ON THE MECHANISM OF PARATHYROID-HORMONE STIMULATION OF CALCIUM-UPTAKE BY MOUSE DISTAL CONVOLUTED TUBULE CELLS

PTH stimulates transcellular Ca2+ absorption in renal distal convoluted tubules. The effect of PTH on membrane voltage, the ionic basis of the change in voltage, and the relations between voltage and calcium entry were determined on immortalized mouse distal convoluted tubule cells. PTH (10(-8) M) significantly increased Ca-45(2+) uptake from basal levels of 2.81 +/- 0.16 to 3.88 +/- 0.19 nmol min-1 mg protein-1. PTH-induced Ca-45(2+) uptake was abolished by the dihydropyridine antagonist, nifedipine (10(-5) M). PTH did not affect Na-22+ uptake. Intracellular calcium activity ([Ca2+]i) was measured in cells loaded with fura-2. Control [Ca2+]i averaged 112 +/- 21 nM. PTH increased [Ca2+]i over the range of 10(11) to 10(-7) M. Maximal stimulation to 326 +/- 31 nM was achieved at 10(-8) M PTH. Resting membrane voltage measured with the potential sensitive dye DiO6(3) averaged -71 +/- 2 mV. PTH hyperpolarized cells by 19 +/- 4 mV. The chloride-channel blocker NPPB prevented PTH-induced hyperpolarization. PTH decreased and NPPB increased intracellular chloride, measured with the fluorescent dye SPQ. Chloride permeability was estimated by measuring the rate of I-125- efflux. PTH increased I-125- efflux and this effect was blocked by NPPB. Clamping voltage with K+/valinomycin; depolarizing membrane voltage by reducing extracellular chloride; or addition of NPPB prevented PTH-induced calcium uptake. In conclusion, PTH increases chloride conductance in distal convoluted tubule cells leading to decreased intracellular chloride activity, membrane hyperpolarization, and increased calcium entry through dihydropyridine-sensitive calcium channels.