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.