Cytosolic Ca2+ and Na+ activities in perfused proximal tubules of Necturus kidney

Abstract

To study the role of intracellular calcium in the regulation of epithelial transport of ions and water, cytosolic calcium ion activity (aiCa) and cytosolic sodium ion activity (aiNa) were measured in cells of isolated perfused proximal tubules of Necturus kidney. aiCa was measured with Ca2+-selective microelectrodes, aiNa with Na+-selective microelectrodes. Under control conditions, i.e., Ringer solution on both sides of the epithelium, aiCa averaged 71 +/- 7 (SE) nM (n = 21) and aiNa was 12.9 +/- 0.6 mmol (n = 56). When peritubular bath sodium was reduced from 100 to 10 mM by choline substitution, aiCa increased from 73 +/- 14 to 382 +/- 69 nmol (paired t test; P less than 0.001; n = 4); in different tubules, aiNa decreased from 12.8 +/- 1.9 to 8.2 +/- 1.8 mM (P less than 0.001; n = 12). Quinidine (10(-4) M) increased aiCa from 87 +/- 19 to 556 +/- 121 nM (P less than 0.02; n = 5) but reduced aiNa from 15.1 +/- 1.2 to 11.8 +/- 0.8 mM (P less than 0.003; n = 8). In contrast, 10(-4) M ouabain increased both aiCa and aiNa; aiCa rose from 71 +/- 9 to 546 +/- 121 nmol (P less than 0.005; n = 9) and aiNa from 15.1 +/- 1.8 to 70.1 +/- 6.3 mM (P less than 0.001; n = 9). The results are consistent with the existence of a Na-Ca exchange process within the contraluminal cell membrane and with the view that increased aiCa inhibits the tubular transport of sodium by decreasing the sodium permeability of the luminal cell membrane.

Fluorescent measurements of intracellular free calcium in isolated toad urinary bladder epithelial cells

“…The proposed role for Ca regulation predicts that an increase in Caf decreases sodium permeability and is based on the observation that sodium transport is reduced by: (i) Ca ionophores in toad [44] and turtle [2] bladders; (ii) maneuvers believed to inhibit cellular Ca efflux mediated by Na-Ca exchange such as reduced serosal Na [15,24] and addition of either quinidine [3,24] or ouabain [24,25]; and (iii) the report that Ca reduces Na permeability in toad bladder plasma membrane vesicles [11]. However, conflicting evidence exists, questioning this simple scheme.…”

“…The plasma membrane Na-Ca exchange has been proposed to play a pivotal role in the regulation of Car [15,24,37]. In this scheme, an increase in intracellular sodium reduces the plasma membrane Na gradient available to drive efflux of Ca via Na-Ca exchange resulting in an increased Cal.…”

Renal electrolyte excretion and renin release during calcium and parathormone infusions in conscious rabbits.

“…Sodium-calcium exchange across the contraluminal cell membrane, a largely passive process dependent upon active sodium-potassium exchange, may be a major regulator of cytosolic calcium (Taylor & Windhager, 1979). There is now mounting micropuncture evidence for sodium-calcium counter transport throughout the nephron (Ullrich et al 1976;Friedman et al 1981;Frindt et al 1982;Lorenzen et al 1984;Costanzo, 1984) and recent evidence from in vitro experiments with renal basolateral (contraluminal) membrane vesicles, largely from the proximal tubule, that PTH may stimulate sodium-calcium exchange activity (Jayakumar, Cheng, Liang & Sacktor, 1984). Thus hypercalcaemia should reduce endogenous PTH secretion (Habener, Rosenblatt & Potts, 1984) and perhaps decrease or even reverse sodium-calcium exchange.…”

“…Originally described in the squid axon (Baker, Blaustein, Hodgkin & Steinhardt, 1969;, this process has become of particular interest in recent years with the suggestion that it may operate in most epithelia, including renal tubular cells, and might be involved in both proximal and distal tubular reabsorption of calcium (Ullrich, Rumrich & Kl6ss, 1976;Friedman, Figueiredo, Maack & Windhager, 1981;Frindt, Windhager & Taylor, 1982;Lorenzen, Lee & Windhager, 1984;Costanzo, 1984). However, its significance in over-all tubular reabsorption of sodium and calcium is unknown (Suki, 1979;Taylor & Windhager, 1979;Roland, Rouse & Suki, 1984).…”

Renal Tubular Transport of Calcium

“…In general, the conclusion has been that maneuvers thought to increase cytosolic free Ca 2 + concentration also inhibit the hydrosmotic response to both vasopressin and cAMP. Thus quinidine, a drug known to raise cytosolic Ca 2 + activity in proximal tubule cells of Necturus (135), inhibits vasopressin-and cAMP-induced water movement in toad urinary bladder (4,201,202,206) and mammalian collecting tubule (134). The calcium ionophares A23187 and X537A also inhibit the hydrosmotic response to vasopressin or cAMP in toad bladder (154).…”