked to cellular metabolism, and K(+) transport channels (Kir1.x, Kir4.x, Kir5.x, and Kir7.x). Inward rectification results from pore block by intracellular substances such as Mg(2+) and polyamines. Kir channel activity can be modulated by ions, phospholipids, and binding proteins. The basic building block of a Kir channel is made up of two transmembrane helices with cytoplasmic NH(2) and COOH termini and an extracellular loop which folds back to form the pore-lining ion selectivity filter. In vivo, functional Kir channels are composed of four such subunits which are either homo- or heterotetramers. Gene targeting and genetic analysis have linked Kir channel dysfunction to diverse pathologies. The crystal structure of different Kir channels is opening the way to understanding the structure-function relationships of this simple but diverse ion channel family.

Regulation of renin release by local and systemic factors

The renin-angiotensin system (RAS) is critically involved in the regulation of the salt and volume status of the body and blood pressure. The activity of the RAS is controlled by the protease renin, which is released from the renal juxtaglomerular epithelioid cells into the circulation. Renin release is regulated in negative feedback-loops by blood pressure, salt intake, and angiotensin II. Moreover, sympathetic nerves and renal autacoids such as prostaglandins and nitric oxide stimulate renin secretion. Despite numerous studies there remained substantial gaps in the understanding of the control of renin release at the organ or cellular level. Some of these gaps have been closed in the last years by means of gene-targeted mice and advanced imaging and electrophysiological methods. In our review, we discuss these recent advances together with the relevant previous literature on the regulation of renin release.

Effect of adenosine on membrane potential and Ca2+ in juxtaglomerular cells. Comparison with angiotensin II

BACKGROUND

Renin is mainly secreted from the juxtaglomerular cells (JGC) in the kidney situated in the afferent arteriole close to the vessel pole. Angiotensin II (ANG II) and adenosine inhibit renin secretion and synergistically constrict the afferent arteriole. ANG II depolarises JGC and increases the cytoplasmic free Ca2+ concentration [Ca2+]i. The responses of JGC to adenosine are less known.

METHODS

Effects of adenosine on membrane potential and [Ca2+]i were studied in afferent arterioles from NaCl-depleted rats and mice.

RESULT

Stimulation of A1 adenosine receptors (A1AR) by adenosine (10 microM) or cyclohexyladenosine (1 microM) increased the spiking frequency of JGC, slightly depolarised the cells and, in < or =50% of the cases, increased [Ca2+]i. These effects were much smaller than those of ANG II (3 nM). Simultaneous application of cyclohexyladenosine and ANG II gave only additive effects on [Ca2+]i; in addition, responses to ANG II in JGC from A1AR knockout mice were similar to those from control mice.

CONCLUSION

The small changes in membrane potential and [Ca2+]i in response to A1AR stimulation as compared to those of ANG II may suggest that these 2 tissue hormones use different signal transduction mechanisms to affect JGC function, including the inhibition of renin release.

Inward rectifier K(+) currents and Kir2.1 expression in renal afferent and efferent arterioles

The afferent and efferent arterioles regulate the inflow and outflow resistance of the glomerulus, acting in concert to control the glomerular capillary pressure and glomerular filtration rate. The myocytes of these two vessels are remarkably different, especially regarding electromechanical coupling. This study investigated the expression and function of inward rectifier K(+) channels in these two vessels using perfused hydronephrotic rat kidneys and arterioles and myocytes isolated from normal rat kidneys. In afferent arterioles pre-constricted with angiotensin II, elevating [K(+)](0) from 5 to 15 mmol/L induced hyperpolarization (-27 +/- 2 to 41 +/- 3 mV) and vasodilation (6.6 +/- 0.9 to 13.1 +/- 0.6 microm). This manipulation also attenuated angiotensin II-induced Ca(2+) signaling, an effect blocked by 100 micromol/LBa(2+). By contrast, elevating [K(+)](o) did not alter angiotensin II-induced Ca2(+) signaling or vasoconstriction in efferent arterioles, even though a significant hyperpolarization was observed (from -30 +/- 1 to 37 +/- 3 mV, P = 0.003). Both vessels expressed mRNA for Kir2.1 and exhibited anti-Kir2.1 antibody labeling.Patch-clamp measurements revealed prominent inwardly rectifying and Ba(2+)-sensitive currents in afferent and efferent arteriolar myocytes. Our findings indicate that both arterioles express an inward rectifier K(+) current, but that modulation of this current alters responsiveness of only the a different arteriole. The expression of Kir in the efferent arteriole, a resistance vessel whose tone is not affected by membrane potential, is intriguing and may suggest a novel function of this channel in the renal microcirculation.

Renin Release

The aspartyl-protease renin is the key regulator of the renin-angiotensin-aldosterone system, which is critically involved in salt, volume, and blood pressure homeostasis of the body. Renin is mainly produced and released into circulation by the so-called juxtaglomerular epithelioid cells, located in the walls of renal afferent arterioles at the entrance of the glomerular capillary network. It has been known for a long time that renin synthesis and secretion are stimulated by the sympathetic nerves and the prostaglandins and are inhibited in negative feedback loops by angiotensin II, high blood pressure, salt, and volume overload. In contrast, the events controlling the function of renin-secreting cells at the organ and cellular level are markedly less clear and remain mysterious in certain aspects. The unravelling of these mysteries has led to new and interesting insights into the process of renin release.

Cloning and expression of cardiac Kir2.1 and Kir2.2 channels in thermally acclimated rainbow trout

Potassium currents are plastic entities that modify electrical activity of the heart in various physiological conditions including chronic thermal stress. We examined the molecular basis of the inward rectifier K+ current (IK1) in rainbow trout acclimated to cold (4 degrees C, CA) and warm (18 degrees C, WA) temperature. Inward rectifier K+ channel (Kir)2.1 and Kir2.2 transcripts were expressed in atrium and ventricle of the trout heart, K(ir)2.1 being the major component in both cardiac chambers. The relative expression of Kir2.2 was, however, higher (P < 0.05) in atrium than ventricle. The density of ventricular IK1 was approximately 25% larger (P < 0.05) in WA than CA trout. Furthermore, the IK1 of the WA trout was 10 times more sensitive to Ba2+ (IC50 0.18 +/- 0.42 microM) than the IK1 of the CA trout (1.17 +/- 0.44 microM) (P < 0.05), and opening kinetics of single Kir2 channels was slower in WA than CA trout (P < 0.05). When expressed in COS-1 cells, the homomeric Kir2.2 channels demonstrated higher Ba2+ sensitivity (2.88 +/- 0.42 microM) than Kir2.1 channels (24.99 +/- 7.40 microM) (P < 0.05). In light of the different Ba2+ sensitivities of rainbow trout (om)Kir2.1 and omKir2.2 channels, it is concluded that warm acclimation increases either number or activity of the omK(ir)2.2 channels in trout ventricular myocytes. The functional changes in I(K1) are independent of omKir2 transcript levels, which remained unaltered by thermal acclimation. Collectively, these findings suggest that thermal acclimation modifies functional properties and subunit composition of the trout Kir2 channels, which may be needed for regulation of cardiac excitability at variable temperatures.

Vasa recta pericytes express a strong inward rectifier K+ conductance

Strong inward rectifier potassium channels are expressed by some vascular smooth muscle cells and facilitate K+-induced hyperpolarization. Using whole cell patch clamp of isolated descending vasa recta (DVR), we tested whether strong inward rectifier K+ currents are present in smooth muscle and pericytes. Increasing extracellular K+ from 5 to 50 and 140 mmol/l induced inward rectifying currents. Those currents were Ba2+ sensitive and reversed at the K+ equilibrium potential imposed by the electrode and extracellular buffers. Ba2+ binding constants in symmetrical K+ varied between 0.24 and 24 micromol/l at -150 and -20 mV, respectively. Ba2+ blockade was time and voltage dependent. Extracellular Cs+ also blocked the inward currents with binding constants between 268 and 4,938 micromol/l at -150 and -50 mV, respectively. Ba2+ (30 micromol/l) and ouabain (1 mmol/l) depolarized pericytes by an average of 11 and 24 mV, respectively. Elevation of extracellular K+ from 5 to 10 mmol/l hyperpolarized pericytes by 6 mV. That hyperpolarization was reversed by Ba2+ (30 micromol/l). We conclude that strong inward rectifier K+ channels and Na+-K+-ATPase contribute to resting potential and that KIR channels can mediate K+-induced hyperpolarization of DVR pericytes.