BK-β1 subunit: immunolocalization in the mammalian connecting tubule and its role in the kaliuretic response to volume expansion

Large conductance voltage-and calcium-activated K+ (BK) channel in health and disease

Large Conductance Voltage- and Calcium-activated K+ (BK) channels are transmembrane pore-forming proteins that regulate cell excitability and are also expressed in non-excitable cells. They play a role in regulating vascular tone, neuronal excitability, neurotransmitter release, and muscle contraction. Dysfunction of the BK channel can lead to arterial hypertension, hearing disorders, epilepsy, and ataxia. Here, we provide an overview of BK channel functioning and the implications of its abnormal functioning in various diseases. Understanding the function of BK channels is crucial for comprehending the mechanisms involved in regulating vital physiological processes, both in normal and pathological conditions, controlled by BK. This understanding may lead to the development of therapeutic interventions to address BK channelopathies.

Aldosterone: Renal Action and Physiological Effects

Aldosterone exerts profound effects on renal and cardiovascular physiology. In the kidney, aldosterone acts to preserve electrolyte and acid-base balance in response to changes in dietary sodium (Na+ ) or potassium (K+ ) intake. These physiological actions, principally through activation of mineralocorticoid receptors (MRs), have important effects particularly in patients with renal and cardiovascular disease as demonstrated by multiple clinical trials. Multiple factors, be they genetic, humoral, dietary, or otherwise, can play a role in influencing the rate of aldosterone synthesis and secretion from the adrenal cortex. Normally, aldosterone secretion and action respond to dietary Na+ intake. In the kidney, the distal nephron and collecting duct are the main targets of aldosterone and MR action, which stimulates Na+ absorption in part via the epithelial Na+ channel (ENaC), the principal channel responsible for the fine-tuning of Na+ balance. Our understanding of the regulatory factors that allow aldosterone, via multiple signaling pathways, to function properly clearly implicates this hormone as central to many pathophysiological effects that become dysfunctional in disease states. Numerous pathologies that affect blood pressure (BP), electrolyte balance, and overall cardiovascular health are due to abnormal secretion of aldosterone, mutations in MR, ENaC, or effectors and modulators of their action. Study of the mechanisms of these pathologies has allowed researchers and clinicians to create novel dietary and pharmacological targets to improve human health. This article covers the regulation of aldosterone synthesis and secretion, receptors, effector molecules, and signaling pathways that modulate its action in the kidney. We also consider the role of aldosterone in disease and the benefit of mineralocorticoid antagonists. © 2023 American Physiological Society. Compr Physiol 13:4409-4491, 2023.

Paracrine and endocrine regulation of renal potassium secretion

The seminal studies conducted by Giebisch and colleagues in the 1960s paved the way for understanding the renal mechanisms involved in K+ homeostasis. It was demonstrated that differential handling of K+ in the distal segments of the nephron is crucial for proper K+ balance. Although aldosterone had been classically ascribed as the major ion transport regulator in the distal nephron, thereby contributing to K+ homeostasis, it became clear that aldosterone per se could not explain the kidney's ability to modulate kaliuresis in both acute and chronic settings. The existence of alternative kaliuretic and antikaliuretic mechanisms was suggested by physiological studies in the 1980s but only gained form and shape with the advent of molecular biology. It is now established that the kidneys recruit several endocrine and paracrine mechanisms for adequate kaliuretic response. These mechanisms include the direct effects of peritubular K+, a gut-kidney regulatory axis sensing dietary K+ levels, the kidney secretion of kallikrein during postprandial periods, the upregulation of angiotensin II receptors in the distal nephron during chronic changes in the K+ diet, and the local increase of prostaglandins by low K+ diet. This review discusses recent advances in the understanding of endocrine and paracrine mechanisms underlying the modulation of K+ secretion and how these mechanisms impact kaliuresis and K+ balance. We also highlight important unknowns about the regulation of renal K+ excretion under physiological circumstances.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Abnormal Expression of Fgf9 during the Development of the Anorectum in Rat Embryos with Anorectal Malformations

The study objective was to investigate the role of fibroblast growth factor 9 (Fgf9) in normal and anorectal malformation (ARM) embryos during the development of the anorectum. Fgf9 expression was assayed in both normal rat embryos and embryos with ARM induced by exposure to ethylenethiourea (ETU) on embryonic day 10 (E10). Fgf9 expression was assayed by immunohistochemical staining, Western blotting, and real-time quantitative polymerase chain reaction (qRT-PCR). Immunohistochemical staining revealed spatiotemporal changes in Fgf9 expression between E13 and E16. Fgf9-positive cells predominated in the mesenchyme of the cloaca on E13 and E14 and at the fusion site of the urorectal septum and cloacal membrane, rectal epithelium, and anal membrane on E15. Fgf9-positive cells were obviously decreased after the anal membrane ruptured on E16. Fgf9-positive staining was significantly decreased in embryos with ARM compared with normal embryos from E13 to E15. The results of Western blots and qRT-PCR were consistent, with significantly increased Fgf9 expression in the hindgut and rectum of normal embryos than in embryos with ARM from E13 to E15. However, there was no difference between the two groups on E16. These results suggested that the anorectal embryogenesis might depend on the induction of Fgf9 signal. The expression of Fgf9 was downregulated in ETU-induced ARM embryos, which might be related to the development of ARM.

Regulation of BK Channels by Beta and Gamma Subunits

Ca²⁺- and voltage-gated K⁺ channels of large conductance (BK channels) are expressed in a diverse variety of both excitable and inexcitable cells, with functional properties presumably uniquely calibrated for the cells in which they are found. Although some diversity in BK channel function, localization, and regulation apparently arises from cell-specific alternative splice variants of the single pore-forming α subunit ( KCa1.1, Kcnma1, Slo1) gene, two families of regulatory subunits, β and γ, define BK channels that span a diverse range of functional properties. We are just beginning to unravel the cell-specific, physiological roles served by BK channels of different subunit composition.

Aldosterone and Ion Channels

Since its discovery, aldosterone and ion modulation have been entwined. While scientific investigations throughout the decades have emphasized aldosterone's connection to Na⁺, K⁺, and H⁺ homeostasis, more recent research has demonstrated a relationship between aldosterone and Mg²⁺, Ca²⁺, and Cl^- homeostasis. The mechanisms connecting aldosterone to ion regulation frequently involve ion channels; the membrane localized proteins containing at least one aqueous pore for ion conduction. In order to precisely control intracellular or intraorganelle ion concentrations, ion channels have evolved highly specific regions within the conduction pore that select ions by charge, size, and/or dehydration energy requirement, meaning aldosterone must be able to modulate multiple ion channels to regulate the many ions described above. The list of ion channels presently connected to aldosterone includes ENaC (Na⁺), ROMK/BK (K⁺), TRPV4/5/6 (Ca²⁺), TRPM7/6 (Mg²⁺), and ClC-K/CFTR (Cl^-), among others. This list is only expected to grow over time, as the promiscuity of aldosterone becomes more understood.

Caveolae facilitate TRPV4-mediated Ca2+ signaling and the hierarchical activation of Ca2+-activated K+ channels in K+-secreting renal collecting duct cells

Transient receptor potential cation channel subfamily V member 4 (TRPV4)-mediated Ca²⁺ signaling induces early activation of small/intermediate Ca²⁺-activated K⁺ channels, SK3 (KCNN3) and IK1 (KCNN4), which leads to membrane hyperpolarization and enhanced Ca²⁺ influx, which is critical for subsequent activation of the large conductance Ca²⁺-activated K⁺ channel BK (KCNMA1) and K⁺ secretion in kidney cortical collecting duct (CCD) cells. The focus of the present study was to determine if such coordinated hierarchical/sequential activation of these channels in CCD was orchestrated within caveolae, a known microcompartment underlying selective Ca²⁺-signaling events in other cells. In K⁺-secreting mouse principal cell (PC) line, mCCDcl1 cells, knockdown of caveolae caveolin-1 (CAV-1) depressed TRPV4-mediated Ca²⁺ signaling and activation of SK3, intermediate conductance channel (IK1), and BK. Immunofluorescence colocalization analysis and coimmunoprecipitation assays demonstrated direct coupling of TRPV4 with each of the KCa channels in both mCCDcl1 and whole mouse kidney homogenates. Likewise, extending this analysis to CAV-1 demonstrates colocalization and direct coupling of CAV-1 with TRPV4, SK3, IK1, and BK, providing strong support for coupling of the channels in caveolae microdomains. Furthermore, differential expression of CAV-1 along the CCD was apparent where CAV-1 was strongly expressed within and along the cell borders of kidney PCs and intercalated cells (ICs), although significantly less in ICs. It is concluded that caveolae provide a key microdomain in PCs and ICs for coupling of TRPV4 with SK3, IK1, and BK that directly contributes to TRPV4-mediated Ca²⁺ signaling in these domains leading to rapid and sequential coupling of TRPV4-SK3/IK1-BK that may play a central role in mediating Ca²⁺-dependent regulation of BK and K⁺ secretion.

Aldosterone, SGK1, and ion channels in the kidney

Hyperaldosteronism, a common cause of hypertension, is strongly connected to Na⁺, K⁺, and Mg²⁺ dysregulation. Owing to its steroidal structure, aldosterone is an active transcriptional modifier when bound to the mineralocorticoid receptor (MR) in cells expressing the enzyme 11β-hydroxysteroid dehydrogenase 2, such as those comprising the aldosterone-sensitive distal nephron (ASDN). One such up-regulated protein, the ubiquitous serum and glucocorticoid regulated kinase 1 (SGK1), has the capacity to modulate the surface expression and function of many classes of renal ion channels, including those that transport Na⁺ (ENaC), K⁺ (ROMK/BK), Ca²⁺ (TRPV4/5/6), Mg²⁺ (TRPM7/6), and Cl⁻ (ClC-K, CFTR). Here, we discuss the mechanisms by which ASDN expressed channels are up-regulated by SGK1, while highlighting newly discovered pathways connecting aldosterone to nonselective cation channels that are permeable to Mg²⁺ (TRPM7) or Ca²⁺ (TRPV4).

Dynamic coupling between TRPV4 and Ca2+-activated SK1/3 and IK1 K+ channels plays a critical role in regulating the K+-secretory BK channel in kidney collecting duct cells

The large-conductance Ca²⁺-activated K⁺ channel, BK (KCNMA1), is expressed along the connecting tubule (CNT) and cortical collecting duct (CCD) where it underlies flow- and Ca²⁺-dependent K⁺ secretion. Its activity is partially under the control of the mechanosensitive transient receptor potential vanilloid type 4 (TRPV4) Ca²⁺-permeable channel. Recently, we identified three small-/intermediate-conductance Ca²⁺-activated K⁺ channels, SK1 (KCNN1), SK3 (KCNN3), and IK1 (KCNN4), with notably high Ca²⁺-binding affinities, that are expressed in CNT/CCD and may be regulated by TRPV4-mediated Ca²⁺ influx. The K⁺-secreting CCD mCCDcl1 cells, which express these channels, were used to determine whether SK1/3 and IK1 are activated on TRPV4 stimulation and whether they contribute to Ca²⁺ influx and activation of BK. Activation of TRPV4 (GSK1016790A) modestly depolarized the membrane potential and robustly increased intracellular Ca²⁺, [Ca²⁺]_i Inhibition of both SK1/3 and IK1 by application of apamin and 1-[(2-chlorophenyl)diphenylmethyl]-1H-pyrazole (TRAM-34), respectively, further depolarized the membrane potential and markedly suppressed the TRPV4-mediated rise in [Ca²⁺]_i Application of BK inhibitor iberiotoxin after activation of TRPV4 without apamin/TRAM-34 also reduced [Ca²⁺]_i and further intensified membrane depolarization, demonstrating BK involvement. However, the BK-dependent effects on [Ca²⁺]_i and membrane potential were largely abolished by pretreatment with apamin and TRAM-34, identical to that observed by separately suppressing TRPV4-mediated Ca²⁺ influx, demonstrating that SK1/3-IK1 channels potently contribute to TRPV4-mediated BK activation. Our data indicate a direct correlation between TRPV4-mediated Ca²⁺ signal and BK activation but where early activation of SK1/3 and IK1 channels are critical to sufficiently enhanced Ca²⁺ entry and [Ca²⁺]_i levels required for activation of BK.

Spatiotemporal expression of Wnt3a during striated muscle complex development in rat embryos with ethylenethiourea-induced anorectal malformations

Numerous patients with anorectal malformations (ARMs) continue to experience fecal incontinence and constipation following surgical procedures. One of the most important factors that influences defecation is the striated muscle complex (SMC). Wnt signaling regulates the expression of myogenic regulatory factors, which serve an important role in muscle development. Therefore, the present study aimed to investigate the expression pattern of Wnt3a protein (by immunohistochemistry and western blot analysis) and mRNA [by reverse transcription-quantitative polymerase chain reaction (RT-qPCR)] in the SMC of ARM model rats that were exposed to ethylenethiourea. Immunostaining revealed that the expression of Wnt3a exhibits space- and time-dependent changes in the developing SMC of ARM model rat embryos. Immunohistochemistry demonstrated that on embryonic day 17 (E17), Wnt3a-positive cells were observed in the SMC in normal embryos, and expression levels gradually increased as the rat embryos developed. Similar changes in Wnt3a protein expression were detected in ARM model rat embryos; however, the expression of Wnt3a was significantly reduced compared with the normal rat embryos. Western blotting and RT-qPCR also revealed lower expression levels of Wnt3a protein and mRNA, respectively, in the SMC of ARMs model rat embryos compared with normal rat embryos. These data revealed that the expression of Wnt3a in ARM embryos was notably reduced, indicating a potential role for Wnt3a in the maldevelopment of the SMC in patients with ARMs.

The renal TRPV4 channel is essential for adaptation to increased dietary potassium

To maintain potassium homeostasis, kidneys exert flow-dependent potassium secretion to facilitate kaliuresis in response to elevated dietary potassium intake. This process involves stimulation of calcium-activated large conductance maxi-K (BK) channels in the distal nephron, namely the connecting tubule and the collecting duct. Recent evidence suggests that the TRPV4 channel is a critical determinant of flow-dependent intracellular calcium elevations in these segments of the renal tubule. Here, we demonstrate that elevated dietary potassium intake (five percent potassium) increases renal TRPV4 mRNA and protein levels in an aldosterone-dependent manner and causes redistribution of the channel to the apical plasma membrane in native collecting duct cells. This, in turn, leads to augmented TRPV4-mediated flow-dependent calcium ion responses in freshly isolated split-opened collecting ducts from mice fed the high potassium diet. Genetic TRPV4 ablation greatly diminished BK channel activity in collecting duct cells pointing to a reduced capacity to excrete potassium. Consistently, elevated potassium intake induced hyperkalemia in TRPV4 knockout mice due to deficient renal potassium excretion. Thus, regulation of TRPV4 activity in the distal nephron by dietary potassium is an indispensable component of whole body potassium balance.

Role and mechanisms of regulation of the basolateral Kir 4.1/Kir 5.1K+ channels in the distal tubules

Epithelial K+ channels are essential for maintaining electrolyte and fluid homeostasis in the kidney. It is recognized that basolateral inward-rectifying K+ (Kir ) channels play an important role in the control of resting membrane potential and transepithelial voltage, thereby modulating water and electrolyte transport in the distal part of nephron and collecting duct. Monomeric Kir 4.1 (encoded by Kcnj10 gene) and heteromeric Kir 4.1/Kir 5.1 (Kir 4.1 together with Kir 5.1 (Kcnj16)) channels are abundantly expressed at the basolateral membranes of the distal convoluted tubule and the cortical collecting duct cells. Loss-of-function mutations in KCNJ10 cause EAST/SeSAME tubulopathy in humans associated with salt wasting, hypomagnesaemia, metabolic alkalosis and hypokalaemia. In contrast, mice lacking Kir 5.1 have severe renal phenotype that, apart from hypokalaemia, is the opposite of the phenotype seen in EAST/SeSAME syndrome. Experimental advances using genetic animal models provided critical insights into the physiological role of these channels in electrolyte homeostasis and the control of kidney function. Here, we discuss current knowledge about K+ channels at the basolateral membrane of the distal tubules with specific focus on the homomeric Kir 4.1 and heteromeric Kir 4.1/Kir 5.1 channels. Recently identified molecular mechanisms regulating expression and activity of these channels, such as cell acidification, dopamine, insulin and insulin-like growth factor-1, Src family protein tyrosine kinases, as well as the role of these channels in NCC-mediated transport in the distal convoluted tubules, are also described.

The mechanosensitive BKα/β1 channel localizes to cilia of principal cells in rabbit cortical collecting duct (CCD)

Within the CCD of the distal nephron of the rabbit, the BK (maxi K) channel mediates Ca²⁺- and/or stretch-dependent flow-induced K⁺ secretion (FIKS) and contributes to K⁺ adaptation in response to dietary K⁺ loading. An unresolved question is whether BK channels in intercalated cells (ICs) and/or principal cells (PCs) in the CCD mediate these K⁺ secretory processes. In support of a role for ICs in FIKS is the higher density of immunoreactive apical BKα (pore-forming subunit) and functional BK channel activity than detected in PCs, and an increase in IC BKα expression in response to a high-K⁺ diet. PCs possess a single apical cilium which has been proposed to serve as a mechanosensor; direct manipulation of cilia leads to increases in cell Ca²⁺ concentration, albeit of nonciliary origin. Immunoperfusion of isolated and fixed CCDs isolated from control K⁺-fed rabbits with channel subunit-specific antibodies revealed colocalization of immunodetectable BKα- and β1-subunits in cilia as well as on the apical membrane of cilia-expressing PCs. Ciliary BK channels were more easily detected in rabbits fed a low-K⁺ vs. high-K⁺ diet. Single-channel recordings of cilia revealed K⁺ channels with conductance and kinetics typical of the BK channel. The observations that 1) FIKS was preserved but 2) the high-amplitude Ca²⁺ peak elicited by flow was reduced in microperfused CCDs subject to pharmacological deciliation suggest that cilia BK channels do not contribute to K⁺ secretion in this segment, but that cilia serve as modulators of cell signaling.

Expression of a Diverse Array of Ca2+-Activated K+ Channels (SK1/3, IK1, BK) that Functionally Couple to the Mechanosensitive TRPV4 Channel in the Collecting Duct System of Kidney

The voltage- and Ca²⁺-activated, large conductance K⁺ channel (BK, maxi-K) is expressed in the collecting duct system of kidney where it underlies flow- and Ca²⁺-dependent K⁺ excretion. To determine if other Ca²⁺-activated K⁺ channels (KCa) may participate in this process, mouse kidney and the K⁺-secreting mouse cortical collecting duct (CCD) cell line, mCCDcl1, were assessed for TRPV4 and KCa channel expression and cross-talk. qPCR mRNA analysis and immunocytochemical staining demonstrated TRPV4 and KCa expression in mCCDcl1 cells and kidney connecting tubule (CNT) and CCD. Three subfamilies of KCa channels were revealed: the high Ca²⁺-binding affinity small-conductance SK channels, SK1and SK3, the intermediate conductance channel, IK1, and the low Ca²⁺-binding affinity, BK channel (BKα subunit). Apparent expression levels varied in CNT/CCD where analysis of CCD principal cells (PC) and intercalated cells (IC) demonstrated differential staining: SK1:PC<IC, and SK3:PC>IC, IK1:PC>IC, BKα:PC = IC, and TRPV4:PC>IC. Patch clamp analysis and fluorescence Ca²⁺ imaging of mCCDcl1 cells demonstrated potent TRPV4-mediated Ca²⁺ entry and strong functional cross-talk between TRPV4 and KCa channels. TRPV4-mediated Ca²⁺ influx activated each KCa channel, as evidenced by selective inhibition of KCa channels, with each active KCa channel enhancing Ca²⁺ entry (due to membrane hyperpolarization). Transepithelial electrical resistance (TEER) analysis of confluent mCCDcl1 cells grown on permeable supports further demonstrated this cross-talk where TRPV4 activation induce a decrease in TEER which was partially restored upon selective inhibition of each KCa channel. It is concluded that SK1/SK3 and IK1 are highly expressed along with BKα in CNT and CCD and are closely coupled to TRPV4 activation as observed in mCCDcl1 cells. The data support a model in CNT/CCD segments where strong cross talk between TRPV4-mediated Ca²⁺ influx and each KCa channel leads to enhance Ca²⁺ entry which will support activation of the low Ca²⁺-binding affinity BK channel to promote BK-mediated K⁺ secretion.

Hyperaldosteronism after decreased renal K+ excretion in KCNMB2 knockout mice

The kidney is the primary organ ensuring K(+) homeostasis. K(+) is secreted into the urine in the distal tubule by two mechanisms: by the renal outer medullary K(+) channel (Kir1.1) and by the Ca(2+)-activated K(+) channel (KCa1.1). Here, we report a novel knockout mouse of the β2-subunit of the KCa1.1 channel (KCNMB2), which displays hyperaldosteronism after decreased renal K(+) excretion. KCNMB2(-/-) mice displayed hyperaldosteronism, normal plasma K(+) concentration, and produced dilute urine with decreased K(+) concentration. The normokalemia indicated that hyperaldosteronism did not result from primary aldosteronism. Activation of the renin-angiotensin-aldosterone system was also ruled out as renal renin mRNA expression was reduced in KCNMB2(-/-) mice. Renal K(+) excretion rates were similar in the two genotypes; however, KCNMB2(-/-) mice required elevated plasma aldosterone to achieve K(+) balance. Blockade of the mineralocorticoid receptor with eplerenone triggered mild hyperkalemia and unmasked reduced renal K(+) excretion in KCNMB2(-/-) mice. Knockout mice for the α-subunit of the KCa1.1 channel (KCNMA1(-/-) mice) have hyperaldosteronism, are hypertensive, and lack flow-induced K(+) secretion. KCNMB2(-/-) mice share the phenotypic traits of normokalemia and hyperaldosteronism with KCNMA1(-/-) mice but were normotensive and displayed intact flow-induced K(+) secretion. Despite elevated plasma aldosterone, KNCMB2(-/-) mice did not display salt-sensitive hypertension and were able to decrease plasma aldosterone on a high-Na(+) diet, although plasma aldosterone remained elevated in KCNMB2(-/-) mice. In summary, KCNMB2(-/-) mice have a reduced ability to excrete K(+) into the urine but achieve K(+) balance through an aldosterone-mediated, β2-independent mechanism. The phenotype of KCNMB2 mice was similar but milder than the phenotype of KCNMA1(-/-) mice.

Maintaining K+ balance on the low-Na+, high-K+ diet

A low-Na⁺, high-K⁺ diet (LNaHK) is considered a healthier alternative to the "Western" high-Na⁺ diet. Because the mechanism for K⁺ secretion involves Na⁺ reabsorptive exchange for secreted K⁺ in the distal nephron, it is not understood how K⁺ is eliminated with such low Na⁺ intake. Animals on a LNaHK diet produce an alkaline load, high urinary flows, and markedly elevated plasma ANG II and aldosterone levels to maintain their K⁺ balance. Recent studies have revealed a potential mechanism involving the actions of alkalosis, urinary flow, elevated ANG II, and aldosterone on two types of K⁺ channels, renal outer medullary K⁺ and large-conductance K⁺ channels, located in principal and intercalated cells. Here, we review these recent advances.

Cell-specific regulation of L-WNK1 by dietary K

Flow-induced K(+) secretion in the aldosterone-sensitive distal nephron is mediated by high-conductance Ca(2+)-activated K(+) (BK) channels. Familial hyperkalemic hypertension (pseudohypoaldosteronism type II) is an inherited form of hypertension with decreased K(+) secretion and increased Na(+) reabsorption. This disorder is linked to mutations in genes encoding with-no-lysine kinase 1 (WNK1), WNK4, and Kelch-like 3/Cullin 3, two components of an E3 ubiquitin ligase complex that degrades WNKs. We examined whether the full-length (or "long") form of WNK1 (L-WNK1) affected the expression of BK α-subunits in HEK cells. Overexpression of L-WNK1 promoted a significant increase in BK α-subunit whole cell abundance and functional channel expression. BK α-subunit abundance also increased with coexpression of a kinase dead L-WNK1 mutant (K233M) and with kidney-specific WNK1 (KS-WNK1), suggesting that the catalytic activity of L-WNK1 was not required to increase BK expression. We examined whether dietary K(+) intake affected L-WNK1 expression in the aldosterone-sensitive distal nephron. We found a paucity of L-WNK1 labeling in cortical collecting ducts (CCDs) from rabbits on a low-K(+) diet but observed robust staining for L-WNK1 primarily in intercalated cells when rabbits were fed a high-K(+) diet. Our results and previous findings suggest that L-WNK1 exerts different effects on renal K(+) secretory channels, inhibiting renal outer medullary K(+) channels and activating BK channels. A high-K(+) diet induced an increase in L-WNK1 expression selectively in intercalated cells and may contribute to enhanced BK channel expression and K(+) secretion in CCDs.

Roles and Regulation of Renal K Channels

More than two dozen types of potassium channels, with different biophysical and regulatory properties, are expressed in the kidney, influencing renal function in many important ways. Recently, a confluence of discoveries in areas from human genetics to physiology, cell biology, and biophysics has cast light on the special function of five different potassium channels in the distal nephron, encoded by the genes KCNJ1, KCNJ10, KCNJ16, KCNMA1, and KCNN3. Research aimed at understanding how these channels work in health and go awry in disease has transformed our understanding of potassium balance and provided new insights into mechanisms of renal sodium handling and the maintenance of blood pressure. This review focuses on recent advances in this rapidly evolving field.

An unexpected journey: conceptual evolution of mechanoregulated potassium transport in the distal nephron

Flow-induced K secretion (FIKS) in the aldosterone-sensitive distal nephron (ASDN) is mediated by large-conductance, Ca(2+)/stretch-activated BK channels composed of pore-forming α-subunits (BKα) and accessory β-subunits. This channel also plays a critical role in the renal adaptation to dietary K loading. Within the ASDN, the cortical collecting duct (CCD) is a major site for the final renal regulation of K homeostasis. Principal cells in the ASDN possess a single apical cilium whereas the surfaces of adjacent intercalated cells, devoid of cilia, are decorated with abundant microvilli and microplicae. Increases in tubular (urinary) flow rate, induced by volume expansion, diuretics, or a high K diet, subject CCD cells to hydrodynamic forces (fluid shear stress, circumferential stretch, and drag/torque on apical cilia and presumably microvilli/microplicae) that are transduced into increases in principal (PC) and intercalated (IC) cell cytoplasmic Ca(2+) concentration that activate apical voltage-, stretch- and Ca(2+)-activated BK channels, which mediate FIKS. This review summarizes studies by ourselves and others that have led to the evolving picture that the BK channel is localized in a macromolecular complex at the apical membrane, composed of mechanosensitive apical Ca(2+) channels and a variety of kinases/phosphatases as well as other signaling molecules anchored to the cytoskeleton, and that an increase in tubular fluid flow rate leads to IC- and PC-specific responses determined, in large part, by the cell-specific composition of the BK channels.

Deciphering physiological role of the mechanosensitive TRPV4 channel in the distal nephron

Long-standing experimental evidence suggests that epithelial cells in the renal tubule are able to sense osmotic and pressure gradients caused by alterations in ultrafiltrate flow by elevating intracellular Ca(2+) concentration. These responses are viewed as critical regulators of a variety of processes ranging from transport of water and solutes to cellular growth and differentiation. A loss in the ability to sense mechanical stimuli has been implicated in numerous pathologies associated with systemic imbalance of electrolytes and to the development of polycystic kidney disease. The molecular mechanisms conferring mechanosensitive properties to epithelial tubular cells involve activation of transient receptor potential (TRP) channels, such as TRPV4, allowing direct Ca(2+) influx to increase intracellular Ca(2+) concentration. In this review, we critically analyze the current evidence about signaling determinants of TRPV4 activation by luminal flow in the distal nephron and discuss how dysfunction of this mechanism contributes to the progression of polycystic kidney disease. We also review the physiological relevance of TRPV4-based mechanosensitivity in controlling flow-dependent K(+) secretion in the distal renal tubule.

Distal convoluted tubule

The distal convoluted tubule (DCT) is a short nephron segment, interposed between the macula densa and collecting duct. Even though it is short, it plays a key role in regulating extracellular fluid volume and electrolyte homeostasis. DCT cells are rich in mitochondria, and possess the highest density of Na+/K+-ATPase along the nephron, where it is expressed on the highly amplified basolateral membranes. DCT cells are largely water impermeable, and reabsorb sodium and chloride across the apical membrane via electroneurtral pathways. Prominent among this is the thiazide-sensitive sodium chloride cotransporter, target of widely used diuretic drugs. These cells also play a key role in magnesium reabsorption, which occurs predominantly, via a transient receptor potential channel (TRPM6). Human genetic diseases in which DCT function is perturbed have provided critical insights into the physiological role of the DCT, and how transport is regulated. These include Familial Hyperkalemic Hypertension, the salt-wasting diseases Gitelman syndrome and EAST syndrome, and hereditary hypomagnesemias. The DCT is also established as an important target for the hormones angiotensin II and aldosterone; it also appears to respond to sympathetic-nerve stimulation and changes in plasma potassium. Here, we discuss what is currently known about DCT physiology. Early studies that determined transport rates of ions by the DCT are described, as are the channels and transporters expressed along the DCT with the advent of molecular cloning. Regulation of expression and activity of these channels and transporters is also described; particular emphasis is placed on the contribution of genetic forms of DCT dysregulation to our understanding.

Chemical and Physical Sensors in the Regulation of Renal Function

In order to assess the status of the volume and composition of the body fluid compartment, the kidney monitors a wide variety of chemical and physical parameters. It has recently become clear that the kidney's sensory capacity extends well beyond its ability to sense ion concentrations in the forming urine. The kidney also keeps track of organic metabolites derived from a surprising variety of sources and uses a complex interplay of physical and chemical sensing mechanisms to measure the rate of fluid flow in the nephron. Recent research has provided new insights into the nature of these sensory mechanisms and their relevance to renal function.

Effects of biomechanical forces on signaling in the cortical collecting duct (CCD)

An increase in tubular fluid flow rate (TFF) stimulates Na reabsorption and K secretion in the cortical collecting duct (CCD) and subjects cells therein to biomechanical forces including fluid shear stress (FSS) and circumferential stretch (CS). Intracellular MAPK and extracellular autocrine/paracrine PGE2 signaling regulate cation transport in the CCD and, at least in other systems, are affected by biomechanical forces. We hypothesized that FSS and CS differentially affect MAPK signaling and PGE2 release to modulate cation transport in the CCD. To validate that CS is a physiological force in vivo, we applied the intravital microscopic approach to rodent kidneys in vivo to show that saline or furosemide injection led to a 46.5 ± 2.0 or 170 ± 32% increase, respectively, in distal tubular diameter. Next, murine CCD (mpkCCD) cells were grown on glass or silicone coated with collagen type IV and subjected to 0 or 0.4 dyne/cm(2) of FSS or 10% CS, respectively, forces chosen based on prior biomechanical modeling of ex vivo microperfused CCDs. Cells exposed to FSS expressed an approximately twofold greater abundance of phospho(p)-ERK and p-p38 vs. static cells, while CS did not alter p-p38 and p-ERK expression compared with unstretched controls. FSS induced whereas CS reduced PGE2 release by ∼40%. In conclusion, FSS and CS differentially affect ERK and p38 activation and PGE2 release in a cell culture model of the CD. We speculate that TFF differentially regulates biomechanical signaling and, in turn, cation transport in the CCD.

Emerging role of the calcium-activated, small conductance, SK3 K+ channel in distal tubule function: regulation by TRPV4

The Ca2+-activated, maxi-K (BK) K+ channel, with low Ca2+-binding affinity, is expressed in the distal tubule of the nephron and contributes to flow-dependent K+ secretion. In the present study we demonstrate that the Ca2+-activated, SK3 (KCa2.3) K+ channel, with high Ca2+-binding affinity, is also expressed in the mouse kidney (RT-PCR, immunoblots). Immunohistochemical evaluations using tubule specific markers demonstrate significant expression of SK3 in the distal tubule and the entire collecting duct system, including the connecting tubule (CNT) and cortical collecting duct (CCD). In CNT and CCD, main sites for K+ secretion, the highest levels of expression were along the apical (luminal) cell membranes, including for both principal cells (PCs) and intercalated cells (ICs), posturing the channel for Ca2+-dependent K+ secretion. Fluorescent assessment of cell membrane potential in native, split-opened CCD, demonstrated that selective activation of the Ca2+-permeable TRPV4 channel, thereby inducing Ca2+ influx and elevating intracellular Ca2+ levels, activated both the SK3 channel and the BK channel leading to hyperpolarization of the cell membrane. The hyperpolarization response was decreased to a similar extent by either inhibition of SK3 channel with the selective SK antagonist, apamin, or by inhibition of the BK channel with the selective antagonist, iberiotoxin (IbTX). Addition of both inhibitors produced a further depolarization, indicating cooperative effects of the two channels on Vm. It is concluded that SK3 is functionally expressed in the distal nephron and collecting ducts where induction of TRPV4-mediated Ca2+ influx, leading to elevated intracellular Ca2+ levels, activates this high Ca2+-affinity K+ channel. Further, with sites of expression localized to the apical cell membrane, especially in the CNT and CCD, SK3 is poised to be a key pathway for Ca2+-dependent regulation of membrane potential and K+ secretion.

Smooth muscle BK channel activity influences blood pressure independent of vascular tone in mice

The large conductance voltage- and Ca(2+)-activated K(+) (BK) channel is an important determinant of vascular tone and contributes to blood pressure regulation. Both activities depend on the ancillary BKβ1 subunit. To determine the significance of smooth muscle BK channel activity for blood pressure regulation, we investigated the potential link between changes in arterial tone and altered blood pressure in BKβ1 knockout (BKβ1(-/-)) mice from three different genetically defined strains. While vascular tone was consistently increased in all BKβ1(-/-) mice independent of genetic background, BKβ1(-/-) strains exhibited increased (strain A), unaltered (strain B) or decreased (strain C) mean arterial blood pressures compared to their corresponding BKβ1(+/+) controls. In agreement with previous data on aldosterone regulation by renal/adrenal BK channel function, BKβ1(-/-) strain A mice have increased plasma aldosterone and increased blood pressure. Consistently, blockade of mineralocorticoid receptors by spironolactone treatment reversibly restored the elevated blood pressure to the BKβ1(+/+) strain A level. In contrast, loss of BKβ1 did not affect plasma aldosterone in strain C mice. Smooth muscle-restricted restoration of BKβ1 expression increased blood pressure in BKβ1(-/-) strain C mice, implying that impaired smooth muscle BK channel activity lowers blood pressure in these animals. We conclude that BK channel activity directly affects vascular tone but influences blood pressure independent of this effect via different pathways.

Regulation of renal potassium secretion: molecular mechanisms

A new understanding of renal potassium balance has emerged as the molecular underpinnings of potassium secretion have become illuminated, highlighting the key roles of apical potassium channels, renal outer medullary potassium channel (ROMK) and Big Potassium (BK), in the aldosterone-sensitive distal nephron and collecting duct. These channels act as the final-regulated components of the renal potassium secretory machinery. Their activity, number, and driving forces are precisely modulated to ensure potassium excretion matches dietary potassium intake. Recent identification of the underlying regulatory mechanisms at the molecular level provides a new appreciation of the physiology and reveals a molecular insight to explain the paradoxic actions of aldosterone on potassium secretion. Here, we review the current state of knowledge in the field.

Interacting influence of diuretics and diet on BK channel-regulated K homeostasis

Large conductance, Ca-activated K channels (BK) are abundantly located in cells of vasculature, glomerulus, and distal nephron, where they are involved in maintaining blood volume, blood pressure, and K homeostasis. In mesangial cells and smooth muscle cells of vessels, the BK-α pore associates with BK-β1 subunits and regulates contraction in a Ca-mediated feedback manner. The BK-β1 also resides in connecting tubule cells of the nephron. BK-β1 knockout mice (β1KO) exhibit fluid retention, hypertension, and compromised K handling. The BK-α/β4 resides in acid/base transporting intercalated cells (IC) of the distal nephron, where they mediate K secretion in mammals on a high K, alkaline diet. BK-α expression in IC is increased by a high K diet via aldosterone. The BK-β4 subunit and alkaline urine are necessary for the luminal expression and function of BK-α in mouse IC. In distal nephron cells, membrane BK-α expression is inhibited by WNK4 in in vitro expression systems, indicating a role in the hyperkalemic phenotype in patients with familial hyperkalemic hypertension type 2 (FHHt2). β1KO and BK-β4 knockout mice (β4KO) are hypertensive because of exaggerated epithelial Na channels (ENaC) mediated Na retention in an effort to secrete K via only renal outer medullary K channels (ROMK). BK hypertension is resistant to thiazides and furosemide, and would be more amenable to ENaC and aldosterone inhibiting drugs. Activators of BK-α/β1 or BK-α/β4 might be effective blood pressure lowering agents for a subset of hypertensive patients. Inhibitors of renal BK would effectively spare K in patients with Bartter Syndrome, a renal K wasting disease.

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.

Regulation of BK-α expression in the distal nephron by aldosterone and urine pH

In the distal nephron, the large-conductance Ca-activated K (BK) channel, comprised of a pore-forming-α (BK-α) and the BK-β4 subunit, promotes K excretion when mice are maintained on a high-K alkaline diet (HK-alk). We examined whether BK-β4 and the acid-base status regulate apical membrane expression of BK-α in the cortical (CCD) and medullary collecting ducts (MCD) using immunohistochemical analysis (IHC) and Western blot. With the use of IHC, BK-α of mice on acontrol diet localized mostly cytoplasmically in intercalated cells (IC) of the CCD and in the perinuclear region of both principle cells (PC) and IC of the MCD. HK-alk wild-type mice (WT), but not BK-β4 knockout mice (β4KO), exhibited increased apical BK-α in both the CCD and MCD. When given a high-K acidic diet (HK-Cl), BK-α expression increased but remained cytoplasmic in the CCD and perinuclear in the MCD of both WT and β4KO. Western blot confirmed that total BK-α expression was enhanced by either HK-alk or HK-Cl but only increased in the plasma membrane with HK-alk. Compared with controls, mice drinking NaHCO3 water exhibited more apical BK-α and total cellular BK-β4. Spironolactone given to mice on HK-alk significantly reduced K secretion and decreased total cellular BK-α but did not affect cellular BK-β4 and apical BK-α. Experiments with MDCK-C11 cells indicated that BK-β4 stabilizes surface BK-α by inhibiting degradation through a lysosomal pathway. These data suggest that aldosterone mediates a high-K-induced increase in BK-α and urinary alkalinization increases BK-β4 expression, which promotes the apical localization of BK-α.

Flow-mediated K(+) secretion in horses intoxicated with lolitrem B (perennial ryegrass staggers)

AIM

To investigate the effects of lolitrem B intoxication on renal K(+) secretion in response to increased tubular flow rates.

METHODS

Results are derived from a repeated measure pilot study of seven horses fed non-perennial ryegrass feed for a week prior to exposing them to perennial ryegrass seed and hay that contained an average of 2 ppm lolitrem B. At the end of the control and treatment period frusemide (1 mg/kg I/V) was administered and serial fractional excretion of K(+)(FEK(+)) and fractional excretion of Na(+)(FENa(+)) calculated. Baseline concentration of aldosterone in plasma, serum K(+)concentration and feed K(+) concentration were also compared.

RESULTS

Key findings included a reduced change in FEK(+) from 0 to 15 minutes in response to frusemide administration (p=0.022, Wilcoxon signed-rank test) and a reduced baseline concentration of aldosterone in plasma (p=0.022, Wilcoxon signed-rank test) during the treatment period compared with the control.

CONCLUSIONS

Results suggest that lolitrem B intoxication reduced flow-mediated K(+) secretion and interfered with aldosterone production or secretion. However, further investigation is required to validate these findings and to further elucidate the underlying pathophysiology.

CLINICAL RELEVANCE

Lolitrem B intoxication in horses may cause disruption to electrolyte handling in addition to neurological deficits.

Novel sensory signaling systems in the kidney

PURPOSE OF REVIEW

This review summarizes recent literature highlighting the roles of chemical and mechanical sensory receptors in renal function.

RECENT FINDINGS

Both chemoreceptors and mechanoreceptors play important roles in renal physiology; here, we discuss specific examples of both chemoreceptors and mechanoreceptors in the kidney.

SUMMARY

In order to maintain homeostasis, the kidney uses sensory receptors to assess the composition and rate of flow of the forming urine. Understanding the roles of these receptors will help us to better understand how the kidney functions both in health and in disease.

Flow-sensitive K+-coupled ATP secretion modulates activity of the epithelial Na+ channel in the distal nephron

The epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN) is under tonic inhibition by a local purinergic signaling system responding to changes in dietary sodium intake. Normal BK(Ca) channel function is required for flow-sensitive ATP secretion in the ASDN. We tested here whether ATP secreted through connexin channels in a coupled manner with K(+) efflux through BK(Ca) channels is required for inhibitory purinergic regulation of ENaC in response to increases in sodium intake. Inhibition of connexin channels relieves purinergic inhibition of ENaC. Deletion of the BK-β4 regulatory subunit, which is required for normal BK(Ca) channel function and flow-sensitive ATP secretion in the ASDN, suppresses increases in urinary ATP in response to increases in sodium intake. As a consequence, ENaC activity, particularly in the presence of high sodium intake, is inappropriately elevated in BK-β4 null mice. ENaC in BK-β4 null mice, however, responds normally to exogenous ATP, indicating that increases in activity do not result from end-organ resistance but rather from lowered urinary ATP. Consistent with this, disruption of purinergic regulation increases ENaC activity in wild type but not BK-β4 null mice. Consequently, sodium excretion is impaired in BK-β4 null mice. These results demonstrate that the ATP secreted in the ASDN in a BK(Ca) channel-dependent manner is physiologically available for purinergic inhibition of ENaC in response to changes in sodium homeostasis. Impaired sodium excretion resulting form loss of normal purinergic regulation of ENaC in BK-β4 null mice likely contributes to their elevated blood pressure.

Bicarbonate promotes BK-α/β4-mediated K excretion in the renal distal nephron

Ca-activated K channels (BK), which are stimulated by high distal nephron flow, are utilized during high-K conditions to remove excess K. Because BK predominantly reside with BK-β4 in acid/base-transporting intercalated cells (IC), we determined whether BK-β4 knockout mice (β4KO) exhibit deficient K excretion when consuming a high-K alkaline diet (HK-alk) vs. high-K chloride diet (HK-Cl). When wild type (WT) were placed on HK-alk, but not HK-Cl, renal BK-β4 expression increased (Western blot). When WT and β4KO were placed on HK-Cl, plasma K concentration ([K]) was elevated compared with control K diets; however, K excretion was not different between WT and β4KO. When HK-alk was consumed, the plasma [K] was lower and K clearance was greater in WT compared with β4KO. The urine was alkaline in mice on HK-alk; however, urinary pH was not different between WT and β4KO. Immunohistochemical analysis of pendrin and V-ATPase revealed the same increases in β-IC, comparing WT and β4KO on HK-alk. We found an amiloride-sensitive reduction in Na excretion in β4KO, compared with WT, on HK-alk, indicating enhanced Na reabsorption as a compensatory mechanism to secrete K. Treating mice with an alkaline, Na-deficient, high-K diet (LNaHK) to minimize Na reabsorption exaggerated the defective K handling of β4KO. When WT on LNaHK were given NH(4)Cl in the drinking water, K excretion was reduced to the magnitude of β4KO on LNaHK. These results show that WT, but not β4KO, efficiently excretes K on HK-alk but not on HK-Cl and suggest that BK-α/β4-mediated K secretion is promoted by bicarbonaturia.

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.

Function of transient receptor potential cation channel subfamily V member 4 (TRPV4) as a mechanical transducer in flow-sensitive segments of renal collecting duct system

The TRPV4 Ca(2+)-permeable channel is sensitive to mechanical stimuli. In the current study we have employed immunocytochemical staining in kidney slices and functional assessments (Ca(2+) imaging) in isolated, split-opened, tubule segments to define TRPV4 sites of expression and flow-dependent function in the collecting duct system. Staining patterns revealed strong expression of TRPV4 along the entire collecting duct system with highest levels at the apical (luminal)/subapical region of the principal cells (PCs), the dominant cell type, with more diffuse staining in intercalated cells (ICs). Using fluorescence Ca(2+) imaging and the selective TRPV4 agonist, GSK1016790A, we demonstrated functional TRPV4 channels in PCs and ICs of split-opened cortical collecting ducts and connecting tubules. The agonist was ineffective in inducing a rise in [Ca(2+)](i) in the absence of extracellular Ca(2+) or in tubules from TRPV4-deficient animals. Most importantly, a 10-fold elevation in luminal (apical) fluid flow induced a rapid and sustained influx of Ca(2+) that was abolished by the TRPV channel inhibitor, ruthenium red, or in tubules isolated from TRPV4 deficient animals. We concluded that TRPV4 is highly expressed along the entire collecting duct system where it appears to function as a sensor/transducer of flow-induce mechanical stresses.

Independent regulation of Na+ and K+ balance by the kidney

The understanding of the independent regulation of sodium and potassium by the kidney has remained elusive. Recent evidence now points to dissimilar regulatory mechanisms in ion handling, dependent on the presence of either aldosterone alone or angiotensin II with aldosterone among other factors. This review summarizes past and present information in an attempt to reconcile the current concepts of differential regulation of sodium and potassium balance through the with-no-lysine (K) kinase (WNK) system and the previous knowledge regarding ion transport mechanisms in the distal nephron.

Role of BK channels in hypertension and potassium secretion

PURPOSE OF REVIEW

This review summarizes recent studies of hypertension associated with a defect in renal K excretion due to genetic deletions of various components of the large, Ca-activated K channel (BK), and describes new evidence and theories regarding K secretory roles of BK in intercalated cells.

RECENT FINDINGS

Isolated perfused tubule methods have revealed the importance of BK in flow-induced K secretion. Subsequently, mice with genetically deleted BK subunits revealed the complexities of BK-mediated K secretion. Deletion of BKα results in extreme aldosteronism, hypertension, and an absence of flow-induced K secretion. Deletion of the BKβ1 ancillary subunit results in decreased handling of a K load, increased plasma K, mild aldosteronism and hypertension that is exacerbated by a high K diet. Deletion of BKβ4 (β4KO) leads to insufficient K handling, high plasma K, fluid retention, but with milder hypertension. Fluid retention in β4KO may be the result of insufficient flow-induced secretion of adenosine triphosphate (ATP), which normally inhibits epithelial Na channels (ENaCs).

SUMMARY

Classical physiological analysis of electrolyte handling in knockout mice has enlightened our understanding of the mechanism of handling K loads by renal K channels. Studies have focused on the different roles of BK-α/β1 and BK-α/β4 in the kidney. BKβ1 hypertension may be a 'three-hit' hypertension, involving a K secretory defect, elevated production of aldosterone, and increased vascular tone. The disorders observed in BK knockout mice have shed new insights on the importance of proper renal K handling for maintaining volume balance and blood pressure.

Role of NKCC in BK channel-mediated net K⁺ secretion in the CCD

Apical SK/ROMK and BK channels mediate baseline and flow-induced K secretion (FIKS), respectively, in the cortical collecting duct (CCD). BK channels are detected in acid-base transporting intercalated (IC) and Na-absorbing principal (PC) cells. Although the density of BK channels is greater in IC than PC, Na-K-ATPase activity in IC is considered inadequate to sustain high rates of urinary K secretion. To test the hypothesis that basolateral NKCC in the CCD contributes to BK channel-mediated FIKS, we measured net K secretion (J(K)) and Na absorption (J(Na)) at slow (∼1) and fast (∼5 nl·min(-1)·mm(-1)) flow rates in rabbit CCDs microperfused in vitro in the absence and presence of bumetanide, an inhibitor of NKCC, added to the bath. Bumetanide inhibited FIKS but not basal J(K), J(Na), or the flow-induced [Ca(2+)](i) transient necessary for BK channel activation. Addition of luminal iberiotoxin, a BK channel inhibitor, to bumetanide-treated CCDs did not further reduce J(K). Basolateral Cl removal reversibly inhibited FIKS but not basal J(K) or J(Na). Quantitative PCR performed on single CCD samples using NKCC1- and 18S-specific primers and probes and the TaqMan assay confirmed the presence of the transcript in this nephron segment. To identify the specific cell type to which basolateral NKCC is localized, we exploited the ability of NKCC to accept NH(4)(+) at its K-binding site to monitor the rate of bumetanide-sensitive cytosolic acidification after NH(4)(+) addition to the bath in CCDs loaded with the pH indicator dye BCECF. Both IC and PC were found to have a basolateral bumetanide-sensitive NH(4)(+) entry step and NKCC1-specific antibodies labeled the basolateral surfaces of both cell types in CCDs. These results suggest that BK channel-mediated FIKS is dependent on a basolateral bumetanide-sensitive, Cl-dependent transport pathway, proposed to be NKCC1, in both IC and PC in the CCD.

Recent advances in distal tubular potassium handling

It is well known that sodium reabsorption and aldosterone play important roles in potassium secretion by the aldosterone-sensitive distal nephron. Sodium- and aldosterone-independent mechanisms also exist. This review focuses on some recent studies that provide novel insights into the sodium- and aldosterone-independent potassium secretion by the aldosterone-sensitive distal nephron. In addition, we discuss a study reporting on the regulation of the mammalian potassium kidney channel ROMK by intracellular and extracellular magnesium, which may be important in the pathogenesis of persistent hypokalemia in patients with concomitant potassium and magnesium deficiency. We also discuss outstanding questions and propose working models for future investigation.

Large-conductance Ca2+-activated K+ channel beta1-subunit knockout mice are not hypertensive

Large-conductance Ca2+-activated K+ (BK) channels are composed of pore-forming α-subunits and accessory β1-subunits that modulate Ca2+ sensitivity. BK channels regulate arterial myogenic tone and renal Na+ clearance/K+ reabsorption. Previous studies using indirect or short-term blood pressure measurements found that BK channel β1-subunit knockout (BK β1-KO) mice were hypertensive. We evaluated 24-h mean arterial pressure (MAP) and heart rate in BK β1-KO mice using radiotelemetry. BK β1-KO mice did not have a higher 24-h average MAP when compared with wild-type (WT) mice, although MAP was ∼10 mmHg higher at night. The dose-dependent peak declines in MAP by nifedipine were only slightly larger in BK β1-KO mice. In BK β1-KO mice, giving 1% NaCl to mice to drink for 7 days caused a transient (5 days) elevation of MAP (∼5 mmHg); MAP returned to pre-saline levels by day 6. BK β1-KO mesenteric arteries in vitro demonstrated diminished contractile responses to paxilline, increased reactivity to Bay K 8644 and norepinephrine (NE), and maintained relaxation to isoproterenol. Paxilline and Bay K 8644 did not constrict WT or BK β1-KO mesenteric veins (MV). BK β1-subunits are not expressed in MV. The results indicate that BK β1-KO mice are not hypertensive on normal or high-salt intake. BK channel deficiency increases arterial reactivity to NE and L-type Ca2+ channel function in vitro, but the L-type Ca2+ channel modulation of MAP is not altered in BK β1-KO mice. BK and L-type Ca(2+) channels do not modulate murine venous tone. It appears that selective loss of BK channel function in arteries only is not sufficient to cause sustained hypertension.

Embryonic development of the internal anal sphincter in rats with anorectal malformations

BACKGROUND/PURPOSE

The embryogenesis of the internal anal sphincter (IAS) in anorectal malformations (ARMs) remains unclear. This study aimed to investigate the development of the smooth muscle in the terminus of the digestive tract in normal and abnormal rats.

METHODS

Rat embryos with ARMs were generated by administration of ethylenethiourea to pregnant rats. The normal rat embryos and embryos with ARMs from E13.5 to E21 were serially sectioned in the sagittal plane and stained immunohistochemically using specific antibody to α-smooth muscle actin (SMA). Temporospatial study was carried out on circular muscle of the distal portion of the hindgut.

RESULTS

α-Smooth muscle actin immunolabeling cells could not be observed in the hindgut on E13.5, E14, and E14.5. On E15, there were α-SMA immunolabeling circular muscle cells in the hindgut; and the distal portion of the circular muscle was not thickened in the normal and ARMs rats. From E16 onward, the smooth muscle with slight dilated terminus, which was characterized by the features of IAS, could be noted in the primitive anorectum. In the normal group, the circular muscle in the distal portion of the hindgut thickened slightly and became the musculature with shutter-like bundles. In the ARMs group, the α-SMA immunolabeling myogenic precursors of the smooth muscle could be observed in the primitive anorectum as well. The musculature was similar to that in the normal group. On E15 and E16, there was no significant difference in the development of the circular muscle in the 2 groups. Moreover, the terminus of the circular muscle in the hindgut did not reach the orificium fistulae in ARMs rats. From E17 onward, in ARMs rats, the funnel-shaped distal hindgut communicated the genitourinary tract with a narrow fistula; the dilated musculature at this portion thinned gradually and formed an acute angled extremity in the ARMs group rather than formed blunt extremity in the normal group; the terminus circular muscle in the dorsal hindgut reached the orificium fistulae. During the following gestational days, the circular muscle of the hindgut in both normal and ARMs rats continued its own tendency.

CONCLUSION

The IAS primordium started to appear at the terminus of the hindgut on E15 in the 2 groups. The IAS in the ARMs group failed to develop as well as that in the normal group. The IAS dysplasia occurred in the late embryonic development (E17-E21).

BK channels and a new form of hypertension

Large, Ca-activated K channels (BK) are comprised of an α pore (BKα) and one of four β subunits (BKβ1-4). When the gene for BKβ1 is knocked out (BKβ1-KO), the result is increased myogenic tone of vascular smooth muscle and hypertension. We reexamined whether the hypertension is entirely due to increased vascular tone, because most monogenic forms of hypertension have renal origins and BKβ1 resides in renal connecting tubule (CNT) cells. Moreover, BKβ1 is localized in the adrenal glands, where it may control production of aldosterone. This review will summarize our report that a majority of the hypertension of BKβ1-KO is the result of insufficient handling of dietary K, resulting in increased plasma K and hyperaldosteronism, the latter promoting Na and fluid retention. The fluid retention and hypertension are exacerbated by a high-K diet and reduced by eplerenone, an aldosterone receptor inhibitor. Genetic knockout of BKβ4 (BKβ4-KO), which resides in intercalated cells, also exhibits deficient K excretion, fluid retention, and mild hypertension that is not exacerbated when animals are treated with a high-K diet. These results show that the hypertension associated with BKβ1-KO occurs because of enhanced fluid retention, as well as because of the previously described vascular dysfunction.

Shear stress-induced volume decrease in C11-MDCK cells by BK-alpha/beta4

Large-conductance, calcium-activated potassium channels (BK) are expressed in principal cells (PC) and intercalated cells (IC) in mammalian nephrons as BK-alpha/beta1 and BK-alpha/beta4, respectively. IC, which protrude into the lumens of tubules, express substantially more BK than PC despite lacking sufficient Na-K-ATPase to support K secretion. We previously showed in mice that IC exhibit size reduction when experiencing high distal flows induced by a high-K diet. We therefore tested the hypothesis that BK-alpha/beta4 are regulators of IC volume via a shear stress (tau)-induced, calcium-dependent mechanism, resulting in a reduction in intracellular K content. We determined by Western blot and immunocytochemical analysis that C11-Madin-Darby canine kidney cells contained a predominance of BK-alpha/beta4. To determine the role of BK-alpha/beta4 in tau-induced volume reduction, we exposed C11 cells to tau and measured K efflux by flame photometry and cell volume by calcein staining, which changes inversely to cell volume. With 10 dynes/cm(2), calcein intensity significantly increased 39% and monovalent cationic content decreased significantly by 37% compared with static conditions. Furthermore, the shear-induced K loss from C11 was abolished by the reduction of extracellular calcium, addition of 5 mM TEA, or BK-beta4 small interfering (si) RNA, but not by addition of nontarget siRNA. These results show that BK-alpha/beta4 plays a role in shear-induced K loss from IC, suggesting that BK-alpha/beta4 regulate IC volume during high-flow conditions. Furthermore, these results support the use of C11 cells as in vitro models for studying BK-related functions in IC of the kidney.

Potassium secretion by voltage-gated potassium channel Kv1.3 in the rat kidney

The fine regulation of Na(+) and K(+) transport takes place in the cortical distal nephron. It is well established that K(+) secretion occurs through apical K(+) channels: the ROMK and the Ca(2+)- and voltage-dependent maxi-K. Previously, we identified the voltage-gated Kv1.3 channel in the inner medulla of the rat kidney (Escobar LI, Martínez-Téllez JC, Salas M, Castilla SA, Carrisoza R, Tapia D, Vázquez M, Bargas J, Bolívar JJ. Am J Physiol Cell Physiol 286: C965-C974, 2004). To examine the role of Kv1.3 in the renal regulation of K(+) homeostasis, we characterized the effect of dietary K(+) on the molecular and functional expression of this channel. We performed real-time-PCR and immunoblot assays in kidneys from rats fed a control (CK; 1.2% wt/wt) or high-K(+) (HK; 10% wt/wt) diet for 5-15 days. Kv1.3 mRNA and protein expression did not change with HK in the whole kidney. However, dietary K(+) loading provoked a change in the cellular distribution of Kv1.3 from the cytoplasm to apical membranes. Immunolocalization of Kv1.3 detected the channel exclusively in the intercalated cells. We investigated whether Kv1.3 mediated K(+) transport in microperfused cortical collecting ducts (CCDs). The HK diet led to an increase in net K(+) transport from 7.4 +/- 1.1 (CK) to 11.4 +/- 1.0 (HK) pmol x min(-1.) mm(-1). Luminal margatoxin, a specific blocker of Kv1.3, decreased net K(+) secretion in HK CCDs to 6.0 +/- 1.6 pmol x min(-1.) mm(-1). Our data provide the first evidence that Kv1.3 channels participate in K(+) secretion and that apical membrane localization of Kv1.3 is enhanced in the intercalated cells by dietary K(+) loading.

Hoxd-13 expression in the development of hindgut in ethylenethiourea-exposed fetal rats

PURPOSE

Hoxd-13, as one of the most posterior genes among Hox genes, was reported to play a critical role in the development of the most posterior alimentary canal in vertebrates. This study investigated the expression pattern of Hoxd-13 in the hindgut development of the normal and ethylenethiourea (ETU)-exposed rat embryos with anorectal malformations (ARMs) to find out the possible role of Hoxd-13 in the hindgut development and anorectal morphogenesis.

MATERIAL AND METHOD

The ETU murine model of ARMs was used via ETU 1% (125 mg/kg) on gestational day (gD) 10. Embryos were harvested via cesarean delivery on gD13 to gD21. Temporal and spatial expression of Hoxd-13 was evaluated in the normal fetal rats (n = 215) and ARMs rats (n = 218) using immunohistochemistry staining, reverse transcriptase polymerase chain reaction, and Western blot analysis.

RESULTS

Immunohistochemistry staining revealed that Hoxd-13 expression was confined to the epithelium of the hindgut, cloacal membrane, and urogenital sinus as well as the mesenchyme of the urorectal septum at all gestations in the normal group; however, in the ARMs group, the signal specific for Hoxd-13 was weak in the epithelium of the hindgut and cloacal membrane as well as the mesenchyme of the urorectal septum. Western blot analysis and reverse transcriptase polymerase chain reaction revealed that the level of Hoxd-13 expression was significantly decreased in the ARMs embryos compared with that in the normal embryos on gD13 to gD16 (P < .05) rather than on gD18 to gD21.

CONCLUSIONS

The aberrations in spatiotemporal expression pattern of Hoxd-13 on gD13 to gD16 suggested that Hoxd-13 may be an essential inductive signal for normal development of the hindgut, and altered expression may contribute to the abnormal development of the hindgut and accordingly lead to ARMs.

A role for BK channels in heart rate regulation in rodents

The heart generates and propagates action potentials through synchronized activation of ion channels allowing inward Na(+) and Ca(2+) and outward K(+) currents. There are a number of K(+) channel types expressed in the heart that play key roles in regulating the cardiac cycle. Large conductance calcium-activated potassium (BK) ion channels are not thought to be directly involved in heart function. Here we present evidence that heart rate can be significantly reduced by inhibiting the activity of BK channels. Agents that specifically inhibit BK channel activity, including paxilline and lolitrem B, slowed heart rate in conscious wild-type mice by 30% and 42%, respectively. Heart rate of BK channel knock-out mice (Kcnma1(-/-)) was not affected by these BK channel inhibitors, suggesting that the changes to heart rate were specifically mediated through BK channels. The possibility that these effects were mediated through BK channels peripheral to the heart was ruled out with experiments using isolated, perfused rat hearts, which showed a significant reduction in heart rate when treated with the BK channel inhibitors paxilline (1 microM), lolitrem B (1 microM), and iberiotoxin (0.23 microM), of 34%, 60%, and 42%, respectively. Furthermore, paxilline was shown to decrease heart rate in a dose-dependent manner. These results implicate BK channels located in the heart to be directly involved in the regulation of heart rate.

Simvastatin reverses podocyte injury but not mesangial expansion in early stage type 2 diabetes mellitus

Statins may confer renal protection in a variety of glomerular diseases, including diabetic nephropathy (DN). However, various glomerular lesions have different etiologies and may have different responses to statins. This study was performed to determine the differential effects of simvastatin (SMV) on glomerular pathology including mesangial expansion and podocyte injury in a mouse model of early stage type 2 diabetes mellitus (DM). Type 2 DM was induced in male C57BL/6 mice by feeding a high fat diet (HF; 45 kcal% fat). After 22 weeks, one group of HF mice was treated with SMV (HF-SMV; 7 mug/day/g BW) and another group was treated with vehicle (HF-vehicle) for 4 weeks via osmotic mini-pump. A third group served as age-matched normal diet vehicle controls (ND-vehicle; 10 kcal% fat). At the end of treatment, glomerular morphology was evaluated in a blind manner to determine the progression of DN. Body weight, blood glucose, insulin, HDL-cholesterol and triglycerides, but not LDL-cholesterol, were increased in HF mice. Over the course of treatment, the 24-hour urinary albumin excretion (UAE) was unchanged in ND-vehicle. HF mice exhibited elevated UAE, which decreased with SMV, but was unchanged with vehicle. The absolute mesangial volume and the relative mesangial volume per glomerular volume increased in HF-vehicle and remained elevated with SMV treatment. The immuno-staining of nephrin, a protein marker of the integrity of podocyte slit diaphragms, was decreased in HF-vehicle; however, the nephrin quantity of the HF-SMV group was not different from ND-vehicle. It is concluded that SMV reverses podocyte damage, but does not affect mesangial expansion in the kidneys of early stage proteinuria of type 2 DM.

Distal potassium handling based on flow modulation of maxi-K channel activity

PURPOSE OF REVIEW

Studies on the mechanisms of distal K+ secretion have highlighted the importance of the renal outer-medullary K+ (ROMK) and maxi-K channels. This review considers several human disorders characterized by hypokalemia and hyperkalemia, as well as mouse models of these disorders, and the mechanisms by which ROMK and maxi-K may be dysregulated.

RECENT FINDINGS

Analysis of knockout mice lacking ROMK, a model for type II Bartter's syndrome, has shown a role for maxi-K in distal K+ secretion. Knockout mice lacking either the alpha or beta1 subunits of maxi-K also show deficits in flow-dependent K+ secretion. Analysis of transgenic and knock-in mouse models of pseudohypoaldosteronism type II, in which mutant forms of with-no-lysine kinase 4 are expressed, suggests ways in which ROMK and maxi-K may be dysregulated to result in hyperkalemia. Modeling studies also provide insights into the role of Na+ delivery vs. flow in K+ secretion.

SUMMARY

The importance of both ROMK and maxi-K to distal K+ secretion is now well established, but the relative role that each of these two channels plays in normal and diseased states has not been definitively established. Analysis of human and animal model data can generate hypotheses for future experiments.