Factors governing sodium and chloride transport across the inner medullary collecting duct

The renal cortical collecting duct: a secreting epithelium?

KEY POINTS

The cortical collecting duct (CCD) plays an essential role in sodium homeostasis by fine-tuning the amount of sodium that is excreted in the urine. Ex vivo, the microperfused CCD reabsorbs sodium in the absence of lumen-to-bath concentration gradients. In the present study, we show that, in the presence of physiological lumen-to-bath concentration gradients, and in the absence of endocrine, paracrine and neural regulation, the mouse CCD secretes sodium, which represents a paradigm shift. This secretion occurs via the paracellular route, as well as a transcellular pathway that is energized by apical H+ /K+ -ATPase type 2 pumps operating as Na+ /K+ exchangers. The newly identified transcellular secretory pathway represents a physiological target for the regulation of sodium handling and for anti-hypertensive therapeutic agents.

ABSTRACT

In vitro microperfusion experiments have demonstrated that cortical collecting ducts (CCDs) reabsorb sodium via principal and type B intercalated cells under sodium-depleted conditions and thereby contribute to sodium and blood pressure homeostasis. However, these experiments were performed in the absence of the transepithelial ion concentration gradients that prevail in vivo and determine paracellular transport. The present study aimed to characterize Na+ , K+ and Cl- fluxes in the mouse CCD in the presence of physiological transepithelial concentration gradients. For this purpose, we combined in vitro measurements of ion fluxes across microperfused CCDs of sodium-depleted mice with the predictions of a mathematical model. When NaCl transport was inhibited in all cells, CCDs secreted Na+ and reabsorbed K+ ; Cl- transport was negligible. Removing inhibitors of type A and B intercalated cells increased Na+ secretion in wild-type (WT) mice but not in H+ /K+ -ATPase type 2 (HKA2) knockout mice. Further inhibition of basolateral NaCl entry via the Na+ -K+ -2Cl- cotransporter in type A intercalated cells reduced Na+ secretion in WT mice to the levels observed in HKA2-/- mice. With no inhibitors, WT mouse CCDs still secreted Na+ and reabsorbed K+ . In vivo, HKA2-/- mice excreted less Na+ than WT mice after switching to a high-salt diet. Taken together, our results indicate that type A intercalated cells secrete Na+ via basolateral Na+ -K+ -2Cl- cotransporters in tandem with apical HKA2 pumps. They also suggest that the CCD can mediate overall Na+ secretion, and that its ability to reabsorb NaCl in vivo depends on the presence of acute regulatory factors.

Acute inhibition of NCC does not activate distal electrogenic Na+ reabsorption or kaliuresis

Na(+) reabsorption from the distal renal tubule involves electroneutral and electrogenic pathways, with the latter promoting K(+) excretion. The relative activities of these two pathways are tightly controlled, participating in the minute-to-minute regulation of systemic K(+) balance. The pathways are interdependent: the activity of the NaCl cotransporter (NCC) in the distal convoluted tubule influences the activity of the epithelial Na(+) channel (ENaC) downstream. This effect might be mediated by changes in distal Na(+) delivery per se or by molecular and structural adaptations in the connecting tubule and collecting ducts. We hypothesized that acute inhibition of NCC activity would cause an immediate increase in Na(+) flux through ENaC, with a concomitant increase in renal K(+) excretion. We tested this using renal clearance methodology in anesthetized mice, by the administration of hydrochlorothiazide (HCTZ) and/or benzamil (BZM) to exert specific blockade of NCC and ENaC, respectively. Bolus HCTZ elicited a natriuresis that was sustained for up to 110 min; urinary K(+) excretion was not affected. Furthermore, the magnitude of the natriuresis was no greater during concomitant BZM administration. This suggests that ENaC-mediated Na(+) reabsorption was not normally limited by Na(+) delivery, accounting for the absence of thiazide-induced kaliuresis. After dietary Na(+) restriction, HCTZ elicited a kaliuresis, but the natiuretic effect of HCTZ was not enhanced by BZM. Our findings support a model in which inhibition of NCC activity does not increase Na(+) reabsorption through ENaC solely by increasing distal Na(+) delivery but rather by inducing a molecular and structural adaptation in downstream nephron segments.

Conservation of Na+ vs. K+ by the rat cortical collecting duct

Regulation of transport by principal cells of the distal nephron contributes to maintenance of Na(+) and K(+) homeostasis. To assess which of these ions is given a higher priority by these cells, we investigated the upregulation of epithelial Na(+) channels (ENaC) in the rat cortical collecting duct (CCD) during Na depletion with and without simultaneous K depletion. ENaC activity, assessed as whole cell amiloride-sensitive current in split-open tubules, was 260 ± 40 pA/cell in K-repleted but virtually undetectable (3 ± 1 pA/cell) in K-depleted animals. This difference was confirmed biochemically by the reduced amounts of the cleaved forms of both the α-ENaC and γ-ENaC subunits measured in immunoblots. In contrast, in K-depleted rats, simultaneously reducing Na intake did not affect the activity of ROMK channels, assessed as tertiapin-Q-sensitive whole cell currents, in the CCDs. The lack of Na current in K-depleted animals was the result of reduced levels of aldosterone in plasma, rather than a reduced sensitivity to the hormone. However, rats on a low-Na, low-K diet for 1 wk did not excrete more Na than those on a low-Na, control-K diet for the same period of time. Immunoblot analysis indicated increased levels of the thiazide-sensitive NaCl cotransporter and the apical Na-H exchanger NHE3. This suggests that with reduced K intake, Na balance is maintained despite reduced aldosterone and Na(+) channel activity by upregulation of Na(+) transport in upstream segments. Under these conditions, Na(+) transport by the aldosterone-sensitive distal nephron is reduced, despite the low-Na intake to minimize K(+) secretion and urinary K losses.

A voltage-dependent Ca2+ influx pathway regulates the Ca2+-dependent Cl(-) conductance of renal IMCD-3 cells

We have previously shown that the membrane conductance of mIMCD-3 cells at a holding potential of 0 mV is dominated by a Ca2+-dependent Cl(-) current (I(CLCA)). Here we report that I(CLCA) activity is also voltage dependent and that this dependence on voltage is linked to the opening of a novel Al3+-sensitive, voltage-dependent, Ca2+ influx pathway. Using whole-cell patch-clamp recordings at a physiological holding potential (-60 mV), ICLCA was found to be inactive and resting currents were predominantly K+ selective. However, membrane depolarization to 0 mV resulted in a slow, sigmoidal, activation of ICLCA (T(0.5) approximately 500 s), while repolarization in turn resulted in a monoexponential decay in I(CLCA) (T (0.5) approximately 100 s). The activation of I(CLCA) by depolarization was reduced by lowering extracellular Ca2+ and completely inhibited by buffering cytosolic Ca2+ with EGTA, suggesting a role for Ca2+ influx in the activation of I(CLCA). However, raising bulk cytosolic Ca2+ at -60 mV did not produce sustained I(CLCA) activity. Therefore I(CLCA) is dependent on both an increase in intracellular Ca2+ and depolarization to be active. We further show that membrane depolarization is coupled to opening of a Ca2+ influx pathway that displays equal permeability to Ca2+ and Ba2+ ions and that is blocked by extracellular Al3+ and La3+. Furthermore, Al3+ completely and reversibly inhibited depolarization-induced activation of ICLCA, thereby directly linking Ca2+ influx to activation of I(CLCA). We speculate that during sustained membrane depolarization, calcium influx activates ICLCA which functions to modulate NaCl transport across the apical membrane of IMCD cells.

Early embryonic renal tubules of wild-type and polycystic kidney disease kidneys respond to cAMP stimulation with cystic fibrosis transmembrane conductance regulator/Na(+),K(+),2Cl(-) Co-transporter-dependent cystic dilation

Metanephric organ culture has been used to determine whether embryonic kidney tubules can be stimulated by cAMP to form cysts. Under basal culture conditions, wild-type kidneys from embryonic day 13.5 to 15.5 mice grow in size and continue ureteric bud branching and tubule formation over a 4- to 5-d period. Treatment of these kidneys with 8-Br-cAMP or the cAMP agonist forskolin induced the formation of dilated tubules within 1 h, which enlarged over several days and resulted in dramatically expanded cyst-like structures of proximal tubule and collecting duct origin. Tubule dilation was reversible upon withdrawal of 8-Br-cAMP and was inhibited by the cAMP-dependent protein kinase inhibitor H89 and the cystic fibrosis transmembrane conductance regulator (CFTR) inhibitor CFTR(inh)172. For further testing of the role of CFTR, metanephric cultures were prepared from mice with a targeted mutation of the Cftr gene. In contrast to kidneys from wild-type mice, those from Cftr -/- mice showed no evidence of tubular dilation in response to 8-Br-cAMP, indicating that CFTR Cl(-) channels are functional in embryonic kidneys and are required for cAMP-driven tubule expansion. A requirement for transepithelial Cl(-) transport was demonstrated by inhibiting the basolateral Na(+),K(+),2Cl(-) co-transporter with bumetanide, which effectively blocked all cAMP-stimulated tubular dilation. For determination of whether cystic dilation occurs to a greater extent in PKD kidneys in response to cAMP, Pkd1(m1Bei) -/- embryonic kidneys were treated with 8-Br-cAMP and were found to form rapidly CFTR- and Na(+),K(+),2Cl(-) co-transporter-dependent cysts that were three- to six-fold larger than those of wild-type kidneys. These results suggest that cAMP can stimulate fluid secretion early in renal tubule development during the time when renal cysts first appear in PKD kidneys and that PKD-deficient renal tubules are predisposed to abnormally increased cyst expansion in response to elevated levels of cAMP.

Extracellular zinc stimulates a calcium-activated chloride conductance through mobilisation of intracellular calcium in renal inner medullary collecting duct cells

We have used the perforated patch clamp and fura-2 fluorescence techniques to study the effect of extracellular Zn(2+) on whole-cell Ca(2+)-activated Cl(-) currents (I (CLCA)) in mouse inner medullary collecting duct cells (mIMCD-3). I (CLCA) was spontaneously active in 74% of cells under basal conditions and displayed time and voltage-independent kinetics and an outwardly rectifying current/voltage relationship (I/V). Addition of zinc chloride (10-400 microM) to the bathing solution resulted in a dose-dependent increase in I (CLCA) with little change in Cl(-) selectivity or biophysical characteristics, whereas gadolinium chloride (30 microM) and lanthanum chloride (100 microM) had no significant effect on the whole-cell current. Using fura-2-loaded mIMCD-3 cells, extracellular Zn(2+) (400 microM) stimulated an increase in intracellular Ca(2+) to an elevated plateau. The Zn(2+)-stimulated [Ca(2+)](i) increase was inhibited by thapsigargin (200 nM), the IP(3) receptor antagonist 2-aminoethoxydiphenyl borate (10 microM) and removal of bath Ca(2+). Pre-exposure to Zn(2+) (400 microM) markedly attenuated the ATP (100 microM)-stimulated [Ca(2+)](i) increase. These data are consistent with the hypothesis that extracellular Zn(2+) stimulates an increase in [Ca(2+)](i) by a release of calcium from thapsigargin/IP(3) sensitive stores. A possible physiological role for a divalent metal ion receptor, distinct from the extracellular Ca(2+)-sensing receptor, in IMCD cells is discussed.

Adrenergic regulation of salt and fluid secretion in human medullary collecting duct cells

Transepithelial salt and fluid secretion mediated by cAMP in initial inner medullary collecting ducts (IMCDi) may be important for making final adjustments to urine composition. We examined in primary cultures of human IMCDi cells the effects of adrenergic receptor (AR) agonists and antagonists on intracellular cAMP levels, short-circuit current (I(SC)), and fluid secretion. Epinephrine (1 microM), norepinephrine (1 microM), and isoproterenol (10 nM) individually increased intracellular cAMP levels 57-, 2-, and 25-fold, respectively, and stimulated I(SC) 3.3-, 2.9-, and 3.4-fold, respectively. beta-AR activation increased net fluid secretion by cultured human IMCDi cell monolayers from 0.09 +/- 0.04 to 0.26 +/- 0.05 microl x h(-1) x cm(-2) and freshly isolated rat IMCDi from 0.02 +/- 0.01 to 0.09 +/- 0.02 nl x h(-1) x mm(-1). In monolayers, these effects were eliminated by blocking beta2-AR, but not beta1-AR. Activation of alpha2-AR with guanabenz inhibited isoproterenol-induced I(SC) by 37% in human IMCDi monolayers and fluid secretion by 91% in rat IMCDi. Immunohistochemistry of human medullary tissue sections revealed greater expression of beta2-AR than beta1-AR; beta2-AR was localized to the basolateral membranes of human IMCDi. Immunoblots identified alpha2A-AR and alpha2B-AR in cultured human IMCDi cell monolayers. We conclude that 1) catecholamines stimulate cAMP-dependent anion and fluid secretion by IMCDi cells primarily through beta2-AR activation and 2) alpha2-AR activation attenuates cAMP-dependent anion secretion.

PGE2 stimulates Cl- secretion in murine M-1 cortical collecting duct cells in an autocrine manner

Prostaglandin E2 (PGE2) is thought to be an important modulator of renal ion and water transport, but its effects remain complex and incompletely understood. Here we examined the effects of PGE2 on transepithelial ion transport of M-1 mouse cortical collecting duct cells using short-circuit current (ISC) measurements. Basolateral addition of PGE2 (1 microM) produced a transient peak increase in ISC of 6.3+/-0.8 microA cm(-2) (n=11), followed by a sustained plateau. The PGE2-evoked response was preserved in the presence of 100 micro M apical amiloride with an average peak increase of 10.6+/-1.0 microA cm(-2) (n=23). However, it was greatly diminished in both the presence of apical diphenylamine-2-carboxylic acid (DPC, 1 mM) and the absence of extracellular Cl-, indicating that Cl- secretion had been stimulated. Basolateral PGE2 induced a concentration dependent response, with an EC50 of about 8 nM. Apical addition of PGE2 elicited an ISC response similar to that observed with basolateral PGE2. Furthermore, apical exposure to arachidonic acid (AA) produced a similar increase in ISC, which could be prevented by the cyclooxygenase inhibitor indomethacin, while AA failed to exert an additional effect in the presence of PGE2. Using RT-PCR, we confirmed the expression of the PGE2 (EP) receptor subtypes EP1, EP3 and EP4 but not of EP2 in cultured M-1 CCD cells. We conclude that M-1 cells express functional cyclooxygenase activity and can generate PGE2 which acts in an autocrine manner, causing Cl- secretion.

Characterization of Ca2+-activated Cl- currents in mouse kidney inner medullary collecting duct cells

Ca2+-activated Cl- (ClCa) channels were characterized biophysically and pharmacologically in a mouse kidney inner medullary collecting duct cell line, IMCD-K2. Whole cell recording was performed with symmetrical N-methyl-d-glucamine chloride (NMDG)-Cl in the intracellular and extracellular solutions, and the intracellular Ca2+ concentration ([Ca2+]i) was adjusted with Ca2+-EGTA buffers. The amplitude of the current was dependent on [Ca2+]i. [Ca2+]i <800 nM strongly activated outwardly rectifying Cl- currents, whereas high Ca2+ (21 microM) elicited time-independent currents that did not rectify. The currents activated at low [Ca2+] exhibited time-dependent activation and deactivation. The affinity of the channel for Ca2+ was voltage dependent. The EC50 for Ca2+ was approximately 0.4 microM at +100 mV and approximately 1.0 microM at -100 mV. The Cl- channel blocker niflumic acid in the bath equally inhibited both inward and outward currents reversibly, with a Ki = 7.6 microM. DIDS, diphenylamine-2-carboxylic acid, and anthracene-9-carboxylic acid reversibly inhibited outward currents in a voltage-dependent manner. DTT slowly inhibited the currents, but tamoxifen did not. Comparing the biophysical and pharmacological properties, we conclude that IMCD-K2 cells express the same type of ClCa channels as those we have described in detail in Xenopus laevis oocytes (Qu Z and Hartzell HC. J Biol Chem 276: 18423-18429, 2001).

Mathematical models of renal fluid and electrolyte transport: acknowledging our uncertainty

Mathematical models of renal tubular function, with detail at the cellular level, have been developed for most nephron segments, and these have generally been successful at capturing the overall bookkeeping of solute and water transport. Nevertheless, considerable uncertainty remains about important transport events along the nephron. The examples presented include the role of proximal tubule tight junctions in water transport and in regulation of Na(+) transport, the mechanism by which axial flow in proximal tubule modulates solute reabsorption, the effect of formate on proximal Cl(-) transport, the assessment of potassium transport along collecting duct segments inaccessible to micropuncture, the assignment of pathways for peritubular Cl(-) exit in outer medullary collecting duct, and the interaction of carbonic anhydrase-sensitive and -insensitive pathways for base exit from inner medullary collecting duct. Some of these uncertainties have had intense experimental interest well before they were cast as modeling problems. Indeed, many of the renal tubular models have been developed based on data acquired over two or three decades. Nevertheless, some uncertainties have been delineated as the result of model exploration and represent communications from the modelers back to the experimental community that certain issues should not be considered closed. With respect to model refinement, incorporating more biophysical detail about individual transporters will certainly enhance model reliability, but ultimate confidence in tubular models will still be contingent on experimental development of critical information at the tubular level.

Electrolyte and fluid secretion by cultured human inner medullary collecting duct cells

Inner medullary collecting ducts (IMCD) are the final nephron segments through which urine flows. To investigate epithelial ion transport in human IMCD, we established primary cell cultures from initial (hIMCD(i)) and terminal (hIMCD(t)) inner medullary regions of human kidneys. AVP, PGE(2), and forskolin increased cAMP in both hIMCD(i) and hIMCD(t) cells. The effects of AVP and PGE2 were greatest in hIMCD(i); however, forskolin increased cAMP to the same extent in hIMCD(i) and hIMCD(t). Basal short-circuit current (I(SC)) of hIMCD(i) monolayers was 1.4 +/- 0.5 microA/cm2 and was inhibited by benzamil, a Na+ channel blocker. 8-Bromo-cAMP, AVP, PGE(2), and forskolin increased I(SC); the current was reduced by blocking PKA, apical Cl- channels, basolateral NKCC1 (a Na+ - K+ - 2Cl- cotransporter), and basolateral Cl-/HCO(3)(-) exchangers. In fluid transport studies, hIMCD(i) monolayers absorbed fluid in the basal state and forskolin reversed net fluid transport to secretion. In hIMCD(t) monolayers, basal current was not different from zero and cAMP had no effect on I(SC). We conclude that AVP and PGE2 stimulate cAMP-dependent Cl- secretion by hIMCD(i) cells, but not hIMCD(t) cells, in vitro. We suggest that salt secretion at specialized sites along human collecting ducts may be important in the formation of the final urine.

Contribution of the Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) to transepithelial transport of H(+), NH(4)(+), K(+), and Na(+) in rat outer medullary collecting duct

In rat kidney, the "secretory" isoform of the Na-K-Cl cotransporter, NKCC1 (BSC-2), localizes to the basolateral membrane of the alpha intercalated cell, the acid secreting cell of the outer medullary collecting duct (OMCD). This laboratory has reported that NKCC1 mediates Cl(-) uptake across the basolateral membrane in series with Cl(-) secretion across the apical membrane in rat OMCD. NKCC1 transports NH(4)(+), K(+), and Na(+) as well as Cl(-); therefore, a role for the cotransporter in the process of HCl, NH(4)Cl, KCl, and NaCl secretion has been suggested. Thus, it was determined if bumetanide, an inhibitor of NKCC1, alters transepithelial cation transport in rat OMCD. OMCD tubules from deoxycorticosterone pivalate (DOCP)-treated rats were perfused in vitro. Hydration of CO(2), rather than NH(4)(+), provides the principle source of H(+) for net acid secretion. In HCO(3)(-)/CO(2)-buffered solutions, no effect of bumetanide on net K(+) flux was detected. Under some conditions, bumetanide addition resulted in a small reduction in secretion of net H(+) equivalents. Transepithelial Na(+) flux, J(Na), was -1.5 +/- 1.7 pmol/mm per min, values not different from zero. However, with the application of bumetanide to the bath, J(Na) was +5.2 +/- 1.3 pmol/mm per min (P < 0.05), which indicates net Na(+) absorption. In conclusion, inhibition of NKCC1 in rat OMCD changes transepithelial movement of Na(+) and Cl(-). The role of NKCC1 in the secretion of net H(+) equivalents is small.

cAMP-dependent fluid secretion in rat inner medullary collecting ducts

We used an unambiguous in vitro method to determine if inner medullary collecting ducts (IMCD) have intrinsic capacities to absorb and secrete solutes and fluid in an isotonic medium. IMCD(1), IMCD(2), and IMCD(3) were dissected from kidneys of young Sprague-Dawley rats. 8-Bromo-3',5'-cyclic monophosphate (8-BrcAMP) stimulated lumen formation and progressive dilation in all IMCD subsegments; lumen formation was greatest in IMCD(1.) Benzamil potentiated the rate of lumen expansion in response to 8-BrcAMP. Fluid entered tubule lumens by transcellular secretion rather than simple translocation of intracellular fluid. Secreted lumen solutes were osmometrically active. Inhibition of protein kinase A with H-89 and Rp diastereomer of adenosine 3',5'-cyclic monophosphorothioate blocked fluid secretion. The rate of lumen expansion was reduced by the selective addition of ouabain, barium, diphenyl-2-carboxylate, bumetanide, glybenclamide, or DIDS, or reduction of extracellular Cl(-). We conclude that IMCD absorb and secrete electrolytes and fluid in vitro and that secretion is accelerated by cAMP. We suggest that salt and fluid secretion by the terminal portions of the renal collecting system may have a role in modulating the composition and volume of the final urine.

Contribution of the Na+-K+-2Cl- cotransporter NKCC1 to Cl- secretion in rat OMCD

In rat kidney the "secretory" isoform of the Na+-K+-2Cl- cotransporter (NKCC1) localizes to the basolateral membrane of the alpha-intercalated cell. The purpose of this study was to determine whether rat outer medullary collecting duct (OMCD) secretes Cl- and whether transepithelial Cl- transport occurs, in part, through Cl- uptake across the basolateral membrane mediated by NKCC1 in series with Cl- efflux across the apical membrane. OMCD tubules from rats treated with deoxycorticosterone pivalate were perfused in vitro in symmetrical HCO/CO2-buffered solutions. Cl- secretion was observed in this segment, accompanied by a lumen positive transepithelial potential. Bumetanide (100 microM), when added to the bath, reduced Cl- secretion by 78%, although the lumen positive transepithelial potential and fluid flux were unchanged. Bumetanide-sensitive Cl- secretion was dependent on extracellular Na+ and either K+ or NH, consistent with the ion dependency of NKCC1-mediated Cl- transport. In conclusion, OMCD tubules from deoxycorticosterone pivalate-treated rats secrete Cl- into the luminal fluid through NKCC1-mediated Cl- uptake across the basolateral membrane in series with Cl- efflux across the apical membrane. The physiological role of NKCC1-mediated Cl- uptake remains to be determined. However, the role of NKCC1 in the process of fluid secretion could not be demonstrated.

Bradykinin regulation of salt transport across mouse inner medullary collecting duct epithelium involves activation of a Ca(2+)-dependent Cl(-) conductance

The mechanism by which bradykinin regulates renal epithelial salt transport has been investigated using a mouse inner medullary renal collecting duct cell-line mIMCD-K2. Using fura-2 loaded mIMCD-K2 cells bradykinin (100 nM) has been shown to induce a transient increase in intracellular Ca(2+) via activation of bradykinin B2 receptors localized to both the apical and basolateral epithelial cell surfaces. In mIMCD-K2 epithelial cell-layers clamped in Ussing chambers, 100 nM bradykinin via apical and basolateral bradykinin B2 receptors stimulated a transient increase in inward short-circuit current (I:(sc)) of similar duration to the increase in intracellular Ca(2+). Replacements of the bathing solution Na(+) by the impermeant cation, N-methyl-D-glucamine and of Cl(-) and HCO(3)(-) by the impermeant anion gluconate at either the apical (no reduction) or basal bathing solutions (abolition of the response) are consistent with the bradykinin-stimulated increase in inward I:(sc) resulting from basal to apical Cl(-) (anion) secretion. Using the slow whole cell configuration of the patch-clamp technique, bradykinin was shown to activate a transient Cl(-) selective whole cell current which showed time-dependent activation at positive membrane potentials and time-dependent inactivation at negative membrane potentials. These currents were distinct from those activated by forskolin (CFTR), but identical to those activated by exogenous ATP and are therefore consistent with bradykinin activation of a Ca(2+)-dependent Cl(-) conductance. The molecular identity of the Ca(2+)-dependent Cl(-) conductance has been investigated by an RT - PCR approach. Expression of an mRNA transcript with 96% identity to mCLCA1/2 was confirmed, however an additional but distinct mRNA transcript with only 81% of the identity to mCLCA1/2 was identified.

The cystic fibrosis transmembrane regulator (CFTR) in the kidney

The cystic fibrosis transmembrane regulator (CFTR) is a Cl - channel. Mutations of this transporter lead to a defect of chloride secretion by epithelial cells causing the Cystic Fibrosis disease (CF). In spite of the high expression of CFTR in the kidney, patients with CF do not show major renal dysfunction, but it is known that both the urinary excretion of drugs and the renal capacity to concentrate and dilute urine is deficient. CFTR mRNA is expressed in all nephron segments and its protein is involved with chloride secretion in the distal tubule, and the principal cells of the cortical (CCD) and medullary (IMCD) collecting ducts. Several studies have demonstrated that CFTR does not only transport Cl - but also secretes ATP and, thus, controls other conductances such as Na+ (ENaC) and K+ (ROMK2) channels, especially in CCD. In the polycystic kidney the secretion of chloride through CFTR contributes to the cyst enlargement. This review is focused on the role of CFTR in the kidney and the implications of extracellular volume regulators, such as hormones, on its function and expression.

Role of PGE(2) in alpha(2)-induced inhibition of AVP- and cAMP-stimulated H(2)O, Na(+), and urea transport in rat IMCD

PGE(2) inhibits osmotic water permeability (P(f)) in the rat inner medullary collecting duct (IMCD) via cellular events occurring after the stimulation of cAMP, i.e., post-cAMP-dependent events. The alpha(2)-agonists also inhibit P(f) in the rat IMCD via post-cAMP-dependent events. The purpose of this study was to determine whether PGE(2) plays a role in alpha(2)-mediated inhibition of P(f), Na(+), and urea transport in the rat IMCD. Isolated terminal IMCDs from Wistar rats were perfused to measure, in separate experiments, P(f), lumen-to-bath (22)Na(+) transport (J(lb)), and urea permeability (P(u)). Transport was stimulated with 220 pM arginine vasopressin (AVP) or 0.1 mM 8-(4-chlorophenylthio)-cAMP (CPT-cAMP). Indomethacin was used to inhibit endogenous prostaglandin synthesis, and the alpha(2)-agonists clonidine, oxymetazoline, and dexmedetomidine were used to test the role of PGE(2) in the alpha(2)-mediated mechanism that inhibits transport. All agents were added to the bath. Indomethacin at 5 microM significantly elevated CPT-cAMP-stimulated P(f), J(lb), and P(u), and subsequent addition of 100 nM PGE(2) reduced these transport parameters. Indomethacin reversed alpha(2) inhibition of CPT-cAMP-stimulated P(f), J(lb), and P(u), and subsequent addition of PGE(2) reduced transport in each case. Indomethacin partially reversed alpha(2) inhibition of AVP-stimulated P(f), J(lb), and P(u), and PGE(2) reduced transport back to the alpha(2)-inhibited level. These results indicate that PGE(2) is a second messenger involved in the mechanism of transport inhibition mediated by alpha(2)-adrenoceptors via post-cAMP-dependent events in the rat IMCD.

Ca2+ and cAMP-activated Cl- conductances mediate Cl- secretion in a mouse renal inner medullary collecting duct cell line

1. The nature of Cl- conductance(s) participating in transepithelial anion secretion by renal inner medullary collecting duct (IMCD, mIMCD-K2 cell line) was investigated. 2. Extracellular ATP (100 microM) stimulated a transient increase in both whole-cell Cl- conductance and intracellular free Ca2+. In contrast, ionomycin (10-100 nM) caused a sustained increase in whole-cell Cl- conductance. Pre-loading cells with the Ca2+ buffer BAPTA abolished the ATP-dependent responses and delayed the onset of the increase observed with ionomycin. 3. The Ca2+-activated whole-cell Cl- current stimulated by ATP (peak) and ionomycin (maximal) displayed (i) a linear steady-state current-voltage relationship and (ii) time and voltage dependence with slow activation at +80 mV and slow inactivation at -80 mV. In BAPTA-loaded cells, ionomycin-elicited whole-cell currents exhibited pronounced outward rectification with time-dependent activation/inactivation. 4. Ca2+-activated and forskolin-activated Cl- conductances co-exist since ATP activation of whole-cell current occurred during a maximal stimulation by forskolin in single cell recordings. 5. In IMCD epithelial layers, ATP and ionomycin stimulated an inward short circuit current (Isc) dependent upon basal medium Na+ and Cl-/HCO3- but independent of the presence of apical bathing medium Na+ and Cl-/HCO3-. This was identical to forskolin stimulation and consistent with transepithelial anion secretion. 6. PCR amplification of reverse-transcribed mRNA using gene-specific primers demonstrated expression of both cystic fibrosis transmembrane conductance regulator (CFTR) mRNA and Ca2+-activated Cl- channel (mCLCA1) mRNA in mIMCD-K2 cells. 7. Ca2+ and forskolin-activated Cl- conductances participate in anion secretion by IMCD.

Thiazide induces water absorption in the inner medullary collecting duct of normal and Brattleboro rats

The reduction of urinary volume after the use of thiazide in the treatment of diabetes insipidus (DI) is known as the "paradoxical effect." Since enhanced proximal solute and water reabsorption only partially account for the reduction in urinary volume, an additional diuretic effect on nephron terminal segments was postulated. Thus the aim of our work was to investigate the effect of hydrochlorothiazide (HCTZ) on water transport in the inner medullary collecting duct (IMCD) of normal and Brattleboro rats. Osmotic water permeability (P(f)) and diffusional water permeability (P(dw)) were studied at 37 degrees C and pH 7.4 by the in vitro microperfusion technique. In the absence of antidiuretic hormone (ADH), HCTZ (10(-6) M) added to the perfused fluid enhanced P(f) from 6.36 +/- 0. 56 to 19.08 +/- 1.70 micro(m)/s (P < 0.01) and P(dw) from 38.01 +/- 4.52 to 52.26 +/- 4.38 x10(-5) cm/s (P < 0.01) in normal rats and also stimulated P(f) in Brattleboro rats from 3.53 +/- 1.41 to 11.16 +/- 1.13 micro(m)/s (P < 0.01). Prostaglandin E(2) (PGE(2)) (10(-5) M) added to the bath fluid inhibited HCTZ-stimulated P(f) (in micro(m)/s) as follows: control, 16.93 +/- 2.64; HCTZ, 29.65 +/- 5.67; HCTZ+PGE(2), 10.46 +/- 1.84 (P < 0.01); recovery, 16.77 +/- 4.07. These data indicate that thiazides enhance water absorption in IMCD from normal rats (in the absence of ADH) and from Brattleboro rats and that the HCTZ-stimulated P(f) was partially blocked by PGE(2). Thus we may conclude that the effect of thiazide in the treatment of DI occurs not only in the Na(+)-Cl(-) cotransport in the distal tubule but also in the IMCD.

Role of natriuretic peptides in ion transport mechanisms

Natriuretic peptides (NP) act as ligands on the guanylyl cyclase family of receptors. The NP binding site on these receptors is extracellular and the guanylyl cyclase and protein kinase domains are intracellular. The guanylyl cyclase receptor catalyzes the synthesis of the second messenger molecule, cGMP, which activates protein kinase. This in turn is involved in the phosphorylation of various ion transport proteins. Ion transport proteins, which are modulated by NP and are thought to underlie the natriuretic and diuretic actions of NP, include: (a) calcium-activated K+ channels; (b) ATP-sensitive K+ channels; (c) inwardly-rectifying K+ channels; (d) outwardly-rectifying K+ channels; (e) L-type Ca2+ channels; (f) Cl- channels including cystic fibrosis transmembrane conductance regulator Cl- channels; (g) Na+- K+ 2Cl- co-transporter; (h) Na+- K+ ATPase; (i) Na+ channels; (j) stretch-activated channels; and (k) water channels. It appears that NP modulate the kinetics, rather than the conductance, of ion channels. Some of these channels, like the Ca2+, ATP-sensitive K+ and stretch-activated channels, are also involved in NP secretion. In addition, the structural properties of the NP, e.g., ovCNP-22 and ovCNP-39, appear to confer on them the ability to form ion channels. These CNP-formed ion channels can modify the trans-membrane signal transduction and second messenger systems underlying NP-induced pathological effects.

Epithelial transport in polycystic kidney disease

In autosomal dominant polycystic kidney disease (ADPKD), the genetic defect results in the slow growth of a multitude of epithelial cysts within the renal parenchyma. Cysts originate within the glomeruli and all tubular structures, and their growth is the result of proliferation of incompletely differentiated epithelial cells and the accumulation of fluid within the cysts. The majority of cysts disconnect from tubular structures as they grow but still accumulate fluid within the lumen. The fluid accumulation is the result of secretion of fluid driven by active transepithelial Cl- secretion. Proliferation of the cells and fluid secretion are activated by agonists of the cAMP signaling pathway. The transport mechanisms involved include the cystic fibrosis transmembrane conductance regulator (CFTR) present in the apical membrane of the cystic cells and a bumetanide-sensitive transporter located in the basolateral membrane. A lipid factor, called cyst activating factor, has been found in the cystic fluid. Cyst activating factor stimulates cAMP production, proliferation, and fluid secretion by cultured renal epithelial cells and also is a chemotactic agent. Cysts also appear in the intrahepatic biliary tree in ADPKD. Normal ductal cells secrete Cl- and HCO3-. The cystic ductal cell also secretes Cl-, but HCO3- secretion is diminished, probably as the result of a lower population of Cl-/HCO3- exchangers in the apical membrane as compared with the normal cells. Some segments of the normal renal tubule are also capable of utilizing CFTR to secrete Cl-, particularly the inner medullary collecting duct. The ability of Madin-Darby canine kidney cells and normal human kidney cortex cells to form cysts in culture and to secrete fluid and the functional similarities between these incompletely differentiated, proliferative cells and developing cells in the intestinal crypt and in the fetal lung have led us to suggest that Cl- and fluid secretion may be a common property of at least some renal epithelial cells in an intermediate stage of development. The genetic defect in ADPKD may not directly affect membrane transport mechanisms but rather may arrest the development of certain renal epithelial cells in an incompletely differentiated, proliferative stage.

A mathematical model of the inner medullary collecting duct of the rat: pathways for Na and K transport

A mathematical model of the inner medullary collecting duct (IMCD) of the rat has been developed representing Na+, K+, Cl-, HCO3-, CO2, H2CO3, phosphate, ammonia, and urea. Novel model features include: finite rates of hydration of CO2, a kinetic representation of the H-K-ATPase within the luminal cell membrane, cellular osmolytes that are regulated in defense of cell volume, and the repeated coalescing of IMCD tubule segments to yield the ducts of Bellini. Model transport is such that when entering Na+ is 4% of filtered Na+, approximately 75% of this load is reabsorbed. This requirement renders the area-specific transport rate for Na+ comparable to that for proximal tubule. With respect to the luminal membrane, there is experimental evidence for both NaCl cotransport and an Na+ channel in parallel. The experimental constraints that transepithelial potential difference is small and that the fractional apical resistance is greater than 85% mandate that more than 75% of luminal Na+ entry be electrically silent. When Na+ delivery is limited, an NaCl cotransporter can be effective at reducing luminal Na+ concentration to the observed low urinary values. Given the rate of transcellular Na+ reabsorption, there is necessarily a high rate of peritubular K+ recycling; also, given the lower bound on luminal membrane Cl- reabsorption, substantial peritubular Cl- flux must be present. Thus, if realistic limits on cell membrane electrical resistance are observed, then this model predicts a requirement for peritubular electroneutral KCl exit.

Cyclic nucleotide-gated cation channels mediate sodium absorption by IMCD (mIMCD-K2) cells

The inner medullary collecting duct cell line, mIMCD-K2, absorbs Na+ by an amiloride-sensitive, electrogenic mechanism. The goal of the present study was to characterize the amiloride-sensitive, Na+ -conducting channels responsible for electrogenic Na+ absorption. To this end, we measured Na+ currents in single cells with the patch-clamp technique and Na+ currents across monolayers mounted in Ussing-type chambers. In whole cell patch-clamp experiments, amiloride-sensitive, inward Na+ currents were mediated by nonselective cation channels. In single-channel patch-clamp experiments, amiloride- and guanosine 3',5'-cyclic monophosphate (cGMP)-sensitive, 20-pS nonselective cation channels (i.e., CNG channels) were identified in the apical membrane. CNG channels were inhibited by amiloride, diltiazem, ethylisopropylamiloride (EIPA), and 8-bromo-cGMP and were permeable to Ca2+ and Mg2+. Epithelial Na+ channels were never observed in whole cell or single-channel recordings. Na+ absorption across confluent monolayers was inhibited with a rank order potency of benzamil > amiloride > phenamil >> EIPA > diltiazem. Our data are most consistent with the view that CNG channels mediate electrogenic Na+ absorption across mIMCD-K2 cells.

Chloride and fluid secretion by cultured human polycystic kidney cells

Epithelial cells cultured from the renal cysts of patients with autosomal dominant polycystic kidney disease (ADPKD) secrete fluid via a process stimulated by adenosine 3',5'-cyclic monophosphate (cAMP). We have investigated the hypothesis that fluid secretion by these cells is dependent on cAMP-mediated chloride secretion. Individual cultured ADPKD cells were suspended within a polymerized collagen matrix and stimulated to form cysts. Individual cultured cysts were placed in a chamber on the stage of an inverted microscope equipped with epifluorescent and video analysis attachments. The rate of fluid secretion, cell volume and changes in intracellular Cl- were measured. In the absence of secretagogues, fluid was absorbed from the cyst cavity (-2.36 +/- 0.64 nl/min/cm2 inner surface area). 8-Bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) plus 3-isobutyl-1-methlyxanthine (IBMX) induced a rapid reversal in the net movement of fluid to secretion (6.79 +/- 1.28 nl/min/cm2). Bumetanide reversibly reduced fluid secretion to 0.95 +/- 0.60 nl/min/cm2. Cell volume rapidly decreased by 7.5 +/- 0.9% with the initiation of secretion and bumetanide caused an additional loss (4.2 +/- 1.0%). Furosemide had a similar effect on forskolin-induced fluid secretion. Cellular chloride concentration was monitored with the use of the indicator, 6-methoxy-N-ethylquinolinium chloride (MEQ). Removal of Cl- from the bath reduced intracellular [Cl-] (MEQ fluorescence increased by 11.4 +/- 2.3%). In cysts pretreated with furosemide to prevent Cl- entry, the application of forskolin caused a decrease in Cl- concentration (MEQ fluorescence increased by 9.3 +/- 2.6%). Using monolayers of cultured ADPKD cells, grown on permeant supports, we compared the changes in short circuit current (ISC) induced by forskolin in the presence and absence of external Cl-. Forskolin increased ISC (from 8.9 +/- 2.7 to 10.6 +/- 2.7 microA/cm2) in the presence of Cl-, but did not significantly affect ISC in its absence. These data indicate that cultured ADPKD cells can direct fluid transport in either the absorptive or the secretory direction, and that cAMP stimulates secretion and this secretion is accompanied by a net loss of cell solute. Inhibition of secretion by bumetanide or furosemide caused an additional loss of cell solute, including Cl-. The ionic transepithelial current induced by forskolin is dependent on the presence of Cl-. These data support the thesis that chloride secretion drives fluid secretion by cultured ADPKD cells.

Properties of a Cl(-)-conductive pathway(s) in microsomes from rat kidney inner medulla. Involvement of cystic fibrosis transmembrane regulator protein

The properties of a protein-kinase-A(PKA)-activated Cl(-)-conductive pathway(s) in alkaline phosphatase-enriched microsomes from the rat inner medulla (IMV) were investigated. Transcripts of cystic fibrosis transmembrane regulator (CFTR) were detected by reverse transcription/polymerase analysis of total RNA from the inner medulla, while immunoblot analysis using anti-CFTR antibodies detected a 170-kDa protein in the IMV. The PKA Cl(-)-conductive pathway(s) was studied by measuring the rate of valinomycin-induced microsomal swelling by light scattering. PKA increased the rate of valinomycin-induced swelling of vesicles consistent with the presence of Cl(-)-conductive pathway(s). The pharmacological properties and anion selectivity of the PKA-activated Cl(-)-conductive pathway(s) were similar to those of the CFTR Cl(-) channel. Our results show that a CFTR Cl(-) channel and possibly another cAMP-activated pathway(s) may participate in Cl(-) secretion in the rat inner medulla.

CFTR mediates electrogenic chloride secretion in mouse inner medullary collecting duct (mIMCD-K2) cells

Previously we demonstrated that the inner medullary collecting duct cell line mIMCD-K2 secretes Cl- by an electrogenic mechanism [N. L. Kizer, B. Lewis, and B. A. Stanton. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F347-F355, 1995; N. L. Kizer, D. Vandorpe, B. Lewis, B. Bunting, J. Russell, and B. A. Stanton. Am. J. Physiol. 268 (Renal Fluid Electrolyte Physiol. 37): F854-F861, 1995]. The goal of the present study was to characterize the Cl- channel responsible for adenosine 3',5'-cyclic monophosphate (cAMP)-stimulated Cl- secretion. To this end, using the patch-clamp technique, we measured Cl- currents. In whole cell patch-clamp experiments, 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (CPT-cAMP) activated Cl- currents that were time and voltage independent, inhibited by diphenylamine 2-carboxylate (DPC), and had a linear current-voltage (I-V) relation. In cell-attached patches of the apical membrane, we identified 7-pS Cl- channels that were stimulated by CPT-cAMP. In inside-out patches with Cl- in the pipette and bath solutions, Cl- currents had a linear I-V relation. The halide permeability sequence was PCl = PBr > PI. The Cl- channel inhibitors DPC, 5-nitro-2-(3-phenylpropylamino)-benzoic acid, and glibenclamide blocked the 7-pS Cl- channel, whereas 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid was ineffective. By reverse transcriptase polymerase chain reaction, we isolated a partial cDNA clone encoding the cystic fibrosis transmembrane conductance regulator in mIMCD-K2 cells. We conclude that cAMP stimulates electrogenic Cl- secretion in inner medullary collecting duct cells by activating cystic fibrosis transmembrane conductance regulator Cl- channels.

Monovalent cation selective channel in the apical membrane of rat inner medullary collecting duct cells in primary culture

Ion channels in the apical membrane of rat inner medullary collecting duct (IMCD) were investigated by the patch clamp technique. Owing to the histological heterogeneity of IMCD, cells were cultured from the lower half of the inner medulla of Wistar rat kidney. Channel activity was rarely seen in cell attached patch, but membrane excision activated multiple units of 28.2 +/- 0.7 pS cation selective channel. A Na or K selective channel was not found. The 28 pS channel showed membrane voltage dependency, no rectification, almost equal permeability to monovalent cations (Na/K/Li/Cs/Rb/NH4 = 1:1.00:0.82:0.97:1.10:1.71) and no significant permeation to anions or divalent cations. Calcium of the cytoplasmic side from 10(-7) M to 10(-4) M affected the mean number of open channels (nPo) dose-dependently in excised patch (IC50 = 5 x 10(-6) M). 1 mM of ATP, ADP, AMP and gadolinium reversibly suppressed nPo to near zero whereas amiloride, cAMP or cGMP had no effect. Multiple conductance substates were frequently observed. These results suggested that this channel belongs to the nonselective cation channels which has been identified in other epithelia and is not responsible for amiloride sensitive Na transport through IMCD cells.