Molecular diversity and regulation of renal potassium channels

Novel diuretic targets

As the molecular revolution continues to inform a deeper understanding of disease mechanisms and pathways, there exist unprecedented opportunities for translating discoveries at the bench into novel therapies for improving human health. Despite the availability of several different classes of antihypertensive medications, only about half of the 67 million Americans with hypertension manage their blood pressure appropriately. A broader selection of structurally diverse antihypertensive drugs acting through different mechanisms would provide clinicians with greater flexibility in developing effective treatment regimens for an increasingly diverse and aging patient population. An emerging body of physiological, genetic, and pharmacological evidence has implicated several renal ion-transport proteins, or regulators thereof, as novel, yet clinically unexploited, diuretic targets. These include the renal outer medullary potassium channel, ROMK (Kir1.1), Kir4.1/5.1 potassium channels, ClC-Ka/b chloride channels, UTA/B urea transporters, the chloride/bicarbonate exchanger pendrin, and the STE20/SPS1-related proline/alanine-rich kinase (SPAK). The molecular pharmacology of these putative targets is poorly developed or lacking altogether; however, recent efforts by a few academic and pharmaceutical laboratories have begun to lessen this critical barrier. Here, we review the evidence in support of the aforementioned proteins as novel diuretic targets and highlight examples where progress toward developing small-molecule pharmacology has been made.

WNK4 inhibits Ca(2+)-activated big-conductance potassium channels (BK) via mitogen-activated protein kinase-dependent pathway

We used the perforated whole-cell recording technique to examine the effect of with-no-lysine kinase 4 (WNK4) on the Ca(2+) activated big-conductance K channels (BK) in HEK293T cells transfected with BK-α subunit (BK-α). Expression of WNK4 inhibited BK channels and decreased the outward K currents. Coexpression of SGK1 abolished the inhibitory effect of WNK4 on BK channels and restored the outward K currents. Expression of WNK4(S1169D//1196D), in which both SGK1-phosphorylation sites (serine 1169 and 1196) were mutated to aspartate, had no effect on BK channels. Moreover, coexpression of SGK1 had no additional effect on K currents in the cells transfected with BKα+WNK4(S1169D//1196D), suggesting that SGK1 reversed WNK4-induced inhibition of BK channels by stimulating WNK4 phosphorylation. Expression of WNK4 but not WNK4(S1169D//1196D) increased the phosphorylation of ERK and p38 mitogen-activated protein kinase (MAPK); an effect was abolished by coexpression of SGK1. The role of ERK and p38 MAPK in mediating the effect of WNK4 on BK channels was further suggested by the finding that the inhibition of ERK and P38 MAPK completely abolished the inhibitory effect of WNK4 on BK channels. In contrast, inhibition of MAPK failed to abolish the inhibitory effect of WNK4 on ROMK channels in both HEK cells and Xenopus oocytes. Expression of dominant negative dynaminK44A (Dyn(K44A)) or treatment of the cells with dynasore, a dynamin inhibitor, not only increased K currents but also largely abolished the inhibitory effect of WNK4 on BK channels. However, inhibition of MAPK still increased the outward K currents in the cells transfected with BKα+WNK4 and treated with dynasore. Similar results were obtained in experiments performed in the native tissue in which inhibition of ERK and p38 MAPK increased BK channel activity in the cortical collecting duct (CCD) treated with dynasore. We concluded that WNK4 inhibited BK channels by stimulating ERK and p38 MAPK and that activation of MAPK by WNK4 may inhibit BK channels partially via a mechanism other than stimulating endocytosis.

Renal β-intercalated cells maintain body fluid and electrolyte balance

Inactivation of the B1 proton pump subunit (ATP6V1B1) in intercalated cells (ICs) leads to type I distal renal tubular acidosis (dRTA), a disease associated with salt- and potassium-losing nephropathy. Here we show that mice deficient in ATP6V1B1 (Atp6v1b1-/- mice) displayed renal loss of NaCl, K+, and water, causing hypovolemia, hypokalemia, and polyuria. We demonstrated that NaCl loss originated from the cortical collecting duct, where activity of both the epithelial sodium channel (ENaC) and the pendrin/Na(+)-driven chloride/bicarbonate exchanger (pendrin/NDCBE) transport system was impaired. ENaC was appropriately increased in the medullary collecting duct, suggesting a localized inhibition in the cortex. We detected high urinary prostaglandin E2 (PGE2) and ATP levels in Atp6v1b1-/- mice. Inhibition of PGE2 synthesis in vivo restored ENaC protein levels specifically in the cortex. It also normalized protein levels of the large conductance calcium-activated potassium channel and the water channel aquaporin 2, and improved polyuria and hypokalemia in mutant mice. Furthermore, pharmacological inactivation of the proton pump in β-ICs induced release of PGE2 through activation of calcium-coupled purinergic receptors. In the present study, we identified ATP-triggered PGE2 paracrine signaling originating from β-ICs as a mechanism in the development of the hydroelectrolytic imbalance associated with dRTA. Our data indicate that in addition to principal cells, ICs are also critical in maintaining sodium balance and, hence, normal vascular volume and blood pressure.

Discovery of a novel sub-class of ROMK channel inhibitors typified by 5-(2-(4-(2-(4-(1H-Tetrazol-1-yl)phenyl)acetyl)piperazin-1-yl)ethyl)isobenzofuran-1(3H)-one

A sub-class of distinct small molecule ROMK inhibitors were developed from the original lead 1. Medicinal chemistry endeavors led to novel ROMK inhibitors with good ROMK functional potency and improved hERG selectivity. Two of the described ROMK inhibitors were characterized for the first in vivo proof-of-concept biology studies, and results from an acute rat diuresis model confirmed the hypothesis that ROMK inhibitors represent new mechanism diuretic and natriuretic agents.

Interleukin-1β suppresses activity of an inwardly rectifying K+ channel in human renal proximal tubule cells

We investigated the effect of interleukin-1β (IL-1β) on activity of an inwardly rectifying K+ channel in cultured human proximal tubule cells (RPTECs), using the patch-clamp technique and Fura-2 Ca2+ imaging. IL-1β (15 pg/ml) acutely reduced K+ channel activity in cell-attached patches. This effect was blocked by the IL-1 receptor antagonist (20 ng/ml), an inhibitor of phospholipase C, neomycin (300 μM), and an inhibitor of protein kinase C (PKC), GF109203X (500 nM). The Fura-2 Ca2+ imaging revealed that IL-1β increased intracellular Ca2+ concentration even after removal of extracellular Ca2+, which was blocked by an inhibitor of inositol 1,4,5-trisphosphate receptors, 2-aminoethoxydiphenyl borate (2-APB, 1 μM). Moreover, IL-1β suppressed channel activity in the presence of 2-APB without extracellular Ca2+. These results suggest that IL-1β suppresses K+ channel activity in RPTECs through binding to its specific receptor and activation of the PKC pathway even though intracellular Ca2+ does not increase.

Ankyrin-3 is a novel binding partner of the voltage-gated potassium channel Kv1.1 implicated in renal magnesium handling

The voltage-gated potassium channel, Kv1.1, was recently identified as a causative gene in isolated dominant hypomagnesemia. The channel is situated in the distal convoluted tubule, where it participates in maintaining a favorable electrical gradient for driving magnesium ion into the cell through the transient receptor potential melastatin 6 channel. Pull-down experiments coupled to mass spectrometry using the carboxy-terminal domain of Kv1.1 as bait were used in mouse kidney lysates. Ankyrin-3 (ANK3) was identified as a binding partner of Kv1.1 and was enriched in isolated distal convoluted tubules as compared to whole kidney. Electrophysiology studies performed in HEK293 cells expressing Kv1.1 showed that ANK3 significantly inhibited Kv1.1-mediated currents (267 compared to 125 pA/pF) for control and ANK3, respectively. Finally, to evaluate a potential role of ANK3 in magnesium handling, the intrarenal abundance of ANK3 was measured in mice fed a low-, normal-, or high-magnesium diet for 10 days. Mice maintained on high dietary magnesium significantly doubled their fractional urinary excretion of magnesium, which coincided with a 1.8-fold increase in the renal expression of ANK3 compared to mice on a normal- or low-magnesium diet. Thus, our observations demonstrate a novel role for ANK3 in modulating the biophysical properties of Kv1.1. Such regulation appears to be particularly important in conditions of high dietary magnesium.

Regulation of large-conductance Ca2+-activated K+ channels by WNK4 kinase

Large-conductance, Ca(2+)-activated K(+) channels, commonly referred to as BK channels, have a major role in flow-induced K(+) secretion in the distal nephron. With-no-lysine kinase 4 (WNK4) is a serine-threonine kinase expressed in the distal nephron that inhibits ROMK activity and renal K(+) secretion. WNK4 mutations have been described in individuals with familial hyperkalemic hypertension (FHHt), a Mendelian disorder characterized by low-renin hypertension and hyperkalemia. As BK channels also have an important role in renal K(+) secretion, we examined whether they are regulated by WNK4 in a manner similar to ROMK. BK channel activity was inhibited in a rabbit intercalated cell line transfected with WNK4 or a WNK4 mutant found in individuals with FHHt. Coexpression of an epitope-tagged BK α-subunit with WNK4 or the WNK4 mutant in HEK293 cells reduced BK α-subunit plasma membrane and whole cell expression. A region within WNK4 encompassing the autoinhibitory domain and a coiled coil domain was required for WNK4 to inhibit BK α-subunit expression. The relative fraction of BK α-subunit that was ubiquitinated was significantly increased in cells expressing WNK4, compared with controls. Our results suggest that WNK4 inhibits BK channel activity, in part, by increasing channel degradation through an ubiquitin-dependent pathway. Based on these results, we propose that WNK4 provides a cellular mechanism for the coordinated regulation of two key secretory K(+) channels in the distal nephron, ROMK and BK.

Src family protein tyrosine kinase regulates the basolateral K channel in the distal convoluted tubule (DCT) by phosphorylation of KCNJ10 protein

The loss of function of the basolateral K channels in the distal nephron causes electrolyte imbalance. The aim of this study is to examine the role of Src family protein tyrosine kinase (SFK) in regulating K channels in the basolateral membrane of the mouse initial distal convoluted tubule (DCT1). Single-channel recordings confirmed that the 40-picosiemen (pS) K channel was the only type of K channel in the basolateral membrane of DCT1. The suppression of SFK reversibly inhibited the basolateral 40-pS K channel activity in cell-attached patches and decreased the Ba(2+)-sensitive whole-cell K currents in DCT1. Inhibition of SFK also shifted the K reversal potential from -65 to -43 mV, suggesting a role of SFK in determining the membrane potential in DCT1. Western blot analysis showed that KCNJ10 (Kir4.1), a key component of the basolateral 40-pS K channel in DCT1, was a tyrosine-phosphorylated protein. LC/MS analysis further confirmed that SFK phosphorylated KCNJ10 at Tyr(8) and Tyr(9). The single-channel recording detected the activity of a 19-pS K channel in KCNJ10-transfected HEK293T cells and a 40-pS K channel in the cells transfected with KCNJ10+KCNJ16 (Kir.5.1) that form a heterotetramer in the basolateral membrane of the DCT. Mutation of Tyr(9) did not alter the channel conductance of the homotetramer and heterotetramer. However, it decreased the whole-cell K currents, the probability of finding K channels, and surface expression of KCNJ10 in comparison to WT KCNJ10. We conclude that SFK stimulates the basolateral K channel activity in DCT1, at least partially, by phosphorylating Tyr(9) on KCNJ10. We speculate that the modulation of tyrosine phosphorylation of KCNJ10 should play a role in regulating membrane transport function in DCT1.

Vasotocin analogues with selective natriuretic, kaliuretic and antidiuretic effects in rats

The aim of the present study was an investigation of mechanisms mediating selective effect of vasotocin analogues on water, sodium, and potassium excretion. We tested vasotocin analogues: Mpa(1)-vasotocin (dAVT), Mpa(1)-Arg(4)-vasotocin (dAAVT) and Mpa(1)-DArg(8)-vasotocin (dDAVT). The effects on water, sodium, and potassium transport were evaluated in experiments using normal and water-loaded Wistar rats. It was shown that all tested peptides exerted antidiuretic activity. Vasotocin and dAVT induced natriuresis and kaliuresis in rats. V1a agonist (Phe(2)-Ile(3)-Orn(8)-vasopressin) reproduced the renal effects of dAVT on sodium and potassium excretion but not on water reabsorption. dAAVT, dDAVT and V2 agonist (desmopressin) induced kaliuresis without any effect on sodium excretion. Natriuresis was associated with increase in cGMP excretion, whereas kaliuresis was correlated with rise of cAMP excretion. V1a antagonist (Pmp(1)-Tyr(Me)(2)-vasopressin) significantly reduced the dAVT-stimulated natriuresis and did not influence on urinary potassium excretion. V2 antagonist (Pmp(1)-DIle(2)-Ile(4)-vasopressin) significantly reduced the dAVT- and dAAVT-induced kaliuresis. It is assumed that effects of the nonapeptides on sodium and potassium transport are independent of their antidiuretic activity and mediated by different subtypes of V receptors (the V1a or V1a-like receptor for natriuretic effect and V2 or V2-like one for kaliuretic). In accordance to the data obtained, there is a possibility of selective regulation of renal water reabsorption and urinary sodium and potassium excretion with involvement of neurohypophysial hormones.

Tissue-specific ionomotive enzyme activity and K+ reabsorption reveal the rectum as an important ionoregulatory organ in larval Chironomus riparius exposed to varying salinity

A role for the rectum in the ionoregulatory homeostasis of larval Chironomus riparius was revealed by rearing animals in different saline environments and examining: (1) the spatial distribution and activity of keystone ionomotive enzymes Na(+)-K(+)-ATPase (NKA) and V-type H(+)-ATPase (VA) in the alimentary canal, and (2) rectal K(+) transport with the scanning ion-selective electrode technique (SIET). NKA and VA activity were measured in four distinct regions of the alimentary canal as follows: the combined foregut and anterior midgut, the posterior midgut, the Malpighian tubules and the hindgut. Both enzymes exhibited 10-20 times greater activity in the hindgut relative to all other areas. When larvae were reared in either ion-poor water (IPW) or freshwater (FW), no significant difference in hindgut enzyme activity was observed. However, in larvae reared in brackish water (BW), NKA and VA activity in the hindgut significantly decreased. Immunolocalization of NKA and VA in the hindgut revealed that the bulk of protein was located in the rectum. Therefore, K(+) transport across the rectum was examined using SIET. Measurement of K(+) flux along the rectum revealed a net K(+) reabsorption that was reduced fourfold in BW-reared larvae versus larvae reared in FW or IPW. Inhibition of NKA with ouabain, VA with bafilomycin and K(+) channels with charybdotoxin diminished rectal K(+) reabsorption in FW- and IPW-reared larvae, but not BW-reared larvae. Data suggest that the rectum of C. riparius plays an important role in allowing these larvae to cope with dilute as well as salinated environmental conditions.

hERG channel function: beyond long QT

To date, research on the human ether-a-go-go related gene (hERG) has focused on this potassium channel's role in cardiac repolarization and Long QT Syndrome (LQTS). However, growing evidence implicates hERG in a diversity of physiologic and pathological processes. Here we discuss these other functions of hERG, particularly their impact on diseases beyond cardiac arrhythmia.

Cloning and functional characterization of inward-rectifying potassium (Kir) channels from Malpighian tubules of the mosquito Aedes aegypti

Inward-rectifying K(+) (Kir) channels play critical physiological roles in a variety of vertebrate cells/tissues, including the regulation of membrane potential in nerve and muscle, and the transepithelial transport of ions in osmoregulatory epithelia, such as kidneys and gills. It remains to be determined whether Kir channels play similar physiological roles in insects. In the present study, we sought to 1) clone the cDNAs of Kir channel subunits expressed in the renal (Malpighian) tubules of the mosquito Aedes aegypti, and 2) characterize the electrophysiological properties of the cloned Kir subunits when expressed heterologously in oocytes of Xenopus laevis. Here, we reveal that three Kir subunits are expressed abundantly in Aedes Malpighian tubules (AeKir1, AeKir2B, and AeKir3); each of their full-length cDNAs was cloned. Heterologous expression of the AeKir1 or the AeKir2B subunits in Xenopus oocytes elicits inward-rectifying K(+) currents that are blocked by barium. Relative to the AeKir2B-expressing oocytes, the AeKir1-expressing oocytes 1) produce larger macroscopic currents, and 2) exhibit a modulation of their conductive properties by extracellular Na(+). Attempts to functionally characterize the AeKir3 subunit in Xenopus oocytes were unsuccessful. Lastly, we show that in isolated Aedes Malpighian tubules, the cation permeability sequence of the basolateral membrane of principal cells (Tl(+) > K(+) > Rb(+) > NH(4)(+)) is consistent with the presence of functional Kir channels. We conclude that in Aedes Malpighian tubules, Kir channels contribute to the majority of the barium-sensitive transepithelial transport of K(+).

Differential diagnosis of hyperkalemia: an update to a complex problem

Hyperkalemia is a relative common and sometimes life threatening electorlyte disorder. Although its symptomatic treatment is relatively easy, since precise therapeutic algorithms are available, its differential diagnosis is more complicated. The present review aims to unfold the differential diagnosis of hypekalemia using a pathophysiological, albeit clinically useful, approach. The basic elements of potassium homeostasis are provided, the causes of hyperkalemia are categorized and analysed and finally the required for the diferrential diagnosis laboratory tests are mentioned.

Identification of uterine ion transporters for mineralisation precursors of the avian eggshell

BACKGROUND

In Gallus gallus, eggshell formation takes place daily in the hen uterus and requires large amounts of the ionic precursors for calcium carbonate (CaCO3). Both elements (Ca2+, HCO3-) are supplied by the blood via trans-epithelial transport. Our aims were to identify genes coding for ion transporters that are upregulated in the uterine portion of the oviduct during eggshell calcification, compared to other tissues and other physiological states, and incorporate these proteins into a general model for mineral transfer across the tubular gland cells during eggshell formation.

RESULTS

A total of 37 candidate ion transport genes were selected from our database of overexpressed uterine genes associated with eggshell calcification, and by analogy with mammalian transporters. Their uterine expression was compared by qRTPCR in the presence and absence of eggshell formation, and with relative expression levels in magnum (low Ca2+/HCO3- movement) and duodenum (high rates of Ca2+/HCO3- trans-epithelial transfer). We identified overexpression of eleven genes related to calcium movement: the TRPV6 Ca2+ channel (basolateral uptake of Ca2+), 28 kDa calbindin (intracellular Ca2+ buffering), the endoplasmic reticulum type 2 and 3 Ca2+ pumps (ER uptake), and the inositol trisphosphate receptors type 1, 2 and 3 (ER release). Ca2+ movement across the apical membrane likely involves membrane Ca2+ pumps and Ca2+/Na+ exchangers. Our data suggests that Na+ transport involved the SCNN1 channel and the Na+/Ca2+ exchangers SLC8A1, 3 for cell uptake, the Na+/K+ ATPase for cell output. K+ uptake resulted from the Na+/K+ ATPase, and its output from the K+ channels (KCNJ2, 15, 16 and KCNMA1).We propose that the HCO3- is mainly produced from CO2 by the carbonic anhydrase 2 (CA2) and that HCO3- is secreted through the HCO3-/Cl- exchanger SLC26A9. HCO3- synthesis and precipitation with Ca2+ produce two H+. Protons are absorbed via the membrane's Ca2+ pumps ATP2B1, 2 in the apical membrane and the vacuolar (H+)-atpases at the basolateral level. Our model incorporate Cl- ions which are absorbed by the HCO3-/Cl- exchanger SLC26A9 and by Cl- channels (CLCN2, CFTR) and might be extruded by Cl-/H+ exchanger (CLCN5), but also by Na+ K+ 2 Cl- and K+ Cl- cotransporters.

CONCLUSIONS

Our Gallus gallus uterine model proposes a large list of ion transfer proteins supplying Ca2+ and HCO3- and maintaining cellular ionic homeostasis. This avian model should contribute towards understanding the mechanisms and regulation for ionic precursors of CaCO3, and provide insight in other species where epithelia transport large amount of calcium or bicarbonate.

Residues at the outer mouth of Kir1.1 determine K-dependent gating

Three residues (E132, F127, and R128) at the outer mouth of Kir1.1b directly affected inward rectifier gating by external K, independent of pH gating. Each of the individual mutations E132Q, F127V, F127D, and R128Y changed the normal K dependence of macroscopic conductance from hyperbolic (Km = 6 ± 2 mM) to linear, up to 500 mM, without changing the hyperbolic K dependence of single-channel conductance. This suggests that E132, F127, and R128 are responsible for maximal Kir1.1b activation by external K. In addition, these same residues were also essential for recovery of Kir1.1b activity after complete removal of external K by 18-Crown-6 polyether. In contrast, charge-altering mutations at neighboring residues (E92A, E104A, D97V, or Q133E) near the outer mouth of the channel did not affect Kir1.1b recovery after chelation of external K. The collective role of E132, R128, and F127 in preventing Kir1.1b inactivation by either cytoplasmic acidification or external K removal implies that pH inactivation and the external K sensor share a common mechanism, whereby E132, R128, and F127 stabilize the Kir1.1b selectivity filter gate in an open conformation, allowing rapid recovery of channel activity after a period of external K depletion.

Differential immunolocalization of sulfonylurea receptors in mouse and rat ureters

The receptors for sulphonylurea (SURs) are known to be expressed in the mouse kidney, but their expression in the ureter is undefined. Owing to the physiological and pharmacological significance of SURs, the localization of SUR in ureters of adult mice and rats was investigated through immunohistochemistry. Animals were perfused transcardially with 4% paraformaldehyde and tissues were processed for immunohistochemistry using polyclonal antisera against SUR2A and SUR2B (SUR1) receptor proteins. Sections were incubated with primary and secondary antisera and developed with aminoethylcarbazole as a chromogen. A differentiated localized staining pattern of SUR proteins in rat and mouse ureters is demonstrated. In the mouse, immunoreactivity of SUR2A was predominantly confined to the cytoplasmic portion of epithelial cells and blood vessels, with comparatively low-level staining found in smooth muscle. In contrast, SUR2B (SUR1) immunoreactivity was absent in mouse ureters. In rats, SUR2A immunoreactivity was localized only in the blood vessels, while SUR2B (SUR1) immunoreactivity was localized in the epithelial cell cytoplasm. Tissue specificity of SUR is demonstrated in the two species of rodents and suggests a role of SUR proteins in urinary metabolism pertaining possibly to salt handling and maintenance of the smooth muscle tone.

Discovery of Selective Small Molecule ROMK Inhibitors as Potential New Mechanism Diuretics

The renal outer medullary potassium channel (ROMK or Kir1.1) is a putative drug target for a novel class of diuretics that could be used for the treatment of hypertension and edematous states such as heart failure. An internal high-throughput screening campaign identified 1,4-bis(4-nitrophenethyl)piperazine (5) as a potent ROMK inhibitor. It is worth noting that this compound was identified as a minor impurity in a screening hit that was responsible for all of the initially observed ROMK activity. Structure-activity studies resulted in analogues with improved rat pharmacokinetic properties and selectivity over the hERG channel, providing tool compounds that can be used for in vivo pharmacological assessment. The featured ROMK inhibitors were also selective against other members of the inward rectifier family of potassium channels.

Advances in WNK signaling of salt and potassium metabolism: clinical implications

Recent evidence due to the discovery of a family of kinases implicated in arterial hypertension now points to the underlying molecular mechanisms that dictate Na(+), K(+) and water handling in the nephron. These new key players need to be understood in order to fully comprehend the pathophysiology, manifestations, and treatment of common clinical entities such as hypovolemic shock, congestive heart failure, primary hyperaldosteronism, nephrotic syndrome and hypertension. It is through the analysis of the volume status and electrolyte abnormalities that commonly present with these diseases that we can begin to create a link between the abstract concept of a kinase regulation and how a patient will respond to a particular treatment. This review is an attempt to bridge that gap.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Non-genomic actions of aldosterone: from receptors and signals to membrane targets

In tissues which express the mineralocorticoid receptor (MR), aldosterone modulates the expression of membrane targets such as the subunits of the epithelial Na(+) channel, in combination with important signalling intermediates such as serum and glucocorticoid-regulated kinase-1. In addition, the rapid 'non-genomic' activation of protein kinases and secondary messenger signalling cascades has also been detected in aldosterone-sensitive tissues of the nephron, distal colon and cardiovascular system. These rapid actions are variously described as being coupled to MR or to an as yet unidentified, membrane-associated aldosterone receptor. The rapidly activated signalling cascades add a level of fine-tuning to the activity of aldosterone-responsive membrane transporters and also modulate the aldosterone-induced changes in gene expression through receptor and transcription factor phosphorylation.

Metabolic complications associated with use of diuretics

Diuretics are commonly used therapeutic agents that act to inhibit sodium transport systems along the length of the renal tubule. The most effective diuretics are inhibitors of sodium chloride transport in the thick ascending limb of Henle. Loop diuretics mobilize large amounts of sodium chloride and water and produce a copious diuresis with a sharp reduction of extracellular fluid volume. As the site of action of diuretics moves downstream (thiazide and potassium-sparing diuretics), their effectiveness declines because the transport systems they inhibit have low transport capacity. Depending on the site of action diuretics can influence the renal handling of electrolyte-free water, calcium, potassium, protons, sodium bicarbonate, and uric acid. As a result, electrolyte and acid-base disorders commonly accompany diuretic use. Glucose and lipid abnormalities also can occur, particularly with the use of thiazide diuretics. This review focuses on the biochemical complications associated with the use of diuretics. The development of these complications can be minimized with careful monitoring, dosage adjustment, and replacement of electrolyte losses.

Potassium excretion through ROMK potassium channel expressed in gill mitochondrion-rich cells of Mozambique tilapia

Despite recent progress in physiology of fish ion homeostasis, the mechanism of plasma K+ regulation has remained unclear. Using Mozambique tilapia, a euryhaline teleost, we demonstrated that gill mitochondrion-rich (MR) cells were responsible for K+ excretion, using a newly invented technique that insolubilized and visualized K+ excreted from the gills. For a better understanding of the molecular mechanism of K+ excretion in the gills, cDNA sequences of renal outer medullary K+ channel (ROMK), potassium large conductance Ca(2+)-activated channel, subfamily M (Maxi-K), K(+)-Cl(-) cotransporters (KCC1, KCC2, and KCC4) were identified in tilapia as the candidate molecules that are involved in K+ handling. Among the cloned candidate molecules, only ROMK showed marked upregulation of mRNA levels in response to high external K+ concentration. In addition, immunofluorescence microscopy revealed that ROMK was localized in the apical opening of gill MR cells, and that the immunosignals were most intense in the fish acclimated to the environment with high K+ concentration. To confirm K+ excretion via ROMK, K+ insolubilization-visualization technique was applied again in combination with K+ channel blockers. The K+ precipitation was prevented in the presence of Ba2+, indicating that ROMK has a pivotal role in K+ excretion. The present study is the first to demonstrate that the fish excrete K+ from the gill MR cells, and that ROMK expressed in the apical opening of the MR cells is a main molecular pathway responsible for K+ excretion.

Basolateral membrane K+ channels in renal epithelial cells

The major function of epithelial tissues is to maintain proper ion, solute, and water homeostasis. The tubule of the renal nephron has an amazingly simple structure, lined by epithelial cells, yet the segments (i.e., proximal tubule vs. collecting duct) of the nephron have unique transport functions. The functional differences are because epithelial cells are polarized and thus possess different patterns (distributions) of membrane transport proteins in the apical and basolateral membranes of the cell. K(+) channels play critical roles in normal physiology. Over 90 different genes for K(+) channels have been identified in the human genome. Epithelial K(+) channels can be located within either or both the apical and basolateral membranes of the cell. One of the primary functions of basolateral K(+) channels is to recycle K(+) across the basolateral membrane for proper function of the Na(+)-K(+)-ATPase, among other functions. Mutations of these channels can cause significant disease. The focus of this review is to provide an overview of the basolateral K(+) channels of the nephron, providing potential physiological functions and pathophysiology of these channels, where appropriate. We have taken a "K(+) channel gene family" approach in presenting the representative basolateral K(+) channels of the nephron. The basolateral K(+) channels of the renal epithelia are represented by members of the KCNK, KCNJ, KCNQ, KCNE, and SLO gene families.

Role of the renal circadian timing system in maintaining water and electrolytes homeostasis

Many basic physiological functions exhibit circadian rhythmicity. These functional rhythms are driven, in part, by the circadian clock, an ubiquitous molecular mechanism allowing cells and tissues to anticipate regular environmental events and to prepare for them. This mechanism has been shown to play a particularly important role in maintaining stability (homeostasis) of internal conditions. Because the homeostatic equilibrium is continuously challenged by environmental changes, the role of the circadian clock is thought to consist in the anticipative adjustment of homeostatic pathways in relation with the 24h environmental cycle. The kidney is the principal organ responsible for the regulation of the composition and volume of extracellular fluids (ECF). Several major parameters of kidney function, including renal plasma flow (RPF), glomerular filtration rate (GFR) and tubular reabsorption and secretion have been shown to exhibit strong circadian oscillations. Recent evidence suggest that the circadian clock can be involved in generation of these rhythms through external circadian time cues (e.g. humoral factors, activity and body temperature rhythms) or, trough the intrinsic renal circadian clock. Here, we discuss the role of renal circadian mechanisms in maintaining homeostasis of water and three major ions, namely, Na(+), K(+) and Cl(-).

Discovery, characterization, and structure-activity relationships of an inhibitor of inward rectifier potassium (Kir) channels with preference for Kir2.3, Kir3.x, and Kir7.1

The inward rectifier family of potassium (Kir) channels is comprised of at least 16 family members exhibiting broad and often overlapping cellular, tissue, or organ distributions. The discovery of disease-causing mutations in humans and experiments on knockout mice has underscored the importance of Kir channels in physiology and in some cases raised questions about their potential as drug targets. However, the paucity of potent and selective small-molecule modulators targeting specific family members has with few exceptions mired efforts to understand their physiology and assess their therapeutic potential. A growing body of evidence suggests that G protein-coupled inward rectifier K (GIRK) channels of the Kir3.X subfamily may represent novel targets for the treatment of atrial fibrillation. In an effort to expand the molecular pharmacology of GIRK, we performed a thallium (Tl⁺) flux-based high-throughput screen of a Kir1.1 inhibitor library for modulators of GIRK. One compound, termed VU573, exhibited 10-fold selectivity for GIRK over Kir1.1 (IC₅₀ = 1.9 and 19 μM, respectively) and was therefore selected for further study. In electrophysiological experiments performed on Xenopus laevis oocytes and mammalian cells, VU573 inhibited Kir3.1/3.2 (neuronal GIRK) and Kir3.1/3.4 (cardiac GIRK) channels with equal potency and preferentially inhibited GIRK, Kir2.3, and Kir7.1 over Kir1.1 and Kir2.1.Tl⁺ flux assays were established for Kir2.3 and the M125R pore mutant of Kir7.1 to support medicinal chemistry efforts to develop more potent and selective analogs for these channels. The structure–activity relationships of VU573 revealed few analogs with improved potency, however two compounds retained most of their activity toward GIRK and Kir2.3 and lost activity toward Kir7.1. We anticipate that the VU573 series will be useful for exploring the physiology and structure–function relationships of these Kir channels.

Renal outer medullary potassium channel knockout models reveal thick ascending limb function and dysfunction

The renal outer medullary potassium channel (ROMK) is an adenosine triphosphate-sensitive inward-rectifier potassium channel (Kir1.1 or KCNJ1) highly expressed in the cortical and medullary thick ascending limbs (TAL), connecting segment (CNT) and cortical collecting duct (CCD) in the mammalian kidney, where it serves to recycle potassium (K(+)) across the apical membrane in TAL and to secrete K(+) in the CNT and CCD. ROMK channel mutations cause type II Bartter's syndrome with salt wasting and dehydration, and ROMK knockout mice display a similar phenotype of Bartter's syndrome in humans. Studies from ROMK null mice indicate that ROMK is required to form both the small-conductance (30pS, SK) K channels and the 70pS (IK) K channels in the TAL. The availability of ROMK(-/-) mice has made it possible to study electrolyte transport along the nephron in order to understand the TAL function under physiological conditions and the compensatory mechanisms of salt and water transport under the conditions of TAL dysfunction. This review summarizes previous progress in the study of K(+) channel activity in the TAL and CCD, ion transporter expression and activities along the nephron, and renal functions under physiological and pathophysiological conditions using ROMK(-/-) mice.

Effects of cytokines on potassium channels in renal tubular epithelia

Renal tubular potassium (K(+)) channels play important roles in the formation of cell-negative potential, K(+) recycling, K(+) secretion, and cell volume regulation. In addition to these physiological roles, it was reported that changes in the activity of renal tubular K(+) channels were involved in exacerbation of renal cell injury during ischemia and endotoxemia. Because ischemia and endotoxemia stimulate production of cytokines in immune cells and renal tubular cells, it is possible that cytokines would affect K(+) channel activity. Although the regulatory mechanisms of renal tubular K(+) channels have extensively been studied, little information is available about the effects of cytokines on these K(+) channels. The first report was that tumor necrosis factor acutely stimulated the single channel activity of the 70 pS K(+) channel in the rat thick ascending limb through activation of tyrosine phosphatase. Recently, it was also reported that interferon-γ (IFN-γ) and interleukin-1β (IL-1β) modulated the activity of the 40 pS K(+) channel in cultured human proximal tubule cells. IFN-γ exhibited a delayed suppression and an acute stimulation of K(+) channel activity, whereas IL-1β acutely suppressed the channel activity. Furthermore, these cytokines suppressed gene expression of the renal outer medullary potassium channel. The renal tubular K(+) channels are functionally coupled to the coexisting transporters. Therefore, the effects of cytokines on renal tubular transporter activity should also be taken into account, when interpreting their effects on K(+) channel activity.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Effects of pH on potassium: new explanations for old observations

Maintenance of extracellular K(+) concentration within a narrow range is vital for numerous cell functions, particularly electrical excitability of heart and muscle. Potassium homeostasis during intermittent ingestion of K(+) involves rapid redistribution of K(+) into the intracellular space to minimize increases in extracellular K(+) concentration, and ultimate elimination of the K(+) load by renal excretion. Recent years have seen great progress in identifying the transporters and channels involved in renal and extrarenal K(+) homeostasis. Here we apply these advances in molecular physiology to understand how acid-base disturbances affect serum potassium.

Effects of ozone on membrane permeability and ultrastructure in Pseudomonas aeruginosa

AIMS

To examine the mechanism of ozone-induced damage to cytoplasmic membrane and cell ultrastructure of Pseudomonas aeruginosa ATCC27853.

METHODS AND RESULTS

Cell suspensions of Ps. aeruginosa ATCC27853 were treated with ozonated water. The leakages of cellular potassium (K⁺), magnesium (Mg²⁺) and adenosine triphosphate (ATP), determined by inductively coupled plasma/mass spectrometry (ICP/MS) and a commercial bioluminescence assay kit, were to assess ozone-induced damage to the cytoplasmic membrane. Maximum leakages of K⁺ and Mg²⁺ were attained, respectively, at 0·53 mg l⁻¹ ozone after 0·5 and 2 min with > 99% inactivation of culturable bacteria, while that of ATP was achieved at 0·67 mg l⁻¹ ozone after 1 min. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) revealed that treated cells retained intact shapes and cytoplasm agglutinations and vacuoles occurred.

CONCLUSIONS

Ozone inactivates Ps. aeruginosa ATCC27853 by the combined results of increased cytoplasmic membrane permeability and cytoplasm coagulation, rather than by severe membrane disruption and cell lysis.

SIGNIFICANCE AND IMPACT OF THE STUDY

Pseudomonas aeruginosa is a common water-related pathogen. These insights into the leakage of cytoplasmic components and ultrastructural changes provide evidence for the mechanisms of ozone-mediated inactivation.

Aldosterone paradox: differential regulation of ion transport in distal nephron

The mechanisms through which aldosterone promotes apparently opposite effects like salt reabsorption and K(+) secretion remain poorly understood. The identification, localization, and physiological analysis of ion transport systems in distal nephron have revealed an intricate network of interactions between several players, revealing the complex mechanism behind the aldosterone paradox. We review the mechanisms involved in differential regulation of ion transport that allow the fine tuning of salt and K(+) balance.

Interaction between Calcineurin and Ca/Calmodulin Kinase-II in Modulating Cellular Functions

Roles of calcineurin (CaN), a Ca²⁺/calmodulin- (CaM-) dependent protein phosphatase, and Ca²⁺/CaM-dependent protein kinase-II (CaMKII) in modulating K⁺ channel activity and the intracellular Ca²⁺ concentration ([Ca²⁺]_i) have been investigated in renal tubule epithelial cells. The channel current through the cell membrane was recorded with the patch-clamp technique, and [Ca²⁺]_i was monitored using fura-2 imaging. We found that a CaN-inhibitor, cyclosporin A (CyA), lowered the K⁺ channel activity and elevated [Ca²⁺]_i, suggesting that CyA closes K⁺ channels and opens Ca²⁺-release channels of the cytosolic Ca²⁺-store. Moreover, both of these responses were blocked by KN-62, an inhibitor of CaMKII. It is suggested that the CyA-mediated response results from the activation of CaMKII. Indeed, Western blot analysis revealed that CyA increased phospho-CaMKII, an active form of CaMKII. These findings suggest that CaN-dependent dephosphorylation inhibits CaMKII-mediated phosphorylation, and the inhibition of CaN increases phospho-CaMKII, which results in the stimulation of CaMKII-dependent cellular actions.

WNK4 kinase inhibits Maxi K channel activity by a kinase-dependent mechanism

WNK [with no lysine (k)] kinase is a serine/threonine kinase subfamily. Mutations in two of the WNK kinases result in pseudohypoaldosteronism type II (PHA II) characterized by hypertension, hyperkalemia, and metabolic acidosis. Recent studies showed that both WNK1 and WNK4 inhibit ROMK activity. However, little is known about the effect of WNK kinases on Maxi K, a large-conductance Ca(2+) and voltage-activated potassium (K) channel. Here, we report that WNK4 wild-type (WT) significantly inhibits Maxi K channel activity in HEK αBK stable cell lines compared with the control group. However, a WNK4 dead-kinase mutant, D321A, has no inhibitory effect on Maxi K activity. We further found that WNK4 inhibits total and cell surface protein expression of Maxi K equally compared with control groups. A dominant-negative dynamin mutant, K44A, did not alter the WNK4-mediated inhibitory effect on Maxi K surface expression. Treatment with bafilomycin A1 (a proton pump inhibitor) and leupeptin (a lysosomal inhibitor) reversed WNK4 WT-mediated inhibition of Maxi K total protein expression. These findings suggest that WNK4 WT inhibits Maxi K activity by reducing Maxi K protein at the membrane, but that the inhibition is not due to an increase in clathrin-mediated endocytosis of Maxi K, but likely due to enhancing its lysosomal degradation. Also, WNK4's inhibitory effect on Maxi K activity is dependent on its kinase activity.

Four cases of autosomal dominant hypocalcaemia with hypercalciuria including two with novel mutations in the calcium-sensing receptor gene

We present four cases with clinical and biochemical hypocalcaemia and evidence supportive of hypoparathyroidism. One case had been previously ascribed a diagnosis of idiopathic hypoparathyroidism. Following the detection of relative hypercalciuria, all cases were found to have autosomal dominant hypocalcaemia with hypercalciuria and mutations of the calcium-sensing receptor gene, of which two were novel. Increased awareness of this condition and access to genotyping enables prompt accurate diagnosis and cascade screening of first-degree relatives.

A novel compound heterozygous ROMK mutation presenting as late onset Bartter syndrome associated with nephrocalcinosis and elevated 1,25(OH)(2) vitamin D levels

Bartter syndrome (BS) is a rare renal tubular disorder presenting with hypokalemic metabolic alkalosis, which is classified into five types. KCNJ1 mutations usually cause the neonatal form of BS, type II BS (OMIM 241200). However, this report concerns a female patient with a novel, compound heterozygous KCNJ1 mutation that causes late-onset BS. The unique clinical findings of this case include persistently elevated 1,25(OH)(2) vitamin D levels, possibly due to increase prostaglandin E(2) levels, and medullary nephrocalcinosis. Treatment with COX-2 inhibitors resolved her hypercalciuria and improved her height and weight; renal function remains stable and there is no progression of nephrocalcinosis.

A Long Affair with Renal Tubules

This essay provides a summary of my professional activities. My interest in renal physiology started as a medical student in Vienna, when I became acquainted with Homer Smith's essays on kidney function. After moving to the United States in 1951, I was fortunate to be mentored by Robert Pitts, in whose Department of Physiology at Cornell Medical College in New York I was given early independence, intellectual stimulation, and the opportunity to pursue experiments on single renal tubules. The problem of how the nephron manages its myriad of transport functions has never lost its fascination for me, and I am profoundly grateful to the many colleagues at Cornell Medical College and at Yale University School of Medicine who shared my passion for the kidney.

Functional and developmental expression of a zebrafish Kir1.1 (ROMK) potassium channel homologue Kcnj1

The zebrafish, Danio rerio, is emerging as an important model organism for the pathophysiological study of some human kidney diseases, but the sites of expression and physiological roles of a number of protein orthologues in the zebrafish nephron remain mostly undefined. Here we show that a zebrafish potassium channel is orthologous to the mammalian kidney potassium channel, ROMK. The cDNA (kcnj1) encodes a protein (Kcnj1) that when expressed in Xenopus laevis oocytes displayed pH- and Ba2+-sensitive K+-selective currents, but unlike the mammalian channel, was completely insensitive to the peptide inhibitor tertiapin-Q. In the pronephros, kcnj1 transcript expression was restricted to a distal region and overlapped with that of sodium–chloride cotransporter Nkcc, chloride channel ClC-Ka, and ClC-Ka/b accessory subunit Barttin, indicating the location of the diluting segment. In a subpopulation of surface cells, kcnj1 was coexpressed with the a1a.4 isoform of the Na+/K+-ATPase, identifying these cells as potential K+ secretory cells in this epithelium. At later stages of development, kcnj1 appeared in cells of the developing gill that also expressed the a1a.4 subunit.Morpholino antisense-mediated knockdown of kcnj1 was accompanied by transient tachycardia followed by bradycardia, effects consistent with alterations in extracellular K+ concentration in the embryo.Our findings indicate that Kcnj1 is expressed in cells associated with osmoregulation and acts as a K+ efflux pathway that is important in maintaining extracellular levels of K+ in the developing embryo.

Stimulation of A(₂a) adenosine receptor abolishes the inhibitory effect of arachidonic acid on the basolateral 50-pS K channel in the thick ascending limb

The basolateral 50-pS K channels are stimulated by a cAMP-dependent pathway and inhibited by cytochrome P-450-omega-hydroxylase-dependent metabolism of arachidonic acid (AA) in the rat thick ascending limb (TAL). We now used the patch-clamp technique to examine whether stimulation of adenosine A(₂a) receptor modulates the inhibitory effect of AA on the basolateral 50-pS K channels in the medullary TAL. Stimulation of adenosine A(₂a) receptor with CGS-21680 or inhibition of phospholipase A₂ (PLA₂) with AACOCF3 increased the 50-pS K channel activity in the TAL. Western blot demonstrated that application of CGS-21680 decreased the phosphorylation of PLA(2) at serine residue 505, an indication of inhibiting PLA₂ activity. In the presence of CGS-21680, inhibition of PLA₂ had no further effect on the basolateral 50-pS K channels. The possibility that CGS-21680-induced stimulation of the basolateral 50-pS K channels was partially achieved by inhibition of PLA₂ in the TAL was also supported by the observation that CGS-21680 had no additional effect in the presence of AACOCF3. Moreover, stimulation of adenosine A(₂a) receptor with CGS-21680 also abolished the inhibitory effect of AA and 20-hydroxyeicosatetraenoic acid (20-HETE) on the 50-pS K channels. The effect of CGS-21680 on AA and 20-HETE-mediated inhibition of the 50-pS K channels was mediated by cAMP because application of membrane-permeable cAMP analog, dibutyryl-cAMP, not only increased the 50-pS K channel activity but also abolished the inhibitory effect of AA and 20-HETE. We conclude that stimulation of adenosine A(₂a) receptor increased the 50-pS K channel activity in the TAL, an effect that is achieved by suppression of PLA₂ activity and 20-HETE-induced inhibition.

Development of a selective small-molecule inhibitor of Kir1.1, the renal outer medullary potassium channel

The renal outer medullary potassium (K+) channel, ROMK (Kir1.1), is a putative drug target for a novel class of loop diuretic that would lower blood volume and pressure without causing hypokalemia. However, the lack of selective ROMK inhibitors has hindered efforts to assess its therapeutic potential. In a high-throughput screen for small-molecule modulators of ROMK, we previously identified a potent and moderately selective ROMK antagonist, 7,13-bis(4-nitrobenzyl)-1,4,10-trioxa-7,13-diazacyclopentadecane (VU590), that also inhibits Kir7.1. Because ROMK and Kir7.1 are coexpressed in the nephron, VU590 is not a good probe of ROMK function in the kidney. Here we describe the development of the structurally related inhibitor 2,2'-oxybis(methylene)bis(5-nitro-1H-benzo[d]imidazole) (VU591), which is as potent as VU590 but is selective for ROMK over Kir7.1 and more than 65 other potential off-targets. VU591 seems to block the intracellular pore of the channel. The development of VU591 may enable studies to explore the viability of ROMK as a diuretic target.

AMP-activated protein kinase inhibits KCNQ1 channels through regulation of the ubiquitin ligase Nedd4-2 in renal epithelial cells

The KCNQ1 K(+) channel plays a key role in the regulation of several physiological functions, including cardiac excitability, cardiovascular tone, and body electrolyte homeostasis. The metabolic sensor AMP-activated protein kinase (AMPK) has been shown to regulate a growing number of ion transport proteins. To determine whether AMPK regulates KCNQ1, we studied the effects of AMPK activation on KCNQ1 currents in Xenopus laevis oocytes and collecting duct epithelial cells. AMPK activation decreased KCNQ1 currents and channel surface expression in X. laevis oocytes, but AMPK did not phosphorylate KCNQ1 in vitro, suggesting an indirect regulatory mechanism. As it has been recently shown that the ubiquitin-protein ligase Nedd4-2 inhibits KCNQ1 plasma membrane expression and that AMPK regulates epithelial Na(+) channels via Nedd4-2, we examined the role of Nedd4-2 in the AMPK-dependent regulation of KCNQ1. Channel inhibition by AMPK was blocked in oocytes coexpressing either a dominant-negative or constitutively active Nedd4-2 mutant, or a Nedd4-2 interaction-deficient KCNQ1 mutant, suggesting that Nedd4-2 participates in the regulation of KCNQ1 by AMPK. KCNQ1 is expressed at the basolateral membrane in mouse polarized kidney cortical collecting duct (mpkCCD(c14)) cells and in rat kidney. Treatment with the AMPK activators AICAR (2 mM) or metformin (1 mM) reduced basolateral KCNQ1 currents in apically permeabilized polarized mpkCCD(c14) cells. Moreover, AICAR treatment of rat kidney slices ex vivo induced AMPK activation and intracellular redistribution of KCNQ1 from the basolateral membrane in collecting duct principal cells. AICAR treatment also induced increased ubiquitination of KCNQ1 immunoprecipitated from kidney slice homogenates. These results indicate that AMPK inhibits KCNQ1 activity by promoting Nedd4-2-dependent channel ubiquitination and retrieval from the plasma membrane.

Physiological carbon dioxide, bicarbonate, and pH sensing

In biological systems, carbon dioxide exists in equilibrium with bicarbonate and protons. The individual components of this equilibrium (i.e., CO₂, HCO₃⁻, and H(+)), which must be sensed to be able to maintain cellular and organismal pH, also function as signals to modulate multiple physiological functions. Yet, the molecular sensors for CO₂/HCO₃⁻/pH remained unknown until recently. Here, we review recent progress in delineating molecular and cellular mechanisms for sensing CO₂, HCO₃⁻, and pH.