A loss-of-function mutation in NaPi-IIa and renal Fanconi's syndrome

We describe two siblings from a consanguineous family with autosomal recessive Fanconi's syndrome and hypophosphatemic rickets. Genetic analysis revealed a homozygous in-frame duplication of 21 bp in SLC34A1, which encodes the renal sodium-inorganic phosphate cotransporter NaPi-IIa, as the causative mutation. Functional studies in Xenopus laevis oocytes and in opossum kidney cells indicated complete loss of function of the mutant NaPi-IIa, resulting from failure of the transporter to reach the plasma membrane. These findings show that disruption of the human NaPi-IIa profoundly impairs overall renal phosphate reabsorption and proximal-tubule function and provide evidence of the critical role of NaPi-IIa in human renal phosphate handling.

Macula densa cell signaling

Macula densa cells are renal sensor elements that detect changes in distal tubular fluid composition and transmit signals to the glomerular vascular elements. This tubuloglomerular feedback mechanism plays an important role in regulating glomerular filtration rate and blood flow. Macula densa cells detect changes in luminal sodium chloride concentration through a complex series of ion transport-related intracellular events. NaCl entry via a Na:K:2Cl cotransporter and Cl exit through a basolateral channel lead to cell depolarization and increases in cytosolic calcium. Na/H exchange (NHE2) results in cell alkalization, whereas intracellular [Na] is regulated by an apically located H(Na)-K ATPase and not by the traditional basolateral Na:K ATPase. Communication from macula densa cells to the glomerular vascular elements involves ATP release across the macula densa basolateral membrane through a maxi-anion channel. The adaptation of multi-photon microscopy is providing new insights into macula densa-glomerular signaling.

Molecular identity and regulation of renal potassium channels

K channels are ubiquitous in animal cells, where they are involved in a variety of physiological functions. In epithelial cells of the kidney, K channels are primarily involved in maintaining membrane potential, recycling and secreting K and regulating cell volume. As many renal K channels have now been studied or identified at the molecular level by means of a variety of approaches, including patch-clamp recordings, cDNA cloning and immunohistochemistry, the purpose of this review is to summarize what is presently known about the molecular identity of renal K channels with an emphasis on their regulatory properties.

Local osmotic gradients drive the water flux associated with Na(+)/glucose cotransport

It recently was proposed [Loo, D. D. F., Zeuthen, T., Chandy, G. & Wright, E. M. (1996) Proc. Natl. Acad. Sci. USA 93, 13367--13370] that SGLT1, the high affinity intestinal and renal sodium/glucose cotransporter carries water molecules along with the cosubstrates with a strict stoichiometry of two Na(+), one glucose, and approximately 220 water molecules per transport cycle. Using electrophysiology together with sensitive volumetric measurements, we investigated the nature of the driving force behind the cotransporter-mediated water flux. The osmotic water permeability of oocytes expressing human SGLT1 (L(p) +/- SE) averaged 3.8 +/- 0.3 x 10(-4) cm x s(-1) (n = 15) and addition of 100 microM phlorizin (a specific SGLT1 inhibitor) reduced the permeability to 2.2 +/- 0.2 x 10(-4) cm x s(-1) (n = 15), confirming the presence of a significant water permeability closely associated with the cotransporter. Addition of 5 mM alpha-methyl-glucose (alpha MG) induced an average inward current of 800 +/- 10 nA at -50 mV and a water influx reaching 120 +/- 20 pL cm(-2) x s(-1) within 5-8 min. After rapidly inhibiting the Na(+)/glucose cotransport with phlorizin, the water flux remained significantly elevated, clearly indicating the presence of a local osmotic gradient (Delta pi) estimated at 16 +/- 2 mOsm. In short-term experiments, a rapid depolarization from -100 to 0 mV in the presence of alpha MG decreased the cotransport current by 94% but failed to produce a comparable reduction in the swelling rate. A mathematical model depicting the intracellular accumulation of transported osmolytes can accurately account for these observations. It is concluded that, in SGLT1-expressing oocytes, alpha MG-dependent water influx is induced by a local osmotic gradient by using both endogenous and SGLT1-dependent water permeability.

Activation of an ATP-dependent K(+) conductance in Xenopus oocytes by expression of adenylate kinase cloned from renal proximal tubules

In rabbit proximal convoluted tubules, an ATP-sensitive K(+) (K(ATP)) channel has been shown to be involved in membrane cross-talk, i.e. the coupling (most likely mediated through intracellular ATP) between transepithelial Na(+) transport and basolateral K(+) conductance. This K(+) conductance is inhibited by taurine. We sought to isolate this K(+) channel by expression cloning in Xenopus oocytes. Injection of renal cortex mRNA into oocytes induced a K(+) conductance, largely inhibited by extracellular Ba(2+) and intracellular taurine. Using this functional test, we isolated from our proximal tubule cDNA library a unique clone, which induced a large K(+) current which was Ba(2+)-, taurine- and glibenclamide-sensitive. Surprisingly, this clone is not a K(+) channel but an adenylate kinase protein (AK3), known to convert NTP+AMP into NDP+ADP (N could be G, I or A). AK3 expression resulted in a large ATP decrease and activation of the whole-cell currents including a previously unknown, endogenous K(+) current. To verify whether ATP decrease was responsible for the current activation, we demonstrated that inhibition of glycolysis greatly reduces oocyte ATP levels and increases an inwardly rectifying K(+) current. The possible involvement of AK in the K(ATP) channel's regulation provides a means of explaining their observed activity in cytosolic environments characterized by high ATP concentrations.

Functional expression of tagged human Na+-glucose cotransporter in Xenopus laevis oocytes

1. High-affinity, secondary active transport of glucose in the intestine and kidney is mediated by an integral membrane protein named SGLT1 (sodium glucose cotransporter). Though basic properties of the transporter are now defined, many questions regarding the structure- function relationship of the protein, its biosynthesis and targeting remain unanswered. In order to better address these questions, we produced a functional hSGLT1 protein (from human) containing a reporter tag. 2. Six constructs, made from three tags (myc, haemaglutinin and poly-His) inserted at both the C- and N-terminal positions, were thus tested using the Xenopus oocyte expression system via electrophysiology and immunohistochemistry. Of these, only the hSGLT1 construct with the myc tag inserted at the N-terminal position proved to be of interest, all other constructs showing no or little transport activity. A systematic comparison of transport properties was therefore performed between the myc-tagged and the untagged hSGLT1 proteins. 3. On the basis of both steady-state (affinities for substrate (glucose) and inhibitor (phlorizin) as well as expression levels) and presteady-state parameters (transient currents) we conclude that the two proteins are functionally indistinguishable, at least under these criteria. Immunological detection confirmed the appropriate targeting of the tagged protein to the plasma membrane of the oocyte with the epitope located at the extracellular side. 4. The myc-tagged hSGLT1 was also successfully expressed in polarized MDCK cells. alpha-Methylglucose uptake studies on transfected cells showed an exclusively apical uptake pathway, thus indicating that the expressed protein was correctly targeted to the apical domain of the cell. 5. These comparative studies demonstrate that the myc epitope inserted at the N-terminus of hSGLT1 produces a fully functional protein while other epitopes of similar size inserted at either end of the protein inactivated the final protein.

Two types of K(+) channels at the basolateral membrane of proximal tubule: inhibitory effect of taurine

The cell-attached configuration of the patch-clamp technique was used to investigate the effects of taurine on the basolateral potassium channels of rabbit proximal convoluted tubule. In the absence of taurine, the previously reported ATP-blockable channel, K(ATP), was observed in 51% of patches. It is characterized by an inwardly rectifying current-voltage curve with an inward slope conductance of 49 +/- 5 pS (n = 15) and an outward slope conductance of 13 +/- 6 pS (n = 15). The K(ATP) channel open probability (P(o)) is low, 0.15 +/- 0.06 (n = 15) at a -V(p) = -100 mV (V(p) is the pipette potential), and increases slightly with depolarization. The gating kinetics are characterized by one open time constant (tau(o) = 5.0 +/- 1.9 ms, n = 6) and two closed time constants (tau(C1) = 5. 2 +/- 1.5 ms, tau(C2) = 140 +/- 40 ms; n = 6). In 34% of patches, a second type of potassium channel, sK, with distinct properties was recorded. Its current-voltage curve is characterized by a sigmoidal shape, with an inward slope conductance of 12 +/- 2 pS (n = 4). Its P(o) is voltage independent and averages 0.67 +/- 0.03 (n = 4) at -V(p) = -80 mV. Both its open time and closed time distributions are described by a single time constant (tau(o) = 96 +/- 19 ms, tau(C) = 10.5 +/- 3.6 ms; n = 4). Extracellular perfusion of 40 mM taurine fails to affect sK channels, whereas K(ATP) channel P(o) decreases by 75% (from 0.17 +/- 0.06 to 0.04 +/- 0.02, n = 7, P < 0.05). In conclusion, the absolute basolateral potassium conductance of rabbit proximal tubules is the resulting combination of, at least, two types of potassium channels of roughly equal importance: a high-conductance low-open probability K(ATP) channel and a low-conductance high-open probability sK channel. The previously described decrease in the basolateral absolute potassium conductance by taurine is, however, mediated by a single type of K channel: the ATP-blockable K channel.

Molecular characterization of an inwardly rectifying K+ channel from HeLa cells

Previous patch-clamp studies have shown that the potassium permeability of the plasma membrane in HeLa cells, a cell line derived from an epidermoid carcinoma of the cervix, is controlled by various K+-selective pores including an IRK1 type inwardly rectifying K+ channel. We used the sequence previously reported for the human heart Kir2.1 channel to design a RT-PCR strategy for cloning the IRK1 channel in HeLa cells. A full-length clone of 1.3 kb was obtained that was identical to the human cardiac Kir2.1 inward rectifier. The nature of the cloned channel was also confirmed in a Northern blot analysis where a signal of 5.3 kb corresponding to the molecular weight expected for a Kir2.1 channel transcript was identified not only in HeLa cells, but also in WI-38, ECV304 and bovine aortic endothelial cells. The HeLa IRK1 channel cDNA was subcloned in an expression vector (pMT21) and injected into Xenopus oocytes. Cell-attached and inside-out single channel recordings obtained from injected oocytes provided evidence for a voltage-independent K+-selective channel with current/voltage characteristics typical of a strong inward rectifier. The single channel conductance for inward currents measured in 200 mm K2SO4 conditions was estimated at 40 +/- 1 pS (n = 3), for applied voltages ranging from -100 to -160 mV, in agreement with the unitary conductance for the IRK1 channel identified in HeLa cells. In addition, the single channel conductance for inward currents, Gamma, was found to vary as a function of alphaK, the external K+ ion activity, according to Gamma = Gamma0 [alphaK]delta with Gamma0 = 3.3 pS and delta = 0.5. Single channel recordings from injected oocytes also provided evidence of a voltage-dependent block by external Cs+ and Ba2+. The presence of 500 micron Cs+ caused a voltage-dependent flickering, typical of a fast channel blocking process which resulted in a reduction of the channel open probability at increasingly negative membrane potential values. The fractional electrical distance computed for the Cs+ blocking site was greater than 1 indicating a multiple ion channel occupation. In contrast, external Ba2+ at concentrations ranging from 25 to 100 micron caused a slow channel block, consistent with the binding of a single Ba2+ ion at a site located at half the membrane span. It is concluded on the basis of these observations that HeLa cells expressed a Kir2.1 type inwardly rectifying channel likely to be involved in maintaining and regulating the cell resting potential.

Transport and regulatory properties of the apical Na-K-2Cl cotransporter of macula densa cells

NH+4/NH3 fluxes were used to probe apical Na-K-2Cl transport activity of macula densa (MD) cells from rabbit kidney. In the presence of 25 mM NaCl and 5 mM Ba2+, addition of 20 mM NH+4 to the lumen produced a profound intracellular acidification, and approximately 80% of the initial acidification rate was bumetanide sensitive. The NH+4-induced acidification rate was dependent on luminal Cl- and Na+ with apparent affinities of 17 +/- 4 mM (Hill number 1.45) and 1.0 +/- 0.3 mM, respectively. In the presence of saturating luminal NaCl concentration ([NaCl]L), blockade of basolateral Cl- efflux with 10 microM 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) reduced the NH+4-induced acidification rate by 51 +/- 6% (P > 0.01, n = 5). Under similar conditions, dibutyryl-cAMP (DBcAMP) + forskolin increased the NH+4-induced acidification rate by 27%, whereas it produced no detectable effect at low luminal NaCl concentration. Most of the observed DBcAMP + forskolin effect was probably due to the stimulation of the basolateral Cl- conductance, since, in the presence of basolateral NPPB, this activation was changed to a 17.1% and 16.6% inhibition of the NH+4-induced acidification rate observed at high or low [NaCl]L, respectively. We conclude that the cotransporter found in MD cells displays, with respect to other Na-K-2Cl cotransporters, a relatively high affinity for luminal Na+ and luminal Cl- and can be specifically inhibited by increases in intracellular Cl- and cAMP concentrations.

Ionic transport in macula densa cells

Recent work has provided substantial insights into functional characteristics of macula densa (MD) cells. Microelectrode and patch-clamp experiments on the rabbit isolated thick ascending limb (TAL)/glomerulus preparation have shown that MD cells possess a furosemide-sensitive Na:K:2Cl cotransporter, an apical 41-pS K+ channel, and a dominant basolateral Cl- conductance. Increasing luminal fluid [NaCl] ([NaCl]L) results in furosemide-sensitive cell depolarization due to a rise in intracellular [Cl-] that stimulates basolateral electrogenic Cl- efflux. Intracellular pH (pHi) measurements show the presence of an apical Na:H exchanger that couples transepithelial Na+ transport to pHi. Experimental results and thermodynamic considerations allow estimation of intracellular [Na+] and [Cl-] ([Na+]i, [Cl-]i) under different conditions. When the Na:K:2Cl cotransporter is equilibrated (or in the presence of furosemide), [Na+]i and [Cl-]i are low (approximately 6 to 7 mM), whereas when the cotransporter is fully activated, [Na+]i and [Cl-]i increase substantially to approximately 70 and 20 mM, respectively. Finally, luminal addition of NH4+ produces cell acidification that depends on NH4+ apical transport rate through the Na:K:2Cl. Using a simple transport model for NH4+, the initial NH4+ influx rate in MD cells is comparable to the corresponding flux in TAL. This challenges the idea that MD cells have a low transport activity but supports our findings about large changes in intracellular concentrations as a function of [NaCl]L.

Effects of Vpu expression on Xenopus oocyte membrane conductance

The HIV-1-specific vpu gene encodes an integral membrane phosphoprotein which affects three aspects of the HIV-1 infectious cycle: it enhances virion release from infected cells; it causes degradation of the CD4 protein in the endoplasmic reticulum; and it delays syncytia formation in HIV-1-infected CD4+ T-cells. Although little is known about how Vpu mediates these effects, it has been proposed to function as a nonspecific cation channel. In this report, voltage clamp measurements of Xenopus oocytes show that Vpu expression is not associated with increased transmembrane currents. Instead, Vpu expression diminishes membrane conductance. Injection of 4.6 ng of Vpu mRNA into these cells reduces endogenous potassium conductance by 50%. Only Vpu mutants which retain the ability to degrade CD4 can diminish K+ conductance. Inhibition by Vpu is not unique to K+ channels as it is also observed on several coexpressed membrane proteins but not on a coexpressed cytoplasmic protein. These results indicate that the CD4 degradative capability of Vpu and the Vpu-mediated modulation of membrane protein expression are mechanistically coupled and that Vpu may contribute to HIV pathogenesis by altering plasma membrane protein expression at the cell surface.

Pharmacological and biochemical evidence for the regulation of osteocalcin secretion by potassium channels in human osteoblast-like MG-63 cells

Previous reports have suggested the involvement of voltage-activated calcium (Ca2+) channels in bone metabolism and in particular on the secretion of osteocalcin by osteoblast-like cells. We now report that potassium (K+) channels can also modulate the secretion of osteocalcin by MG-63 cells, a human osteosarcoma cell line. When 1,25-dihydroxyvitamin D3(1,25(OH)2D3)-treated MG-63 cells were depolarized by step increases of the extracellular K+ concentration ([K+]out) from 5-30 mM, osteocalcin (OC) secretion increased from a control value of 218 +/- 13 to 369 +/- 18 ng/mg of protein/48 h (p < 0.005 by analysis of variance). In contrast, in the absence of 1,25(OH)2D3, there is no osteocalcin secretion nor any effect of cell depolarization on this activity. The depolarization-induced increase in 1,25(OH)2D3-dependent osteocalcin secretion was totally inhibited in the presence of 10 microM Nitrendipine (a Ca2+ channel blocker, p < 0.005) without affecting cellular alkaline phosphatase nor cell growth. Charybdotoxin, a selective blocker of Ca2+-dependent K+ channels (maxi-K) present in MG-63 cells, stimulated 1,25(OH)2D3-induced osteocalcin synthesis about 2-fold (p < 0.005) after either 30, 60, or 120 minutes of treatment. However, Charybdotoxin was without effect on basal release of osteocalcin in the absence of 1,25(OH)2D3 pretreatment. Using patch clamp technique, we occasionally observed the presence of a small conductance K+ channel, compatible with an ATP-dependent K+ channel (GK[ATP]) in nonstimulated cells, whereas multiple channel openings were observed when cells were treated with Diazoxide, a sulfonamide derivative which opens GK(ATP). Western blot analysis revealed the presence of the N-terminal peptide of GK(ATP) in MG-63 cells, and its expression was regulated with the proliferation rate of these cells, maximal detection by Western blots being observed during the logarithmic phase of the cycle. Glipizide and Glybenclamide, selective sulfonylureas which can block GK(ATP), dose-dependently enhanced 1,25(OH)2D3-induced OC secretion (p < 0.005). Reducing the extracellular calcium concentration with EGTA (microM range) totally inhibited the effect of Glipizide and Glybenclamide on osteocalcin secretion (p < 0.005), which remained at the same levels as controls. Diazoxide totally prevented the effect of these sulfonylureas. These results suggest that voltage-activated Ca2+ channels triggered via cell depolarization can enhance 1,25(OH)2D3-induced OC release by MG-63 cells. In addition, OC secretion is increased by blocking two types of K+ channels: maxi-K channels, which normally hyperpolarize cells and close Ca2+ channels, and GK(ATP) channels. The role of these channels is closely linked to the extracellular Ca2+ concentration.

Sodium leak pathway and substrate binding order in the Na+-glucose cotransporter

The Na+-glucose cotransporter (SGLT1) expressed in Xenopus laevis oocytes was shown to generate a phlorizin-sensitive sodium leak in the absence of sugars. Using the current model for SGLT1, where the sodium leak was presumed to occur after two sodium ions are bound to the free carrier before glucose binding, a characteristic concentration constant (Kc) was introduced to describe the relative importance of the sodium leak versus Na+-glucose cotransport currents. Kc represents the glucose concentration at which the Na+-glucose cotransport current is equal to the sodium leak. As both the sodium leak and the Na+-glucose cotransport current are predicted to occur after the binding of two sodium ions, the model predicted that Kc should be sodium-independent. However, by using a two-microelectrode voltage-clamp technique, the observed Kc was shown to depend strongly on the external sodium concentration ([Na+]o): it was four times higher at 5 mM [Na+]o than at 20 mM [Na+]o. In addition, the magnitude of the sodium leak varied as a function of [Na+]o in a Michaelian fashion, and the sodium affinity constant for the sodium leak was 2-4 times lower than that for cotransport in the presence of low external glucose concentrations (50 or 100 microM), whereas the current model predicted a sigmoidal sodium dependence of the sodium leak and identical sodium affinities for the sodium leak and the Na+-glucose cotransport. These observations indicate that the sodium leak occurs after one sodium ion is associated with the carrier and agree with predictions from a model with the binding order sodium-glucose-sodium. This conclusion was also supported by experiments performed where protons replaced Na+ as a "driving cation."

Determination of NH4+/NH3 fluxes across apical membrane of macula densa cells: a quantitative analysis

Luminal addition of 20 mM NH4+ produced a rapid acidification of rabbit macula densa (MD) cells from 7.50 +/- 0.06 to 6.91 +/- 0.05 at an initial rate of 0.071 +/- 0.008 pH unit/s. In the luminal presence of 5 microM bumetanide, 5 mM Ba2+ or both, the acidification rate was reduced by 57%, 35% and 93% of control levels. In contrast, intracellular pH (pHi) recovery after removing luminal NH4+ was unaffected by bumetanide and Ba2+ but was sensitive to 1 mM luminal amiloride (71% inhibition). The bumetanide-sensitive acidification rate represents most certainly the NH4+ flux mediated by the apical Na+:K+ (NH4+):2Cl- cotransporter, but the Ba(2+)-sensitive portion does not seem to be associated with the apical K+ channels previously characterized by us. The effects of NH4+ entry across the apical membrane were simulated using a simple model involving five adjustable parameters: apical and basolateral permeabilities for NH4+ and NH3 and a parameter describing a pH-regulating mechanism. The model shows that the apical membrane of MD cells is much more permeable to NH3 than it is to NH4+ and, under control conditions, the apical NH4+ flux appears surprisingly high (11-20 mM/s) and challenges the notion that MD cells present a low intensity of ionic transport.

Characteristics of membrane transport processes of macula densa cells

1. Macula densa (MD) cells are located within the thick ascending limb (TAL) and have their apical surface in contact with tubular fluid and their basilar region in contact with the glomerulus. These cells sense changes in luminal fluid sodium chloride concentration ([NaCl]) and transmit signals resulting in changes in vascular resistance (tubuloglomerular feedback) and renin release. 2. Current efforts have focused on understanding the cellular transport mechanisms of MD cells. Progress in this area has benefited from the use of the isolated perfused TAL-glomerular preparation, which permits direct access to MD cells. 3. Using microelectrodes to measure basolateral membrane potential (VBL) of MD cells, it was found that VBL was very sensitive to changes in luminal fluid [NaCl]. As [NaCl] was elevated from 20 to 150 mmol/L, VBL was found to depolarize by over 30 mV. 4. Basolateral membrane potential measurements were also used to identify an apical Na+:2Cl-:K+ cotransport pathway in MD cells that is the major pathway for NaCl entry into these cells. 5. Other work identified a basolateral chloride channel that is presumed to be responsible for changes in VBL during alterations in luminal [NaCl]. This channel, which is the predominant conductance across the basolateral membrane, may be regulated by intracellular Ca2+ and cAMP. 6. An apical Na+:H+ exchanger in MD cells was detected by measuring changes in intracellular pH using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein. 7. Using patch-clamp techniques, a high density of pH- and Ca(2+)-sensitive K+ channels was observed at the apical membrane of MD cells. 8. Other studies found that, at the normal physiological conditions prevailing at the end of the TAL (luminal [NaCl] of 20-60 mmol/L), reabsorption mediated by MD cells is very sensitive to changes in luminal [NaCl].

Inhibition of basolateral potassium conductance by taurine in the proximal convoluted tubule

The effect of taurine on the electrophysiological properties of the basolateral membrane of the rabbit proximal convoluted tubule was examined. Short-duration isosmotic pulses of 40 mM taurine in the bath solution induced basolateral membrane depolarizations (delta Vbl) of 6.44 +/- 0.5 mV, which were reduced by 58% in absence of Na+. In presence of barium and quinine, delta Vbl values were reduced by 55% in Na(+)-containing bath solutions and were completely abolished in Na(+)-free solutions. Continuous addition of taurine into the bath solutions for a period of 5 min induced 1) a decrease in the partial conductance of the basolateral membrane to K+ (tK) from 0.39 to 0.23; 2) an increase in the partial conductance to Cl- (tcl) from 0.055 to 0.172; 3) a rapid and transient increase of the partial conductance mediated by the Na-HCO3 cotransporter (tNaHCO3) from 0.37 to 0.52, followed by a progressive decrease to 0.29; and 4) a depolarization of the basolateral membrane of 16 mV. The absolute membrane conductance mediated by the Na-HCO3 cotransporter was not initially affected by taurine, whereas that to K (GK) initially decreased by a maximal factor of 2 after 1 min, followed by a partial recovery after 5 min, and that to Cl (GCl) increased by a factor of 4. Addition of taurine after a hypotonicity-induced cell swelling produced an inhibition of GK comparable to the one observed under isotonic conditions. These results demonstrate the presence of an electrogenic Na-dependent transport of taurine and indicate that taurine inhibits GK.

Fast voltage clamp discloses a new component of presteady-state currents from the Na(+)-glucose cotransporter

The human Na(+)-glucose cotransporter (hSGLT1) has been shown to generate, in the absence of sugar, presteady-state currents in response to a change in potential, which could be fitted with single exponentials once the voltage had reached a new constant value. By the cut-open oocyte technique (voltage rising-speed approximately 1 mV/microsecond), phlorizin-sensitive transient currents could be detected with a higher time resolution during continuous intracellular perfusion. In the absence of sugar and internal Na+, and with 90 mM external Na+ concentration ([Na+]o), phlorizin-sensitive currents exhibited two relaxation time-constants: tau 1 increased from 2 to 10 ms when Vm decreased from +60 mV to -80 mV and remained at 10 ms for more negative Vm; tau 2 ranged from 0.4 to 0.8 ms in a weakly voltage-dependent manner. According to a previously proposed model, these two time constants could be accounted for by 1) Na+ crossing a fraction of the membrane electrical field to reach its binding site on the carrier and 2) conformational change of the free carrier. To test this hypothesis, the time constants were measured as [Na+]o was progressively reduced to 0 mM. At 30 and 10 mM external Na+, tau 1 reached the same plateau value of 10 ms but at more negative potentials (-120 and -160 mV, respectively). Contrary to the prediction of the model, two time constants continued to be detected in the bilateral absence of Na+ (at pH 8.0). Under these conditions, tau 1 continuously increased through the whole voltage range and did not reach the 10 ms level even when Vm had attained -200 mV while tau 2 remained in the range of 0.4-0.8 ms. These results indicate that 1) conformational change of the free carrier across the membrane must occur in more than one step and 2) Na+ binding/debinding is not responsible for either of the two observed exponential components of transient currents. By use of the simplest kinetic model accounting for the portion of the hSGLT1 transport cycle involving extracellular Na+ binding/debinding and the dual-step conformational change of the free carrier, tau 1 and tau 2 were fitted throughout the voltage range, and a few sets of parameters were found to reproduce the data satisfactorily. This study shows that 1) tau 1 and tau 2 correspond to two steps in the conformational change of the free carrier, 2) Na+ binding/debinding modulates the slow time constant (tau 1) and 3) a voltage-independent slow conformational change of the free carrier accounts for the observed plateau value of 10 ms.

Cell volume increases of physiologic amplitude activate basolateral K and CI conductances in the rabbit proximal convoluted tubule

The effects of increases in cell volume (CV) of physiologic amplitude, induced either hypotonically or isotonically, were studied on the three major basolateral conductances of rabbit isolated proximal convoluted tubules. CV increases were produced by a 40 mosmol/kg H2O hypotonic shock or by the isotonic replacement of mannitol by 40 mM glucose or alanine. The hypotonic shock led to an increase in CV of 17 +/- 3% (N = 8), whereas additions of glucose and alanine led to increases in CV of 22.6 +/- 2.5% (N = 7) and 28.3 +/- 3.5 (N = 5), respectively. Under all of these conditions, the absolute conductance mediated by the NaHCO3 cotransporter did not vary appreciably. This allowed determination of the variations of the absolute conductances to potassium (GK) and chloride (GCl) from their measured partial conductances. All three protocols induced significant increases in GK by factors of 2.37 +/- 0.3, 1.43 +/- 0.16, and 1.69 +/- 0.40, and in GCI by factors of 3.32 +/- 0.57, 3.68 +/- 0.75, and 3.90 +/- 1.0 during the hypotonic, glucose, and alanine protocols, respectively. These increases in GK and GCl occurred with a delay compared with the variations in CV, indicating a more elaborate signaling mechanism than stretch-activation of channels that are known to activate channels within seconds. Intracellular pH increased from 7.19 +/- 0.03 to 7.23 +/- 0.03, 7.17 +/- 0.02 to 7.20 +/- 0.02, and 7.13 +/- 0.01 to 7.16 +/- 0.01 after the hypotonic shock and the glucose and alanine additions, respectively. The study presented here demonstrates that there is a close relationship between CV and GK and GCl, independent of the means used (hypotonically or isotonically) to increase CV in rabbit proximal convoluted tubules. CV activation of GK is proposed to account for part of the increase in GK reported previously during activation of transepithelial transport.

Volume regulation and ion transport in renal cells

Volume regulatory mechanisms are reviewed for both short- and long-term adaptation of renal cells to anisotonic media. Within minutes after exposure to hypotonic solutions a common feature of many renal cells is the increase in plasma membrane potassium and chloride conductances. Although extrusion of intracellular potassium certainly contributes to a regulatory volume decrease, the role of chloride efflux itself is probably modest, given the relatively low intracellular chloride concentration. Indeed, other intracellular osmolytes such as taurine and other amino acids are transported out of the cell to achieve a regulatory volume decrease. On a longer-term basis cells from the renal medulla have to adapt to an extracellular milieu which can become extremely hypertonic under certain conditions. In tissue culture models, cells exposed to hypertonic media react by actively taking up small molecules, such as betaine, taurine and myo-inositol, and by synthesizing more sorbitol and glycerophosphocholine. For each of these osmolytes hypertonicity was shown to activate the transcription of a specific enzyme or transporter gene, the expression of which reaches a peak within 18 h of hypertonicity.

Kinetic separation and characterization of three sugar transport modes in Caco-2 cells

The question of sugar transport heterogeneity in the human intestinal Caco-2 cell line was addressed using alpha-methyl-D-glucose (AMG) and 2-deoxy-D-glucose (DG) as substrate analogues for D-glucose, the transport inhibitors phlorizin (PZ) and phloretin (PT), and NaCl or choline chloride uptake media. The data are compatible with the existence of three distinct pathways that can be isolated kinetically according to specific characteristics: 1) an "AMG-strict" system, strictly Na+ dependent and specific for AMG [Michaelis-Menten constant value (K(m)) = 2.0 +/- 0.3 mM] but sensitive to both PZ and PT, with PZ being more potent than PT, 2) a "DG-strict" system, strictly Na+ independent and specific for both DG (K(m) = 5.2 +/- 0.5 mM) and PT; and 3) a "DG/AMG-mixed" system, strictly Na+ dependent, with loose specificities for the glucose analogues DG (K(m) = 0.81 +/- 0.07 mM) and AMG (K(m) = 8.1 +/- 0.8 mM), and the inhibitors PZ and PT, but with PT being more potent than PZ. Since SGLT-1 obtained by polymerase chain reaction from either Caco-2 cells or normal human jejunum demonstrated identical transport properties when expressed in Xenopus laevis oocytes, we conclude that the "AMG-strict" system represents the expression of human SGLT-1 activity in this cell line. Moreover, Western blot analysis revealed that SGLT-1 is located exclusively in the apical membrane. In contrast, neither the nature nor the membrane location of both the DG-strict and DG/AMG-mixed pathways could be resolved unambiguously. Still it has been demonstrated that expression of the latter system is constitutive to all Caco-2 cells and that its Na+ dependence is not the consequence of H(+)-dependent transport activity. Aside from the presence of the DG/AMG-mixed system, a salient feature of Caco-2 cells is that the GLUT-3 protein is located exclusively in the brush-border membrane. Due to these limitations, it is concluded that the Caco-2 cell line cannot be considered as equivalent to either fetal colonic cells or normal enterocytes.

Activation of maxi-K channels by parathyroid hormone and prostaglandin E2 in human osteoblast bone cells

Patch clamp experiments were performed on two human osteosarcoma cell lines (MG-63 and SaOS-2 cells) that show an osteoblasticlike phenotype to identify and characterize the specific K channels present in these cells. In case of MG-63 cells, in the cell-attached patch configuration (CAP) no channel activity was observed in 2 mM Ca Ringer (control condition) at resting potential. In contrast, a maxi-K channel was observed in previously silent CAP upon addition of 50 nM parathyroid hormone (PTH), 5 nM prostaglandin E2 (PGE2) or 0.1 mM dibutyryl cAMP + 1 microM forskolin to the bath solution. However, maxi-K channels were present in excised patches from both stimulated and nonstimulated cells in 50% of total patches tested. A similar K channel was also observed in SaOS-2 cells. Characterization of this maxi-K channel showed that in symmetrical solutions (140 mM K) the channel has a conductance of 246 +/- 4.5 pS (n = 7 patches) and, when Na was added to the bath solution, the permeability ratio (PK/PNa) was 10 and 11 for MG-63 and SaOS-2 cells respectively. In excised patches from MG-63 cells, the channel open probability (Po) is both voltage- (channel opening with depolarization) and Ca-dependent; the presence of Ca shifts the Po vs. voltage curve toward negative membrane potential. Direct modulation of this maxi-K channel via protein kinase A (PKA) is very unlikely since in excised patches the activity of this channel is not sensitive to the addition of 1 mM ATP + 20 U/ml catalytic subunit of PKA. We next evaluated the possibility that PGE2 or PTH stimulated the channel through a rise in intracellular calcium. First, calcium uptake (45Ca2+) by MG-63 cells was stimulated in the presence of PTH and PGE2 an effect inhibited by Nitrendipine (10 microM). Second, whereas PGE2 stimulated the calcium-activated maxi-K channel in 2 mM Ca Ringer in 60% of patches studied, in Ca-free Ringer bath solution, PGE2 did not open any channels (n = 10 patches) nor did cAMP + forskolin (n = 3 patches), although K channels were present under the patch upon excision. In addition, in the presence of 2 mM Ca Ringer and 10 microM Nitrendipine in CAP configuration, PGE2 (n = 5 patches) and cAMP + forskolin (n = 2 patches) failed to open K channels present under the patch. As channel activation by phosphorylation with the catalytic subunit of PKA was not observed, and Nitrendipine addition to the bath or the absence of calcium prevented the opening of this channel, it is concluded that activation of this channel by PTH, PGE2 or dibutyryl cAMP + forskolin is due to an increase in intracellular calcium concentration via Ca influx.

rBAT is an amino acid exchanger with variable stoichiometry

The rBAT protein, when expressed in Xenopus oocytes, was previously shown to reproduce the selectivity of the Na(+)-independent neutral and basic amino acid transport system called bo,+. More recently, the capacity of rBAT to generate a transmembrane current was demonstrated when addition of neutral amino acids stimulated the efflux of cations (presumably basic amino acids) in rBAT-injected oocytes. In the present paper, aminoisobutyric acid (AIB), a neutral amino acid analogue, was shown to induce outward currents (efflux of basic amino acids) through rBAT similar to those caused by alanine in terms of affinity, maximal currents and I-V curves. Despite generating similar currents, the AIB transport rate was more than 30 times lower than that of alanine, thus challenging the assumption that rBAT functions as a classical exchanger. Experiments using a cut-open oocyte voltage clamp demonstrated that AIB was capable of stimulating rBAT-mediated currents from either side of the membrane. AIB, like alanine, was able to stimulate the efflux of radiolabeled alanine and arginine while no rBAT-mediated efflux was measurable in the absence of external rBAT substrates. These results demonstrate that (i) the presence of amino acids is required on both sides of the membrane for rBAT to mediate amino acid flux and thus rBAT must be some type of exchanger but (ii) rBAT-mediated amino acid influx is not stoichiometrically related to the efflux. A model of a "double gated pore" is proposed to account for these properties of rBAT, which contravene standard models of exchangers and other transporters.

Thermodynamic determination of the Na+: glucose coupling ratio for the human SGLT1 cotransporter

Phlorizin-sensitive currents mediated by a Na-glucose cotransporter were measured using intact or internally perfused Xenopus laevis oocytes expressing human SGLT1 cDNA. Using a two-microelectrode voltage clamp technique, measured reversal potentials (Vr) at high external alpha-methylglucose (alpha MG) concentrations were linearly related to In[alpha MG]o, and the observed slope of 26.1 +/- 0.8 mV/decade indicated a coupling ratio of 2.25 +/- 0.07 Na ions per alpha MG molecule. As [alpha MG]o decreased below 0.1 mM, Vr was no longer a linear function of In[alpha MG]o, in accordance with the suggested capacity of SGLT1 to carry Na in the absence of sugar (the "Na leak"). A generalized kinetic model for SGLT1 transport introduces a new parameter, Kc, which corresponds to the [alpha MG]o at which the Na leak is equal in magnitude to the coupled Na-alpha MG flux. Using this kinetic model, the curve of Vr as a function of In[alpha MG]o could be fitted over the entire range of [alpha MG]o if Kc is adjusted to 40 +/- 12 microM. Experiments using internally perfused oocytes revealed a number of previously unknown facets of SGLT1 transport. In the bilateral absence of alpha MG, the phlorizin-sensitive Na leak demonstrated a strong inward rectification. The affinity of alpha MG for its internal site was low; the Km was estimated to be between 25 and 50 mM, an order of magnitude higher than that found for the extracellular site. Furthermore, Vr determinations at varying alpha MG concentrations indicate a transport stoichiometry of 2 Na ions per alpha MG molecule: the slope of Vr versus In[alpha MG]o averaged 30.0 +/- 0.7 mV/decade (corresponding to a stoichiometry of 1.96 +/- 0.04 Na ions per alpha MG molecule) whenever [alpha MG]o was higher than 0.1 mM. These direct observations firmly establish that Na ions can utilize the SGLT1 protein to cross the membrane either alone or in a coupled manner with a stoichiometry of 2 Na ions per sugar, molecule.

Activation of Na:2Cl:K cotransport by luminal chloride in macula densa cells

Changes in macula densa intracellular pH (pHi) were used to monitor the direction of flux mediated by the apical Na:2Cl:K cotransporter. At the macula densa, a decrease in luminal [Cl] ([Cl]1) from 60 to 1 mM produced cellular alkalinization secondary to a cascade of events involving a decrease in apical Na:2Cl:K cotransport, a fall in intracellular [Na] ([Na]i) and a stimulation of Na:H exchange. This is supported by the fact that 97% of the change in macula densa pHi with reduction in [Cl]1 was bumetanide-sensitive whereas 92% of this pH change was amiloride-sensitive. We found that, in the presence of 20 mM Na and 5 mM K, a [Cl]1 of 14.3 +/- 2.4 mM (N = 7) produced equilibrium of the apical cotransporter since the pHi obtained under this condition was identical to the pHi found after reducing the net ionic flux to zero with bumetanide. Using this value together with the expected stoichiometry for the bumetanide-sensitive cotransporter, it was estimated that the intracellular [Cl] ([Cl]i) at equilibrium (or in the presence of bumetanide) could be as low as 5 mM. Also, using a Hill number of 2 which is consistent with the present data, the affinity for [Cl]1 was found to be 32.5 mM. Under physiological luminal conditions prevailing at the end of the thick ascending limb (approximately 3.5 mM K, and approximately 25 to 30 mM NaCl), macula densa cells are probably operating close to equilibrium while maintaining a small net reabsorption of Na/K and Cl. Since macula densa cells appear capable of reducing [Cl]i to very low levels, a reabsorptive flux should continue to occur until [NaCl]1 is reduced to 18 mM.

Evidence for apical sodium proton exchange in macula densa cells

These studies were performed to determine if changes in luminal sodium chloride concentration ([NaCl]) might alter macula densa intracellular pH. Isolated thick ascending limbs with attached glomeruli were bathed in a 150 mM NaCl Ringer's solution and perfused in vitro with a 25 mM NaCl solution; N-methyl-D-glucamine cyclamate was used to substitute for NaCl. Macula densa cells were loaded with BCECF and intracellular pH was monitored using a microscope based-dual excitation photometer system. Control intracellular pH for all experiments in which tubules were initially perfused with 25 mM NaCl averaged 7.22 +/- 0.06; N = 28. Increasing luminal [NaCl] from 25 to 150 mM elevated macula densa pH by 0.15 +/- 0.03 (N = 6; P < 0.05) while increasing just luminal [Na] from 25 to 150 mM alkalinized macula densa cells by 0.17 +/- 0.05 (N = 6; P < 0.05). In addition, there was a highly significant linear relationship between luminal [Na] and intracellular pH between 25 and 150 mM NaCl. Other studies were performed to assess the effects of amiloride, an inhibitor of Na:H exchange, on macula densa intracellular pH. Addition of amiloride, to the 25 mM NaCl perfusate acidified macula densa cells by 0.09 +/- 0.03 (N = 6; P < 0.001) and significantly attenuated the increase in pH obtained when luminal [NaCl] was raised from 25 to 150 mM. Other studies evaluated the effects of inhibition of Na:2Cl:K cotransport on macula densa pH.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for coupling between Na+ pump activity and TEA-sensitive K+ currents in Xenopus laevis oocytes

Using the two-microelectrode voltage clamp technique in Xenopus laevis oocytes, we estimated Na(+)-K(+)-ATPase activity from the dihydroouabain-sensitive current (IDHO) in the presence of increasing concentrations of tetraethylammonium (TEA+; 0, 5, 10, 20, 40 mM), a well-known blocker of K+ channels. The effects of TEA+ on the total oocyte currents could be separated into two distinct parts: generation of a nonsaturating inward current increasing with negative membrane potentials (VM) and a saturable inhibitory component affecting an outward current easily detectable at positive VM. The nonsaturating component appears to be a barium-sensitive electrodiffusion of TEA+ which can be described by the Goldman-Hodgkin-Katz equation, while the saturating component is consistent with the expected blocking effect of TEA+ on K+ channels. Interestingly, this latter component disappears when the Na(+)-K(+)-ATPase is inhibited by 10 microM DHO. Conversely, TEA+ inhibits a component of IDHO with a KD of 25 +/- 4 mM at +50 mV. As the TEA(+)-sensitive current present in IDHO reversed at -75 mV, we hypothesized that it could come from an inhibition of K+ channels whose activity varies in parallel with the Na(+)-K(+)-ATPase activity. Supporting this hypothesis, the inward portion of this TEA(+)-sensitive current can be completely abolished by the addition of 1 mM Ba2+ to the bath. This study suggests that, in X. laevis oocytes, a close link exists between the Na-K-ATPase activity and TEA(+)-sensitive K+ currents and indicates that, in the absence of effective K+ channel inhibitors, IDHO does not exclusively represent the Na(+)-K(+)-ATPase-generated current.

Electrogenic amino acid exchange via the rBAT transporter

A cDNA clone was isolated from rabbit renal cortex using DNA-mediated expression cloning, which caused alanine-dependent outward currents when expressed in Xenopus oocytes. The cDNA encodes rBAT, a Na-independent amino acid transporter previously cloned elsewhere. Exposure of cDNA-injected oocytes to neutral amino acids led to voltage-dependent outward currents, but inward currents were seen upon exposure to basic amino acids. Assuming one charge/alanine, the outward current represented 38% of the rate of uptake of radiolabelled alanine, and was significantly reduced by prolonged preincubation of oocytes in 5 mM alanine. The currents were shown to be due to countertransport of basic amino acids for external amino acids using the cut-open oocyte system. This transport represents a major mode of action of this protein, and may help in defining a physiological role for rBAT in the apical membrane of renal and intestinal cells.

Basic properties and potential regulators of the apical K+ channel in macula densa cells

These studies examine the properties of an apical potassium (K+) channel in macula densa cells, a specialized group of cells involved in tubuloglomerular feedback signal transmission. To this end, individual glomeruli with thick ascending limbs (TAL) and macula densa cells were dissected from rabbit kidney and the TAL covering macula densa cells was removed. Using patch clamp techniques, we found a high density (up to 54 channels per patch) of K+ channels in the apical membrane of macula densa cells. An inward conductance of 41.1 +/- 4.8 pS was obtained in cell-attached patches (patch pipette, 140 mM K+). In inside-out patches (patch pipette, 140 mM; bath, 5 mM K+), inward currents of 1.1 +/- 0.1 pA (n = 11) were observed at 0 mV and single channel current reversed at a pipette potential of -84 mV giving a permeability ratio (PK/PNa) of over 100. In cell-attached patches, mean channel open probability (N,Po, where N is number of channels in the patch and Po is single channel open probability) was unaffected by bumetanide, but was reduced from 11.3 +/- 2.7 to 1.6 +/- 1.3 (n = 5, p < 0.02) by removal of bath sodium (Na+). Simultaneous removal of bath Na+ and calcium (Ca2+) prevented the Na(+)-induced decrease in N.Po indicating that the effect of Na+ removal on N.Po was probably mediated by stimulation of Ca2+ entry. This interpretation was supported by studies where ionomycin, which directly increases intracellular Ca2+, produced a fall in N.Po from 17.8 +/- 4.0 to 5.9 +/- 4.1 (n = 7, p < 0.02). In inside-out patches, the apical K+ channel was not sensitive to ATP but was directly blocked by 2 mM Ca2+ and by lowering bath pH from 7.4 to 6.8. These studies constitute the first single channel observations on macula densa cells and establish some of the characteristics and regulators of this apical K+ channel. This channel is likely to be involved in macula densa transepithelial Cl- transport and perhaps in the tubuloglomerular feedback signaling process.

Coupling between transepithelial Na transport and basolateral K conductance in renal proximal tubule

A common feature of sodium-reabsorbing epithelia is their ability to match salt entry to salt exit. It is recognized that a key strategy to perform this feat involves the coupling between basolateral sodium pump and potassium conductance (pump-leak coupling). In the renal proximal tubule this coupling is of major importance, as regions of this nephron segment are faced with ever-changing reabsorptive loads. An understanding of this coupling can be facilitated by critically examining those studies that have looked at the problem from the point of view of the whole cell (macroscopic studies) and of single channels (microscopic studies). An overview of such work suggests that the transduction mechanisms which are likely to effect pump-leak coupling in the renal proximal tubule involve cell volume, ATP, and pH (but not calcium). Although the relationship between ATP and potassium conductance may be relatively straightforward, the involvement of pH is likely to be only transient and that of volume remains controversial, occurring either directly though stretch-activated channels in amphibian preparations or indirectly through an as yet unidentified second messenger system in mammalian preparations.

Isolation of single mammalian proximal tubule cells: effects of hypotonic shocks on cell yield and function

Nord et al. [Am J Physiol 1986; 250:F539-F550] proposed a method to give a high yield of proximal tubule cells by exposing a suspension of rabbit cortical tubules to a hypotonic shock in calcium-free media. The present study describes the effects of both amplitude and duration of the hypotonic treatment on some transport-related characteristics of individual cells as compared to the starting tubule suspension. The averaged cell yield increased by an order of magnitude when the osmolality of the hypotonic solution was varied in four steps from 200 (C200 cells) to 70 mosm/kg H2O (C70 cells) while the proportion of trypan blue-positive cells progressively decreased from 33% for C200 cells to 9.5% for C70 cells. An increase in duration of the hypotonic shock from 0.5 to 6 min did not change the cell yield of C200 cells while it significantly increased that of C70 cells by 61%. Basal and ouabain-sensitive oxygen consumption (QO2) increased by 57 and 155%, respectively, from C70 to C200 cells but was approximately one order of magnitude smaller than the QO2 measured for tubule suspension. Intracellular ATP content averaged 5.5 +/- 0.8 nmol/mg for the starting tubule suspension, 4.6 +/- 0.8 nmol/mg for C70 cells but only 1.3 +/- 0.1 nmol/mg for C200 cells. The maximal velocity for phloridzin-sensitive alpha-methyl glucose transport averaged 13.7 +/- 1.7 nmol min-1 mg-1 for C70 cells and only 6.3 +/- 1.3 nmol min-1 mg-1 for C200 cells which is approximately one order of magnitude smaller than what can be expected from a tubule presenting a good access to luminal membrane. We conclude from these results that, in the process of isolating individual cells from a polarized epithelium, membrane transport rates have decreased by one order of magnitude and this reduction is intensified by a large hypotonic shock. In comparison with C200 cells, the cells obtained with a large hypotonic shock give a high yield, a larger proportion of trypan blue-negative cells and their lower overall transport rate allows the cells to maintain a better electrochemical gradient for Na and a higher intracellular ATP level.

Na+ pump inhibition downregulates an ATP-sensitive K+ channel in rabbit proximal convoluted tubule

In several epithelial and nonepithelial tissues a functional link between the basolateral Na(+)-K(+)-adenosinetriphosphatase (Na(+)-K(+)-ATPase) and a basolateral K+ conductance has been established. However, the nature of this link is unclear. We have previously identified a K+ channel on the basolateral membrane of the proximal convoluted tubule perfused in vitro, the activity of which is increased by stimulation of Na+ transport [J. S. Beck, A. M. Hurst, J.-Y. Lapointe, and R. Laprade. Am. J. Physiol. 264 (Renal Fluid Electrolyte Physiol. 33): F496-F501, 1993]. In the present study we investigate whether basolateral membrane K+ channel activity is tightly coupled to Na(+)-K(+)-ATPase activity. In cell-attached patches (150 mM K+ pipette), following stimulation of channel activity by addition of Na(+)-cotransported solutes to the tubule lumen, mean channel open probability (NPo) was reduced from 0.35 +/- 0.09 to 0.14 +/- 0.06 (n = 7, P < 0.05) by blocking the Na(+)-K(+)-ATPase with 100 microM strophanthidin. In excised patches the channel was reversibly blocked by 2 mM ATP from the cytosolic face of the patch, such that NPo fell to 20.1 +/- 7.0% (n = 5, P < 0.001) of control and recovered to 52.2 +/- 11.2% (n = 5, P < 0.05) after washout of ATP. Diazoxide, a putative opener of ATP-sensitive K+ channels, when added to the bathing solution of an unstimulated tubule (microperfused in the absence of Na(+)-cotransported solutes), increased NPo from 0.046 +/- 0.035 to 0.44 +/- 0.2 (n = 6, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of basolateral K channels in proximal tubule studied during continuous microperfusion

Potassium channel activity of the basolateral membrane of the collagenase-treated rabbit proximal convoluted tubule (PCT) was studied during continuous luminal microperfusion. In cell-attached patches (high-K pipette) an inwardly rectifying potassium channel was observed with an inward slope conductance of 60.8 +/- 3.3 pS (n = 12) and outward slope conductance of 17.1 +/- 2.7 pS (n = 6). Stimulation of transcellular sodium transport with luminal glucose and alanine increased channel activity [measured as single-channel open probability (NPo)] from 0.19 +/- 0.11 to 0.44 +/- 0.09 (n = 8). This increase in channel activity was not likely to be mediated by either cell depolarization or cell swelling, because channel activity was voltage insensitive over physiological potentials and because the channel was not activated by stretch. However, channel activity was pH sensitive; reducing luminal pH from 7.4 to 6.5 reduced NPo from 0.63 +/- 0.24 to 0.26 +/- 0.16 (n = 5). Our work demonstrates the feasibility of patch clamping the basolateral membrane of microperfused nephron segments. This has allowed us to follow the activity of this potassium channel during an increase in sodium transport and show that its activity does increase during this maneuver. We conclude that: 1) it is possible to patch clamp the basolateral membrane of microperfused nephron segments, and 2) basolateral membrane of the rabbit PCT contains an inwardly rectifying, pH-sensitive potassium channel. The behavior of this channel on stimulation of transcellular sodium transport could explain the macroscopic increase in basolateral potassium conductance observed under similar conditions.

Basolateral membrane potassium channels in rabbit cortical thick ascending limb

The nature of K exit across the basolateral membrane of rabbit cortical thick ascending limb (CTAL) was investigated using the patch clamp technique. The basolateral membrane was exposed by mild collagenase treatment (0.1 U/ml), and a K-selective inwardly rectifying channel was identified. In cell-attached patches (140 mM K pipette) the inward conductance was 35.0 +/- 1.3 pS (n = 9) compared with an outward conductance of 7.0 +/- 0.9 pS (n = 5), and the current reversed at a pipette potential of -63.5 +/- 3.1 mV (n = 9). The channel is strongly voltage dependent, showing an e-fold increase in open probability per 18-mV depolarization. Barium blocked the channel, reducing both mean open probability and single-channel current amplitude; however, the channel was not Ca sensitive. On excision the channel exhibited rundown, which could not be prevented by 0.1 mM ATP or ATP plus 20 U/ml catalytic subunit of protein kinase A. A few excised patch recordings were possible, which confirmed the presence of a highly K-selective channel with a K-to-Na permeability ratio of 100. In conclusion, 1) it is possible to obtain patch clamp recordings from the rabbit CTAL basolateral membrane using a very mild collagenase treatment, and 2) the exit of K across the basolateral membrane is mediated at least in part by the presence of voltage-sensitive K channels.

Basolateral ionic permeabilities of macula densa cells

It has recently been shown that membrane ionic transport pathways of macula densa cells can be measured using conventional microelectrodes. To determine if conductances could be identified at the basolateral membrane of macula densa cells, cortical thick ascending limbs (CTAL) with attached glomeruli were continuously perfused with a 25 mM NaCl bicarbonate-free Ringer solution. Individual basolateral Na+, Cl-, NaCl, and K+ concentrations were altered by isosmotic replacement with N-methyl-D-glucamine and/or cyclamate. Reduction in basolateral [Na+] from 150 to 25 mM hyperpolarized basolateral membrane potential (Vbl) by 9.9 +/- 1.3 mV (n = 10; all data are corrected for changes in liquid junction potential at bath electrode). A decrease in bath [Cl-] from 150 to 25 mM depolarized Vbl by 20 +/- 2.4 mV (n = 13), whereas decreases in bath [NaCl] from 150 to 25 mM depolarized Vbl by 29 +/- 6.8 mV (n = 5). In the presence of 150 mM NaCl bathing solution, a stepwise increase in [K+] from 5 to 15 mM (by replacement of 10 mM NaCl with 10 mM KCl) depolarized Vbl by 3.3 +/- 1.1 mV (n = 8). After correction for individual transepithelial diffusion potentials, Cl conductance averaged 59 +/- 19% of the total basolateral conductance, whereas K+ (23 +/- 8%) and Na+ (17 +/- 10%) contributed significantly less to the overall basolateral conductance. These results indicate that membrane potential of macula densa cells may be very sensitive to alterations in intracellular Cl- activity and suggest that apical transport of NaCl through a furosemide-sensitive Na(+)-K(+)-2Cl- transporter may affect membrane potential in macula densa cells via a change in intracellular Cl- activity.

Intracellular potassium activity in mammalian proximal tubule: effect of perturbations in transepithelial sodium transport

Intracellular potassium activity (alpha Ki) was measured in control conditions in mid-cortical rabbit proximal convoluted tubule using two methods: (i) by determination of the K+ equilibrium potential (EK) using Ba(2+)-induced variations in the basolateral membrane potential (VBL) during transepithelial current injections and (ii) with double-barrel K-selective microelectrodes. Using the first method, the mean VBL was -48.5 +/- 3.2 mV (n = 16) and the mean EK was -78.4 +/- 4.1 mV corresponding to alpha Ki of 68.7 mM. With K-selective microelectrodes, VBL was -36.6 +/- 1.1 mV (n = 19), EK was -64.0 +/- 1.1 mV and alpha Ki averaged 40.6 +/- 1.7 mM. While these last EK and VBL values are significantly lower than the corresponding values obtained with the first method (P less than 0.001 and P less than 0.01, respectively), the electrochemical driving force for K transport across the basolateral membrane (microK = VBL-EK) is not significantly different for both techniques (30.1 +/- 3.3 mV for the first technique and 27.6 +/- 1.8 mV for ion-selective electrodes). This suggests an adequate functioning of the selective barrel but an underestimation of VBL by the reference barrel of the double-barrel microelectrode. Such double-barrel microelectrodes were used to measure temporal changes in alpha Ki and microK in different experimental conditions where Na reabsorption rate (JNa) was reduced. alpha Ki was shown to increase by 12.2 +/- 2.7 (n = 5) and 14.1 +/- 4.4 mM (n = 5), respectively, when JNa was reduced by omitting in the luminal perfusate: (i) 5.5 mM glucose and 6 mM alanine and (ii) glucose, alanine, other Na-cotransported solutes and 110 mM Na. In terms of the electrochemical driving force for K exit across the basolateral membrane, microK, a decrease of 5.4 +/- 2.0 mV (P less than 0.05, n = 5) was measured when glucose and alanine were omitted in the luminal perfusate while microK remained unchanged when JNa was more severely reduced (mean change = -1.7 +/- 2.1 mV, NS, n = 5). In the latter case, this means that the electrochemical driving force for K efflux across the basolateral membrane has not changed while both the active influx through the Na-K pump and the passive efflux in steady state are certainly reduced. If the main pathway for K transport is through the basolateral K conductance, this implies that this conductance must have decreased in the same proportion as that of the reduction in the Na-K pump activity.

Regulation of basolateral membrane potential after stimulation of Na+ transport in proximal tubules

We have previously shown that stimulation of apical Na-coupled glucose and alanine transport produces a transient depolarization of basolateral membrane potential (Vbl) in rabbit proximal convoluted tubule (PCT, S1 segment). The present study is aimed at understanding the origin of the membrane repolarization following the initial effect of addition of luminal cotransported solutes. Luminal addition of 10-15 mM L-alanine produced a rapid and highly significant depolarization of Vbl (20.3 +/- 1.1 mV, n = 15) which was transient and associated with an increase in the fractional K+ conductance of the basolateral membrane (tK) from 8 to 29% (P less than 0.01, n = 6). Despite the significant increase in tK, the repolarization was only slightly reduced by the presence of basolateral Ba2+ (2 mM, n = 6) or quinine (0.5 mM, n = 5). The repolarization was greatly reduced in the presence of 0.1 mM 4-acetamino-4'isothiocyamostilbene-2,2'-disulfonic acid (SITS) and blunted by bicarbonate-free solutions. Intracellular pH (pHi) determined with the fluorescent dye 2',7'-bis-2-carboxyethyl-5(and -6)-carboxyfluorescein (BCECF), averaged 7.39 +/- 0.02 in control solution (n = 9) and increased to 7.50 +/- 0.03 in the first 15 sec after the luminal application of alanine. This was followed by a significant acidification averaging 0.16 +/- 0.01 pH unit in the next 3 min. In conclusion, we believe that, contrary to other leaky epithelia, rabbit PCT can regulate its basolateral membrane potential not only through an increase in K+ conductance but also through a cellular acidification reducing the basolateral HCO3- exit through the electrogenic Na-3(HCO3) cotransport mechanism.

Direct evidence for apical Na+:2Cl-:K+ cotransport in macula densa cells

Previous studies by our laboratory indicate that increases in apical NaCl concentration ([NaCl]) depolarize macula densa (MD) cells, although the mechanism for apical NaCl transport was not identified. To determine the pathway for MD apical NaCl transport, we utilized microdissected cortical thick ascending limbs (CTAL) with attached glomeruli and conventional microelectrode techniques. Addition of 50 microM furosemide in the presence of 150 mM NaCl produced a variable hyperpolarization of basolateral membrane voltage (delta Vbl, -14 +/- 8.2 mV, NS P = 0.15, n = 6) and completely blocked the expected repolarization on reducing luminal [NaCl] from 150 to 25 mM. Addition of furosemide in the presence of 25 mM NaCl depolarized Vbl by 22 +/- 6.8 mV (P less than 0.05, n = 6) indicating that the direction of the NaCl transport can be reversed in low luminal [NaCl]. In other studies, luminal concentration of Na or Cl was increased from 25 to 150 mM. Increased [Na] produced a 6.9 +/- 1.2 mV (n = 9) depolarization, whereas Cl addition depolarized Vbl by 8.2 +/- 1.7 mV (n = 5), suggesting that both ions are involved in the NaCl-induced MD depolarization. Removal of K from the luminal perfusate elicited a hyperpolarization of -14 +/- 2.9 mV (n = 9). These results are all consistent with the existence of an apical Na+:2Cl-:K+ transporter that would result in NaCl reabsorption in the presence of 150 mM luminal NaCl but would produce NaCl secretion at low luminal NaCl concentrations.

Membrane crosstalk in the mammalian proximal tubule during alterations in transepithelial sodium transport

The present paper examines the effects of reduced transepithelial Na transport (JNa) on membrane electrophysiological parameters in proximal convoluted tubules and the possible role of cytosolic calcium concentration ([Ca]i) in the regulation of basolateral membrane K conductance (GK). When JNa was reduced by elimination of glucose and alanine and replacement of 100 mM sodium with N-methyl-D-glucamine from the luminal perfusate, basolateral membrane potential (VBL) hyperpolarized transiently by 12.6 mV and the ratio of apical to basolateral membrane resistance (RA/RBL) doubled. The apparent transference number for K at the basolateral membrane (GK/Gcell) decreased from 0.13 to 0.08 in the first 4 min following reductions in JNa. The elimination of Na-alanine and Na-glucose cotransport was responsible for the initial hyperpolarization and increase in RA/RBL, whereas the resultant decrease in the cellular concentrations of glucose and alanine, together with the reductions in GK, could elicit the secondary VBL depolarization. Measurement of [Ca]i with the fluorescent probe fura-2 during reductions in JNa revealed that [Ca]i increased by an average of 12%, a value very similar to the average reduction in cellular volume (13%) measured using morphometric techniques. The observation that [Ca]i increased while GK was decreasing is inconsistent with the effect of [Ca]i on putative basolateral Ca-activated K channel. We believe that [Ca]i changes passively (at least in the first few minutes) in response to a decrease in cell volume occurring as a consequence of reductions in JNa and that some as yet unidentified volume-sensitive mechanism is responsible for the regulation of GK.

Direct measurement of basolateral membrane potentials from cells of the macula densa

At the present time, little is known concerning the electrophysiology of the cells of the macula densa and whether or not these cells are electrically responsive to alterations in luminal fluid composition. To investigate this issue, cortical thick ascending limbs (CTAL) containing macula densa and attached glomeruli were dissected from rabbit kidney and the CTAL perfused in vitro. Basolateral membrane potential (Vbl) was measured with microelectrodes in macula densa cells and, for comparison, in cells of the CTAL. Macula densa Vbl averaged -56.5 +/- 7.6 mV (n = 4) at a (n = 22) at 20 mM NaCl, -35.6 +/- 3.9 mV (n = 16) at 45 mM NaCl, and -25.5 +/- 2.6 mV (n = 32) at 150 mm NaCl. Thus macula densa Vbl depolarized markedly (31 mV) when luminal perfusate [NaCl] was increased from low to high values. In contrast, Vbl measured in CTAL cells averaged -62 +/- 6.1 mV (n = 6) in 45 mM NaCl and did not change significantly as perfusate NaCl was increased to 150 mM. In the presence of 150 mM NaCl, luminal application of furosemide (50 microM) produced a small (3.5 +/- 1.1 mV, n = 16) but statistically significant (P less than 0.02) hyperpolarization in macula densa cells, whereas CTAL cell Vbl hyperpolarized markedly (20 +/- 5.7 mV, n = 6) with addition of furosemide. Finally, neither macula densa cells nor the CTAL cells changed Vbl when 45 mM NaCl solution was made hypotonic by removing mannitol.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel holder allowing internal perfusion of patch-clamp pipettes

We describe a simple pipette holder which allows, within a single experiment, multiple exchanges of the solution inside "gigaseal" glass pipettes commonly used for electrical studies of single cells or isolated membrane patches. The design minimizes electrostatic and mechanical perturbations associated with perfusion by integrating into the holder a reservoir which is connected to a perfusion pipette fabricated from flexible, resilient quartz tubing. The tip of the perfusion pipette can be pulled to any diameter and positioned precisely within the main patch-pipette by sliding the reservoir along a guide in the holder. An open reservoir for suction driven solution exchange, and a closed reservoir for pressure driven solution exchange were developed. For the open system, the speed of solution exchange was studied as a function of the tip diameter of the perfusion pipette (approximately 22 s for a 40 micron tip diameter). Both systems were characterized using atrial myocytes (a) by examining the effects of intracellular applications of cAMP or of the catalytic subunit of protein kinase A on calcium currents in the whole cell recording mode and (b) by studying the effects of local applications of acetylcholine (ACh) on single channel currents in the isolated membrane patch mode.

Current-voltage relations of sodium-coupled sugar transport across the apical membrane of Necturus small intestine

The current-voltage (I-V) relations of the rheogenic Na-sugar cotransport mechanism at the apical membrane of Necturus small intestine were determined from the relations between the electrical potential difference across the apical membrane, psi mc, and that across the entire epithelium, psi ms, when the latter was varied over the range +/- 200 mV, under steady conditions in the presence of galactose and after the current across the apical membrane carried by the cotransporter, ImSNa, is blocked by the addition of phloridzin to the mucosal solution. ImSNa was found to be strongly dependent upon psi mc over the range -50 mV less than psi mc less than EmSNa where EmSNa is the "zero current" or "reversal" potential. Over the range of values of psi mc encountered under physiological conditions the cotransporter may be modeled as a conductance in series with an electromotive force so that ImSNa = gmSNa (EmSNa - psi mc) where gmSNa is the contribution of this mechanism to the conductance of the apical membrane and is "near constant." In several instances ImSNa "saturated" at large hyperpolarizing or depolarizing values of psi mc. The values of EmSNa determined in the presence of 1, 5, and 15 mM galactose strongly suggest that if the Na-galactose cotransporters are kinetically homogeneous, the stoichiometry of this coupled process is unity. Finally, the shapes of the observed I-V relations are consistent with the predictions of a simple kinetic model which conforms with current notions regarding the mechanico-kinetic properties of this cotransport process.

Characterization of the apical membrane ionic permeability of the rabbit proximal convoluted tubule

Basolateral membrane potential (psi BL), transepithelial potential (psi T), and the ratio of apical to basolateral membrane resistance (RA/RBL) were measured in rabbit proximal convoluted tubules (PCT) perfused in vitro. Analysis of RA/RBL changes using several luminal perfusates indicates that the cotransport of Na with glucose and alanine would represent 19% of the apical conductance in normal conditions; the cotransport of Na with acetate, citrate, sulfate, and phosphate would represent 7%, whereas Na, K, and Cl diffusion would represent 10, 4, and 0% of this apical conductance, respectively. On the other hand, psi BL values can also be analyzed using the equivalent circuit of the epithelium to obtain the apical membrane equivalent electromotive force (EA) in the presence of each perfusate. These values, as well as the preceding values obtained from RA/RBL measurements, indicate that in the absence of cotransported solutes the transference number for Na diffusion is several times larger than for K diffusion. Among the conductance pathways studied, the transference number sequence would be as follows: Na cotransport with alanine and glucose greater than Na cotransport with anions greater than Na diffusion greater than K diffusion greater than Cl diffusion. This study also suggests the presence of another important but unidentified apical ionic permeation pathway, since the total of the transference numbers obtained from RA/RBL analysis represents only 40% of the total apical membrane conductance and the absolute values of EA are difficult to account for using only the tested apical membrane permeation pathways.

Electrophysiological studies of sodium cotransport in epithelia: toward a cellular model

During the past two decades, microelectrophysiological studies of small intestine and renal proximal tubule employing conventional as well as ion-selective microelectrodes have contributed significantly to our understanding of the nature of Na-coupled entry processes at the apical membrane as well as the overall workings of the simple model illustrated in FIGURE 1. These studies have unequivocally established the rheogenic and conductive nature of the Na-coupled sugar and amino-acid entry processes across the apical membrane of small intestine (and renal proximal tubule) and have, in addition, disclosed that the properties of the basolateral membrane respond to an increase in Na-coupled solute entry with an increase in the ability of the Na-K pump to extrude Na with little or no change in (Na)c32 and a parallel increase in the conductance of that barrier to K. Although these responses may be "triggered" by cell swelling, it is unclear how a cell "recognizes" minimal swelling and how this recognition, in turn, culminates in the observed changes in basolateral membrane pump-leak properties. Clearly, these findings have brought us to the interfaces between cell physiology and cell and molecular biology and have raised a number of intriguing questions that focus on the more global question: How do epithelial cells work?

Transepithelial and cell membrane electrical resistances of the rabbit proximal convoluted tubule

A technique using double-barreled perfusion pipettes and intracellular microelectrodes was developed to measure transepithelial, apical, and basolateral membrane electrical resistances in isolated rabbit proximal convoluted tubules (PCT). This technique has been tested successfully with respect to cable analysis: the transepithelial resistance (RT) did not change with tubule length and the measured core resistance of the lumen (RC) varied according to prediction with lumen diameter and perfusate resistivity. In control solutions, a linear I-V relationship was observed at the entry of the tubule for current varying from -300 to +300 nA. The mean RT was 1,050 +/- 70 omega X cm (n = 33) (a specific resistance of 8.2 omega X cm2). Bath proteins and large variations in transtubular hydrostatic pressure had no significant effect on RT, whereas RT was not systematically related to transepithelial PD or to the sodium-to-chloride permeability ratio (n = 22). Perfusate substitution of 50 mM NaCl by mannitol increased RT by 21% (n = 7) but the same maneuver in the peritubular solution had no significant effect after a 5-min equilibration period. The ratio of apical to basolateral cell membrane resistance (RA/RBL) determined with intracellular microelectrodes was 3.1 +/- 0.3 (n = 27) in control solutions and increased within 1 min by 36% (n = 8) when glucose and alanine were replaced by mannitol in the perfusate solution. Using simultaneous initial changes in transepithelial and basolateral potential differences when glucose and alanine were removed, the individual values of RA and RBL were determined. Mean RBL was 4,900 +/- 990 omega X cm (39 +/- 1.3 omega X cm2) and mean RA was 15,000 +/- 4,300 omega X cm (118 +/- 33 omega X cm2).

Luminal and peritubular ionic substitutions and intracellular potential of the rabbit proximal convoluted tubule

Transepithelial (psi T) and basolateral (psi BL) potential difference was measured in rabbit proximal convoluted tubules perfused in vitro. In control solution without protein, the mean psi BL was -54 +/- 2.2 mV (n = 57). Luminal substitution of K by Na had no effect. Complete luminal substitution of glucose and alanine, 110 mM substitution of Na or NaCl produced transient hyperpolarizations of psi BL of 14, 10, and 13 mV, respectively, with a return close to the control value within 4-8 min in all cases. Returning to control solution produced similar time-course transient depolarizations of psi BL of 17, 11, and 16 mV, respectively, again with a return to the control value in 4-10 min. Omission of glucose and alanine in the perfusate produced a decrease in cell volume of 14% that was maximal in 4 min with a complete recovery in the post-control period. A 110 mM luminal or peritubular substitution of Cl by cyclamate produced no significant effect on psi BL after taking into account the large psi T generated by the diffusion of Cl across the paracellular pathway. On the other hand, complete peritubular substitution of K by Na and 110 mM substitution of Na or NaCl produced sustained but reversible depolarizations of psi BL of 37.5, 10.2, and 20.4 mV, respectively. The transient nature of the hyperpolarization following luminal substitution of glucose, alanine, or Na can be interpreted in terms of changes in the intracellular sodium activity that would affect the Na-K-ATPase pump. Similarly, the sustained depolarization seen after a peritubular substitution of K and Na would also be compatible with a decrease in the basolateral ionic pump activity.

Effects of variation of ion and methylation of carrier on the rate constants of macrotetralide-mediated ion transport in lipid bilayers

The effects of methylation on the rate constants of carrier-mediated ion transport have been studied on monooleindecane bilayers with K+, Rb+, NH4+, and Tl+ ions, using the series of homologue carriers, nonactin, monactin, dinactin, trinactin, and tetranactin, each member of the series differing from the previous one by only one methyl group. Measurements of the amplitude and time constant of the current relaxation after a voltage jump over a large domain of voltage and permeant ion concentration, together with a computer curve-fitting procedure, have allowed us, without the help of steady-state current-voltage data, to deduce and compare the values of the various rate constants for ion transport: formation (kRi) and dissociation (kDi) of the ion-carrier complex at the interface, translocation across the membrane interior of the carrier (ks) and the complex (kis). With the additional information from steady-state low-voltage conductance measurements, we have obtained the value of the aqueous phase-membrane and torus-membrane partition coefficient of the carrier (gammas and gammas). From nonactin to tetranactin with the NH4+ ion, kis, and gammas are found to increase by factors of 5 and 3, respectively, kDi and gammas to decrease respectively by factors 8 and 2, while kRi and ks are practically invariant. Nearly identical results are found for K+, Rb+, and Tl+ ions. kRi, ks and kis are quite invariant from one ion to the other except for Tl+ were kRi is about five times larger. On the other hand, kDi depends strongly on the ion, indicating that dissociation is the determining step of the ionic selectivity of a given carrier. The systematic variations in the values of the rate constants with increasing methylation are interpreted in terms of modification of energy barriers induced by the carrier increasing size. Within this framework, we have been able to establish and verify a fundamental relationship between the variations of kis and kDi with methylation.