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The renal epithelium is sensitive to changes in blood potassium (K+). We identify the basolateral K+ channel, Kir4.2, as a mediator of the proximal tubule response to K+ deficiency. Mice lacking Kir4.2 have a compensated baseline phenotype whereby they increase their distal transport burden to maintain homeostasis. Upon dietary K+ depletion, knockout animals decompensate as evidenced by increased urinary K+ excretion and development of a proximal renal tubular acidosis. Potassium wasting is not proximal in origin but is caused by higher ENaC activity and depends upon increased distal sodium delivery. Three-dimensional imaging reveals Kir4.2 knockouts fail to undergo proximal tubule expansion, while the distal convoluted tubule response is exaggerated. AKT signaling mediates the dietary K+ response, which is blunted in Kir4.2 knockouts. Lastly, we demonstrate in isolated tubules that AKT phosphorylation in response to low K+ depends upon mTORC2 activation by secondary changes in Cl- transport. Data support a proximal role for cell Cl- which, as it does along the distal nephron, responds to K+ changes to activate kinase signaling. The renal epithelium is sensitive to changes in blood K+. Here, Zhang et al. identify low K+ as a potent activator of proximal tubule mTOR/AKT signaling, which occurs through the K+ channel, Kir4.2 to modulate epithelial cell growth and Na+ transport. [ABSTRACT FROM AUTHOR]

High K+ supplementation is correlated with a lower risk of the composite of death, major cardiovascular events, and ameliorated blood pressure, but the exact mechanisms have not been established. Inwardly rectifying K+ (Kir) channels expressed in the basolateral membrane of the distal nephron play an essential role in maintaining electrolyte homeostasis. Mutations in this channel family have been shown to result in strong disturbances in electrolyte homeostasis, among other symptoms. Kir7.1 is a member of the ATP-regulated subfamily of Kir channels. However, its role in renal ion transport and its effect on blood pressure have yet to be established. Our results indicate the localization of Kir7.1 to the basolateral membrane of aldosterone-sensitive distal nephron cells. To examine the physiological implications of Kir7.1, we generated a knockout of Kir7.1 (Kcnj13) in Dahl salt-sensitive (SS) rats and deployed chronic infusion of a specific Kir7.1 inhibitor, ML418, in the wild-type Dahl SS strain. Knockout of Kcnj13 (Kcnj13-/-) resulted in embryonic lethality. Heterozygous Kcnj13þ/- rats revealed an increase in K+ excretion on a normal-salt diet but did not exhibit a difference in blood pressure development or plasma electrolytes after 3 wk of a high-salt diet. Wild-type Dahl SS rats exhibited increased renal Kir7.1 expression when dietary K+ was increased. K+ supplementation also demonstrated that Kcnj13þ/- rats excreted more K+ on normal salt. The development of hypertension was not different when rats were challenged with high salt for 3 wk, although Kcnj13þ/- rats excrete less Na+. Interestingly, chronic infusion of ML418 significantly increased Na+ and Cl- excretion after 14 days of high salt but did not alter salt-induced hypertension development. Here, we found that reduction of Kir7.1 function, either through genetic ablation or pharmacological inhibition, can influence renal electrolyte excretion but not to a sufficient degree to impact the development of SS hypertension. [ABSTRACT FROM AUTHOR]

The regulated release of anorexigenic α-melanocyte stimulating hormone (α-MSH) and orexigenic Agouti-related protein (AgRP) from discrete hypothalamic arcuate neurons onto common target sites in the central nervous system has a fundamental role in the regulation of energy homeostasis. Both peptides bind with high affinity to the melanocortin-4 receptor (MC4R); existing data show that α-MSH is an agonist that couples the receptor to the [Gα.sub.s] signalling pathway (1), while AgRP binds competitively to block α-MSH binding (2) and blocks the constitutive activity mediated by the ligand-mimetic amino-terminal domain of the receptor (3). Here we show that, in mice, regulation of firing activity of neurons from the paraventricular nucleus of the hypothalamus (PVN) by α-MSH and AgRP can be mediated independently of [Gα.sub.s] signalling by ligand-induced coupling of MC4R to closure of inwardly rectifying potassium channel, Kir7.1. Furthermore, AgRP is a biased agonist that hyperpolarizes neurons by binding to MC4R and opening Kir7.1, independently of its inhibition of α-MSH binding. Consequently, Kir7.1 signalling appears to be central to melanocortin-mediated regulation of energy homeostasis within the PVN. Coupling of MC4R to Kir7.1 may explain unusual aspects of the control of energy homeostasis by melanocortin signalling, including the gene dosage effect of MC4R (4) and the sustained effects of AgRP on food intake (5)., To better understand the diametrically opposed regulation of food intake by α-MSH and AgRP, we sought to identify mechanism(s) by which these peptides control firing activity of MC4R neurons in [...]

Background and Purpose: Inwardly rectifying K+ (Kir) channels located on the basolateral membrane of epithelial cells of the distal nephron play a crucial role in K+ handling and BP control, making these channels an attractive target for the treatment of hypertension. The purpose of the present study was to determine how the inhibition of basolateral Kir4.1/Kir5.1 heteromeric K+ channel affects epithelial sodium channel (ENaC)‐mediated Na+ transport in the principal cells of cortical collecting duct (CCD). Experimental Approach The effect of fluoxetine, amitriptyline and recently developed Kir inhibitor, VU0134992, on the activity of Kir4.1, Kir4.1/Kir5.1 and ENaC were tested using electrophysiological approaches in CHO cells transfected with respective channel subunits, cultured polarized epithelial mCCDcl1 cells and freshly isolated rat and human CCD tubules. To test the effect of pharmacological Kir4.1/Kir5.1 inhibition on electrolyte homeostasis in vivo and corresponding changes in distal tubule transport, Dahl salt‐sensitive rats were injected with amitriptyline (15 mg·kg−1·day−1) for 3 days. Key Results: We found that inhibition of Kir4.1/Kir5.1, but not the Kir4.1 channel, depolarizes the cell membrane, induces the elevation of intracellular Ca2+ concentration and suppresses ENaC activity. Furthermore, we demonstrate that amitriptyline administration leads to a significant drop in plasma K+ level, triggering sodium excretion and diuresis. Conclusion and Implications: The present data uncover a specific role of the Kir4.1/Kir5.1 channel in the modulation of ENaC activity and emphasize the potential for using Kir4.1/Kir5.1 inhibitors to regulate electrolyte homeostasis and BP. [ABSTRACT FROM AUTHOR]

We created a single-compartment computer model of a C[O.sub.2] chemosensory neuron using differential equations adapted from the Hodgkin-Huxley model and measurements of currents in C[O.sub.2] chemosensory neurons from Helix aspersa. We incorporated into the model two inward currents, a sodium current and a calcium current, three outward potassium currents, an A-type current ([I.sub.KA]), a delayed rectifier current ([I.sub.KDR]), a calcium-activated potassium current ([I.sub.KCa]), and a proton conductance found in invertebrate cells. All of the potassium channels were inhibited by reduced pH. We also included the pH regulatory process to mimic the effect of the sodium-hydrogen exchanger (NHE) described in these cells during hypercapnic stimulation. The model displayed chemosensory behavior (increased spike frequency during acid stimulation), and all three potassium channels participated in the chemosensory response and shaped the temporal characteristics of the response to acid stimulation, pH-dependent inhibition of [I.sub.KA] initiated the response to C[O.sub.2], but hypercapnic inhibition of [I.sub.KDR] and [I.sub.KCa] affected the duration of the excitatory response to hypercapnia. The presence or absence of NHE activity altered the chemosensory response over time and demonstrated the inadvisability of effective intracellular pH ([pH.sub.i]) regulation in cells designed to act as chemostats for acid-base regulation. The results of the model indicate that multiple channels contribute to C[O.sub.2] chemosensitivity, but the primary sensor is probably [I.sub.KA]. [pH.sub.i] may be a sufficient chemosensory stimulus, but it may not be a necessary stimulus: either [pH.sub.i] or extracellular pH can be an effective stimuli if chemosensory neurons express appropriate pH-sensitive channels. The lack of [pH.sub.i] regulation is a key feature determining the neuronal activity of chemosensory cells over time, and the balanced lack of [pH.sub.i] regulation during hypercapnia probably depends on intracellular activation of phi regulation but extracellular inhibition of [pH.sub.i] regulation. These general principles are applicable to all C[O.sub.2] chemosensory cells in vertebrate and invertebrate neurons. hypercapnia; potassium channels; computer modeling; central chemoreceptors

Elevated levels of carbon dioxide increase lung ventilation in Helix aspersa. The hypercapnic response originates from a discrete respiratory chemosensory region in the dorsal subesophageal ganglia that contains C[O.sub.2]-sensitive neurons. We tested the hypothesis that pH-dependent inhibition of potassium channels in neurons in this region mediated the chemosensory response to C[O.sub.2]. Cells isolated from the dorsal subesophageal ganglia retained C[O.sub.2] chemosensitivity and exhibited membrane depolarization and/or an increase in input resistance during an acid challenge. Isolated somata expressed two voltage-dependent potassium channels, an A-type and a delayed-rectifier-type channel ([I.sub.KA] and [I.sub.KDR]). Both conductances were inhibited during hypercapnia. The pattern of voltage dependence indicated that [I.sub.KA] was affected by extracellular or intracellular pH, but the activity of [I.sub.KDR] was modulated by extracellular pH only. Application of inhibitors of either channel mimicked many of the effects of acidification in isolated cells and neurons in situ. We also detected evidence of a pH-sensitive calcium-activated potassium channel ([I.sub.KCa]) in neurons in situ. The results of these studies support the hypothesis that [I.sub.KA] initiates the chemosensory response, and [I.sub.KDR] and [I.sub.KCa] prolong the period of activation of C[O.sub.2]-sensitive neurons. Thus multiple potassium channels are inhibited by acidosis, and the combined effect of pH-dependent inhibition of these channels enhances neuronal excitability and mediates C[O.sub.2] chemosensory responses in H. aspersa. We did not find a single 'chemosensory channel,' and the chemosensitive channels that we did find were not unique in any way that we could detect. The protein 'machinery' of C[O.sub.2] chemosensitivity is probably widespread among neurons, and the selection process whereby a neuron acts or does not act as a respiratory C[O.sub.2] chemosensor probably depends on the resting membrane potential and synaptic connectivity. carbon dioxide