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.