Electrophysiologic study of the cortical collecting tubule of the rabbit

Regulation of paracellular transport in the distal nephron

PURPOSE OF REVIEW

Claudins play a major role in the regulation of paracellular electrolyte reabsorption in the kidney. This review describes the recent findings of the physiological function of claudins underlying the paracellular transport mechanisms for Cl(-) reabsorption in the collecting duct.

RECENT FINDINGS

There are two parallel mechanisms for transepithelial Cl(-) reabsorption in the collecting duct that utilize the Na-driven Cl-bicarbonate exchanger (NDCBE) and the claudin-based paracellular channel. Histological studies have demonstrated the renal localization of claudin-3, claudin-4, claudin-7, and claudin-8 in the collecting duct. Molecular analyses using several collecting duct cell models have come to the conclusion that claudin-4 functions as a paracellular Cl(-) channel. The channel function of claudin-4 is conferred by a charged lysine residue (K65) in its extracellular loop. Claudin-8 is required for paracellular Cl(-) permeation through its interaction with and recruitment of claudin-4 during tight junction assembly. Claudin-7 provides the basic barrier function to the collecting duct. Genetic ablation of claudin-7 in animals results in systemic dehydration owing to the loss of extracellular ions and fluid in the kidney.

SUMMARY

The paracellular pathway in the collecting duct is an important route for transepithelial Cl(-) reabsorption that determines the extracellular NaCl content and the blood pressure. In the collecting duct cells, claudin-4 and claudin-8 interact to form a paracellular Cl(-) channel, whereas claudin-7 maintains the transepithelial resistance. Different subsets of the claudin family proteins fulfill diverse aspects of the tight junction function that will be fundamental to understanding the physiology of the paracellular pathway.

Modulation of renal calcium handling by 11 beta-hydroxysteroid dehydrogenase type 2

Reduced concentration of serum ionized calcium and increased urinary calcium excretion have been reported in primary aldosteronism and glucocorticoid-treated patients. A reduced activity of the 11 beta-hydroxysteroid dehydrogenase type 2 (11 beta HSD2) results in overstimulation of the mineralocorticoid receptor by cortisol. Whether inhibition of the 11 beta HSD2 by glycyrrhetinic acid (GA) may increase renal calcium excretion is unknown. Serum and urinary electrolyte and creatinine, serum ionized calcium, urinary calcium excretion, and the steroid metabolites (THF+5 alpha THF)/THE as a parameter of 11 beta HSD2 activity were repeatedly measured in 20 healthy subjects during baseline conditions and during 1 wk of 500 mg/d GA. One week of GA induced a maximal increment of 93% in (THF+5 alpha THF)/THE. Ambulatory BP was significantly higher at day 7 of GA than at baseline (126/77 +/- 10/7 versus 115/73 +/- 8/6 mmHg; P < 0.001 for systolic; P < 0.05 for diastolic). During GA administration, serum ionized calcium decreased from 1.26 +/- 0.05 to 1.18 +/- 0.04 mmol/L (P < 0.0001), and absolute urinary calcium excretion was enhanced from 29.2 +/- 3.6 to 31.9 +/- 3.1 micromol/L GFR (P < 0.01). Fractional calcium excretion increased from 2.4 +/- 0.3 to 2.7 +/- 0.3% (P < 0.01) and was negatively correlated to the fractional sodium excretion during GA (R = -0.35; P < 0.001). Moreover, serum potassium correlated positively with serum ionized calcium (R = 0.66; P < 0.0001). Inhibition of 11 beta HSD2 activity is sufficient to significantly increase the fractional excretion of calcium and decrease serum ionized calcium, suggesting decreased tubular reabsorption of this divalent cation under conditions of renal glucocorticoid/mineralocorticoid excess. The likely site of steroid-regulated renal calcium handling appears to be the distal tubule.

Morphological and functional characterization of a conditionally immortalized collecting tubule cell line

A conditionally immortalized collecting tubule cell line, mCT1, was derived from the H-2Kb-ts A58 transgenic mouse (ImmortoMouse), which harbors a temperature-sensitive mutant of the SV40 large T antigen oncogene. Cells maintained under permissive conditions [33 degreesC with interferon-gamma (IFN-gamma)] form epithelial monolayers, express large T antigen, and proliferate (>50 passages). The cells retain properties characteristic of the renal collecting tubule (CT) including: vasopressin (VP)-stimulated cAMP accumulation, aquaporin-2 expression, high transepithelial electrical resistance, VP-stimulated ion transport, and amiloride-sensitive sodium absorption. When the cells are transferred to nonpermissive conditions (39 degreesC without IFN-gamma), the steady-state level of large T antigen protein declines (>95% decrease) and cell proliferation is arrested. This conditionally immortalized, murine renal cell line should prove useful for studies of CT physiology and large T antigen biology.

Prostaglandin E2 increases 7-pS Cl- channel density in the apical membrane of A6 distal nephron cells

In A6 distal nephron cells, short-circuit current (Isc) was increased by basolateral exposure to prostaglandin E2 (PGE2; peak response at 1 microM). The effect was only partially abolished by either apical amiloride, an Na+ channel blocker, or 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), a Cl- channel blocker. In apical cell-attached patches, we observed a 7-pS Cl- channel with a linear current-voltage relationship, a reversal potential near resting membrane potential, and open probability > 0.5. The channel was blocked by diphenylamine-2-carboxylate, glibenclamide, and NPPB but not by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid. The frequency of observed Cl- channel activity increased 7-fold with 10-min exposure to PGE2 and 3.7-fold with longer (10-50 min) exposure to PGE2. The PGE2-induced increase in Cl- channel activity was due primarily to an increase in the number of functional channels. The following conclusions were made: 1) activation of apical, 7-pS Cl- channels in A6 cells accounts for the PGE2-induced increase in the amiloride-insensitive Isc, and 2) 7-pS Cl- channel activation was mediated via an increase in channel density without substantial effects on channel kinetics.

Prostaglandin E2 activates clusters of apical Cl- channels in principal cells via a cyclic adenosine monophosphate-dependent pathway

We examined cell-attached patches on principal cells of primary cultured, rabbit cortical collecting tubules. Under basal conditions, apical 9-pS Cl(-)-selective channels were observed in 9% of patches (11/126), and number of channels times open probability (NP0) was 0.56 +/- 0.21. The channel had a linear current-voltage relationship, reversal potential (Erev) near resting membrane potential, a P0 (0.30-0.70) that was independent of voltage, and complicated kinetics (i.e., bursting) at hyperpolarized potentials. NP0 and channel frequency were increased after 30 min of basolateral exposure to 0.5 microM PGE2 (18/56), 10 microM forskolin (23/36), or 0.5 mM dibutyryl cyclic adenosine monophosphate (cAMP) (25/41). Increases in NP0 appeared to be mediated primarily through an increase in the number of observed channels per patch (N), not changes in P0. After these cAMP-increasing maneuvers, N was inconsistent with a uniform distribution of channels in the apical membrane (P < 0.001), but rather the channels appeared to be clustered in pairs. Apical 0.5 microM PGE2 (12/91), apical or basolateral 0.5 microM PGF2 alpha (8/110), or 0.25 microM thapsigargin (releaser of intracellular Ca2+ stores) (7/73) did not increase NP0 or channel frequency.

CONCLUSIONS

(a) 9-pS Cl- channels provide a conductive pathway for apical membrane Cl- transport across principal cells. (b) Channel activation by basolateral PGE2 is mediated via a cAMP-, but not a Ca(2+)-dependent mechanism. (c) Apical channels are clustered in pairs. (d) With its low baseline frequency and Erev near resting membrane potential, this channel would not contribute significantly to transcellular Cl- flux under basal conditions. (e) However, cAMP-producing agonists (i.e., PGE2, arginine vasopressin) would increase apical Cl- transport with the direction determined by the apical membrane potential.

Ouabain-induced cell swelling in rabbit cortical collecting tubule: NaCl transport by principal cells

Ouabain had no effect on the volume of intercalated cells of DOCA-stimulated rabbit cortical collecting tubules, but caused principal cells to swell rapidly at an initial rate of 67%/min. Principal cells swelled 133% then activated regulatory volume decrease mechanisms and shrank at an initial rate of -3%/min to a new volume 13% above control. The initial rate of ouabain swelling was completely inhibited by perfusate Na+ removal or reduced 95% by luminal addition of 10(-5) M amiloride. Luminal, peritubular, or bilateral Cl- removal each caused cell shrinkages of 10% and reduced the rate of ouabain swelling by 70, 85, and 99%, respectively. The presence of an apical Cl- transport step in principal cells was confirmed by increasing luminal K+ from 5 to 53 mM, which caused cell swelling of 22%. This volume increase was completely blocked by luminal Cl- removal, but was unaffected by peritubular Cl- substitution. Perfusion of CCT with 0.1 mM acetazolomide, 0.1 mM DPC or 0.5 mM SITS caused principal cell shrinkages of 7-9% and reduced the rate of ouabain swelling by 60, 70, and 40%, respectively. The initial rate of ouabain swelling was inhibited 70% by bilateral CO2/HCO3 removal and 50% by whole animal acid loading. Taken together these results demonstrate that ouabain swelling is due to cellular NaCl accumulation and that Na+ enters the cell primarily through apical Na+ channels. Cellular Cl- entry occurs at least partially through the apical membrane and may be mediated by a Cl-/HCO3- exchanger. Brief (45-90 sec) exposure of principal cells to ouabain is associated with a rapid inhibition of Na+ and/or Cl- entry steps, whereas long-term (greater than 5 min) ouabain exposure completely blocks one or both of these transport pathways.

Ionic conductances of cultured principal cell epithelium of renal collecting duct

The ionic conductive properties were studied of epithelia of collecting duct principal cells which had been grown in primary tissue culture from renal cortex/capsule explants. When pretreated with aldosterone (10(-6) mol/l) and bathed on either surface with isotonic HCO3(-)-free Ringer's solution, the transepithelial voltage, Vte, varied between -21 and -72 mV (apical surface negative) while the transepithelial resistance, Rte, ranged from 0.4 to 1.5 k omega cm2. By 10:1 step-changes in Na+ concentration the apical cell membrane was shown to have a high conductivity for sodium, inhibitable by amiloride, 10(-6) mol/l. However, contrary to observations in natural collecting duct under control conditions, amiloride never reversed the polarity of Vte even at 10(-4) mol/l. Both the apical and the basolateral cell membranes were conductive for potassium and both conductivities were inhibitable by Ba2+ (5 mmol/l). 10:1 reduction of apical Cl- concentration strongly hyperpolarized Vte with a monophasic time course suggesting the presence of a paracellular shunt conductance for Cl-. In addition there may be a small Cl- conductance present in the apical cell membrane since apical application of the chloride channel blocker 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPAB) at 10(-7) mol/l produced a minute but significant hyperpolarization. On the other hand, 10:1 reduction of basolateral Cl- concentration caused a biphasic change in Vte (initial depolarization, followed by repolarization) which indicates the presence of a large Cl- conductance in the basolateral cell membrane. The latter was not inhibitable by 10(-7) mol/l NPPAB. Higher concentrations of this and of an other Cl-channel blocker produced non-specific effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell and luminal activities of chloride, potassium, sodium and protons in the late distal tubule of Necturus kidney

1. Double-barrelled (selective vs. conventional) microelectrodes were used to assess the steady-state activities (a) of the ions Cl-, K+, Na+ and H+ in peritubular blood capillaries (abld) and in cell (acell) and lumen (alum) of the late distal tubule (l.d.t.) of Necturus. 2. a(cell)cl, a(lum)cl and a(bld)cl were 5.5 +/- 0.3, 11.8 +/- 1.0 and 70.5 +/- 0.1 mM, respectively. They were used to compute the chemical potentials for Cl- across the three diffusive barriers of the tissue. Basolateral and apical membrane potentials were -74.3 +/- 1.1 and -60.1 +/- 2.0 mV, respectively (cell negative); the lumen was thus negative with respect to blood, by 13.6 +/- 1.5 mV. The electrochemical potential difference (e.p.d.) for Cl- of 42 mV across the apical membrane opposes Cl- absorption, implying active apical Cl- uptake, since Cl- is known to be absorbed in the l.d.t. Basolateral Cl- exit is favoured by an e.p.d. of 10 mV. 3. a(cell)K, a(lum)K and a(bld)K were 65.8 +/- 0.8, 2.5 +/- 0.1 and 2.5 +/- 0.1 mm, respectively. The electrochemical distribution of K+ indicates that K+ absorption, if present, proceeds against an adverse apical e.p.d. of 18 mV. Basolateral K+ distribution is close to its electrochemical equilibrium, suggesting high K+ permeability at this membrane. 4. a(cell)Na was 9.0 +/- 0.4 mM, a(bld)Na 71.0 +/- 0.3 mM, and a(lum)Na was approximated at about 9 mM. Diffusive Na+ entry from lumen to cell is favoured by an e.p.d. close to 65 mV. Basolateral Na+ exit must be active, since it proceeds against an e.p.d. of 130 mV. 5. Cell, luminal and blood pH were 7.14 +/- 0.03, 6.52 +/- 0.08 and 7.37 +/- 0.04, respectively. The luminal electrochemical potential of H+ is higher than that of cell (by 91 mV) and blood (by 34 mV) indicating that proton secretion into the lumen must be active. 6. The e.p.d. of each ion across the epithelium opposes, by its orientation, the established direction of net transepithelial ion transport, suggesting that the shunt pathway may serve only for back-diffusion.

The role of the collecting duct in urinary concentration

In summary, the three major segments of the collecting duct subserve three different functions in the urinary concentrating mechanism. The main function of the cortical collecting tubule is to raise the fractional solute contribution and absolute concentration of urea in fluid that it delivers to the outer medullary collecting duct. The function of the outer medullary collecting duct is to raise further the absolute intraluminal urea concentration. Finally, the inner medullary collecting duct has two major functions in urinary concentration: first, it adds net urea to the papillary interstitium, and second, it allows the generation of maximally concentrated urine due to osmotic water equilibration. Indeed, the urine osmolality can rise to levels higher than the papillary interstitial osmolality as a consequence of inequalities of the reflection coefficients of urea and sodium chloride.

The regulation of renal acid secretion: new observations from studies of distal nephron segments

In this review we have attempted to present for the general reader the new information on renal acidification that has emerged from the study of discrete segments of the distal nephron. We have structured our presentation in the context of the cation exchange hypothesis which has strongly influenced modern thinking of acid-base regulation. We have shown that distal nephron acidification is active and can proceed even in the absence of sodium. We have also shown beyond doubt, that pH or the determinants of pH can influence the rate of proton secretion in probably all of the distal nephron segments. We have drawn attention to an exciting new means by which chloride (or its substitution) could alter the rate of net bicarbonate transport. A possible role for bicarbonate secretory activity in the mammalian distal nephron has been discussed as has the influence of mineralocorticoids on acid secretion. There is no question that all of this new information has created the need for a reassessment of the validity of the cation exchange hypothesis. After all, this is a view which specifically denies that renal acid excretion is modulated by pH of the blood, and affirms that it is intrarenal sodium handling that is the "driving force", so to speak, behind acidification responses. However, it seems inappropriate at this time to insist that current data do not allow for a component of sodium transport by the distal nephron to modulate the rate of acid secretion. It is also possible, as we have suggested, that an important effect of chloride gradients, independent of blood pH, could alter bicarbonate retrieval. Most importantly, we wish to stress that much of the in vitro perfusion data does not derive from animals subjected to the chronic acid-base derangements which were precisely those situations to which the cation exchange hypothesis was directed. Simply put, the whole animal studies of Schwartz and his colleagues provided no experimental observations on intrarenal sodium handling or acidification mechanisms, just as the microperfusion studies, both in vivo and in vitro, provide insufficient data that can be applied to whole animals subjected to chronic disturbances.(ABSTRACT TRUNCATED AT 400 WORDS)

Bicarbonate secretion and chloride absorption by rabbit cortical collecting ducts. Role of chloride/bicarbonate exchange

Cortical collecting ducts (CCD) from rabbits treated with deoxycorticosterone (DOC) actively secrete bicarbonate at high rates. To investigate the mechanism of bicarbonate secretion, we measured bicarbonate and chloride transport in CCD from rabbits treated with DOC for 9-24 d. Removal of chloride (replaced with gluconate) from both perfusate and bath inhibited bicarbonate secretion without changing transepithelial voltage. Removal of chloride only from the bath increased bicarbonate secretion, while removal of chloride only from the perfusate inhibited secretion. In contrast to the effect of removing chloride, removal of sodium from both the perfusate and bath (replacement with N-methyl-D-glucamine) did not change the rate of bicarbonate secretion. The rate of bicarbonate secretion equaled the rate of chloride absorption in tubules bathed with 0.1 mM ouabain to inhibit any cation-dependent chloride transport. Under these conditions, chloride absorption occurred against an electrochemical gradient. Removal of bicarbonate from both the perfusate and bath inhibited chloride absorption. Removal of bicarbonate only from the bath inhibited chloride absorption, while removal of bicarbonate from the lumen stimulated chloride absorption. We conclude that CCD from DOC-treated rabbits actively secrete bicarbonate and actively absorb chloride by an electroneutral mechanism involving 1:1 chloride/bicarbonate exchange. The process is independent of sodium.

Disorders of distal nephron function

In this review, the distal nephron is considered to be that portion of the renal tubule commencing with the thick ascending limb of the loop of Henle and ending with the papillary collecting duct. The collecting duct, including its subdivisions in the cortex and medulla, originates from a different embryologic anlage than more proximal nephron segments, which may explain its morphologic and functional dissimilarities from the thick ascending limb and the distal convoluted tubule. This review summarizes selected aspects of the physiology of the distal nephron, with particular emphasis on the physiology of distal nephron transport of sodium, potassium, chloride and hydrogen ion. The pathophysiologic features of the following disorders of distal nephron function are reviewed: (1) pseudohypoaldosteronism, a heterogenous group of disorders in which the signs and symptoms are suggestive of aldosterone deficiency, but in which aldosterone levels are supernormal and administration of exogenous mineralocorticoid is not ameliorative; (2) pseudohyperaldosteronism (Liddle syndrome), a familial disorder in which the clinical manifestations closely resemble those resulting from an aldosterone-producing adenoma of the adrenal gland (primary aldosteronism), but in which the measured rate of aldosterone secretion and excretion is greatly subnormal; (3) Bartter syndrome and related syndromes of renal potassium wasting; (4) type 1 renal tubular acidosis (classic, distal); (5) type 4 renal tubular acidosis (hyperkalemic). Reference citations are generally to articles reporting recent advances in these areas and to review articles that contain comprehensive bibliographies.

Heterogeneity of the rabbit collecting tubule: localization of mineralocorticoid hormone action to the cortical portion

This study was designed to examine the sodium, potassium, and chloride transport rates across the cortical and outer medullary collecting tubule and to localize the action of mineralocorticoid hormone. Rabbit collecting tubules were dissected from the cortex (CCT), the outer stripe of the outer medulla (OMCT0), or the inner stripe of the outer medulla (OMCTi). From normal rabbits, the transepithelial voltage was -20.8 +/- 2.7 mV in CCT, -4.2 +/- 1.4 mV in OMCT0, and +10.6 +/- 2.3 mV in OMCTi. From DOCA-treated rabbits, only the CCT voltage was different (-42.9 mV). Net sodium absorption across the CCT increased ith DOCA-treatment from 23 +/- 5 to 54 +/- 8 pEq . mm-1 . min-1, whereas net potassium secretion increased from 15 +/- 2 to 43 +/- 5 pEq . min-1 . min-1. Net chloride absorption was significant only in DOCA-treated rabbits. The OMCTi, in contrast, displayed no net transport of sodium, potassium, or chloride and had a lower rate coefficient for sodium efflux than did the CCT. DOCA treatment had no effect on any transport rate measured in this segment. The OMCT0 has small potassium secretion, which did not increase with DOCA treatment. The collecting tubule is heterogeneous along its length with respect to ion transport. Mineralocorticoid-hormone-sensitive sodium absorption is present predominantly, if not exclusively, in the cortical collecting tubule.

Effects of catecholamines on electrolyte transport in cortical collecting tubule

We examined the direct effects of isoproterenol (ISO) and L-norepinephrine (NE) on electrolyte transport in isolated rabbit cortical collecting tubules (CCT) perfused in vitro. The addition of either ISO (10(-6) M) or NE (10(-6) M) to the bath decreased transepithelial potential difference (PD), on average by 51 and 25%, respectively. These effects of ISO and NE were abolished by prior addition of the beta-adrenergic blocker, L-propranolol. ISO (10(-5) M) had no effect from lumen. Also, osmotic water permeability was not influenced by ISO. Ouabain and ISO had additive effects on PD. Elimination of chloride from both perfusate and bath, or addition of acetazolamide, abolished the effect of ISO on PD. Although isotopic sodium flux for lumen to bath was not influenced by ISO, chemical net chloride absorption increased from 1.1 +/- 0.4 to 2.7 +/- 0.6 peq . cm-1 . sec-1 (n = 8, p less than 0.005). In conclusion, both ISO and NE and capable of decreasing PD in rabbit CCT perfused in vitro. This effect is mediated by beta-adrenergic receptors and is accompanied by the increase in net chloride absorption. Although the mechanism responsible for this decrease in PD with ISO in unclear, active chloride absorption, active hydrogen secretion, or membrane chloride permeability changes may account for the effects of ISO.