Sodium gradient-dependent calcium uptake in renal basolateral membrane vesicles. Effect of parathyroid hormone

The Na+/Ca2+ exchange system in rat renal cortex basolateral membrane vesicles was studied. Uptake and efflux of Ca2+ in the membrane vesicles were stimulated by trans-Na+. The enhancement of Ca2+ uptake by the intravesicular greater than extravesicular Na+ gradient was inhibited by ionophores that dissipated the gradient, and was increased by an outside negative membrane potential. Na+-dependent Ca2+ uptake was saturable with respect to both Ca2+ and Na+. A [Ca2+]0.5 of 8 microM was calculated. The relationship between Na+ concentration and rate of Ca2+ efflux was sigmoidal; a [Na+]0.5 of 15 mM and a Hill coefficient of 2.5 were estimated. Removal of parathyroid glands from the rats resulted in a 40% decrease in the Na+-dependent Ca2+ uptake. Infusion of parathyroid hormone (the synthetic tetratriacontapeptide) into these animals fully restored the activity. The isolated basolateral membrane possessed parathyroid hormone-sensitive adenylate cyclase. These findings may suggest a mechanism by which parathyroid hormone regulates the reabsorption of Ca2+ in the kidney.