Sodium-calcium interactions in the renal proximal convoluted tubule of the rabbit

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Abstract
The effect of experimental maneuvers believed to raise cytosolic [Ca2+] on Na and fluid absorption by isolated perfused proximal convoluted tubules of rabbit kidneys was examined. For this purpose experiments were carried out in which either 1) peritubular [Na] was lowered from 145 mM in controls to 40 mM in experimental periods by isosmotic replacement with Li, tetraethylammonium, or choline; or 2) quinidine (10(-4) M) or A 23187 (5 X 10(-6) M) was added to the peritubular bath containing 145 mM Na. Fluid absorption (Jv), transepithelial unidirectional Na efflux (JNa 1 leads to b), and Na influx (JNa b leads to 1) were measured. Lowering peritubular [Na] inhibited JNa 1 leads to b by 28% and Jv by 61% of the control values. The degree of inhibition of Jv by low peritubular [Na] was dependent on the concentration of ultrafilterable calcium ([Ca]UF) over the range of 0.2-1.0 mM: as perfusate and bath ultrafilterable calcium was reduced, there was an attenuation of the low [Na]-induced inhibition of Jv. Above 1.0 mM [Ca]UF no further increase in inhibition of Jv was observed. Quinidine in the bath inhibited Jv by 37% and JNa 1 leads to b by 28%; A 23187 reduced Jv by 37% and JNa 1 leads to b by 15%. These results are consistent with the view that cytosolic [Ca2+], in turn dependent on a Na-Ca exchange mechanism located at the basolateral cell membrane, regulates, in part, the rate of proximal tubular efflux of sodium, calcium, and water.