Na+-coupled sugar transport: membrane potential-dependent Km and Ki for Na+

Degree of SGLT1 phosphorylation is associated with but does not determine segment-specific glucose transport features in the porcine small intestines

Glucose‐induced electrogenic ion transport is higher in the porcine ileum compared with the jejunum despite equal apical abundance of SGLT1. The objective of this study was a detailed determination of SGLT1 and GLUT2 expressions at mRNA and protein levels along the porcine small intestinal axis. Phosphorylation of SGLT1 at serine 418 was assessed as a potential modulator of activity. Porcine intestinal tissues taken along the intestinal axis 1 h or 3 h after feeding were analyzed for relative mRNA (RT‐PCR) and protein levels (immunoblot) of SGLT1, pSGLT1, GLUT2, (p)AMPK, β₂‐receptor, and PKA substrates. Functional studies on electrogenic glucose transport were done (Ussing chambers: short circuit currents (I_sc)). Additionally, effects of epinephrine (Epi) administration on segment‐specific glucose transport and pSGLT1 content were examined. SGLT1 and GLUT2 expression was similar throughout the small intestines but lower in the duodenum and distal ileum. pSGLT1 abundance was significantly lower in the ileum compared with the jejunum associated with significantly higher glucose‐induced I_sc. SGLT1 phosphorylation was not inducible by Epi. Epi treatment decreased glucose‐induced I_sc and glucose flux rates in the jejunum but increased basal I_sc in the ileum. Epi‐induced PKA activation was detectable in jejunal tissue. These results may indicate that SGLT1 phosphorylation at Ser418 represents a structural change to compensate for certain conditions that may decrease glucose transport (unfavorable driving forces/changed apical membrane potential) rather than being the cause for the overall differences in glucose transport characteristics between the jejunum and ileum.

Mechanism of bradykinin-induced Ca2+ mobilization in murine proximal tubule epithelial cells

Despite the recognized physiological role of bradykinin (BK) in the kidney in maintaining glomerular and tubule function and its role in pathological states such as endotoxemia, diabetes, and other diseases, relatively little is known about the mechanisms by which BK can impact kidney function. Furthermore, the signaling of BK receptors in the murine nephron has not been fully characterized. The present studies were undertaken to examine BK-stimulated Ca(2+) signaling using Fura-2 in the murine proximal tubule epithelial cell line TKPTS. BK produced a concentration-dependent rise in intracellular Ca(2+) ([Ca(2+)])(i) (pEC(50) = 8.39 +/- 0.04). Selective antagonists showed the rise in [Ca(2+)](i) was mediated through B2 receptors. The rise in [Ca(2+)](i) was rapid and reversible and was maximally stimulated at 1 microM (697 +/- 70 nM above basal level of 115 +/- 6 nM). Studies with thapsigargin and EGTA showed Ca(2+) mobilization was dependent on two events: release and influx. Both U73122 (1-[6-[[17-beta-3-methoxyestra-1,3,5(10)-trien-17-yl]amino]hexyl]-1H-pyrrole-2,5-dione) [a phospholipase C (PLC) inhibitor] and genistein (a tyrosine kinase inhibitor) partially inhibited BK-stimulated rise in [Ca(2+)](i). When combined, both agents produced a further decrease, suggesting multiple pathways for PLC activation may be involved. The ability of Ni(2+) to inhibit influx indicated the activation of a Ca(2+) release-activated channel (CRAC). Ca(2+) mobilization did not seem to be affected by cyclic nucleotides or protein kinase C. In summary, the TKPTS murine proximal tubule cell line expresses functional B2 receptors linked to Ca(2+) mobilization that is dependent on phospholipase C and activation of CRAC.

Protein kinase C activation downregulates human organic anion transporter 1-mediated transport through carrier internalization

Organic anion transport in intact renal proximal tubule cells in animal model systems is downregulated by treatments that activate protein kinase C (PKC). How this downregulation is achieved is not yet known. Stimulation of PKC with sn-1,2-dioctanoylglycerol resulted in strong inhibition of p-aminohippurate transport mediated by the cloned human organic anion transporter 1 (hOAT1) expressed in Xenopus oocytes and HEK293 cells, as well as hOAT1 internalization in both expression systems. The sn-1,2-dioctanoylglycerol-induced transport inhibition was partially prevented by staurosporine. It was independent of the conserved canonical PKC consensus sites in hOAT1, however, and was unaffected by agents that destabilize actin filaments or microtubules, which altered baseline hOAT1-mediated p-aminohippurate uptake activity in oocytes. It is concluded that PKC-induced hOAT1 downregulation is achieved through carrier retrieval from the cell membrane and does not involve phosphorylation of the predicted classic hOAT1 PKC consensus sites.

Epstein-Barr virus infection of renal proximal tubule cells: possible role in chronic interstitial nephritis

Chronic interstitial nephritis frequently accompanies renal diseases of different etiologies. Far less common is the entity of primary interstitial nephritis wherein the glomerular and vascular structures of the kidney are not the primary focus of the disease process. Using in situ hybridization and the polymerase chain reaction, we detected DNA from the Epstein-Barr Virus (EBV) exclusively in renal tissue of patients with the idiopathic variety of chronic interstitial nephritis. The EBV genome, but not that of cytomegalovirus or adenovirus, was detected primarily in renal proximal tubule cells. Furthermore, the CD21 antigen, which serves as the receptor for EBV in B lymphocytes, was detected by immunocytochemistry primarily on proximal tubule cells and was markedly upregulated in the EBV-infected tissue. Western blot analysis of primary cultures of normal proximal tubule cells identified a 140-kDa protein, confirming the expression of the CD21 antigen. Colocalization experiments using proximal and distal tubule markers confirmed that EBV DNA and the CD21 antigen are found primarily in proximal tubule cells. EBV infection of renal proximal tubular cells may participate in evoking a cellular immune response that results in a damaged renal interstitium.

Voltage and cosubstrate dependence of the Na-HCO3 cotransporter kinetics in renal proximal tubule cells

The voltage dependence of the kinetics of the sodium bicarbonate cotransporter was studied in proximal tubule cells. This electrogenic cotransporter transports one Na+, three HCO3-, and two negative charges. Cells were grown to confluence on a permeable support, mounted on a Ussing-type chamber, and permeabilized apically to small monovalent ions with amphotericin B. The steady-state, di-nitro-stilbene-di-sulfonate-sensitive current was shown to be sodium and bicarbonate dependent and therefore was taken as flux through the cotransporter. Voltage-current relations were measured as a function of Na+ and HCO3- concentrations between -160 and +160 mV under zero-trans and symmetrical conditions. The kinetics could be described by a Michaelis-Menten behavior with a Hill coefficient of 3 for HCO3- and 1 for Na+. The data were fitted to six-state ordered binding models without restrictions with respect to the rate-limiting step. All ordered models could quantitatively account for the observed current-voltage relationships and the transinhibition by high bicarbonate concentration. The models indicate that 1) the unloaded transporter carries a positive charge; 2) the binding of cytosolic bicarbonate to the transporter "senses" 12% of the electric field in the membrane, whereas its translocation across the membrane "senses" 88% of the field; 3) the binding of Na+ to the cotransporter is voltage independent.

Reduction of an eight-state mechanism of cotransport to a six-state model using a new computer program

A computer program was developed to allow easy derivation of steady-state velocity and binding equations for multireactant mechanisms including or without rapid equilibrium segments. Its usefulness is illustrated by deriving the rate equation of the most general sequential iso ordered ter ter mechanism of cotransport in which two Na+ ions bind first to the carrier and mirror symmetry is assumed. It is demonstrated that this mechanism cannot be easily reduced to a previously proposed six-state model of Na+-D-glucose cotransport, which also includes a number of implicit assumptions. In fact, the latter model may only be valid over a restricted range of Na+ concentrations or when assuming very strong positive cooperativity for Na+ binding to the glucose symporter within a rapid equilibrium segment. We thus propose an equivalent eight-state model in which the concept of positive cooperativity is best explained within the framework of a polymeric structure of the transport protein involving a minimum number of two transport-competent and identical subunits. This model also includes an obligatory slow isomerization step between the Na+ and glucose-binding sequences, the nature of which might reflect the presence of functionally asymmetrical subunits.

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."

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.

The molecular mechanism and potential dependence of the Na+/glucose cotransporter

Activity of the Na+/glucose cotransporter endogenously expressed in LLC-PK1 cells was measured using whole cell recording techniques under three different sodium concentration conditions: 1) externally saturating, zero trans; 2) 40 mM external, zero trans; and 3) externally saturating, 50 mM trans. Activity of the transporter with increasing concentrations of sugar was measured for each set of conditions, from which the maximal current for saturating sugar, Im, was determined. The Im measured shows substantial potential dependence for each set of conditions. The absolute Im and the relative potential dependence of Im compared among the various solute conditions were used to identify which loci in the transport cycle are responsible for potential-dependent changes in function. The experimental data were compared with the predicted Im values calculated from an eight-state, sequential, reversible model of a transport reaction kinetic scheme. Predictions derived from assignment of rate limitation and/or potential dependence to each of the 16 transitions in the transport pathway were derived and compared with the measured data. Most putative models were dismissed because of lack of agreement with the measured data, indicating that several steps along the transport pathway are not rate limiting and/or not potential dependent. Only two models were found that can completely account for the measured data. In one case, translocation of the free carrier must be rate limiting, and both extracellular sodium-binding events as well as translocation of both free and fully loaded carrier forms must be potential-dependent transitions. In the second case, translocation of the free carrier and dissociation of the first sodium to be released intracellularly must be equivalently rate limiting. In this case only the two translocation events are required to be potential dependent. The two external sodium-binding events might still be potential dependent, but this is not required to fit the data. Previous reports suggest that the first model is correct; however, no direct experimental data compel us to dismiss the second option as a feasible model.

A multi-substrate single-file model for ion-coupled transporters

Ion-coupled transporters are simulated by a model that differs from contemporary alternating-access schemes. Beginning with concepts derived from multi-ion pores, the model assumes that substrates (both inorganic ions and small organic molecules) hop a) between the solutions and binding sites and b) between binding sites within a single-file pore. No two substrates can simultaneously occupy the same site. Rate constants for hopping can be increased both a) when substrates in two sites attract each other into a vacant site between them and b) when substrates in adjacent sites repel each other. Hopping rate constants for charged substrates are also modified by the membrane field. For a three-site model, simulated annealing yields parameters to fit steady-state measurements of flux coupling, transport-associated currents, and charge movements for the GABA transporter GAT1. The model then accounts for some GAT1 kinetic data as well. The model also yields parameters that describe the available data for the rat 5-HT transporter and for the rabbit Na(+)-glucose transporter. The simulations show that coupled fluxes and other aspects of ion transport can be explained by a model that includes local substrate-substrate interactions but no explicit global conformational changes.

Oleic acid inhibition of Na+/D-glucose transport in isolated renal brush-border membranes: role of lipid physical parameters and trans Na(+)-inhibition

Inhibition of Na+/D-glucose transport by oleic acid was investigated in renal brush-border membrane vesicles (BBMV). Lipid physical parameters were determined by spectrofluorometry. cis-Unsaturated C16-C22 long-chain fatty acids (LCFA) as oleic acid reduced nonzero limiting anisotropy r infinity with DPH and 12-AS as probes and decreased rotational correlation time phi of 12-AS. At 8 s and 15 s Na+/D-glucose transport was competitively inhibited. A positive correlation existed between decrease in r infinity (acyl chain order) or decrease in rotational correlation time phi (= increase in 'fluidity') and inhibition of Na+/D-glucose transport. Except elaidic acid trans unsaturated and saturated LCFA had no effect on fluorescence anisotropy and Na+/D-glucose transport. Per cent transport inhibition was unaffected by 0 voltage clamping and by FCCP. Ki for trans Na(+)-inhibition of D-glucose transport was 29 mmol/l. Na(+)-transport was stimulated by oleic acid, exceeding the Ki value for trans Na+ inhibition.

CONCLUSION

oleic acid inhibits Na+/D-glucose transport by a decrease in lipid acyl chain order and an increase in 'fluidity', by trans Na(+)-inhibition and presumably by a third unknown mechanism.

Nucleotides mobilize intracellular calcium stores of renal proximal cells in primary culture: existence of a suramin-sensitive mechanism

Changes of intracellular calcium concentrations [Ca2+]i were measured in primary cultured rabbit proximal convoluted tubules (PCT). A dual-excitation, digital-imaging inverted microscope was used to monitor the fura-2 fluorescence. The basal calcium level was 106 +/- 11 nM (n = 36). The stimulatory effects of adenosine triphosphate (ATP), adenosine diphosphate (ADP) and adenosine were studied. ATP and ADP induced transient increases of [Ca2+]i (1059 +/- 115% of the resting level (n = 29), and 659 +/- 134% (n = 10), respectively) by releasing calcium from cytoplasmic stores. Adenosine had less effect (279 +/- 48% of the resting level, n = 3). In the same conditions the ATP antagonist suramin (100 microM) inhibited the action of ATP and ADP to 231 +/- 52% (n = 3), and 308 +/- 29% (n = 4) of the resting level, respectively, but did not modify that of adenosine (281 +/- 72%, n = 3). A pretreatment (500 ng/ml for 2 h at 37 degrees C) of the culture with the toxin of Bordetella pertussis completely blocked the ATP response. Our results are evidence for the presence of a functional suramin-sensitive ATP and ADP puriceptor in cultured renal proximal cells. A pertussis-toxin-sensitive G protein is linked to the transduction mechanism. This receptor is distinct from an adenosine puriceptor also found in the proximal monolayer.

Hepatocyte growth factor stimulates phosphoinositide hydrolysis and mitogenesis in cultured renal epithelial cells

Hepatocyte growth factor (HGF), a novel heparin-binding peptide growth factor of MW 97-kDa, is a potent mitogen for parenchymal hepatocytes. HGF is present in normal serum and increases following liver injury or partial hepatectomy. In addition to liver, HGF mRNA has been detected in kidney. In cultured rabbit proximal tubule cells, recombinant human HGF (10(-10) M) increased DNA synthesis, measured as [3H] thymidine incorporation, from 1345 +/- 213 to 2931 +/- 636 cpm/10(6) cells; n = 9; p < 0.005). HGF was found to exert mitogenic effects at lower concentrations than epidermal growth factor (EGF), with half maximal effects seen at 6 x 10(-11) M compared to 7 x 10(-10) M for EGF. HGF was additive with EGF in stimulating [3H] thymidine incorporation. In addition to rabbit proximal tubule cells, HGF increased proliferation in a cultured mouse proximal tubule cell line, MCT, and in rat glomerular epithelial cells. In contrast, HGF did not stimulate proliferation of either rat mesangial cells or a rat aortic smooth muscle cell line, A7r5. The HGF receptor is the product of the c-met proto-oncogene. C-met mRNA was detected in total kidney and in cultured proximal tubule cells but was not detected in cultured mesangial cells. In contrast, HGF mRNA was detected in mesangial cells but not in cultured proximal tubule cells. Preincubation of rabbit proximal tubule cells with the tyrosine kinase inhibitor, genistein (50 microM), prevented HGF-stimulation of [3H] thymidine incorporation. In LiCl pretreated rabbit proximal tubule cells loaded with [3H] myoinositol, HGF increased total inositol phosphate release, measured by anion exchange chromatography (control: 2181 +/- 414 vs HGF: 2609 +/- 478 cpm/10(6) cells; n = 6; p < 0.05). Although genistein did not affect baseline phosphoinositide hydrolysis, it inhibited the HGF stimulation. Thus, HGF is mitogenic for cultured proximal tubule cells as well as glomerular epithelial cells. Inhibition of proliferation and PI turnover by genistein suggests that HGF's actions are mediated in part by tyrosine kinase activity. In mammalian kidney, HGF released from mesangial cells may serve as a paracrine activator of the adjacent epithelial cells.

Heterogeneity in the effects of membrane potentials on pantothenate and glucose uptakes by rabbit renal apical membranes

1. Previous studies using renal brush-border membrane vesicles have established that both the pantothenate and the low Km (Michaelis-Menten constant), low Vmax (maximal rate) D-glucose systems have a stoichiometry of 2 Na+: 1 organic molecule. In this study, we compared the mechanisms by which the membrane potential energizes pantothenate and D-glucose uptakes by brush-border membrane vesicles isolated from the whole cortex of rabbit kidney. 2. In the absence of Na+, varying the membrane potential from +60 to -60 mV decreased pantothenate uptake, whereas D-glucose uptake was increased in a linear manner. These results suggested the existence of a conductive pathway for pantothenate in these membranes. They also suggested that the pantothenate free carrier is electroneutral, while the glucose free carrier is negatively charged. 3. In the presence of an inwardly directed Na+ gradient, varying the membrane potential from +60 to -60 mV increased Na(+)-dependent pantothenate influx linearly. In contrast, a shift from +60 to +40 mV in the membrane potential had no influence on Na(+)-dependent D-glucose influx, whereas influx was a linear function of the membrane potential from +40 to -60 mV, indicating that there is a threshold membrane potential required for membrane potential-dependent D-glucose movement to occur. 4. Kinetic studies revealed that the effect of membrane potential on pantothenate uptake is through changes in the Km, while Vmax was unchanged. On the other hand, the membrane potential exerted its effect on D-glucose transport solely on the Vmax. 5. Finally, binding studies revealed that membrane potential, both in the presence and absence of a Na+ gradient, elicited effects on phlorizin binding qualitatively similar to those observed for D-glucose transport. 6. Implications of these findings for tubular regulation of these electrogenic secondary active transport systems are discussed.

Calcium-dependent control of volume regulation in renal proximal tubule cells: I. Swelling-activated Ca2+ entry and release

The mechanism of Ca(2+)-dependent control of hypotonic cell volume regulation was investigated in the isolated, nonperfused renal proximal straight tubule. When proximal tubules were exposed to hypotonic solution with 1 mM Ca2+, cells swelled rapidly and then underwent regulatory volume decrease (RVD). This treatment resulted in an increase in intracellular free calcium concentration ([Ca2+]i) by a mechanism that had two phases: the first was a transient increase from baseline (136 nM) to a peak (413 nM) that occurred in the first 15-20 sec, but was followed by a rapid decay toward the pre-swelling levels. The second phase was characterized by a sustained elevation of [Ca2+]i above the baseline (269 nM), which was maintained over several minutes. The dependence of these two phases on extracellular Ca2+ was determined. Reduction of bath [Ca2+] to 10 or 1 microM partially diminished the transient phase, but abolished the sustained phase completely, such that [Ca2+]i fell below the baseline levels during RVD. It was concluded that the transient increase resulted predominantly from swelling-activated release of intracellular Ca2+ stores and that the sustained phase was due to swelling-activated Ca2+ entry across the plasma membrane. Ca2+ entry probably also contributed to the transient increase in [Ca2+]i. The time dependence of swelling-activated Ca2+ entry was also investigated, since it was previously shown that RVD was characterized by a "calcium window" period (less than 60 sec), during which extracellular Ca2+ was required. Outside of this time period, RVD would inactivate and could not be reactivated by subsequent addition of Ca2+. It was found that the Ca2+ permeability did not inactivate over several minutes, indicating that the temporal dependence of RVD on extracellular Ca2+ is not due to the transient activation of a Ca2+ entry pathway.

Calcium-dependent control of volume regulation in renal proximal tubule cells: II. Roles of dihydropyridine-sensitive and -insensitive Ca2+ entry pathways

The Ca2+ entry pathways in the basolateral plasma membrane of the isolated, nonperfused proximal straight tubule (PST) of rabbit kidney were investigated using fura-2 fluorescence microscopy. Under isotonic conditions, reduction of bath [Ca2+] from 1 mM to 1 microM caused intracellular free calcium concentration ([Ca2+]i) to fall close to zero. Treatment with 10 microM verapamil, a calcium channel blocker, had a similar effect. Treatment with verapamil or low Ca2+ also induced fluctuations in cell volume. However, isotonic treatment with 10 microM nifedipine, a dihydropyridine (DHP)-type calcium channel blocker, did not affect [Ca2+]i or cell volume, indicating that the endogenous Ca2+ entry pathway is verapamil-sensitive but DHP-insensitive. When cells were exposed to hypotonic solutions in the presence of 1 mM Ca2+, they swelled and underwent normal RVD while [Ca2+]i increased transiently to a peak before decreasing to a late phase plateau level above the baseline level (see McCarty, N.A., O'Neil, R.G. 1991. J. Membrane Biol. 123:149-160). When cells were swollen in the presence of verapamil or low bath [Ca2+], RVD was abolished and [Ca2+]i fell well below the baseline during the late phase response. In contrast, when cells were swollen in the presence of nifedipine, RVD and the late phase rise in [Ca2+]i were abolished, but [Ca2+]i did not fall below the baseline level in the late phase, indicating that nifedipine inhibited the swelling-induced Ca2+ entry but that Ca2+ entry by another pathway was undisturbed. It was concluded that PST cells are characterized by two Ca2+ permeability pathways in the basolateral membrane. Under both isotonic and hypotonic conditions, Ca2+ entry occurs at a slow rate via a verapamil-sensitive, DHP-insensitive "baseline" Ca2+ entry pathway. Cell swelling activates a separate DHP-sensitive, verapamil-sensitive Ca2+ entry pathway, which is responsible for the supply of Ca ions to the Ca(2+)-dependent mechanism by which cell volume regulation is achieved.

Sodium-alanine cotransport in renal proximal tubule cells investigated by whole-cell current recording

Sodium-alanine cotransport was investigated in single isolated proximal tubule cells from rabbit kidney with the whole-cell current recording technique. Addition of L-alanine at the extracellular side induced an inward-directed sodium current and a cell depolarization. The sodium-alanine cotransport current was stereospecific and sodium dependent. Competition experiments suggested a common cotransport system for L-alanine and L-phenylalanine. Sodium-alanine cotransport current followed simple Michaelis-Menten kinetics, with an apparent Km of 6.6 mM alanine and 11.6 mM sodium and a maximal cotransport current of 0.98 pA/pF at -60 mV clamp potential. Hill plots of cotransport current suggested a potential-independent coupling ratio of one sodium and one alanine. The apparent Km for sodium and the maximal cotransport current were potential dependent, whereas the apparent Km for L-alanine was not affected by transmembrane potential. The increase in Km for alanine with decreasing inward-directed sodium gradients suggested a simultaneous transport mechanism. These results are consistent with a cotransport model with potential-dependent binding or unbinding of sodium (high-field access channel) and a potential-dependent translocation step.

Cellular mechanisms of bradykinin-induced hyperpolarization in renal epitheloid MDCK-cells

Previous studies have demonstrated that bradykinin hyperpolarizes the cell membrane of subconfluent MDCK cells by increase of the potassium conductance. The present study has been performed to elucidate the intracellular mechanisms involved. To this end, the effects of bradykinin on the potential difference across the cell membrane (PD), on formation of inositol phosphates, and on intracellular calcium concentration (Cai) have been analyzed in cells without or with pretreatment with pertussis toxin or 12-O-tetradecanoylphorbol 13-acetate diester (TPA). In untreated cells, bradykinin leads to a transient increase of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate, increase of Cai, activation of potassium channels and hyperpolarization of the cell membrane. The effects of bradykinin on PD and Cai are still present in the absence of extracellular calcium. In cells pretreated with pertussis toxin the effect of bradykinin on inositol trisphosphate formation is almost abolished but bradykinin still leads to a transient increase of Cai and PD in the presence and absence of extracellular calcium. In cells pretreated with TPA the bradykinin-induced increase of inositol trisphosphate formation is blunted, the bradykinin-induced increase of Cai abolished, but the bradykinin-induced hyperpolarization still present. The observations indicate that bradykinin increases Cai in part by phorbol ester and pertussis toxin sensitive activation of phospholipase C. In addition, bradykinin is capable of enhancing Cai by utilizing pertussis toxin insensitive mechanisms. Furthermore, bradykinin is able to transiently enhance the potassium conductance without a general increase of intracellular calcium.

Potassium-selective channels in the basolateral membrane of single proximal tubule cells of frog kidney

The membrane potential of proximal tubule cells is dominated by the potassium conductance of the basolateral membrane. In the present paper the nature of this conductance is investigated by the patch-clamp technique. Only one type of K channel was found in the basolateral membranes of freshly isolated proximal cells. In cell-attached patches, the current/voltage relationship is markedly non-linear with much larger inward (30 pS) than outward (approximately 6 pS) conductances, even in the presence of roughly symmetrical K concentrations. Thus the channels show inward rectification. The determination of the conductance for outward current flow is complicated since the current/voltage curves show an area of negative conductance. Nevertheless, taking the conductance for outward current flow and the density of the channels it is possible to account for all of the previously reported potassium conductance of amphibian proximal tubule cells. The open probability of the channels was found not to depend upon the membrane potential. However, the non-linearity of the current/voltage relationships will confer upon the channel the same voltage dependence as that seen in intact proximal tubules, i.e. the conductance decreases with depolarisation. Incubation of cells in Ringer with no substrates or in the presence of alanine and/or glucose showed no change in the activity of the channels. These findings suggest that, although these channels may represent the basolateral conductance of frog proximal tubule cells, they are not involved in the well-established coupling between transport rate and potassium conductance.

Voltage-clamp studies of the Na+/glucose cotransporter cloned from rabbit small intestine

Inward Na+ currents associated with the cloned intestinal Na+/glucose cotransporter expressed in Xenopus oocytes have been studied using the two-microelectrode voltage-clamp method. The steady-state current/voltage relations showed voltage-dependent (Vm from +20 to -75 mV) and relatively voltage-independent (Vm from -75 to -150 mV) regions. The apparent Imax for Na+ and glucose increased with negative membrane potentials, and the apparent K0.5 for glucose (K(Glc)0.5) depended on Vm and [Na]o. Increasing [Na]o from 7 to 110 mmol/l had the same effect in decreasing K(Glc)0.5 from 0.44 to 0.03 mmol/l as increasing the Vm from -40 to -150 mV. The I/V curves under saturating conditions (20 mmol/l external sugars and 110 mmol/l [Na]o) were identical for D-glucose, D-galactose, alpha-methyl D-glucopyranoside and 3-O-methyl D-glucoside. The specificity of the cotransporter for sugars was: D-glucose, D-galactose, alpha-methyl D-glucopyranoside greater than 3-O-methyl D-glucoside much greater than D-xylose greater than D-allose much greater than D-mannose. Ki for phlorizin (approximately 10 mumol/l) was independent of Vm at saturating [Na]o. We conclude that a variety of sugars are transported by the cloned Na+/glucose cotransporter at the same maximal rate and that membrane potential affects both the maximal current and the apparent K0.5 of the cotransporter for Na+ and glucose.

Intestinal Na+/glucose cotransporter expressed in Xenopus oocytes is electrogenic

The cloned rabbit intestinal Na+/glucose cotransporter was expressed in Xenopus oocytes, and transmembrane currents associated with this transporter were monitored using a two-electrode voltage clamp. Addition of D-glucose to a Na(+)-containing solution bathing these oocytes generated a current which was blocked by phlorizin. Water-injected control oocytes did not exhibit any currents under these conditions. The magnitude and shape of the currents were dependent on the extracellular glucose and Na+ concentrations and the membrane potential. At Vhold = -50 mV, the Km values for glucose and Na+ were 14 +/- 2 (N = 4) microM and 17 +/- 1 (N = 3) mM, respectively. These Km values and imax exhibited voltage dependence: increasing the membrane potential from -30 to -150 mV increased KGlcm and imax threefold and decreased KNam eightfold. The reversal potential (VR) of the phlorizin-sensitive, glucose-dependent current varied with log Nao+ (slope 46 +/- 6 [N = 9] mV). In the absence of sugar, a Na(+)-dependent, phlorizin-sensitive (Ki = 3 +/- 0.5 microM) current was detected only in RNA-injected oocytes. The amplitude of this current at -50 mV was 6 +/- 1% (N = 13) of the maximum current measured in the presence of D-glucose. The VR of this sugar-independent current varied with log Nao+ (slope 63 +/- 1 [N = 4] mV), indicating that the cotransporter may carry Na+ in the absence of sugar. We conclude that the Na+/glucose cotransporter is electrogenic and that investigations of currents associated with its operation can yield valuable insights into the mechanisms of solute translocation.

Two substrate sites in the renal Na(+)-D-glucose cotransporter studied by model analysis of phlorizin binding and D-glucose transport measurements

Time courses of phlorizin binding to the outside of membrane vesicles from porcine renal outer cortex and outer medulla were measured and the obtained families of binding curves were fitted to different binding models. To fit the experimental data a model with two binding sites was required. Optimal fits were obtained if a ratio of low and high affinity phlorizin binding sites of 1:1 was assumed. Na+ increased the affinity of both binding sites. By an inside-negative membrane potential the affinity of the high affinity binding site (measured in the presence of 3 mM Na+) and of the low affinity binding site (measured in the presence of 3 or 90 mM Na+) was increased. Optimal fits were obtained when the rate constants of dissociation were not changed by the membrane potential. In the presence of 90 mM Na+ on both membrane sides and with a clamped membrane potential, KD values of 0.4 and 7.9 microM were calculated for the low and high affinity phlorizin binding sites which were observed in outer cortex and in outer medulla. Apparent low and high affinity transport sites were detected by measuring the substrate dependence of D-glucose uptake in membrane vesicles from outer cortex and outer medulla which is stimulated by an initial gradient of 90 mM Na+ (out greater than in). Low and high affinity transport could be fitted with identical Km values in outer cortex and outer medulla. An inside-negative membrane potential decreased the apparent Km of high affinity transport whereas the apparent Km of low affinity transport was not changed. The data show that in outer cortex and outer medulla of pig high and low affinity Na(+)-D-glucose cotransporters are present which contain low and high affinity phlorizin binding sites, respectively. It has to be elucidated from future experiments whether equal amounts of low and high affinity transporters are expressed in both kidney regions or whether the low and high affinity transporter are parts of the same glucose transport molecule.

Endogenous D-glucose transport in oocytes of Xenopus laevis

Endogenous glucose uptake by the oocytes of Xenopus laevis consists of two distinct components: one that is independent of extracellular Na+, and the other one that represents Na+-glucose cotransport. The latter shows similar characteristics as 2 Na+-1 glucose cotransport of epithelial cells: The similarities include the dependencies on external concentrations of Na+, glucose, and phlorizin, and on pH. As in epithelial cells, the glucose uptake in oocytes can also be stimulated by lanthanides. Both the electrogenic cotransport and the inhibition by phlorizin are voltage-dependent; the data are compatible with the assumption that the membrane potential acts as a driving force for the reaction cycle of the transport process. In particular, hyperpolarization seems to stimulate transport by recruitment of substrate binding sites to the outer membrane surface. The results described pertain to oocytes arrested in the prophase of the first meiotic division; maturation of the oocytes leads to a downregulation of both the Na+-independent and the Na+-dependent transport systems. The effect on the Na+-dependent cotransport is the consequence of a change of driving force due to membrane depolarization associated with the maturation process.

Peptide-dependent regulation of epithelial nephron functions

It has become evident that the nephron is an important target organ of many of the regulatory peptides; this brief overview will not attempt to consider the vast amount of work on peptide-dependent kidney functions; instead, it will emphasize recent work directed towards understanding intracellular signal pathways between peptide ligand-receptor interaction and expression of physiological transport responses in renal epithelial cells. The awareness that peptide hormones of differing origin, e.g., intestinal and cardiac, share at least some of the signal steps in nephron cells, has stimulated work on nephron segmental analysis of receptor binding, of second messengers, of membrane G proteins, of protein phosphorylation, and of final membrane transport responses, such as peptide-dependent ion channel regulation. Peptides involved in cell growth and differentiation, e.g., growth factors, appear to act through part of the signal pathway shared by other peptides. The peptides selected for the purpose of this review, then, are those that have been linked, by experimental evidence, to intracellular messenger systems in nephron epithelia.

Effect of saccharin on the ATP-induced increase in Na+ permeability in isolated chicken intestinal epithelial cells

When isolated intestinal cells from 3-wk-old chickens are treated with exogenous ATP they undergo a dramatic increase in permeability towards Na+. The increase occurs instantaneously and maximum cell loading with Na+ occurs within 2 min. The response is dose dependent (0.1-1.0 mM-ATP) and results in as much as a 10-fold increase in unidirectional influx of 22Na+ into the cells. The resting cellular Na+ gradient and membrane potential are partially dissipated and consequently Na+-dependent transport of sugars and amino acids is inhibited. Sodium saccharin (20 mM), added at the same time as ATP, completely blocks the effect of ATP on Na+ permeability and preserves the functional capacity of the cells for Na+-dependent sugar or amino acid transport. Partial protection is afforded by 10 mM-saccharin. Saccharin added 2 min after ATP will reverse the enhanced Na+ permeability that has already been induced. In cells that have not been treated with ATP, saccharin induces enhanced sugar and amino acid gradients (P less than 0.05 in paired comparisons from the same cell preparation), indicating that it may also inhibit Na+ permeability of the unperturbed membrane and allow cells to establish higher Na+ gradients and/or membrane potentials. The effect of saccharin in blocking ATP-induced Na+ permeability occurs within 10 sec and at a much lower dose than that required for blockade of facilitated diffusional sugar transfer in these cells.