CALCIUM-TRANSPORT THROUGH THE LUMINAL MEMBRANE OF THE DISTAL TUBULE .1. INTERRELATIONSHIP WITH SODIUM

Calcium (Ca2+) transport by isolated luminal membranes from rabbit renal distal tubule has been characterized. Ca2+ uptake by these membrane vesicles exhibited saturation kinetics. In the absence of sodium (Na+) in the incubation medium, a low affinity system was observed with a KmCa2+ of 2.83 +/- 0.64 mM and V(max) of 3.03 +/- 0.48 pmol/mu-g/10 sec. A second type of kinetics was also detected with a high affinity and a low velocity (KmCa2+ 0.04 +/- 0.01 mM, V(max) 1.18 +/- 0.22 pmol/mu-g/10 sec). The luminal membranes from proximal tubules showed a single system with a KmCa2+ of 0.49 +/- 0.20 mM and V(max) of 1.26 +/- 0.17 pmol/mu-g/10 sec. The presence of Na+ sharply decreased Ca2+ uptake by the high affinity system of the membranes from distal tubules, increasing the KmCa2+ to 0.07 mM +/- 0.01 (P < 0.01) and decreasing the V(max) to 0.27 pmol/mu-g/10 sec (P < 0.005). This effect of Na+ was concentration-dependent, with a half-maximal effect at 38 mM Na+ and a Hill coefficient of 0.9. In contrast, Na+ had no effect on Ca2+ transport through the luminal membranes of proximal tubules nor on the low affinity system of the distal tubule. The composition of the intravesicular medium also influenced Ca2+ uptake by the membranes from distal tubules. Compared to mannitol, trans-Na+ or K+ significantly reduced Ca2+ transport. Finally, cis-K+ induced an increase in this transport. As found with Na+, the effect of K+ was concentration-dependent, with a Hill coefficient of 0.42. It is concluded that: 1) the luminal membrane of the distal tubule fundamentally differs from the brush border membrane of the proximal tubule; 2) Na+ has an inhibitory effect on Ca2+ uptake when applied on either side of the distal tubule membrane, and therefore probably binds at two different sites of the carriers in the membrane; 3) trans-K+ inhibits whereas cis-K+ enhances Ca2+ transport in this membrane; and 4) the actions of Na+ and K+ are not dependent upon any exchange mechanism.