H+/Ca2+ exchange in rabbit renal cortical endosomes.

We have examined the effect of second messengers on ATP-driven H+ transport in an H+ ATPase-bearing endosomal fraction isolated from rabbit renal cortex. cAMP (0.1 mM) had no effect on H+ transport. Acridine orange fluorescence in the presence of 0.5 mM Ca2+ (+1 mM EGTA) was 19 +/- 6% of control. Inhibition of ATP-driven H+ transport by Ca2+ was concentration dependent; 0.25 and 0.5 mM Ca2+ (+1 mM EGTA) inhibited acridine orange fluorescence by approximately 50 and approximately 80%, respectively. Ca2+ also produced a concentration-dependent increase in the rate of pH-gradient dissipation. Ca2+ did not affect ATP hydrolysis. ATP-dependent Br- uptake was virtually unchanged in the presence of 0.5 mM Ca2+ (+1 mM EGTA). These vesicles were also shown to transport Ca2+ in an ATP-dependent mode. Inositol 1,4,5-trisphosphate had no effect on ATP-dependent Ca2+ uptake. These results are consistent with the co-existence of an H+ ATPase and an H+/Ca2+ exchanger on these endosomes, the latter transport system using the H+ gradient to energize Ca2+ uptake. Attempts to demonstrate an H+/Ca2+ antiporter in the absence of ATP have been unsuccessful. Yet, when a pH gradient was established by preincubation with ATP and residual ATP was subsequently removed by hexokinase + glucose, stimulation of Ca2+ uptake could be demonstrated. A Ca2(+)-dependent increase in H+ permeability and an ATP-dependent Ca2+ uptake might have important implications for the regulation of vacuolar H+ ATPase activity as well as the homeostasis of cytosolic Ca2+ concentration.