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G Frindt

Publications and source records attributed to G Frindt.

At least 55 records · Page 3Linked to original sources
PubMed1988

Potential role of cytoplasmic calcium ions in the regulation of sodium transport in renal tubules.

Experimental maneuvers that increase intracellular calcium ion levels inhibit sodium transport by renal tubules. In the isolated perfused renal tubule, intracellular calcium ion activity (aiCa) changes in response to alterations in the magnitude of the electrochemical potential gradient for sodium ions across the basolateral cell membrane. However, a potassium-induced depolarization of this cell boundary does not cause a rise but rather a fall in intracellular calcium ion levels. Ionomycin raises aiCa without causing intracellular acidification. This observation does not support the view that high cytosolic calcium produces intracellular acidification. At least in the case of ionomycin, the inhibition of sodium transport appears to be due to ionophore-induced increases in aiCa. The changes in intracellular calcium ion concentration found in the different experimental conditions studied were consistent with the notion that cytosolic calcium ions may mediate a feedback mechanism that links the luminal entry to the peritubular extrusion of sodium ions. The mechanisms by which cytosolic calcium alters entry is not yet clear but recent experiments suggest an indirect effect on sodium channel activity.

Animals↗
PubMed1987

Ca-activated K channels in apical membrane of mammalian CCT, and their role in K secretion.

High conductance, Ca-activated K channels were studied in the apical membrane of the rat cortical collecting tubule (CCT) using the patch-clamp technique. In cell-attached patches the channels were found mainly in the closed state at the spontaneous apical membrane potential. They spent progressively more time in the open state as the pipette potential was made negative relative to the bath. In excised patches these channels had a high selectivity for K over Na and were activated by micromolar concentrations of Ca2+ on the cytoplasmic side of the membrane in a voltage-dependent manner. They had a low conductance to Rb and were blocked by Ba (1-100 microM) from the cytoplasmic side and tetraethylammonium (TEA) (0.2-1 mM) from the luminal side. Block by external TEA and small conductance to Rb were used to investigate the role of these channels in K transport by the isolated perfused rabbit CCT. Ba (2.5 mM), a well-studied blocker of apical K conductance in this segment, hyperpolarized the transepithelial voltage (VT) by 3.7 +/- 0.9 mV when added to the luminal solution of the perfused tubule. Addition of TEA (5 mM) to the luminal solution has no effect on VT. When Na transport was abolished by luminal amiloride, perfusion with 30 mM K (replacing Na) resulted in a lumen-negative VT (18-34 mV). Under these conditions, VT was reduced by 6.0 +/- 1.5 mV by 2.5 mM Ba, whereas TEA had no effect. Perfusion with 30 mM Rb (replacing Na) also caused a lumen-negative VT that was approximately 50% of that observed with 30 mM K. The apical K conductance of the perfused CCT appears to be insensitive to luminal TEA and only modestly selective for K over Rb. This conductance, at least under the conditions of our studies, is probably not mediated by the high conductance Ca-activated K channel.

Animals↗
PubMed1987

Quinidine effect on hydrosmotic response of collecting tubules to vasopressin and cAMP.

Quinidine, a compound thought to increase cytosolic calcium ion activity, has been found to inhibit the hydrosmotic response to vasopressin (VP) and adenosine 3',5'-cyclic monophosphate (cAMP) in the toad urinary bladder. To test whether this drug has a similar action in the mammalian nephron, the effect of quinidine on the hydraulic conductivity of the isolated perfused rabbit cortical collecting tubule (CCT) exposed to either 20 microU/ml VP or 10(-4) M 8-(p-chlorophenylthio) - adenosine 3',5' - cyclic monophosphate (8-CPT-cAMP) was studied. Quinidine had no effect on the basal water permeability of the CCT. Quinidine sulfate (10(-4) M) reduced the VP-stimulated water permeability from 280 +/- 50 X 10(-7) to 115 +/- 41 X 10(-7) cm X s-1 X atm-1 (P less than 0.05). The hydrosmotic response to 8-CPTcAMP was likewise reduced following exposure to quinidine. This effect was shown to be dose dependent. In paired experiments, inhibition of the response to 10(-4) M 8-CPTcAMP averaged 11% at 10(-6) M, 27% at 5 X 10(-6) M, 53% at 5 X 10(-5) M, and 50% at 10(-4) M quinidine. Inhibition of the response to 8-CPTcAMP was estimated to be half maximal at approximately 5 X 10(-6) M quinidine. Tubules were protected against the quinidine-induced inhibition by the addition of 6.5 X 10(-5) M quin 2-acetoxymethylester in the presence of low peritubular Ca concentration. These results are consistent with the view that elevated cytosolic Ca ion levels inhibit the increase in water permeability elicited by VP or exogenous cAMP in the mammalian CCT.

Animals↗
PubMed1987

Effects of cell Ca and pH on Na channels from rat cortical collecting tubule.

The patch-clamp technique was used to identify individual Na channels in the apical membrane of the rat cortical collecting tubule and to evaluate the effects of cytoplasmic Ca2+ and pH on channel activity. In excised, inside-out patches, the probability of a channels's being open (P0) increased with alkalinization of the solution bathing the cytoplasmic side of the patch. Estimates of P0 were 0.05 at pH 6.4, 0.19 at pH 6.9, and 0.41 at pH 7.4. Varying the free Ca2+ concentration of the solution bathing the cytoplasmic side of the patch had no measurable effect on P0. In cell-attached patches, addition of the Ca2+ ionophore ionomycin to the solution bathing the tubules to a final concentration of either 1 or 10 microM decreased channel activity measured as the mean number of open channels (no. open) = n X P0 where n is the number of channels in the membrane. (no. open) was significantly decreased at 3 min after addition of ionomycin and fell to less than 10% of control values after 10 min incubation. There was no fall in (no. open) either in time controls or in tubules exposed to ionomycin in the presence of low bath Ca2+ concentrations [no added Ca2+ with 1 mM ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid (EGTA)]. The results suggest that cytoplasmic pH can directly influence channel activity. Cytoplasmic Ca2+ does not interact directly with the channels, but increased cytoplasmic Ca2+ produces a fall in channel activity through an indirect process.

Animals↗
PubMed1986

Intracellular calcium ions as regulators of renal tubular sodium transport.

This review addresses the putative role of intracellular calcium ions in the regulation of sodium transport by renal tubules. Cytoplasmic calcium-ion activities in proximal tubules of Necturus are less than 10(-7) M and can be increased by lowering the electrochemical potential gradient for sodium ions across the peritubular cell membrane, or by addition of quinidine or ionomycin to peritubular fluid. Whereas lowering of the peritubular Na concentration increases cytosolic [Ca++] and [H+], ionomycin, a calcium ionophore, raises intracellular [Ca++] without decreasing pHi. The intracellular calcium-ion level is maintained by transport processes in the plasma membrane and membranes of intracellular organelles, as well as by calcium-binding proteins. Calcium ions inhibit net transport of sodium by reducing the rate of sodium entry across the luminal cell membrane. In the collecting tubule this inhibition is caused, at least in part, by an indirect reduction in the activity of the amiloride-sensitive sodium channel.

Adenosine Triphosphate↗
PubMed1986

Amiloride-sensitive Na channels from the apical membrane of the rat cortical collecting tubule.

Currents through individual Na channels in the apical membrane of the rat cortical collecting tubule were resolved by using the patch-clamp technique. In cell-attached patches, the channels had a conductance of 5 pS with 140 mM NaCl in the pipet. The conductance was a saturable function of external Na, with a maximal value of about 8 pS and a half saturation at about 75 mM Na. In excised inside-out patches, the selectivity of the channels for Na over K was estimated from reversal potentials to be at least 10:1. The channels underwent spontaneous transitions between open and closed states. Both states had mean lifetimes of 3-4 sec. Amiloride (0.5 microM) added to the pipet induced more frequent closures and openings of the channels and a reduction in the mean open time. These channels are presumed to mediate Na reabsorption by this nephron segment in vivo.

Amiloride↗
PubMed1986

Epithelial sodium channels: characterization by using the patch-clamp technique.

The patch-clamp technique was used to resolve currents through individual Na-selective ion channels in the apical membrane of the rat cortical collecting tubule. The channels had a single unit conductance of 5 pS under control conditions (cell-attached patches, room temperature, 140 mM NaCl in the pipette). They appeared to be highly selective for Na, as K conduction through them was not measurable in inside-out patches. The channels underwent spontaneous transitions between open and closed states, both states being long-lived. At physiological temperature (37C) the conductance increased to 9 pS and the spontaneous transitions became more rapid. In the presence of amiloride on the luminal side of the membrane, the channel flickered rapidly between open and blocked states. The size of the current transitions did not change. This channel activity was observed only in rats that were fed a low-Na diet to elevate aldosterone secretion. In addition to mineralocorticoid control, the activity of the channels in inside-out patches were modulated by the pH on the cytoplasmic side of the membrane. Alkalinization from pH 6.4 to 7.4 increased the probability of channels' being open by eightfold. Changes in Ca concentration on the cytoplasmic side of the membrane did not directly affect the channels. However, addition of ionomycin, a Ca ionophore, to the bath resulted in a decrease in channel activity measured in cell-attached patches. This suggests that high cytoplasmic Ca may indirectly down-regulate Na channels in this tissue.

Amiloride↗
PubMed1982

Hydroosmotic response of collecting tubules to ADH or cAMP at reduced peritubular sodium.

Changes in cytosolic Ca2+ activity have been implicated in the hydroosmotic response to vasopressin (VP) in amphibian urinary bladder; the level of cytosolic free Ca2+ may be regulated, in part, by a process of Na-Ca exchange across the basolateral cell membrane. To assess whether similar mechanisms operate in the mammalian nephron, the effect of low peritubular [NA] on the hydraulic conductivity (Lp) of the isolated perfused rabbit collecting tubule (CT) exposed to either 20 microunits/ml VP or 5 X 10(-5) M 8-[p-chlorophenylthio]-cyclic 3',5'-adenosine monophosphate (ClPheS-cAMP) was studied. Low peritubular [Na] had no effect on the basal water permeability of the CT. After exposure to VP, CTs bathed in 145 mM Na Ringer solution developed an Lp of 324 +/- 27 X 10(-7) cm.s-1.atm-1, while tubules bathed in 4 mM Na and 141 mM tetramethylammonium Ringer solution achieved an Lp of only 112 +/- 13 X 10(-7) cm.s-1.atm-1 (P less than 0.001). Inhibition of the VP response was estimated to be half-maximal when peritubular Na was 120 mM. The hydroosmotic response to ClPheS-cAMP was diminished by 44% of the control values when CTs were bathed in a 4 mM Na medium; this inhibition was greatly attenuated when the peritubular Ca concentration was reduced to 0.05 mM. These results are consistent with the view that 1) a Na-Ca exchange process operates at the basolateral surface of the mammalian cortical collecting tubule cells, and 2) elevated cytosolic Ca ion activity inhibits the increase in water permeability elicited by VP or cAMP in this nephron segment.

Animals↗
PubMed1979

Effects of fluorocitrate on renal ammoniagenesis and glutamine metabolism in the intact dog kidney.

Renal glutamine metabolism was studied in vivo following infusions of fluorocitrate into chronically acidotic and alkalotic dogs. Coincident with a dramatic rise in renal cortical citrate concentrations, there was a significant fall in tissue glutamate in both acid-base states. This was accompanied by a significant increase in total renal ammonia production. Glutamine metabolism and ammoningenesis in alkalotic dogs receiving fluorocitrate simulated that achieved in acidotic dogs. The simultaneous administration of alpha-ketoglutarate and fluorocitrate significantly diminished the fall in tissue glutamate and the rise in ammoniagenesis induced by fluorocitrate alone. These results are compatible with the hypothesis that ammonia production from glutamine is enhanced secondary to increased glutamate deamination. We postulate that this chain of events may be the consequence of impaired alpha-ketoglutarate production from citrate.

Acidosis↗
PubMed1977

The effect of fluorocitrate on urinary calcium and citrate excretion.

The renal handling of calcium and citrate was studied in dogs after the administration of fluorocitrate. The drug produced a significant increase in urinary calcium and citrate excretion. Net renal secretion of citrate occurred during the infusion of fluorocitrate since citrate clearances exceeded the glomerular filtration rate.

Acidosis↗