Cell rubidium uptake: a method for studying functional heterogeneity in the nephron

Mechanism of Na-K-ATPase Inhibition by PGE2 in Intestinal Epithelial Cells

The primary means of intestinal absorption of nutrients by villus cells is via Na-dependent nutrient co-transporters located in the brush border membrane (BBM). These secondary active co-transport processes require a favorable transcellular Na gradient that is provided by Na-K-ATPase. In chronic enteritis, malabsorption of essential nutrients is partially due to inhibition of villus Na-K-ATPase activity mediated by specific immune inflammatory mediators that are known to be elevated in the inflamed mucosa. However, how Prostaglandin E2 (PGE2), a specific mediator of nutrient malabsorption in the villus BBM, may mediate the inhibition of Na-K-ATPase is not known. Therefore, this study aimed to determine the effect of PGE2 on Na-K-ATPase in villus cells and define its mechanism of action. In vitro, in IEC-18 cells, PGE2 treatment significantly reduced Na-K-ATPase activity, accompanied by a significant increase in the intracellular levels of cyclic Adenosine Monophosphate (cAMP). The treatment with cAMP analog 8-Bromo-cAMP mimicked the PGE2-mediated effect on Na-K-ATPase activity, while Rp-cAMP (PKA inhibitor) pretreatment reversed the same. The mechanism of inhibition of PGE2 was secondary to a transcriptional reduction in the Na-K-ATPase α1 and β1 subunit genes, which was reversed by the Rp-cAMP pretreatment. Thus, the PGE2-mediated activation of the PKA pathway mediates the transcriptional inhibition of Na-K-ATPase activity in vitro.

TRPV1 activation stimulates NKCC1 and increases hydrostatic pressure in the mouse lens

The porcine lens response to a hyperosmotic stimulus involves an increase in the activity of an ion cotransporter sodium-potassium/two-chloride cotransporter 1 (NKCC1). Recent studies with agonists and antagonists pointed to a mechanism that appears to depend on activation of transient receptor potential vanilloid 1 (TRPV1) ion channels. Here, we compare responses in lenses and cultured lens epithelium obtained from TRPV1^-/- and wild type (WT) mice. Hydrostatic pressure (HP) in lens surface cells was determined using a manometer-coupled microelectrode approach. The TRPV1 agonist capsaicin (100 nM) caused a transient HP increase in WT lenses that peaked after ∼30 min and then returned toward baseline. Capsaicin did not cause a detectable change of HP in TRPV1^-/- lenses. The NKCC inhibitor bumetanide prevented the HP response to capsaicin in WT lenses. Potassium transport was examined by measuring Rb⁺ uptake. Capsaicin increased Rb⁺ uptake in cultured WT lens epithelial cells but not in TRPV1^-/- cells. Bumetanide, A889425, and the Akt inhibitor Akti prevented the Rb⁺ uptake response to capsaicin. The bumetanide-sensitive (NKCC-dependent) component of Rb⁺ uptake more than doubled in response to capsaicin. Capsaicin also elicited rapid (<2 min) NKCC1 phosphorylation in WT but not TRPV1^-/- cells. HP recovery was shown to be absent in TRPV1^-/- lenses exposed to hyperosmotic solution. Bumetanide and Akti prevented HP recovery in WT lenses exposed to hyperosmotic solution. Taken together, responses to capsaicin and hyperosmotic solution point to a functional role for TRPV1 channels in mouse lens. Lack of NKCC1 phosphorylation and Rb⁺ uptake responses in TRPV1^-/- mouse epithelium reinforces the notion that a hyperosmotic challenge causes TRPV1-dependent NKCC1 activation. The results are consistent with a role for the TRPV1-activated signaling pathway leading to NKCC1 stimulation in lens osmotic homeostasis.

A geochemical perspective on the natural abundance of trace elements in beaver (Castor canadensis) from a rural region of southern Ontario, Canada

Chalcophile (Ag, Cd, Co, Cu, Mo, Ni, Pb, Se, Tl, Zn) and lithophile (Al, Ba, Ce, Cr, Cs, Fe, La, Li, Mn, Nd, Rb, Sr, V, Y) trace elements (TEs) were determined in kidney, liver and muscle of beaver (Castor canadensis) from a rural watershed in southern Ontario, Canada. To estimate the relative bioavailability of TEs in the landscape, they were also determined in the dissolved (<0.45 μm) fraction of water from the river where the animals were harvested. Concentration ratios (tissue/water) always showed the greatest enrichments for Cd (kidney, 1.1 × 10⁷; liver, 2.4 × 10⁶; muscle, 7.2 × 10⁵), most likely due to the metal binding properties of metallothioneins. Despite its potential toxicity, Tl also showed considerable enrichment: kidney, 4.2 × 10⁴; liver 1.2 × 10⁴; muscle 1.5 × 10⁴. Enrichments of Cs and Rb exceeded those of Tl in all three tissues, suggesting that the chemical similarity of their ionic species (Cs⁺, Rb⁺, Tl⁺) to K⁺ may be the key to their uptake. Lithophile elements of limited solubility in natural waters (Al, Ce, La, Nd) show moderate enrichments, despite the lack of physiological role. The smallest enrichments were found for Sr and Ba, the two TEs which are most abundant in the river. Of the TEs considered essential for animal nutrition, V was the least enriched in tissue relative to water (liver 19×, kidney, 33× and muscle 28×). Despite the lack of physiological function and absence of any known sources of contamination, Al, Ag, Cd, Ce, Cs, La, Pb, Rb, and Tl, are all enriched in beaver tissue, relative to water, by at least three orders of magnitude, due to natural processes. The widespread abundance of beaver in Canada combined with the growing need to manage their numbers in populated regions offer a unique opportunity for monitoring environmental quality in the riparian zone.

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

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

Cell-specific regulation of L-WNK1 by dietary K

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

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

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

Role of BK channels in hypertension and potassium secretion

PURPOSE OF REVIEW

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

RECENT FINDINGS

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

SUMMARY

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

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

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

A Long Affair with Renal Tubules

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

Recent advances in distal tubular potassium handling

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

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.

K+ secretion in the rat kidney: Na+ channel-dependent and -independent mechanisms

Renal Na(+) and K(+) excretion was measured in rats with varying dietary K(+) intake. The requirement for channel-mediated distal nephron Na(+) reabsorption was assessed by infusing the animals with the K(+)-sparing diuretic amiloride via osmotic minipumps. At infusion rates of 2 nmol/min, the concentration of amiloride in the urine was 38 microM, corresponding to concentrations of 9-23 microM in the distal tubular fluid, sufficient to block >98% of Na(+) transport through apical Na(+) channels (ENaC). With a control K(+) intake (0.6% KCl), amiloride reduced K(+) excretion rates (U(K)V) from 0.85 +/- 0.15 to 0.05 +/- 0.01 micromol/min during the first 2 h of infusion, suggesting that distal nephron K(+) secretion was completely dependent on the activity of Na(+) channels. When K(+) intake was increased by feeding overnight with a diet containing 10% KCl, amiloride reduced U(K)V from 7.5 +/- 0.7 to 1.3 +/- 0.1 micromol/min despite an increased plasma K(+) of 9 mM, again suggesting a major but not exclusive role for the Na(+) channel-dependent pathway of K(+) secretion. The maximal measured rates of amiloride-sensitive K(+) excretion correspond well with estimates based on apical K(+) channel activity in distal nephron segments. However, when the animals were adapted to the high-K(+) diet for 7-9 days, the diuretic decreased U(K)V less, from 6.1 +/- 0.6 to 3.0 +/- 0.8 micromol/min, indicating an increasing fraction of K(+) excretion that was independent of Na(+) channels. This indicates the upregulation of a Na(+) channel-independent mechanism for secreting K(+).

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.

Inhibition of MAPK stimulates the Ca2+ -dependent big-conductance K channels in cortical collecting duct

The kidney plays a key role in maintaining potassium (K) homeostasis. K excretion is determined by the balance between K secretion and absorption in distal tubule segments such as the connecting tubule and cortical collecting duct. K secretion takes place by K entering principal cells (PC) from blood side through Na+, K+ -ATPase and being secreted into the lumen via both ROMK-like small-conductance K (SK) channels and Ca2+ -activated big-conductance K (BK) channels. K reabsorption occurs by stimulation of apical K/H-ATPase and inhibition of K recycling across the apical membrane in intercalated cells (IC). The role of ROMK channels in K secretion is well documented. However, the importance of BK channels in mediating K secretion is incompletely understood. It has been shown that their activity increases with high tubule flow rate and augmented K intake. However, BK channels have a low open probability and are mainly located in IC, which lack appropriate transporters for effective K secretion. Here we demonstrate that inhibition of ERK and P38 MAPKs stimulates BK channels in both PC and IC in the cortical collecting duct and that changes in K intake modulate their activity. Under control conditions, BK channel activity in PC was low but increased significantly by inhibition of both ERK and P38. Blocking MAPKs also increased channel open probability of BK in IC and thereby it may affect K backflux and net K absorption Thus, modulation of ERK and P38 MAPK activity is involved in controlling net K secretion in the distal nephron.

High-conductance K channels in intercalated cells of the rat distal nephron

High-conductance (BK or maxi) K(+) channels were observed in cell-attached patches of the apical membrane of the isolated split-open rat connecting tubule (CNT). These channels were quite rare in cells identified visually as principal cells (PCs; 5/162 patches) but common in intercalated cells (ICs; 24/26 patches). The BK-expressing intercalated cells in the CNT and cortical collecting duct (CCD) were characterized by a low membrane potential (-36 mV) under short-circuit conditions, measured from the reversal potential of the channel currents with similar K(+) concentrations on both sides of the membrane. Under whole-cell clamp conditions with low intracellular Ca(2+), ICs had a very low K(+) conductance. When cell Ca(2+) was increased to 200 nM, a voltage-dependent, tetraethylammonium (TEA)-sensitive outward conductance was activated with a limiting value of 90 and 140 nS/cell in the CNT and CCD, respectively. Feeding animals a high-K diet for 1 wk did not increase these currents. TEA-sensitive currents were much smaller in PCs and usually below detection limits. To examine the possibility that the ICs participate in transepithelial K(+) secretion, we measured Na/K pump activity as a ouabain-sensitive current. Although these currents were easily observed in PCs, averaging 79 +/- 14 and 250 +/- 50 pA/cell in the CCD and CNT, respectively, they were below the level of detection in the ICs. We conclude that ICs have BK channel densities that are sufficient to support renal secretion of K(+) if cell Ca(2+) is elevated. However. a pathway for K(+) entry into these cells has not been identified.

Intracellular Na concentration and Rb uptake in proximal convoluted tubule cells and abundance of Na/K-ATPase alpha1-subunit in NHE3-/- mice

Proximal solute and fluid absorption is greatly reduced in mice in which the gene encoding the Na/H exchanger isoform 3 has been ablated (NHE3-/-). To obtain information on the intracellular functional consequences of such selective NHE3 deficiency, Na, Cl and K concentrations and cell Rb uptake were measured using electron microprobe analysis after a 30-s infusion of Rb (an index of basolateral Na/K-ATPase activity) in proximal convoluted tubule (PCT) cells of NHE3-/- and wild-type (NHE3+/+) mice. In addition, the relative abundance of the alpha1-subunit of the Na/K-ATPase in the outer cortex was determined by Western blot analysis. PCT cell Na concentration in NHE3-/- mice was slightly but significantly lower than in NHE3+/+ [13.1+/-0.6 ( n=64) vs. 14.9+/-0.6 ( n=62) mmol/kg wet wt.; means +/-SEM]. The lower intracellular Na concentration was associated with significantly reduced Rb uptake rates [9.7+/-0.6 ( n=59) vs. 14.8+/-0.8 ( n=50) mmol/kg wet wt./30 s], but the abundance of the alpha1-subunit of the Na/K-ATPase was not different between NHE3-/- and NHE3+/+ mice. Intracellular Cl concentration was higher (14.2+/-0.4 vs. 12.8+/-0.4 mmol/kg wet wt.) and K concentration unchanged (122.7+/-2.7 vs. 121.6+/-2.5 mmol/kg wet wt.) in PCT cells in NHE3-/- compared with NHE3+/+ mice. These findings suggest that the elimination of apical NHE3 in PCT cells of NHE3-/- mice reduces apical Na entry and, due to lower cell Na concentrations, Na/K-ATPase activity. The observed changes in intracellular Na concentration did not affect the expression of Na/K-ATPase in the renal cortex of NHE3-/- mice. There were no significant changes of cell Na concentration and Rb uptake in distal convoluted tubule, connecting tubule, principal and intercalated cells.

A trail of research on potassium

A complex pump-leak system involving both active and passive transport mechanisms is responsible for the appropriate distribution of potassium (K) between the intra- and extracellular fluid compartments. In addition, the kidneys, and to a lesser extent the colon, safeguard maintenance of the narrow range of low K concentrations in the extracellular fluid. Early renal clearance studies showed that K is normally both reabsorbed and secreted by renal tubules, and that regulated secretion is the major source of K excretion. Studies at the tubule and cell level have localized secretion and reabsorption of K to principal and intercalated cells in the collecting ducts. Measurements of the electrochemical driving forces across individual cell membranes have permitted the characterization of specific ATPases, K channels and K cotransporters and also provided insights into the molecular structure of individual transporters that regulate K excretion.

Use of Rb(+) and Br(-) as tracers for investigating ion transport by X-ray microanalysis in the Malpighian tubules of the black field cricket Teleogryllus oceanicus

Substitution of Rb(+) for K(+) in the incubation saline for in vitro preparations of Malpighian tubules had little effect on tubule function. Secretion rates increased by 10% for whole tubules, 9% for distal segments and 10% for main segments. In the secreted fluids Rb(+) almost completely replaced K(+). Within the cells of the main segment of the tubules Rb replaced the majority of the intracellular K. Treatment by ouabain in Rb saline resulted in a considerable increase in intracellular Na and Cl concentrations but no change in Rb concentration. This suggests that Rb(+) did not enter the cell via Na K ATPase and that the latter was not directly involved in Rb(+) secretion and by inference K(+) secretion. Substitution of Br(-) for Cl(-) in the incubation saline resulted in a 30% reduction in secretion rate from the distal segments but only a 10% reduction for the main segment. Cl(-) was almost completely replaced by Br(-) in fluid from both main and distal segments. In cells of the main segment the intracellular concentration of Br(-) did not exceed 30mmol kg(-1) dry weight and the Cl(-) concentration was unchanged in the apical region of the cell and increased in the basal region. These data suggest that Br(-) was transported across the tubule epithelium by a paracellular route and that the basal cell membrane is relatively impermeable to Cl(-). By inference Cl(-) may also be transported by a paracellular route.

Renal potassium transport: mechanisms and regulation

The regulation of potassium metabolism involves mechanisms for the appropriate distribution between the intra- and extracellular fluid compartments and for the excretion by the kidney. Clearance and single nephron studies show that renal excretion is determined by regulated potassium secretion and potassium reabsorption, respectively, in principal and intercalated cells of the distal nephron. Measurement of the electrochemical driving forces acting on potassium transport across individual cell membranes and characterization of several ATPases and potassium channels provide insights into the transport and regulation of renal potassium excretion.

Inhibition of angiotensin-converting enzyme modulates structural and functional adaptation to loop diuretic-induced diuresis

The roles of elevated cell sodium concentrations and the angiotensin-aldosterone system (AAS) in the structural and functional adaptation of the distal tubule and collecting duct system to a chronic increase of sodium delivery were examined using electron microprobe and quantitative morphologic/stereologic analyses. Studies were performed on rats given the loop diuretic torasemide acutely (20 min) or chronically (12 days), either alone or in combination with the angiotensin-converting enzyme (ACE) inhibitor, enalapril. In the sodium-absorbing cells of the distal tubule and cortical collecting duct-that is, in distal convoluted tubule (DCT), connecting tubule (CNT) and principal cells-an acute increase in sodium delivery caused a significant rise in intracellular sodium concentration and rubidium uptake, the latter an index of in vivo Na,K(Rb)-ATPase activity. The elevated cell sodium concentrations returned to, or close to, control values during chronic torasemide treatment. Intracellular rubidium concentrations, measured after a 30-second rubidium exposure, were not different from controls in DCT and CNT cells but were still higher in principal cells. Since, however, the distribution space for rubidium was significantly increased in chronic torasemide animals, rubidium uptake, and hence Na,K-ATPase activity, must have increased in proportion to cell volume in DCT and CNT cells, but more than proportionately in principal cells. When ACE was inhibited during chronic torasemide, the epithelial volume of DCT and cortical collecting duct (CCD) was increased mainly by lengthening and not, as was the case in rats given torasemide alone, by thickening of the tubule wall. Adaptation of the proximal tubule exclusively by lengthening was not affected by inhibition of the ACE. These data indicate that changes in cell ion composition may participate in initiating cell processes leading to adaptation of distal nephron segments to chronically increased salt delivery. Inhibition of the ACE reverses the torasemide-induced increase in apparent Na pump density in principal cells and seems to shift the relationship between hypertrophy and hyperplasia noted in DCT and CCD after chronic torasemide in favor of hyperplasia.

Effects of chronic hyperfiltration on proximal tubule bicarbonate transport and cell electrolytes

The compensatory response to unilateral nephrectomy (UNX) was investigated by a combination of renal clearance, microperfusion, electron microprobe, and morphological techniques. Filtration rate was significantly elevated 21 days following UNX and associated with a marked stimulation of bicarbonate and fluid absorption in the proximal tubule. Analysis of kinetic data of bicarbonate transport demonstrated strong flow-dependent activation of bicarbonate absorption in both control and experimental condition. The bicarbonate level at which half-saturation (apparent Kd) of transport occurred decreased uniformly at higher flow rates, but maximal transport rates (apparent Vmax) in the proximal tubule doubled in the remnant kidney. The flow dependence of bicarbonate transport in control and experimental conditions can be explained by an apparent unstirred layer effect modifying radial bicarbonate gradients in the tubule. Both Na/H-exchange and electrogenic H secretion contribute to bicarbonate absorption, but only Na/H-exchange increased significantly in proximal tubules of UNX rats. Cell ion concentrations after UNX were unchanged in cortical tubules, consistent with proportionately enhanced apical and basolateral ion transport. Proximal tubule cell rubidium concentration measured after a 30-second rubidium infusion as an index of basolateral Na,K-ATPase activity was unchanged in UNX rats. Inasmuch as cell volume increased significantly (25%), these data are consistent with a proportionate and similar stimulation of rubidium uptake and Na,K-ATPase activity.

Proximal tubular cell electrolytes during volume expansion in the rat

1. Proximal tubular intracellular elements were measured by electron microprobe X-ray analysis (a) in rats volume-expanded with albumin-saline in which peritubular oncotic pressure remained normal and (b) in rats in which the renal artery was snared before volume expansion (the early snare model). Glomerular filtration rate and urine Na+ excretion were measured in addition to intracellular Rb+ following a 30 s infusion of RbCl as a marker for K+ transport. 2. In albumin-saline volume-expanded rats, intracellular levels of Na+ ([Na+]i) at 21.5 +/- 0.6 mmol (kg wet wt)-1, Cl- ([Cl-]i) at 18.0 +/- 0.4 mmol (kg wet wt)-1 and Rb+ ([Rb+]i) at 9.4 +/- 0.4 mmol (kg wet wt)-1 were significantly higher (P < 0.0001) than the levels in non-expanded rats ([Na+]i, [Cl-]i and [Rb+]i at 17.7 +/- 0.4, 14.6 +/- 0.3 and 4.7 +/- 0.4 mmol (kg wet wt)-1, respectively; means +/- S.E.M.). The data are consistent with Na+ pump inhibition in the proximal tubule, although this cannot be directly derived from intracellular element measurements. 3. In an early snare model of volume expansion, [Na+]i, intracellular K+ ([K+]i) and [Rb+]i remained unchanged (16.1 +/- 0.4, 131.0 +/- 2.0 and 5.2 +/- 0.3 mmol (kg wet wt)-1, respectively) compared to non-expanded snared kidneys (15.9 +/- 0.6, 131.3 +/- 1.8 and 4.8 +/- 0.3 mmol (kg wet wt)-1, respectively). [Cl-]i at 18.3 +/- 0.5 mmol (kg wet wt)-1 increased (P < 0.0008) compared to controls at 15.8 +/- 0.5 mmol (kg wet wt)-1. Thus, in these rats, evidence for an inhibition of the Na+ pump was no longer observed. This points to a major intrinsic mechanism within the kidney for mediating natriuresis, since circulating factors were identical to those in the unsnared kidney, where significant natriuresis occurred.

Transcellular sodium transport and basolateral rubidium uptake in the isolated perfused cortical collecting duct

The relation between transcellular Na+ absorption, intracellular Na+ concentration and Na+/K(+)-ATPase activity (the last estimated by the rubidium uptake across the basolateral cell membrane) was examined in the different cell types of the rabbit cortical collecting duct (CCD). Experiments were performed on isolated perfused CCD in which Na+ absorption was varied by perfusing the tubule with solutions containing different Na+ concentrations (nominally Na(+)-free, 30 mM and 144 mM). Experiments were terminated by shock-freezing the tubules during perfusion. Precisely 30 s before shock-freezing, the K+ in the bathing solution was exchanged for Rb+. Intracellular element concentrations, including Rb+, were determined in freeze-dried cryosections of the tubules using energy-dispersive X-ray analysis. Increasing Na+ concentration in the perfusion solution caused significant rises in intracellular Na+ concentration and Rb+ uptake of principal cells. Principal cell Na+ and Rb+ concentrations were 7.8 +/- 0.9 and 7.0 +/- 0.8 mmol/kg wet weight respectively, when the perfusion solution was Na(+)-free, 10.1 +/- 0.7 and 11.6 +/- 0.6 mmol/kg wet weight with 30 mM Na+ in the perfusion solution, and 14.5 +/- 1.5 and 14.9 +/- 0.9 mmol/kg wet weight with 144 mM Na+ in the perfusion solution. In contrast, a comparable relationship between lumen Na+ concentration, intracellular Na+ concentration and basolateral Rb+ uptake was not seen in intercalated cells. These results support the notion that principal, but not intercalated, cells are involved in transepithelial Na+ absorption. In addition, the data demonstrate that apical Na+ entry and basolateral Na+/K(+)-ATPase activity are closely coupled in principal cells of the rabbit CCD.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of loop diuretics on organic osmolytes and cell electrolytes in the renal outer medulla

Electron microprobe analysis on freeze-dried cryosections was used to determine the effect of the loop diuretics torasemide and furosemide on intracellular electrolyte concentrations in individual cells of the outer and inner stripe of the outer medulla and on cell rubidium uptake, the latter a measure of basolateral Na-K-ATPase activity. In addition, the organic osmolytes glycerophosphorylcholine (GPC), betaine, inositol and sorbitol in cortex, outer medulla and inner medulla were measured using HPLC. Both loop diuretics significantly reduced sodium and chloride concentrations and rubidium uptake in thick ascending limb cells, but did not affect sodium concentration or rubidium uptake in the proximal straight tubule (PST) cells or in the light or dark cells of the outer medullary collecting duct (OMCD). Chloride concentrations in these cells (that is, PST cells, OMCD light and dark cells) were lowered by loop diuretics, albeit less than in thick ascending limb cells. Administration of both loop diuretics for only 20 minutes was sufficient to significantly depress tissue concentrations of GPC, betaine, and myo-inositol in the outer medulla and of GPC, betaine and sorbitol at the papillary tip. These results indicate that loop diuretics, presumably by blocking apical sodium entry, decrease thick ascending limb cellular sodium concentration and, as a consequence, reduce Na-K-ATPase activity as assessed by cell rubidium uptake. Although this has been shown previously in in vitro preparations, the present study confirms this for the first time in vivo.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of K depletion on renal K and Rb excretion: evidence for activation of K reabsorption

Prolonged potassium depletion activates tubular transport mechanisms mediating potassium absorption. To study ion specificity and factors that modulate the activity of potassium transport pathways, fractional potassium excretion (FEK) was compared with that of rubidium (FERb) in control and potassium-depleted rats subjected to various experimental maneuvers. In control rats FEK considerably surpassed FERb (FEK/FERb 1.54 +/- 0.08; mean +/- SEM), whereas in potassium-depleted rats FEK was significantly lower than FERb (FEK/FERb 0.72 +/- 0.05). Preferential retention of potassium compared to rubidium in potassium-depleted rats was accentuated (FEK/FERb 0.33 +/- 0.01) when residual potassium secretion was inhibited by amiloride and K-H exchange stimulated by increased distal buffer delivery (metabolic alkalosis). When distal fluid and buffer delivery were increased in control animals by acetazolamide, FEK and FERb rose in parallel. In potassium-depleted rats only FERb but not FEK was enhanced by acetazolamide. These data demonstrate that both potassium secretory and potassium absorptive transport pathways prefer potassium to its congener rubidium. Prolonged potassium depletion activates a potassium absorptive mechanism which is stimulated by increased distal buffer delivery and which transports potassium more effectively than rubidium.

Tubular sodium handling and tubuloglomerular feedback in compensatory renal hypertrophy

Tubular sodium handling and tubuloglomerular feedback (TGF) activity were assessed in established compensatory renal hypertrophy in Sprague Dawley rats. Hyperfiltration at the level of the single nephron was confirmed 4-6 weeks following a reduction in renal mass. TGF activity, determined as the difference between late proximal and early distal measurements of single-nephron glomerular filtration rate (SNGFR), was significantly increased in compensatory renal hypertrophy, being 7.8 +/- 1.0 vs 23.3 +/- 1.9 vs 25.5 +/- 2.6 nl/min (P for analysis of variance less than 0.05) following sham operation, unilateral nephrectomy, and 1 1/3 nephrectomy, respectively. Enhanced net tubular Na transport was also observed, with total Na reabsorption up to the late proximal site being 1.8 +/- 0.2 vs 2.7 +/- 0.1 vs 3.1 +/- 0.3 nmol/min (P less than 0.05), and to the early distal site being 3.4 +/- 0.5 vs 5.8 +/- 0.6 vs 7.9 +/- 0.8 nmol/min (P less than 0.05) in the three animal groups respectively. Comparison of proximal tubular length demonstrated a 71.9 +/- 8.1% increase in uninephrectomised vs sham-operated animals. This increase was proportionately greater than the increase in proximal Na reabsorption (50.0 +/- 4.0%) observed in the corresponding animal groups. Concurrent electron microprobe experiments in uninephrectomised and sham-operated animals demonstrated that the proximal tubular intracellular Na concentration was significantly lower following uninephrectomy (16.8 +/- 0.6 vs 18.9 +/- 0.5 mmol/kg wet weight, P less than 0.01), in association with evidence of reduced basolateral Na/K-ATPase activity.(ABSTRACT TRUNCATED AT 250 WORDS)

Proximal tubular cell sodium concentration in early diabetic nephropathy assessed by electron microprobe analysis

Electron microprobe X-ray analysis techniques were employed in order to assess the changes that occur in proximal tubular cell sodium concentration during the hyperfiltration phase of early diabetes mellitus induced by streptozotocin in Sprague Dawley rats. Intracellular rubidium accumulation following intravenous infusion of rubidium chloride was used as a marker of basolateral Na/K-ATPase activity. The diabetic animals studied had a significantly higher glomerular filtration rate compared with controls [1.44 +/- 0.07 vs. 1.00 +/- 0.07 ml min-1 (100 g body weight)-1; mean +/- SEM, P less than 0.001]. Intracellular Na concentration was significantly higher in diabetic animals (19.5 +/- 0.6 vs. 17.8 +/- 0.4 mmol/kg wet weight; P less than 0.01). Concurrent measurement of Rb demonstrated significantly higher intracellular accumulation in the proximal tubules of diabetic animals compared with control (7.9 +/- 0.5 vs. 5.5 +/- 0.5 mmol/kg wet weight; P less than 0.001). These results indicate that proximal tubular Na/K-ATPase activity is enhanced in the hyperfiltration phase of diabetes mellitus. Since, however, intracellular Na concentration is increased under these conditions, it may be inferred that apical Na entry into proximal tubular cells is stimulated beyond the rate of basal exit during the initial development of hyperfiltration. The reasons for these alterations in cellular Na transport are unclear but similar changes have been implicated in the pathogenesis of cell growth.

Effect of diuretics on cell potassium transport: an electron microprobe study

To study the short-term uptake of potassium across the basolateral membrane into individual tubule cells, rubidium was used and measured by electron microprobe analysis. Changes of rubidium uptake were interpreted to reflect altered sodium entry and basolateral Na-K-ATPase activity. The effects of hydrochlorothiazide, amiloride and furosemide were determined in saline-loaded animals. Hydrochlorothiazide inhibited rubidium uptake in proximal convoluted and distal convoluted tubule cells. The effect was largest in distal convoluted tubule cells. Amiloride reduced rubidium uptake in principal cells as well as in proximal convoluted, distal convoluted and connecting tubule cells. Furosemide depressed rubidium uptake in distal convoluted tubule cells, but increased uptake in principal cells. Rubidium uptake into intercalated cells was not affected by any of the diuretics used. Hydrochlorothiazide and amiloride altered rubidium uptake also in cells not associated with the main diuretic action. These effects of hydrochlorothiazide and amiloride may be due to interference with cell transport mechanisms of Na-H and anion exchange.

Effect of amiloride on electrolyte concentrations and rubidium uptake in principal and mitochondria-rich cells of frog skin

The role of mitochondria-rich cells (MR cells) in transepithelial Na transport was investigated by determining electrolyte concentrations and Rb uptake in individual cells of frog skin epithelium using electron microprobe analysis. Measurements were performed under control conditions and after blocking the transepithelial Na transport with amiloride. Under control conditions, Na and Cl concentrations of MR cells scattered much more than those of principal cells and ranged from a few up to more than 30 mmol/kg wet weight. Rb uptake from the basal side into individual MR cells also showed a large variation and was, on the average, much less pronounced than into the principal cells. In principal cells, amiloride reduced the Na concentration and Rb accumulation. In contrast, no effect was observed upon electrolyte concentration and Rb uptake of MR cells. Rb uptake was correlated to the Na concentration of MR cells both under control conditions and after amiloride. It is concluded that, in contrast to the principal cells, MR cells are not involved in amiloride-sensitive transepithelial Na transport and that their Na/K-pump activity is very low.

Sodium entry routes in principal and intercalated cells of the isolated perfused cortical collecting duct

Transmembrane sodium transport pathways were studied in principal and intercalated cells of the isolated perfused rabbit cortical collecting duct. Intracellular electrolyte concentrations in individual collecting duct cells were measured by electron microprobe analysis during blockage of basolateral Na-K-ATPase by ouabain and simultaneous inhibition of sodium entry across the apical and/or basolateral cell membrane. In principal cells the ouabain-induced rise in cell sodium concentration could only partially be blocked by amiloride (10(-4) mol/l) in the perfusion fluid. Amiloride (10(-3) mol/l) added to the bathing solution produced a further, significant reduction of sodium influx. In principal cells the ouabain-induced increase in sodium concentration was completely prevented by amiloride in the perfusion solution in combination with omission of sodium from the peritubular bathing solution. In intercalated cells ouabain caused a less pronounced increase in sodium concentration than in principal cells. Neither amiloride in the perfusate, nor amiloride in both bathing and perfusion solution, significantly reduced the ouabain-induced rise in intercalated cell sodium concentration. These results indicate that in principal cells amiloride-sensitive sodium channels constitute the predominant pathway for sodium entry across the apical cell membrane. In addition, substantial amounts of sodium enter principal cells across the basolateral cell membrane, probably via Na-H exchange. Finally, the data suggest that in intercalated cells sodium channels and the Na-H exchange are sparse or even absent.

Studies on the mechanism of rubidium-induced kaliuresis

Renal clearance and electron microprobe methods were used 1) to elucidate the effects of chronic rubidium administration on potassium transport and 2) to localize, by the use of amiloride in acute experiments, the tubule site of interaction between rubidium and potassium. Substitution of drinking water by a 50 mM rubidium chloride solution for 9 to 11 days led to significant hypokalemia (plasma potassium 2.5 +/- 0.1 mM; plasma potassium plus rubidium 3.3 +/- 0.1 mM). Compared to a control group (reduction of plasma potassium to 3.4 +/- 0.1 mM by short-term potassium depletion) with a fractional potassium excretion of 2.1 +/- 0.3%, rubidium-treated rats excreted potassium at a much higher rate of 14.6 +/- 3.0%. The potassium content of principal cells was, however, significantly lower in rubidium-treated than in potassium-deprived animals. Similar to experiments in which rubidium was given acutely (3 hours), chronic rubidium administration was associated with preferential accumulation of rubidium in all tubule cells relative to potassium. Rubidium clearances were uniformly below those of potassium. Amiloride abolished the difference between rubidium and potassium clearances and sharply reduced the excretion of both cations. In view of the known site of action of amiloride, this suggests a distal tubule site of rubidium action on potassium transport. Amiloride also reduced or abolished the preferential uptake of rubidium into all but intercalated tubule cells. Marked cell heterogeneity of rubidium accumulation into intercalated cells was observed: One subpopulation, with low cell chloride, retained rubidium more effectively than another subpopulation with high cell chloride.

Effect of ouabain on electrolyte concentrations in principal and intercalated cells of the isolated perfused cortical collecting duct

Sodium, phosphorus, chloride and potassium concentrations were measured by a new method in individual principal and intercalated cells in the cortical collecting duct in vitro. Electron microprobe analysis was applied to freeze-dried cryosections of the isolated perfused rabbit cortical collecting duct. Cell analyses were performed under control conditions and after addition of ouabain to the bath. Under control conditions similar sodium, potassium, chloride, and phosphorus concentration (means +/- SEM) were observed in principal (10.0 +/- 0.6, 126.5 +/- 2.7, 24.6 +/- 1.0, and 121.5 +/- 3.5 mmol/kg wet weight, respectively) and intercalated cells (9.0 +/- 0.9, 127.1 +/- 4.2, 27.4 +/- 1.8, and 118.7 +/- 4.9 mmol/kg wet weight, respectively). In principal cells ouabain (10 min) caused an increase in sodium and chloride concentrations by 104 and 13 mmol/kg wet weight, and a decrease in potassium and phosphorus concentrations by 106 and 32 mmol/kg wet weight. These changes in cell element concentrations can be ascribed to an exchange of intracellular potassium against extracellular sodium and to cell swelling due to influx of extracellular fluid. The effects of ouabain on intercalated cells were far less pronounced than on principal cells. This different susceptibility to ouabain of principal and intercalated cells can be ascribed to differences in active and passive transmembrane ion transport pathways.

Renal excretion of rubidium and potassium: an electron microprobe and clearance study

A combination of clearance and electron microprobe studies was carried out to investigate renal rubidium excretion and rubidium distribution between plasma and individual tubule cells. Saline-infused animals were compared with potassium-loaded rats and another group in which rubidium was given in such amounts that the sum of plasma rubidium plus potassium equalled the potassium concentration in the potassium-loaded rats. The renal clearance of rubidium was uniformly less than that of potassium. Nevertheless, rubidium stimulated fractional potassium excretion above the levels observed in both saline- and potassium-loaded animals. When compared with their plasma concentrations, rubidium was concentrated in all tubule cell types more than potassium, and this is most likely due to restriction of passive diffusion of rubidium from cells to extracellular fluid. In addition, heterogeneity of intercalated cell ion composition was observed: one cell group had high chloride and potassium, but low rubidium contents, whereas the other was characterized by low chloride and potassium, but high rubidium contents.

Effect of acute metabolic acidosis on transmembrane electrolyte gradients in individual renal tubule cells

We studied the effect of acute metabolic acidosis on potassium, sodium and chloride gradients across the apical membrane of proximal and distal tubule cells by determining electrolyte concentrations in individual cells and in tubule fluid employing electron microprobe analysis. Cellular measurements were performed on freeze-dried cryosections of the renal cortex, analysis of tubule fluid electrolyte concentrations on freeze-dried microdroplets of micropuncture samples obtained from proximal and from early and late distal collection sites. Acidosis (NH4Cl i.v. and i.g.) induced a substantial rise in plasma potassium concentration without significant effects on cell potassium concentrations. Potassium concentrations along the surface distal tubule were also unaltered; thus the chemical driving force for potassium exit from cell to lumen was not affected by acidosis. In all but intercalated cells acidosis markedly increased cell phosphorus concentration and cell dry weight indicating cell shrinkage and thus diminution of cell potassium content. Because the increase in intracellular chloride concentration exceeded the increase in plasma chloride concentration, the chemical chloride gradient across the contraluminal membrane was markedly depressed by acidosis.

The distribution of potassium, sodium and chloride across the apical membrane of renal tubular cells: effect of acute metabolic alkalosis

Studies were undertaken to define the effect of acute metabolic alkalosis (hypertonic sodium bicarbonate i.v.) on the chemical gradients for potassium, sodium and chloride across the apical membrane of individual renal tubule cells. Electron microprobe analysis was used on freeze-dried cryosections of the rat renal cortex to measure electrolyte concentrations in proximal tubule cells and in the various cell types of the superficial distal tubule. Analyses were also performed in fluid samples obtained by micropuncture from proximal and early and late distal collection sites. Compared with the appropriate controls (hypertonic sodium chloride i.v.), administration of sodium bicarbonate resulted only in small and mostly insignificant increases in cell potassium concentrations and induced only minor alterations in the cell/tubule fluid potassium concentration gradient for all cell types analysed. This observation suggests that under this condition factors other than an increase in cell potassium concentration are important in modulating potassium transfer across the apical membrane of potassium secreting cells. Nevertheless, since in alkalosis phosphorus and cell dry weight were decreased, and hence cell volume increased, in all but the intercalated cells, actually the potassium content of most tubular cells was higher under this condition. In comparison with animals infused with isotonic saline at low rates (hydropenic controls), infusion of either hypertonic sodium chloride or sodium bicarbonate led to a sharp increase in distal tubule fluid sodium concentrations and in the sodium concentrations of distal convoluted tubule, connecting tubule and principal cells, indicating that under both conditions the primary event causing enhanced transepithelial sodium absorption is stimulation of the sodium entry step.(ABSTRACT TRUNCATED AT 250 WORDS)