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.

Mechanoregulation of BK channel activity in the mammalian cortical collecting duct: role of protein kinases A and C

Flow-stimulated net K secretion (J(K)) in the cortical collecting duct (CCD) is mediated by an iberiotoxin (IBX)-sensitive BK channel, and requires an increase in intracellular Ca2+ concentration ([Ca2+](i)). The alpha-subunit of the reconstituted BK channel is phosphorylated by PKA and PKC. To test whether the BK channel in the native CCD is regulated by these kinases, J(K) and net Na absorption (J(Na)) were measured at slow (approximately 1) and fast (approximately 5 nl x min(-1) x mm(-1)) flow rates in rabbit CCDs microperfused in the presence of mPKI, an inhibitor of PKA; calphostin C, which inhibits diacylglycerol binding proteins, including PKC; or bisindolylmaleimide (BIM) and Gö6976, inhibitors of classic and novel PKC isoforms, added to luminal (L) and/or basolateral (B) solutions. L but not B mPKI increased J(K) in CCDs perfused at a slow flow rate; a subsequent increase in flow rate augmented J(K) modestly. B mPKI alone or with L inhibitor abolished flow stimulation of J(K). Similarly, L calphostin C increased J(K) in CCDs perfused at slow flow rates, as did calphostin C in both L and B solutions. The observation that IBX inhibited the L mPKI- and calphostin C-mediated increases in J(K) at slow flow rates implicated the BK channel in this K flux, a notion suggested by patch-clamp analysis of principal cells. The kinase inhibited by calphostin C was not PKC as L and/or B BIM and Gö6976 failed to enhance J(K) at the slow flow rate. However, addition of these PKC inhibitors to the B solution alone or with L inhibitor blocked flow stimulation of J(K). Interpretation of these results in light of the effects of these inhibitors on the flow-induced elevation of [Ca2+](i) suggests that the principal cell apical BK channel is tonically inhibited by PKA and that flow stimulation of J(K) in the CCD is PKA and PKC dependent. The specific targets of the kinases remain to be identified.

Electrochemical cues regulate assembly of the Frizzled/Dishevelled complex at the plasma membrane during planar epithelial polarization

Dishevelled (Dsh) is a cytoplasmic multidomain protein that is required for all known branches of the Wnt signalling pathway. The Frizzled/planar cell polarity (Fz/PCP) signalling branch requires an asymmetric cortical localization of Dsh, but this process remains poorly understood. Using a genome-wide RNA interference (RNAi) screen in Drosophila melanogaster cells, we show that Dsh membrane localization is dependent on the Na(+)/H(+) exchange activity of the plasma membrane exchanger Nhe2. Manipulating Nhe2 expression levels in the eye causes PCP defects, and Nhe2 interacts genetically with Fz. Our data show that the binding and surface recruitment of Dsh by Fz is pH- and charge-dependent. We identify a polybasic stretch within the Dsh DEP domain that binds to negatively charged phospholipids and appears to be mechanistically important. Dsh recruitment by Fz can be abolished by converting these basic amino-acid residues into acidic ones, as in the mutant, DshKR/E. In vivo, the DshKR/E(2x) mutant with two substituted residues fails to associate with the membrane during active PCP signalling but rescues canonical Wnt signalling defects in a dsh-background. These results suggest that direct interaction between Fz and Dsh is stabilized by a pH and charge-dependent interaction of the DEP domain with phospholipids. This stabilization is particularly important for the PCP signalling branch and, thus, promotes specific pathway selection in Wnt signalling.

Epoxyeicosatrienoic acid activates BK channels in the cortical collecting duct

The cortical collecting duct (CCD), which is involved in renal potassium (K) excretion, expresses cytochrome P450 (CYP)-epoxygenase. Here, we examined the effect of high dietary K on renal expression of CYP2C23 and CYP2J2 in the rat, as well as the role of CYP-epoxygenase-dependent metabolism of arachidonic acid in the regulation of Ca(2+)-activated big-conductance K (BK) channels. By Western blot analysis, high dietary K stimulated the expression of CYP2C23 but not CYP2J2 and increased 11,12-epoxyeicosatrienoic acid (11,12-EET) levels in isolated rat CCD tubules. Application of arachidonic acid increased BK channel activity, and this occurred to a greater extent in rats on a high-K diet compared with a normal-K diet. This effect was unlikely due to arachidonic acid-induced changes in membrane fluidity, because 11,14,17-eicosatrienoic acid did not alter BK channel activity. Inhibiting CYP-epoxygenase but not cyclooxygenase- or CYP-omega-hydroxylase-dependent pathways completely abolished the stimulatory effect of arachidonic acid on BK channel activity. In addition, application of 11,12-EET mimicked the effect of arachidonic acid on BK channel activity, even in the presence of CYP-epoxygenase inhibition. This effect seemed specific to 11,12-EET, because both 8,9- and 14,15-EET failed to stimulate BK channels. Finally, inhibition of CYP-epoxygenase abolished iberiotoxin-sensitive and flow-stimulated but not basal net K secretion in isolated microperfused CCD. In conclusion, high dietary K stimulates the renal CYP-epoxygenase pathway, which plays an important role in activating BK channels and flow-stimulated K secretion in the CCD.

Pathways of apoptosis in human autosomal recessive and autosomal dominant polycystic kidney diseases

Autosomal dominant polycystic kidney disease (ADPKD) is a major cause of end-stage renal disease in adults. Autosomal recessive (AR) PKD affects approximately 1:20,000 live-born children with high perinatal mortality. Both diseases have abnormalities in epithelial proliferation, secretion, and cell-matrix interactions, leading to progressive cystic expansion and associated interstitial fibrosis. Cell number in a kidney reflects the balance between proliferation and apoptosis. Apoptosis results from extrinsic (ligand-induced, expression of caspase-8) and intrinsic (mitochondrial damage, expression of caspase-9) triggers. Previous studies have suggested a role for apoptosis in PKD cyst formation and parenchymal destruction. Mechanisms underlying apoptosis in human ADPKD and ARPKD were examined by quantitative immunohistochemistry and Western immunoblot analyses of age-matched normal and PKD tissues. Caspase-8 expression was significantly greater in small cysts and normal-appearing tubules than in larger cysts in ADPKD kidneys. Caspase-8 also appeared early in the disease process of ADPKD. In ARPKD, expression of caspase-8 was most pronounced in later stages of the disease and was not confined to a specific cyst size. In conclusion, apoptosis in human ADPKD is an early event, occurring predominantly in normal-appearing tubules and small cysts, and is triggered by an extrinsic factor, but it occurs later in ARPKD.

Effect of aldosterone on BK channel expression in mammalian cortical collecting duct

Apical large-conductance Ca(2+)-activated K(+) (BK) channels in the cortical collecting duct (CCD) mediate flow-stimulated K(+) secretion. Dietary K(+) loading for 10-14 days leads to an increase in BK channel mRNA abundance, enhanced flow-stimulated K(+) secretion in microperfused CCDs, and a redistribution of immunodetectable channels from an intracellular pool to the apical membrane (Najjar F, Zhou H, Morimoto T, Bruns JB, Li HS, Liu W, Kleyman TR, Satlin LM. Am J Physiol Renal Physiol 289: F922-F932, 2005). To test whether this adaptation was mediated by a K(+)-induced increase in aldosterone, New Zealand White rabbits were fed a low-Na(+) (LS) or high-Na(+) (HS) diet for 7-10 days to alter circulating levels of aldosterone but not serum K(+) concentration. Single CCDs were isolated for quantitation of BK channel subunit (total, alpha-splice variants, beta-isoforms) mRNA abundance by real-time PCR and measurement of net transepithelial Na(+) (J(Na)) and K(+) (J(K)) transport by microperfusion; kidneys were processed for immunolocalization of BK alpha-subunit by immunofluorescence microscopy. At the time of death, LS rabbits excreted no urinary Na(+) and had higher circulating levels of aldosterone than HS animals. The relative abundance of BK alpha-, beta(2)-, and beta(4)-subunit mRNA and localization of immunodetectable alpha-subunit were similar in CCDs from LS and HS animals. In response to an increase in tubular flow rate from approximately 1 to 5 nl.min(-1).mm(-1), the increase in J(Na) was greater in LS vs. HS rabbits, yet the flow-stimulated increase in J(K) was similar in both groups. These data suggest that aldosterone does not contribute to the regulation of BK channel expression/activity in response to dietary K(+) loading.

Mechanoregulation of intracellular Ca2+ in human autosomal recessive polycystic kidney disease cyst-lining renal epithelial cells

Mutations of cilia-expressed proteins are associated with an attenuated shear-induced increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) in renal epithelial cell lines derived from murine models of autosomal recessive polycystic kidney disease (ARPKD). We hypothesized that human ARPKD cyst-lining renal epithelial cells also exhibited dysregulated mechanosensation. To test this, conditionally immortalized cell lines derived from human fetal ARPKD cyst-lining (pool and clone 5E) cell lines with low levels of fibrocystin/polyductin expression and age-matched normal collecting tubule [human fetal collecting tubule (HFCT) pool and clone 2C] cell lines were grown in culture, loaded with a Ca(2+) indicator dye, and subjected to laminar shear. Clonal cell lines were derived from single cells present in pools of cells from cyst-lining and collecting tubules, microdissected from human kidney. Resting and peak [Ca(2+)](i) were similar between ARPKD 5E and pool, and HFCT 2C and pool; however, the flow-induced peak [Ca(2+)](i) was greater in ARPKD 5E (700 +/- 87 nM, n = 21) than in HFCT 2C (315 +/- 58 nM, n = 12; P < 0.01) cells. ARPKD 5E cells treated with Gd(3+), an inhibitor of nonselective cation channels, inhibited but did not abolish the shear-induced [Ca(2+)](i) transient. Cilia were approximately 20% shorter in ARPKD than HFCT cells, but no difference in ciliary localization or total cellular expression of polycystin-2, a mechanosenory Gd(3+)-sensitive cation channel, was detected between ARPKD and HFCT cells. The intracellular Ca(2+) stores were similar between cells. In summary, human ARPKD cells exhibit an exaggerated Gd(3+)-sensitive mechano-induced Ca(2+) response compared with controls; whether this represents dysregulated polycystin-2 activity in ARPKD cells remains to be explored.

Loss of apical monocilia on collecting duct principal cells impairs ATP secretion across the apical cell surface and ATP-dependent and flow-induced calcium signals

Renal epithelial cells release ATP constitutively under basal conditions and release higher quantities of purine nucleotide in response to stimuli. ATP filtered at the glomerulus, secreted by epithelial cells along the nephron, and released serosally by macula densa cells for feedback signaling to afferent arterioles within the glomerulus has important physiological signaling roles within kidneys. In autosomal recessive polycystic kidney disease (ARPKD) mice and humans, collecting duct epithelial cells lack an apical central cilium or express dysfunctional proteins within that monocilium. Collecting duct principal cells derived from an Oak Ridge polycystic kidney (orpk ( Tg737 ) ) mouse model of ARPKD lack a well-formed apical central cilium, thought to be a sensory organelle. We compared these cells grown as polarized cell monolayers on permeable supports to the same cells where the apical monocilium was genetically rescued with the wild-type Tg737 gene that encodes Polaris, a protein essential to cilia formation. Constitutive ATP release under basal conditions was low and not different in mutant versus rescued monolayers. However, genetically rescued principal cell monolayers released ATP three- to fivefold more robustly in response to ionomycin. Principal cell monolayers with fully formed apical monocilia responded three- to fivefold greater to hypotonicity than mutant monolayers lacking monocilia. In support of the idea that monocilia are sensory organelles, intentionally harsh pipetting of medium directly onto the center of the monolayer induced ATP release in genetically rescued monolayers that possessed apical monocilia. Mechanical stimulation was much less effective, however, on mutant orpk collecting duct principal cell monolayers that lacked apical central monocilia. Our data also show that an increase in cytosolic free Ca(2+) primes the ATP pool that is released in response to mechanical stimuli. It also appears that hypotonic cell swelling and mechanical pipetting stimuli trigger release of a common ATP pool. Cilium-competent monolayers responded to flow with an increase in cell Ca(2+) derived from both extracellular and intracellular stores. This flow-induced Ca(2+) signal was less robust in cilium-deficient monolayers. Flow-induced Ca(2+) signals in both preparations were attenuated by extracellular gadolinium and by extracellular apyrase, an ATPase/ADPase. Taken together, these data suggest that apical monocilia are sensory organelles and that their presence in the apical membrane facilitates the formation of a mature ATP secretion apparatus responsive to chemical, osmotic, and mechanical stimuli. The cilium and autocrine ATP signaling appear to work in concert to control cell Ca(2+). Loss of a cilium-dedicated autocrine purinergic signaling system may be a critical underlying etiology for ARPKD and may lead to disinhibition and/or upregulation of multiple sodium (Na(+)) absorptive mechanisms and a resultant severe hypertensive phenotype in ARPKD and, possibly, other diseases.

Potassium transport in the maturing kidney

The distal nephron and colon are the primary sites of regulation of potassium (K(+)) homeostasis, responsible for maintaining a zero balance in adults and net positive balance in growing infants and children. Distal nephron segments can either secrete or reabsorb K(+) depending on the metabolic needs of the organism. In the healthy adult kidney, K(+) secretion predominates over K(+) absorption. Baseline K(+) secretion occurs via the apical low-conductance secretory K(+) (SK) channel, whereas the maxi-K channel mediates flow-stimulated net urinary K(+) secretion. The K(+) retention characteristic of the neonatal kidney appears to be due not only to the absence of apical secretory K(+) channels in the distal nephron but also to a predominance of apical H-K-adenosine triphosphatase (ATPase), which presumably mediates K(+) absorption. Both luminal and peritubular factors regulate the balance between K(+) secretion and absorption. Perturbation in any of these factors can lead to K(+) imbalance. In turn, these factors may serve as effective targets for the treatment of both hyper-and hypokalemia. The purpose of this review is to present an overview of recent advances in our understanding of mechanisms of K(+) transport in the maturing kidney.

Multiple receptor interactions trigger release of membrane and intracellular calcium stores critical for herpes simplex virus entry

Herpes simplex viruses (HSV) harness cellular calcium signaling pathways to facilitate viral entry. Confocal microscopy and small interfering RNA (siRNA) were used to identify the source of the calcium and to dissect the requisite viral-cell interactions. Binding of HSV to human epithelial cells induced no calcium response, but shifting the cells to temperatures permissive for penetration triggered increases in plasma membrane calcium followed by a global release of intracellular calcium. Transfection with siRNA targeting the proteoglycan syndecan-2 blocked viral binding and abrogated any calcium response. Transfection with siRNA targeting nectin-1, a glycoprotein D receptor, also prevented both membrane and intracellular calcium responses. In contrast, the membrane response was preserved after transfection with siRNA targeting integrinalphav, a novel glycoprotein H receptor. The membrane response, however, was not sufficient for viral entry, which required interactions with integrinalphav and release of inositol-triphosphate receptor-dependent intracellular calcium stores. Thus, calcium plays a critical, complex role in HSV entry.

Lack of a role of membrane-protein interactions in flow-dependent activation of ENaC

Rates of Na(+) absorption in the distal nephron increase proportionally with the rates of tubular flow. We tested the hypothesis that the deformation or tension generated in the plasma membrane in response to flow activates the epithelial sodium channel (ENaC). We modified the physical properties of the membrane by changing the temperature and the content of cholesterol. Rates of net Na(+) absorption measured in cortical collecting ducts (CCDs) perfused at room temperature at slow (approximately 1) and fast (approximately 5 nl.min(-1).mm(-1)) flow rates were less than those measured at 37 degrees C at the same flow rates, although increases in tubular fluid flow rates led to comparable relative increases in net Na(+) absorption at both temperatures. Xenopus laevis oocytes expressing ENaC responded to an increase in shear stress at 22-25 degrees C with a discrete delay followed by a monoexponential increase in whole-cell Na(+) currents. We observed that temperature affected 1) basal currents, 2) delay times, 3) kinetics of activation, and 4) fold-increase in macroscopic currents in response to flow. The magnitude of the response to flow displayed biphasic behavior as a function of temperature, with a minimal value at 25 degrees C. Steady-state fluorescence anisotropic measurements of purified plasma membranes did not show any obvious phase transition behavior over a temperature range from 8.3 degrees C to 36.5 degrees C. Modification of the content of membrane cholesterol did not affect the response to flow. Our results suggest that the flow-dependent activation of ENaC is not influenced by modifications in the intrinsic properties of the plasma membrane.

Ca2+ dependence of flow-stimulated K secretion in the mammalian cortical collecting duct

Apical low-conductance SK and high-conductance Ca(2+)-activated BK channels are present in distal nephron, including the cortical collecting duct (CCD). Flow-stimulated net K secretion (J(K)) in the CCD is 1) blocked by iberiotoxin, an inhibitor of BK but not SK channels, and 2) associated with an increase in [Ca(2+)](i), leading us to conclude that BK channels mediate flow-stimulated J(K). To examine the Ca(2+) dependence and sources of Ca(2+) contributing to flow-stimulated J(K), J(K) and net Na absorption (J(Na)) were measured at slow (approximately 1) and fast (approximately 5 nl.min(-1).mm(-1)) flow rates in rabbit CCDs microperfused in the absence of luminal Ca(2+) or after pretreatment with BAPTA-AM to chelate intracellular Ca(2+), 2-aminoethoxydiphenyl borate (2-APB), to inhibit the inositol 1,4,5-trisphosphate (IP(3)) receptor or thapsigargin to deplete internal stores. These treatments, which do not affect flow-stimulated J(Na) (Morimoto et al. Am J Physiol Renal Physiol 291: F663-F669, 2006), inhibited flow-stimulated J(K). Increases in [Ca(2+)](i) stimulate exocytosis. To test whether flow induces exocytic insertion of preformed BK channels into the apical membrane, CCDs were pretreated with 10 microM colchicine (COL) to disrupt microtubule function or 5 microg/ml brefeldin-A (BFA) to inhibit delivery of channels from the intracellular pool to the plasma membrane. Both agents inhibited flow-stimulated J(K) but not J(Na) (Morimoto et al. Am J Physiol Renal Physiol 291: F663-F669, 2006), although COL but not BFA also blocked the flow-induced [Ca(2+)](i) transient. We thus speculate that BK channel-mediated, flow-stimulated J(K) requires an increase in [Ca(2+)](i) due, in part, to luminal Ca(2+) entry and ER Ca(2+) release, microtubule integrity, and exocytic insertion of preformed channels into the apical membrane.

Angiotensin II inhibits the ROMK-like small conductance K channel in renal cortical collecting duct during dietary potassium restriction

Base-line urinary potassium secretion in the distal nephron is mediated by small conductance rat outer medullary K (ROMK)-like channels. We used the patch clamp technique applied to split-open cortical collecting ducts (CCDs) isolated from rats fed a normal potassium (NK) or low potassium (LK) diet to test the hypothesis that AngII directly inhibits ROMK channel activity. We found that AngII inhibited ROMK channel activity in LK but not NK rats in a dose-dependent manner. The AngII-induced reduction in channel activity was mediated by AT1 receptor (AT1R) binding, because pretreatment of CCDs with losartan but not PD123319 AT1 and AT2 receptor antagonists, respectively, blocked the response. Pretreatment of CCDs with U73122 and calphostin C, inhibitors of phospholipase C (PLC) and protein kinase C (PKC), respectively, abolished the AngII-induced decrease in ROMK channel activity, confirming a role of the PLC-PKC pathway in this response. Studies by others suggest that AngII stimulates an Src family protein-tyrosine kinase (PTK) via PKC-NADPH oxidase. PTK has been shown to regulate the ROMK channel. Inhibition of NADPH oxidase with diphenyliodonium abolished the inhibitory effect of AngII or the PKC activator phorbol 12-myristate 13-acetate on ROMK channels. Suppression of PTK by herbimycin A significantly attenuated the inhibitory effect of AngII on ROMK channel activity. We conclude that AngII inhibits ROMK channel activity through PKC-, NADPH oxidase-, and PTK-dependent pathways under conditions of dietary potassium restriction.

Regulation of cation transport in the distal nephron by mechanical forces

Thiazide and loop diuretics induce renal K(+) secretion, often leading to renal K(+) wasting and hypokalemia. This phenomenon has been proposed to reflect an increase in delivery to and reabsorption of Na(+) by the distal nephron, with a resultant increase in the driving force for passive K(+) efflux across the apical membrane. Recent studies suggest that cellular mechanisms that lead to enhanced rates of Na(+) reabsorption as well as K(+) secretion in response to increases tubular flow rates are more complex. Increases in tubular flow rates directly enhance the activity of apical membrane Na(+) channels and indirectly activate a class of K(+) channels, referred to as maxi-K, that are functionally inactive under low flow states. This review addresses the role of biomechanical forces, generated by variations in urinary flow rate and tubular fluid volume, in the regulation of transepithelial Na(+) and K(+) transport in the distal nephron. The question of why the distal nephron has evolved to include a component of flow-dependent K(+) secretion is also addressed.

Predominant extrahepatic biliary disease in autosomal recessive polycystic kidney disease: a new association

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by dilation of ectatic renal collecting ducts, intrahepatic biliary dysgenesis, and portal fibrosis. Portal hypertension and recurrent bacterial cholangitis can dominate the clinical picture in long-term survivors. Predominant extrahepatic bile duct disease was revealed in four patients who underwent magnetic resonance cholangiopancreatography. All four patients had portal hypertension, although liver biochemistries did not suggest biliary disease. In two of the patients, cholangitis was clinically ascribed to the bile duct disease. Western blot analysis of plasma membranes from normal rat extrahepatic bile duct and kidney revealed the presence of polyductin as a single approximately 440 kDa protein. Although the exact function of polyductin in the extrahepatic duct is unknown, it may have a role in the development and control of lumenal size. Clinical management of patients with ARPKD should include consideration of potential problems related to extrahepatic bile duct disease.

Mechanism underlying flow stimulation of sodium absorption in the mammalian collecting duct

Vectorial Na(+) absorption across the aldosterone-sensitive distal nephron plays a key role in the regulation of extracellular fluid volume and blood pressure. Within this nephron segment, Na(+) diffuses from the urinary fluid into principal cells through an apical, amiloride-sensitive, epithelial Na(+) channel (ENaC), which is considered to be the rate-limiting step for Na(+) absorption. We have reported that increases in tubular flow rate in microperfused rabbit cortical collecting ducts (CCDs) lead to increases in net Na(+) absorption and that increases in laminar shear stress activate ENaC expressed in oocytes by increasing channel open probability. We therefore examined whether flow stimulates net Na(+) absorption (J(Na)) in CCDs by increasing channel open probability or by increasing the number of channels at the apical membrane. Both baseline and flow-stimulated J(Na) in CCDs were mediated by ENaC, as J(Na) was inhibited by benzamil. Flow-dependent increases in J(Na) were observed following treatment of tubules with reagents that altered membrane trafficking by disrupting microtubules (colchicine) or Golgi (brefeldin A). Furthermore, reducing luminal Ca(2+) concentration ([Ca(2+)]) or chelating intracellular [Ca(2+)] with BAPTA did not prevent the flow-dependent increase in J(Na). Extracellular trypsin has been shown to activate ENaC by increasing channel open probability, and we observed that trypsin significantly enhanced J(Na) when tubules were perfused at a slow flow rate. However, trypsin did not further enhance J(Na) in CCDs perfused at fast flow rates. Similarly, the shear-induced increase in benzamil-sensitive J(Na) in oocytes expressing protease resistance ENaC mutants was similar to that of controls. Our results suggest the rise in J(Na) accompanying increases in luminal flow rates reflects an increase in channel open probability.

Heightened epithelial Na+ channel-mediated Na+ absorption in a murine polycystic kidney disease model epithelium lacking apical monocilia

The Tg737 degrees (rpk) autosomal recessive polycystic kidney disease (ARPKD) mouse carries a hypomorphic mutation in the Tg737 gene. Because of the absence of its protein product Polaris, the nonmotile primary monocilium central to the luminal membrane of ductal epithelia, such as the cortical collecting duct (CCD) principal cell (PC), is malformed. Although the functions of the renal monocilium remain elusive, primary monocilia or flagella on neurons act as sensory organelles. Thus we hypothesized that the PC monocilium functions as a cellular sensor. To test this hypothesis, we assessed the contribution of Polaris and cilium structure and function to renal epithelial ion transport electrophysiology. Properties of Tg737 degrees (rpk) mutant CCD PC clones were compared with clones genetically rescued with wild-type Tg737 cDNA. All cells were grown as polarized cell monolayers with similarly high transepithelial resistance on permeable filter supports. Three- to fourfold elevated transepithelial voltage (V(te)) and short-circuit current (I(sc)) were measured in mutant orpk monolayers vs. rescued controls. Pharmacological and cell biological examination of this enhanced electrical end point in mutant monolayers revealed that epithelial Na(+) channels (ENaCs) were upregulated. Amiloride, ENaC-selective amiloride analogs (benzamil and phenamil), and protease inhibitors (aprotinin and leupeptin) attenuated heightened V(te) and I(sc). Higher concentrations of additional amiloride analogs (ethylisopropylamiloride and dimethylamiloride) also revealed inhibition of V(te). Cell culture requirements and manipulations were also consistent with heightened ENaC expression and function. Together, these data suggest that ENaC expression and/or function are upregulated in the luminal membrane of mutant, cilium-deficient orpk CCD PC monolayers vs. cilium-competent controls. When the genetic lesion causes loss or malformation of the monocilium, ENaC-driven Na(+) hyperabsorption may explain the rapid emergence of severe hypertension in a majority of patients with ARPKD.

Focal Adhesion Kinase Plays a Pivotal Role in Herpes Simplex Virus Entry

Development of strategies to prevent herpes simplex virus (HSV) infection requires knowledge of cellular pathways harnessed by the virus for invasion. This study demonstrates that HSV induces rapid phosphorylation of focal adhesion kinase (FAK) in several human target cells and that phosphorylation is important for entry post-binding. Nuclear transport of the viral tegument protein VP16, transport of viral capsids to the nuclear pore, and downstream events (including expression of immediate-early genes and viral plaque formation) were substantially reduced in cells transfected with dominant-negative mutants of FAK or small interfering RNA designed to inhibit FAK expression. These observations were substantiated using mouse embryonic fibroblast cells derived from embryonic FAK-deficient mice. Infection was reduced by >90% in knockout cells relative to control cells and was further reduced if the knockout cells were transfected with small interfering RNA targeting proline-rich tyrosine kinase-2, which was also phosphorylated in response to HSV. The knockout cells were permissive for viral binding, and virus triggered an intracellular calcium response, but nuclear transport was inhibited. Together, these results support a novel model for invasion that implicates FAK phosphorylation as important for delivery of viral capsids to the nuclear pore.

Mechanoregulation of intracellular Ca2+ concentration is attenuated in collecting duct of monocilium-impaired orpk mice

Autosomal recessive polycystic kidney disease (ARPKD) is characterized by the progressive dilatation of collecting ducts, the nephron segments responsible for the final renal regulation of sodium, potassium, acid-base, and water balance. Murine models of ARPKD possess mutations in genes encoding cilia-associated proteins, including Tg737 in orpk mice. New findings implicate defects in structure/function of primary cilia as central to the development of polycystic kidney disease. Our group (Liu W, Xu S, Woda C, Kim P, Weinbaum S, and Satlin LM, Am J Physiol Renal Physiol 285: F998-F1012, 2003) recently reported that increases in luminal flow rate in rabbit collecting ducts increase intracellular Ca(2+) concentration ([Ca(2+)](i)) in cells therein. We thus hypothesized that fluid shear acting on the apical membrane or hydrodynamic bending moments acting on the cilium increase renal epithelial [Ca(2+)](i). To further explore this, we tested whether flow-induced [Ca(2+)](i) transients in collecting ducts from mutant orpk mice, which possess structurally abnormal cilia, differ from those in controls. Isolated segments from 1- and 2-wk-old mice were microperfused in vitro and loaded with fura 2; [Ca(2+)](i) was measured by digital ratio fluorometry before and after the rate of luminal flow was increased. All collecting ducts responded to an increase in flow with an increase in [Ca(2+)](i), a response that appeared to be dependent on luminal Ca(2+) entry. However, the magnitude of the increase in [Ca(2+)](i) in 2- but not 1-wk-old mutant orpk animals was blunted. We speculate that this defect in mechano-induced Ca(2+) signaling in orpk mice leads to aberrant structure and function of the collecting duct in ARPKD.

Dietary K+ regulates apical membrane expression of maxi-K channels in rabbit cortical collecting duct

The cortical collecting duct (CCD) is a final site for regulation of K(+) homeostasis. CCD K(+) secretion is determined by the electrochemical gradient and apical permeability to K(+). Conducting secretory K(+) (SK/ROMK) and maxi-K channels are present in the apical membrane of the CCD, the former in principal cells and the latter in both principal and intercalated cells. Whereas SK channels mediate baseline K(+) secretion, maxi-K channels appear to participate in flow-stimulated K(+) secretion. Chronic dietary K(+) loading enhances the CCD K(+) secretory capacity due, in part, to an increase in SK channel density (Palmer et al., J Gen Physiol 104: 693-710, 1994). Long-term exposure of Ambystoma tigrinum to elevated K(+) increases renal K(+) excretion due to an increase in apical maxi-K channel density in their CDs (Stoner and Viggiano, J Membr Biol 162: 107-116, 1998). The purpose of the present study was to test whether K(+) adaptation in the mammalian CCD is associated with upregulation of maxi-K channel expression. New Zealand White rabbits were fed a low (LK), control (CK), or high (HK) K(+) diet for 10-14 days. Real-time PCR quantitation of message encoding maxi-K alpha- and beta(2-4)-subunits in single CCDs from HK animals was greater than that detected in CK and LK animals (P < 0.05); beta(1)-subunit was not detected in any CCD sample but was present in whole kidney. Indirect immunofluorescence microscopy revealed a predominantly intracellular distribution of alpha-subunits in LK kidneys. In contrast, robust apical labeling was detected primarily in alpha-intercalated cells in HK kidneys. In summary, K(+) adaptation is associated with an increase in steady-state abundance of maxi-K channel subunit-specific mRNAs and immunodetectable apical alpha-subunit, the latter observation consistent with redistribution from an intracellular pool to the plasma membrane.

Developmental regulation of expression of renal potassium secretory channels

PURPOSE OF REVIEW

Somatic growth is associated with an increase in total body K content. K homeostasis is regulated, in large part, by urinary K excretion. Within the adult kidney and specifically the cortical collecting duct, K secretion is accomplished by the passive diffusion of cell K into the urinary fluid down a favorable electrochemical gradient through K selective channels. The purpose of this review is to summarize the results of recent studies that provide insight into how the cortical collecting duct is uniquely adapted for K retention early in life.

RECENT FINDINGS

Electrophysiological analyses have identified two types of apical K channels in the mammalian cortical collecting duct. The prevalence of the secretory K channel and its high open probability at the resting membrane potential in the adult has led to the belief that this channel mediates baseline K secretion. The Ca and stretch-activated maxi-K channel has been proposed to mediate flow-stimulated K secretion. In contrast to the high rates of K secretion observed in adult cortical collecting ducts microperfused in vitro, segments isolated from neonatal animals show no significant net K transport until after the third week of postnatal life. The temporal delay between expression of conducting secretory K channels (baseline K secretion) and maxi-K channels (flow-stimulated K secretion) in the maturing cortical collecting duct reflect unique developmental programs regulating the transcription and/or translation of ROMK (rat outer medullary K channel) and slo, the molecular correlates of the secretory K and maxi-K channels, respectively.

SUMMARY

The K retention characteristic of the neonatal kidney is due, in part, to a paucity of distinct K channels mediating baseline and flow-stimulated K secretion in the collecting duct. The signals directing the developmental regulation of channel expression are as yet unknown.

Renal potassium handling in healthy and sick newborns

Growing infants must maintain a state of positive K+ balance, a task accomplished, in large part, by the kidney. The distal nephron is uniquely adapted to retain total body K+ early in life. The magnitude and direction of net K+ transport in the cortical collecting duct (CCD), the segment responsible for the final renal regulation of K+ balance in the adult, reflect the balance of opposing fluxes of K+ secretion and K+ absorption. Evidence now indicates that the low capacity of the neonatal CCD for K+ secretion is due, at least in part, to a relative paucity of conducting K+ channels in the urinary membrane. A relative excess of K+ absorption in this nephron segment may further reduce net urinary K+ secretion. Under conditions prevailing in vivo, the balance of fluxes in the CCD likely contributes to the relative K+ retention characteristic of the neonatal kidney.

Effect of flow and stretch on the [Ca2+]i response of principal and intercalated cells in cortical collecting duct

An acute increase in tubular fluid flow rate in the microperfused cortical collecting duct (CCD), associated with a approximately 20% increase in tubular diameter, leads to an increase in intracellular Ca2+ concentration ([Ca2+]i)in both principal and intercalated cells (Woda CB, Leite M Jr, Rohatgi R, and Satlin LM. Am J Physiol Renal Physiol 283: F437-F446, 2002). The apical cilium present in principal but not intercalated cells has been proposed to be a flow sensor. To determine whether flow across the cilium and/or epithelial stretch mediates the [Ca2+]i response, CCDs from New Zealand White rabbits were microperfused in vitro, split-open (to isolate the effect of flow across cilia), or occluded (to examine the effect of stretch and duration/magnitude of the flow impulse), and [Ca2+]i was measured using fura 2. In perfused and occluded CCDs, a rapid (<1 s) but not slow (>3 min) increase in luminal flow rate and/or circumferential stretch led to an approximately threefold increase in [Ca2+]i in both principal and intercalated cells within approximately 10 s. This response was mediated by external Ca2+ entry and inositol 1,4,5-trisphosphate-mediated release of cell Ca2+ stores. In split-open CCDs, an increase in superfusate flow led to an approximately twofold increase in [Ca2+]i in both cell types within approximately 30 s. These experimental findings are interpreted using mathematical models to predict the fluid stress on the apical membranes of the CCD and the forces and torques on and deformation of the cilia. We conclude that rapid increases in luminal flow rate and circumferential stretch, leading to shear or hydrodynamic impulses at the cilium or apical membrane, lead to increases in [Ca2+]i in both principal and intercalated cells.

Herpes simplex virus triggers activation of calcium-signaling pathways

The cellular pathways required for herpes simplex virus (HSV) invasion have not been defined. To test the hypothesis that HSV entry triggers activation of Ca2+-signaling pathways, the effects on intracellular calcium concentration ([Ca2+]i) after exposure of cells to HSV were examined. Exposure to virus results in a rapid and transient increase in [Ca2+]i. Pretreatment of cells with pharmacological agents that block release of inositol 1,4,5-triphosphate (IP3)-sensitive endoplasmic reticulum stores abrogates the response. Moreover, treatment of cells with these pharmacological agents inhibits HSV infection and prevents focal adhesion kinase (FAK) phosphorylation, which occurs within 5 min after viral infection. Viruses deleted in glycoprotein L or glycoprotein D, which bind but do not penetrate, fail to induce a [Ca2+]i response or trigger FAK phosphorylation. Together, these results support a model for HSV infection that requires activation of IP3-responsive Ca2+-signaling pathways and that is associated with FAK phosphorylation. Defining the pathway of viral invasion may lead to new targets for anti-viral therapy.

Ontogeny of flow-stimulated potassium secretion in rabbit cortical collecting duct: functional and molecular aspects

High urinary flow rates stimulate K secretion in the fully differentiated but not neonatal or weanling rabbit cortical collecting duct (CCD). Both small-conductance secretory K and high-conductance Ca2+/stretch-activated maxi-K channels have been identified in the apical membrane of the mature CCD by patch-clamp analysis. We reported that flow-stimulated net K secretion in the adult rabbit CCD is 1) blocked by TEA and charybdotoxin, inhibitors of intermediate- and high-conductance (maxi-K) Ca2+-activated K channels, and 2) associated with increases in net Na absorption and intracellular Ca2+ concentration ([Ca2+]i). The present study examined whether the absence of flow-stimulated K secretion early in life is due to a 1) limited flow-induced rise in net Na absorption and/or [Ca2+]i and/or 2) paucity of apical maxi-K channels. An approximately sixfold increase in tubular fluid flow rate in CCDs isolated from 4-wk-old rabbits and microperfused in vitro led to an increase in net Na absorption and [Ca2+]i, similar in magnitude to the response observed in 6-wk-old tubules, but it failed to generate an increase in net K secretion. By 5 wk of age, there was a small, but significant, flow-stimulated rise in net K secretion that increased further by 6 wk of life. Luminal perfusion with iberiotoxin blocked the flow stimulation of net K secretion in the adult CCD, confirming the identity of the maxi-K channel in this response. Maxi-K channel alpha-subunit message was consistently detected in single CCDs from animals >/=4 wk of age by RT-PCR. Indirect immunofluorescence microscopy using antibodies directed against the alpha-subunit revealed apical labeling of intercalated cells in cryosections from animals >/=5 wk of age; principal cell labeling was generally intracellular and punctate. We speculate that the postnatal appearance of flow-dependent K secretion is determined by the transcriptional/translational regulation of expression of maxi-K channels. Furthermore, our studies suggest a novel function for intercalated cells in mediating flow-stimulated K secretion.

Sodium and potassium clearances by the maturing kidney: clinical-molecular correlates

A temporal dissociation exists between the early appearance of sodium absorptive and later detection of potassium secretory processes in the maturing rabbit collecting duct. To extend the latter findings to the human, we sought to correlate developmental changes in renal sodium and potassium clearances with the molecular expression of corresponding ion channels in kidneys of premature infants. In a longitudinal prospective study of 23- to 31-week gestational age (GA) infants, sodium, potassium, and creatinine clearances were measured weekly for 5 weeks and the absolute and fractional excretions of sodium (FE(Na)) and potassium (FE(K)) calculated. Gene-specific probes were used to assess steady-state abundance of mRNA encoding the sodium channel ENaC and potassium channel ROMK in homogenates of human kidneys (obtained from the Anatomic Gift Foundation). Although urinary losses of sodium in infants <approximately 28 weeks GA exceeded intake, leading to a state of negative sodium balance, most infants >/=28 weeks and all infants >approximately 32 weeks GA achieved a state of positive balance, a maturational process associated with a decrease in FE(Na )and increase in ENaC. Infants >approximately 30 weeks GA maintained a state of positive potassium balance. We noted a twofold reduction in FE(K )after approximately 26 weeks GA and no change in ROMK abundance during the developmental window studied. We speculate that the developmental regulation of renal ENaC expression contributes, at least in part, to the decrease in FE(Na )observed with advancing GA, and that in the human, as in the rabbit, there is a delay between the maturation of sodium absorptive and potassium secretory pathways.

Na transport in autosomal recessive polycystic kidney disease (ARPKD) cyst lining epithelial cells

Autosomal dominant (ADPKD) and recessive (ARPKD) polycystic kidney disease are characterized by the progressive growth and expansion of cysts or ectatic collecting ducts, respectively, that ultimately destroy the normal renal parenchyma. Evidence from experimental models of ADPKD suggests that transepithelial Na and fluid secretion contribute to cyst growth, yet little is known about solute transport in ARPKD. This purpose of this study was to begin to characterize the expression and polarity of transport proteins involved in vectorial Na movement in ARPKD epithelium. Immunodetectable alpha1 and beta2 subunits of the Na/K-ATPase localized to the apical membrane of collecting duct cysts in tissue sections of human fetal ARPKD nephrectomy specimens and conditionally immortalized cells derived from these cysts. Measurements of transepithelial (22)Na transport performed on monolayers of ARPKD and age-matched collecting tubule (HFCT) cells grown on permeable supports revealed net Na absorption in both models. However, ARPKD cells absorbed Na at a rate approximately 50% greater than that of HFCT. Furthermore, Na absorption in ARPKD cells was partially inhibited by 100 micro M apical amiloride or 1 mM basolateral but not apical ouabain. Northern blot analyses of ARPKD whole kidney and Western immunoblot of ARPKD cells showed approximately twofold greater expression of the alpha-subunit of the epithelial Na channel (ENaC) compared with age-matched controls. These results suggest that, despite the presence of apical Na/K-ATPase, ARPKD cyst-lining cells absorb Na by a pathway that is modestly amiloride-sensitive. Whether Na absorption is mediated by ENaC, perhaps of nonclassical subunit composition, or another amiloride-sensitive transporter remains to be determined.

Effects of luminal flow and nucleotides on [Ca(2+)](i) in rabbit cortical collecting duct

Nucleotide binding to purinergic P2 receptors contributes to the regulation of a variety of physiological functions in renal epithelial cells. Whereas P2 receptors have been functionally identified at the basolateral membrane of the cortical collecting duct (CCD), a final regulatory site of urinary Na(+), K(+), and acid-base excretion, controversy exists as to whether apical purinoceptors exist in this segment. Nor has the distribution of receptor subtypes present on the unique cell populations that constitute Ca(2+) the CCD been established. To examine this, we measured nucleotide-induced changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) in fura 2-loaded rabbit CCDs microperfused in vitro. Resting [Ca(2+)](i) did not differ between principal and intercalated cells, averaging approximately 120 nM. An acute increase in tubular fluid flow rate, associated with a 20% increase in tubular diameter, led to increases in [Ca(2+)](i) in both cell types. Luminal perfusion of 100 microM UTP or ATP-gamma-S, in the absence of change in flow rate, caused a rapid and transient approximately fourfold increase in [Ca(2+)](i) in both cell types (P < 0.05). Luminal suramin, a nonspecific P2 receptor antagonist, blocked the nucleotide- but not flow-induced [Ca(2+)](i) transients. Luminal perfusion with a P2X (alpha,beta-methylene-ATP), P2X(7) (benzoyl-benzoyl-ATP), P2Y(1) (2-methylthio-ATP), or P2Y(4)/P2Y(6) (UDP) receptor agonist had no effect on [Ca(2+)](i). The nucleotide-induced [Ca(2+)](i) transients were inhibited by the inositol-1,4,5-triphosphate receptor blocker 2-aminoethoxydiphenyl borate, thapsigargin, which depletes internal Ca(2+) stores, luminal perfusion with a Ca(2+)-free perfusate, or the L-type Ca(2+) channel blocker nifedipine. These results suggest that luminal nucleotides activate apical P2Y(2) receptors in the CCD via pathways that require both internal Ca(2+) mobilization and extracellular Ca(2+) entry. The flow-induced rise in [Ca(2+)](i) is apparently not mediated by apical P2 purinergic receptor signaling.

Herpes simplex virus as a model vector system for gene therapy in renal disease

The past decade has been marked by significant advances in the application of gene transfer into living cells of animals and humans. These approaches have been tested in a few animal models of inherited and acquired renal diseases, including carbonic anhydrase II deficiency [1] and experimental glomerulonephritis [2, 3]. Gene transfer into proximal tubular cells has been successfully accomplished by intrarenal arterial infusion of a liposomal complex [4] or an adenoviral vector [5]. Tubular cells from the papilla and medulla have been selectively transduced by retrograde infusion into the pelvi-calyceal system of an adenoviral vector containing a reporter for beta-galactosidase [5]. Although the results of these initial studies are promising, further studies to optimize viral vectors, maximize gene delivery, minimize side-effects, and develop cell-specific and long-term regulated gene expression are critical to the success of gene therapy targeted to specific compartments of the kidney. Our recent efforts have focused on defining the cellular pathways responsible for viral entry and infection into renal epithelial cells using herpes simplex virus (HSV) as a model vector. We anticipate that a solid understanding of the basic scientific principles underlying viral entry and gene expression into specific populations of renal cells will facilitate the design of successful therapeutic viral-based gene transfer strategies.

Epithelial Na(+) channels are regulated by flow

Na(+) absorption in the renal cortical collecting duct (CCD) is mediated by apical epithelial Na(+) channels (ENaCs). The CCD is subject to continuous variations in intraluminal flow rate that we speculate alters hydrostatic pressure, membrane stretch, and shear stress. Although ENaCs share limited sequence homology with putative mechanosensitive ion channels in Caenorhabditis elegans, controversy exists as to whether ENaCs are regulated by biomechanical forces. We examined the effect of varying the rate of fluid flow on whole cell Na(+) currents (I(Na)) in oocytes expressing mouse alpha,beta,gamma-ENaC (mENaC) and on net Na(+) absorption in microperfused rabbit CCDs. Oocytes injected with mENaC but not water responded to the initiation of superfusate flow (to 4-6 ml/min) with a reversible threefold stimulation of I(Na) without a change in reversal potential. The increase in I(Na) was variable among oocytes. CCDs responded to a threefold increase in rate of luminal flow with a twofold increase in the rate of net Na(+) absorption. An increase in luminal viscosity achieved by addition of 5% dextran to the luminal perfusate did not alter the rate of net Na(+) absorption, suggesting that shear stress does not influence Na(+) transport in the CCD. In sum, our data suggest that flow stimulation of ENaC activity and Na(+) absorption is mediated by an increase in hydrostatic pressure and/or membrane stretch. We propose that intraluminal flow rate may be an important regulator of channel activity in the CCD.

Flow-dependent K+ secretion in the cortical collecting duct is mediated by a maxi-K channel

K+ secretion by the cortical collecting duct (CCD) is stimulated at high flow rates. Patch-clamp analysis has identified a small-conductance secretory K+ (SK) and a high-conductance Ca(2+)-activated K+ (maxi-K) channel in the apical membrane of the CCD. The SK channel, encoded by ROMK, is believed to mediate baseline K+ secretion. The role of the stretch- and Ca2+-activated maxi-K channel is still uncertain. The purpose of this study was to identify the K+ channel mediating flow-dependent K+ secretion in the CCD. Segments isolated from New Zealand White rabbits were microperfused in the absence and presence of luminal tetraethylammonium (TEA) or charybdotoxin, both inhibitors of maxi-K but not SK channels, or apamin, an inhibitor of small-conductance maxi-K+ channels. Net K+ secretion and Na+ absorption were measured at varying flow rates. In the absence of TEA, net K+ secretion increased from 8.3 +/- 1.0 to 23.4 +/- 4.7 pmol. min(-1). mm(-1) (P < 0.03) as the tubular flow rate was increased from 0.5 to 6 nl. min(-1). mm(-1). Flow stimulation of net K+ secretion was blocked by luminal TEA (8.2 +/- 1.2 vs. 9.9 +/- 2.7 pmol. min(-1). mm(-1) at 0.6 and 6 nl. min(-1). mm(-1) flow rates, respectively) or charybdotoxin (6.8 +/- 1.6 vs. 8.3 +/- 1.6 pmol. min(-1). mm(-1) at 1 and 4 nl. min(-1). mm(-1) flow rates, respectively) but not by apamin. These results suggest that flow-dependent K+ secretion is mediated by a maxi-K channel, whereas baseline K+ secretion occurs through a TEA- and charybdotoxin-insensitive SK (ROMK) channel.

Na(+)-K(+)-ATPase-mediated basolateral rubidium uptake in the maturing rabbit cortical collecting duct

Within the renal cortical collecting duct (CCD), transepithelial Na(+) absorption and K(+) secretion are linked to basolateral Na(+)-K(+)-ATPase activity. Our purpose was to examine the developmental changes in basolateral Na(+)-K(+)-ATPase-mediated (86)rubidium (Rb) uptake, its inhibitor sensitivity and relationship to pump hydrolytic activity and Na(+) transport. Multiple CCDs ( approximately 6 mm) from maturing rabbits were affixed to coverslips, preincubated at 37 degrees C for 10 min (+/-1-2.5 mM ouabain or 10 or 100 micro M Schering-28080, an inhibitor of H(+)-K(+)-ATPase), and then transferred to prewarmed incubation solution containing tracer amounts of (86)Rb (+/-inhibitors). After 1 min at 37 degrees C, tubular samples were rinsed and permeabilized and isotope counts were measured to calculate basolateral Rb uptake. Ouabain-inhibitable Rb uptake, an index of basolateral Na(+)-K(+) pump activity, increased approximately 3-fold during the 1st 8 wk of postnatal life (P < 0.03). The approximately 2-fold increase in absolute rate of Rb uptake between 1 and 6 wk (2.64 +/- 0.45 to 5.02 +/- 0.32 pmol. min(-1). mm(-1)) did not reach statistical significance. The rate of basolateral Rb uptake increased further after the 6th wk of life to 7.29 +/- 0.53 pmol. min(-1). mm(-1) in adult animals (P < 0.03 vs. 6 wk). Schering-28080 failed to inhibit Rb uptake, implying that functional H(+)-K(+)-ATPase is absent at the basolateral membrane. Na(+)-K(+)-ATPase hydrolytic activity, determined by using a microassay that measured inorganic phosphate release from [gamma-(32)P]ATP under maximum velocity (V(max)) conditions, also increased in the differentiating CCD (from 316.2 +/- 44.4 pmol. h(-1). mm(-1) at 2 wk to 555.9 +/- 105.1 at 4 wk to 789.7 +/- 145.0 at 6 wk; r = 1.0 by linear regression analysis; P < 0.005). The parallel approximately 2.5-fold increases in Na(+)-K(+)-ATPase activity and ouabain-sensitive Rb uptake between 2- and 6-wk postnatal age suggest that the developmental increase in basolateral transport capacity is due predominantly to an increase in enzyme abundance. The signals mediating the developmental increase in Na(+)-K(+)-ATPase activity in the CCD remain to be defined.

Developmental expression of ROMK mRNA in rabbit cortical collecting duct

The cortical collecting duct (CCD) is a major site of regulation of K+ homeostasis in the fully differentiated mammalian kidney. CCDs isolated from adult rabbits and microperfused in vitro secrete K+ into the tubular fluid at high rates. However, CCDs dissected from newborn animals show no significant net K+ secretion until the 3rd wk of life, at least in part because of a paucity of conducting apical secretory K+ (SK) channels. To determine whether the abundance of genes encoding the SK channel is developmentally regulated, we used reverse transcriptase-polymerase chain reaction (RT-PCR) and Northern blot analysis to test for the presence of mRNA encoding rat outer medullary K+ channel (ROMK), considered to be a major subunit of the SK channel, in kidney and single CCDs isolated from maturing rabbits. Using rat ROMK-specific primers, RT-PCR of rabbit kidney yielded an amplification product of expected size and sequence. Northern blot analysis identified a single band at approximately 2.9 kb in kidney at all ages. Densitometric analysis revealed a progressive increase in steady state expression of ROMK message in kidney after birth. RT-PCR of individual CCDs yielded a single band of predicted size for ROMK in all segments isolated from animals > or =3 wk old. In contrast, transcripts were not detected in any CCD samples obtained from 1-wk-old animals and were identified in only 30% of CCD samples isolated from 2-wk-old rabbits. In all of the latter tubular samples, a specific PCR product of correct size for beta-actin mRNA was detected. These results suggest that an increase in steady state expression of ROMK mRNA contributes to the developmental appearance of conducting secretory K+ channels in the CCD.

Developmental expression of ROMK in rat kidney

The apical secretory K+ (SK) channel in the principal cell represents the rate-limiting step for K+ secretion across the cortical collecting duct (CCD). Patch clamp analysis of maturing rabbit principal cells identifies an increase in number of conducting SK channels after the 2nd week of life [L. M. Satlin and L. G. Palmer. Am. J. Physiol. 272 (Renal Physiol. 41): F397-F404, 1997], approximately 1 wk after an increase in activity of the amiloride-sensitive epithelial Na+ channel (ENaC) is detected. To correlate the postnatal increase in channel activity with developmental expression of ROMK, the molecular correlate of the SK channel, we used gene-specific probes to show a developmental increase in abundance of renal ROMK mRNA and a ROMK-specific antibody to examine the nephron distribution, localization, and abundance of this protein in developing rat kidney. Using antibodies directed against aquaporin-3 (AQP-3) and Tamm-Horsfall protein (THP), we confirmed that ROMK was expressed along the apical membranes of principal cells and thick ascending limbs of Henle (TALH) in adult kidney. Within the midcortex of the neonatal kidney, ROMK-positive segments revealed weak coincident staining with the TALH-specific antibody but did not colabel with an antibody directed against distal and connecting tubule (CNT)-specific kallikrein or the lectin Dolichos biflorus agglutinin (DBA), which labels proximal tubules and collecting ducts. In inner cortex and outer medulla of kidneys from 1-wk-old animals, ROMK protein was identified in medullary TALH (MTALH) and DBA-positive collecting ducts. By 3 wk of age, coincident ROMK and DBA expression was detected in midcortical and outer cortical CNTs and CCDs. Immunoblot analysis of plasma membrane-enriched fractions of maturing rat kidney revealed a developmental increase in a approximately 40-kDa band, the expected size for ROMK. Immunolocalization of alpha-ENaC showed apical staining of a majority of cells in distal nephron segments after the 1st week of postnatal life. The beta- and gamma-ENaC subunit expression was routinely detected in a mostly cytoplasmic distribution immediately after birth, albeit in low abundance; gamma-ENaC showed some apical polarization. These results suggest that the postnatal increases in a principal cell apical SK and Na+ channel activity are mediated, at least in part, by increases in abundance of ROMK message and protein and ENaC subunit proteins.

Regulation of potassium transport in the maturing kidney

Kidneys of full-term newborn humans and animals conserve potassium (K+), a condition essential for growth. The cortical collecting duct (CCD) is uniquely adapted to accomplish this task early in life. CCDs isolated from newborn rabbits and microperfused in vitro show no net K+ secretion until after the third week of life; in contrast, segments isolated from adult animals secrete net K+ at high rates. The magnitude and direction of net K+ transport in the CCD reflect the balance of opposing fluxes of K+ secretion and K+ absorption mediated by principal and intercalated cells, respectively. The absence of net K+ secretion in the CCD early in life may thus be caused by a limited capacity of principal cells for K+ secretion and/or an excess of K+ absorption by intercalated cells. Recent studies provide data to support both possibilities. Patch-clamp analysis detects few conducting apical K+-secretory channels in neonatal principal cells, whereas fluorescent functional assays identify significant activity of the apical hydrogen, potassium adenosine triphosphatase (H+,K+-ATPase), a pump that reabsorbs K+ in exchange for H+s, in adjacent intercalated cells. Under conditions prevailing in vivo, the sum of the fluxes mediated by these two cell types likely contributes to the relative K+ retention characteristic of the neonatal kidney.

Characterization and regulation of H-K-ATPase in intercalated cells of rabbit cortical collecting duct

K-dependent H+ extrusion was investigated using fluorescence techniques in rabbit cortical collecting tubules (CCTs). Experiments were performed in split-open tubules from normal animals exposed to the intracellular pH indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF). This preparation permitted the study of individual intercalated cells (ICs). In the ICs, partial recovery of pH(i) was observed in response to an acute acid load upon readdition of 5 mM K to the superfusate. This recovery was SCH 28080-inhibitable (10(-5) M) and ouabain-insensitive suggesting the process is mediated by a gastric-type H-K ATPase. To see if H-K ATPase plays a role in acid secretion its function was evaluated under chronic metabolic acidosis (CMA) conditions. CMA was induced by replacing drinking water with 75 mM NH4Cl in 5% sucrose for 10-14 days. The SCH 28080-inhibitable K-dependent pH(i) recovery rate was three-fold higher in CMA ICs compared to controls. To determine the location of the H-K ATPase, CCTs were microperfused and individual peanut lectin binding (PNA) ICs studied. K-dependent pH(i) recovery was measured in response to an NH4Cl pulse. An apical SCH 28080-inhibited K-dependent pH(i) recovery process was observed in control and CMA ICs. Taken together these data confirm the existence of a gastric-type H-K ATPase in ICs of rabbit CCT. Based on our findings the H-K ATPase is found on the apical side of the cell and is stimulated under conditions of CMA.

Organic anion transporting polypeptide mediates organic anion/HCO3- exchange

Organic anion transporting polypeptide (oatp) is an integral membrane protein cloned from rat liver that mediates Na+-independent transport of organic anions such as sulfobromophthalein and taurocholic acid. Previous studies in rat hepatocytes suggested that organic anion uptake is associated with base exchange. To better characterize the mechanism of oatp-mediated organic anion uptake, we examined transport of taurocholate in a HeLa cell line stably transfected with oatp under the regulation of a zinc-inducible promoter (Shi, X., Bai, S., Ford, A. C., Burk, R. D., Jacquemin, E., Hagenbuch, B., Meier, P. J., and Wolkoff, A. W. (1995) J. Biol. Chem. 270, 25591-25595). Whereas noninduced transfected cells showed virtually no uptake of [3H]taurocholate, taurocholate uptake by induced cells was Na+-independent and saturable (Km = 19.4 +/- 3.3 microM; Vmax = 62.2 +/- 1.4 pmol/min/mg protein; n = 3). To test whether organic anion transport is coupled to HCO3- extrusion, we compared the rates of taurocholate-dependent HCO3- efflux from alkali-loaded noninduced and induced cells. Monolayers grown on glass coverslips were loaded with the pH-sensitive dye 2', 7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; intracellular pH (pHi) was measured by excitation ratio fluorometry. Noninduced and induced cells were alkalinized to an equivalent pHi ( approximately 7.7) by transient exposure to a 50 mM HCO3-, Cl--free solution. In the absence of extracellular Cl- and taurocholate, isohydric reduction of superfusate HCO3- concentration from 50 to 25 mM resulted in an insignificant change in pHi over time (dpHi/dt) in both groups. Addition of 25 microM taurocholate to the superfusate led to a rapid fall in pHi in induced (-0.037 +/- 0.011 pH units/min to pHi of 7.41 +/- 0.14) but not in noninduced (0.003 +/- 0.006 pH units/min to pHi of 7.61 +/- 0.08) cells (p < 0.03). These data indicate that oatp-mediated taurocholate transport is Na+-independent, saturable, and accompanied by HCO3- exchange. We conclude that organic anion/base exchange is an important, potentially regulatable component of oatp function.

Expression of carbonic anhydrase IV in carbonic anhydrase II-deficient mice

Chronic metabolic acidosis (CMA) in the rabbit upregulates carbonic anhydrase (CA) IV in the proximal convoluted tubule (PCT). This study was designed to assess CA IV expression in a model of CMA in the mouse, i.e., congenital deficiency in CA II [CA(II)D]. In female CA(II)D mice, CA IV specific activity but not CA IV immunoreactivity was upregulated in the renal cortex, specifically in microdissected PCTs. Western blot analysis showed higher expression of CA IV immunoreactive protein in renal membranes from males than in those from females.

Apical K+ conductance in maturing rabbit principal cell

Net K+ secretion is not detected in cortical collecting ducts (CCDs) isolated from newborn rabbits and perfused in vitro. To establish whether a low apical K+ permeability of the neonatal principal cell limits K+ secretion early in life, we used the patch-clamp technique in split-open CCDs isolated from maturing rabbits to study the properties and density of conducting K+ channels in principal cells. With KCl in the pipette and a NaCl solution warmed to 37 degrees C in the bath, inward currents with a conductance of approximately 42 pS were observed in 0% (0 out of 13 or 0/13), 10% (2/21), 18% (5/28), 29% (4/14), and 56% (10/18) of cell-attached patches obtained in 1-, 2-, 3-, 4-, and 5-wk-old animals, respectively. The conductance and reversal potential of this channel led us to suspect that it represented the low-conductance K+ channel previously described in the rat CCD by L. G. Palmer, L. Antonian, and G. Frindt (J. Gen. Physiol. 104: 693-710, 1994). The mean number of open channels per patch (NPo) increased progressively (P < 0.05) after birth, from 0 at 1 wk, to 0.06 +/- 0.04 at 2 wk, to 0.40 +/- 0.18 at 3 wk, to 0.74 +/- 0.41 at 4 wk, and to 1.06 +/- 0.28 at 5 wk. The increase in NPo appeared to be due primarily to a developmental increase in N, which is the number of channels; open probability, Po, remained constant at approximately 0.5 for all channels identified after the 2nd wk of life. The increase in number of conducting K+ channels during postnatal life is likely to contribute to the maturational increase in net K+ secretion in the CCDs.

H-K-ATPase activity in PNA-binding intercalated cells of newborn rabbit cortical collecting duct

Functional and immunocytochemical studies indicate that intercalated cells in the adult rabbit cortical collecting duct (CCD) possess an H-K-adenosinetriphosphatase (H-K-ATPase). Because growing subjects must retain K+ and excrete H+, we sought to determine whether H-K-ATPase is present in the CCD early in life and, if so, to assess its activity and polarity. H-K-ATPase activity was defined as the initial rate of Sch-28080-inhibitable K+-dependent cell pH (pHi) recovery observed, in the absence of Na+, in response to an in vitro acid load. Transporter activity was assayed in intercalated cells labeled with the pH-sensitive dye 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein and apical cell surface marker rhodamine peanut lectin (PNA) in split-open CCDs isolated from neonatal and adult New Zealand White rabbits. In Na+-free N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solutions (nominal absence of CO2/HCO3-), the rate of K+-dependent pH(i) recovery from a NH4Cl-induced acid load was similar in newborn (0.056 +/- 0.015 pH U/min, n = 9) and adult (0.060 +/- 0.019 pH U/min; n = 9, P = not significant) cells. This rate of K+-dependent pH(i) recovery was significantly reduced by 10-20 pM Sch-28080, an inhibitor of gastric H-K-ATPase, in both newborns (0.009 +/- 0.003 pH U/min, n = 7) and adults (0.013 +/- 0.007 pH U/min, n = 9) (P < 0.05 compared with rates in absence of inhibitor). To determine whether the location of the transporter is consistent with a role in K+ absorption and H+ secretion, pH(i) recovery of acutely acid-loaded intercalated cells in neonatal CCDs (n = 7) microperfused and bathed in the absence of Na+ and K+ was monitored after selective addition of K+ to either the luminal or basolateral membrane. Addition of 5 mM K+ led to a significantly greater rate of pH(i) recovery when it was added to the luminal rather than the peritubular solution (0.049 +/- 0.005 vs. 0.018 +/- 0.005 pH U/min, P < 0.05). We conclude that PNA-binding intercalated cells of the neonatal CCD possess H-K-ATPase activity, predominantly located in the apical membrane. This provides a mechanism for H secretion and K+ retention, processes required for growth.

Stimulation of apical H-K-ATPase in intercalated cells of cortical collecting duct with chronic metabolic acidosis

This study evaluated the role of H-K-adenosinetriphosphatase (H-K-ATPase) with chronic metabolic acidosis (CMA) in intercalated cells (ICs) of rabbit cortical collecting duct (CCD). CMA was induced by replacing drinking water with 75 mM NH4Cl in 5% sucrose for 10-14 days. CCDs isolated from CMA and control rabbits were split open and exposed to the intracellular pH (pHi) indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein. In N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered solutions, the resting pHi in ICs was similar for both groups. K-dependent pHi recovery (5 mM K, 140 mM N-methyl-D-glucamine) was monitored in response to a pulse of NH4Cl (10 mM). The K-dependent pHi recovery rate was threefold higher in CMAICs compared with controls and was abolished with the gastric H-K-ATPase inhibitor, Sch-28080 (10(-5) M). Polarity of the H-K-ATPase was studied in microperfused CMA and control CCDs. Luminal K-dependent pHi recovery was monitored in response to an acute pulse of NH4Cl in individual peanut lectin agglutinin (PNA)-binding ICs. The apical Sch-28080-inhibitable K-dependent pHi recovery rate was significantly greater in CMA ICs than control ICs. In summary, CMA enhances functional activity of an apical H-K-ATPase in PNA-binding ICs of rabbit CCD.

Apical Na+ conductance in maturing rabbit principal cell

Net Na+ absorption in microperfused rabbit cortical collecting ducts (CCDs) is low during the 1st wk of postnatal life, increasing substantially thereafter [L. M. Satlin. Am. J. Physiol. 266 (Renal Fluid Electrolyte Physiol. 35): F57-F65, 1994]. To establish whether the low rate of Na+ absorption observed immediately after birth is due to a low apical Na+ permeability of the neonatal principal cell, we used the patch-clamp technique in split-open CCDs isolated from maturing rabbits to estimate conductance, number (N), and open probability (Po) of apical Na+ channels in principal cells. With LiCl in the pipette and a NaCl or potassium gluconate solution, warmed to 37 degrees C, in the bath, inward currents with a conductance of approximately 11 pS (n = 23) were observed in 17% of cell-attached patches at 1 wk, 41% of patches at 2 wk, and 43% of patches at 5 wk. The mean N per patch in the 1st wk (0.22 +/- 0.09; n = 36) was significantly less than that observed in the 2nd (1.38 +/- 0.39; n = 34) and 5th (1.24 +/- 0.37; n = 21) wk of life. Po, studied at positive pipette voltages, was significantly lower in the 1st wk (0.085 +/- 0.035; n = 5) than in the 2nd wk (0.345 +/- 0.063; n = 9) and 5th wk (0.291 +/- 0.058; n = 4). To confirm that the 11-pS channel represented the amiloride-sensitive apical Na+ channel, cell-attached patches in CCDs isolated from 2-wk-old rabbits were studied with 0.5 microM amiloride added to the LiCl pipette solution. Amiloride led to > 90% reductions in mean open and closed times of the 11-pS conductance, consistent with blockade of the channel. These data indicate that N and Po of apical amiloride-sensitive Na+ channels in principal cells increase significantly after birth.

H+ secretion in rabbit mesonephric collecting tubule

The mesonephros, the precursor of the metanephros, the definitive kidney, is the functional excretory organ in the 12- to 20-day-old rabbit fetus. It is believed to acidify allantoic fluid. To determine whether H+ excretion occurs in the distal nephron, we examined isolated perfused mesonephric collecting tubules by microcalorimetry and pH-sensitive fluorescent dyes. Collecting tubules secreted H+ (absorbed HCO3-) at rates twice those observed in the mature outer medullary collecting duct (OMCD) of the metanephric kidney. H+ secretion was not inhibited by ouabain (18.5 +/- 2.2 vs. 16.7 +/- 4.0 pmol.min-1.mm-1; n = 7, P = NS) but was reversibly inhibited by removing Cl- from the bathing solution (15.1 +/- 2.3 to -0.6 +/- 3.7 to 15.5 +/- 1.1 pmol.min-1.mm-1; n = 5, P < 0.05); luminal application of N-ethylmaleimide (NEM), an inhibitor of the H(+)-ATPase, also inhibited H+ secretion (n = 2). These results suggested that H+ secretion in the mesonephric collecting tubule is mediated by transporters similar to those of the OMCD. To test this hypothesis, we stained collecting tubules from 15-20 day embryos with 6-carboxyfluorescein (6-CF) diacetate to identify intercalated-like cells or perfused them with 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM) to measure intracellular pH (pHi). We found that 139 +/- 15 cells/mm concentrated 6-CF or BCECF, consistent with the phenotype of metanephric intercalated cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Inhibition of bicarbonate transport in peanut lectin-positive intercalated cells by a monoclonal antibody

Intercalated cells (ICC) of the collecting duct are believed to secrete acid (alpha-type) or HCO3 (beta-type). Although these two types of ICC are functionally mirror images of each other, several components in their cell membranes are clearly unique. As a first step in defining the molecular composition of beta-ICC membranes, we raised cell-specific monoclonal antibodies (MAb) against surface antigens. One of these MAb, designated B63, reacts with the apical membrane of peanut lectin agglutinin (PNA)-positive cells of the kidney cortex. B63-positive cells do not react with antibodies against band 3 (the basolateral C1/HCO3 exchanger) or ST.48, markers for alpha-ICC and principal cells, respectively. Despite a significant positive correlation between PNA and B63 staining intensities, determined by flow cytometry, these markers react with separate antigens, as indicated by competition studies and the different distribution of the two antigens. In addition to renal beta-ICC, B63 antigen is present in tissues that are involved in HCO3 secretion, such as the pancreas, salivary glands, and the small intestine, suggesting that it might play a role in HCO3 secretion. To test this hypothesis more directly, we tested the effect of MAb B63 on HCO3 secretion and on changes in intracellular pH (pHi) in isolated perfused cortical collecting ducts. Luminal Cl removal in the presence of luminal 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid resulted in a reversible increase in pHi. Luminal application of MAb B63 prevented this change in pHi. MAb B63 also significantly inhibited (by 37.7 +/- 7.3%) HCO3 secretion by isolated perfused tubules, whereas another MAb (MAb 601), which reacts with a separate antigen on beta-ICC, did not alter HCO3 secretion or pHi. These results indicate that B63 antigen plays an important role in HCO3 secretion: it might be either the apical anion exchanger of beta-ICC or an associated regulatory protein.

Postnatal maturation of potassium transport in rabbit cortical collecting duct

Clearance studies in newborns demonstrate low rates of urinary excretion of potassium, suggesting that the neonatal kidney contributes to the conservation of potassium necessary for growth. Because the cortical collecting duct (CCD) is a primary site for potassium secretion in the adult, we sought to examine the transport capacity of this segment for potassium during postnatal maturation. CCDs isolated from rabbits of various ages (5-6 animals/age group) were microperfused in vitro with solutions simulating plasma. The concentrations of potassium in samples of collected fluid, measured by helium glow photometry, were used to calculate net transport. At a flow rate of approximately 1.6 nl.min-1 x mm-1 net potassium secretion was absent at birth, first became evident at 4 wk of age (-11.08 +/- 2.39 pmol.min-1 x mm-1), and increased sharply thereafter to reach mature rates (-23.08 +/- 3.47 pmol.min-1 x mm-1; P < 0.05) by 6 wk of age. To determine whether low distal tubular flow rates limit net potassium secretion in the neonate, we perfused CCDs at two or more flow rates in the 0.5-5 nl.min-1 x mm-1 range. In CCDs taken from animals > or = 6 wk of age, potassium secretion showed a significant linear correlation with flow rate (y = -10.0x - 7.45; r = 0.87; n = 12).(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation of rabbit cortical collecting duct to in vitro acid incubation

Cortical collecting ducts (CCDs) isolated from acid-loaded rabbits and perfused in vitro absorb HCO3-, whereas CCDs from normal animals secrete HCO3-. We have previously shown that CCDs incubated in vitro for 3 h at pH 6.9 show a reduction in net (baseline and stimulated) HCO3- secretion. In this study we ascertained the minimum duration of an acidic stimulus necessary to induce adaptive changes in stimulated HCO3- secretion (determined in the absence of basolateral Cl-) and the roles of protein synthesis and cytoskeletal function in this process. CCDs incubated in acid (pH 6.8, HCO3- 6 mM) for 1 h followed by incubation at pH 7.4 (HCO3- 25 mM) for 2 h showed a 41% reduction in stimulated HCO3- secretion (P < 0.001), similar to that observed after 3 h of incubation at pH 6.8. However, this incubation protocol failed to enhance stimulated HCO3- absorption (determined in the absence of luminal Cl-). Addition of 10 microM anisomycin, a reversible inhibitor of protein synthesis, throughout the entire period of incubation (1 h at pH 6.8 plus 2 h at pH 7.4) blocked adaptive reduction in HCO3- secretion, as did exposure to anisomycin only during the initial 1 h of acid incubation. In contrast, anisomycin application during the 2-h incubation at pH 7.4 failed to block this adaptation of HCO3- secretion. Application of 4 microM actinomycin D, an inhibitor of DNA transcription, during the acid incubation also prevented the adaptive response, as did application during the total or during the 2-h pH 7.4 incubation period of 0.2 microM cytochalasin D, an inhibitor of actin filament function.(ABSTRACT TRUNCATED AT 250 WORDS)