BIDIRECTIONAL ACTIVITY OF THE ENDOPLASMIC-RETICULUM CA2-ATPASE OF BOVINE ADRENAL-CORTEX()

It is generally accepted that the ryanodine receptor and the inositol 1,4,5-trisphosphate receptor play major roles in the complex mechanism s by which agonists increase intracellular Ca2+ concentration. In thes e mechanisms, the endoplasmic reticulum Ca2+-ATPase has been attribute d an accessory role of refilling the intracellular Ca2+ store. In the present study, the activity of the microsomal Ca2+-ATPase of bovine ad renal cortex was investigated. We show that the Ca2+-pumping activity of the Ca2+-ATPase is related to the ADP/ATP ratio. Our results also s how that a brisk increase of the ADP/ATP ratio upon addition of exogen ous ADP triggered a rapid release of Ca2+ from preloaded microsomes. A DP released Ca2+ in a dose-dependent manner with an EC(50) of 2.98 +/- 0.78 mM. ADP-induced Ca2+ release was not prevented by heparin, rulin g out the participation of the inositol 1,4,5-trisphosphate receptor. ADP-induced Ca2+ release could not be attributed to the mere inhibitio n of the Ca2+-ATPase, since the rate of ADP-induced Ca2+ release was 2 0 times faster than the rate of Ca2+ release induced by a maximal conc entration of thapsigargin (2 mu M). ADP-induced Ca2+ release experimen ts performed in the presence of [P-32]PO4 revealed a concomitant produ ction of [P-32]ATP. ADP-induced [P-32]ATP production was dose-dependen t, with an EC(50) of 5.50 +/- 0.70 mM, ADP-induced [P-32]ATP productio n was prevented by ionomycin (10 mu M) and by high concentrations of e xtramicrosomal Ca2+. These results demonstrate that the microsomal Ca2 +-ATPase of adrenal cortex possesses a bidirectional activity that dep ends on ADP concentrations, the Ca2+ gradient across the microsomal me mbrane, and probably also ATP concentrations. The reverse activity of the endoplasmic reticulum Ca2+-ATPase could therefore play an addition al role in cellular Ca2+ homeostasis. Indeed, under physiological or p athological conditions, where local ADP concentrations are increasing (and/or ATP concentrations are decreasing), the reverse activity of th e Ca2+ pump should liberate Ca2+ from intracellular stores.