Bidirectional activity of the endoplasmic reticulum Ca(2+)-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 mechanisms by which agonists increase intracellular Ca2+ concentration. In these mechanisms, the endoplasmic reticulum Ca(2+)-ATPase has been attributed an accessory role of refilling the intracellular Ca2+ store. In the present study, the activity of the microsomal Ca(2+)-ATPase of bovine adrenal cortex was investigated. We show that the Ca(2+)-pumping activity of the Ca(2+)-ATPase is related to the ADP/ATP ratio. Our results also show that a brisk increase of the ADP/ATP ratio upon addition of exogenous ADP triggered a rapid release of Ca2+ from preloaded microsomes. ADP released Ca2+ in a dose-dependent manner with an EC50 of 2.98 +/- 0.78 mM. ADP-induced Ca2+ release was not prevented by heparin, ruling out the participation of the inositol 1,4,5-trisphosphate receptor. ADP-induced Ca2+ release could not be attributed to the mere inhibition of the Ca(2+)-ATPase, since the rate of ADP-induced Ca2+ release was 20 times faster than the rate of Ca2+ release induced by a maximal concentration of thapsigargin (2 microM). ADP-induced Ca2+ release experiments performed in the presence of [32P]PO4 revealed a concomitant production of [32P]ATP. ADP-induced [32P]ATP production was dose-dependent, with an EC50 of 5.50 +/- 0.70 mM. ADP-induced [32P]ATP production was prevented by ionomycin (10 microM) and by high concentrations of extramicrosomal Ca2+. These results demonstrate that the microsomal Ca(2+)-ATPase of adrenal cortex possesses a bidirectional activity that depends on ADP concentrations, the Ca2+ gradient across the microsomal membrane, and probably also ATP concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)
Poitras
M
M
Department of Pharmacology, Faculty of Medicine, University of Sherbrooke, Québec, Canada.
Ribeiro-Do-Valle
R M
RM
Poirier
S N
SN
Guillemette
G
G
D000244:Adenosine Diphosphate / Q000378:metabolism / Q000494:pharmacology; D000255:Adenosine Triphosphate / Q000378:metabolism; D000302:Adrenal Cortex / Q000201:enzymology* / Q000648:ultrastructure; D000818:Animals; D002118:Calcium / Q000378:metabolism / Q000494:pharmacology; D015220:Calcium Channels / Q000502:physiology; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002417:Cattle; D004721:Endoplasmic Reticulum / Q000201:enzymology*; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism; D053496:Inositol 1,4,5-Trisphosphate Receptors; D015759:Ionomycin / Q000494:pharmacology; D008861:Microsomes / Q000201:enzymology; D009124:Muscle Proteins / Q000502:physiology; D010710:Phosphates / Q000494:pharmacology; D018160:Receptors, Cytoplasmic and Nuclear / Q000502:physiology; D019837:Ryanodine Receptor Calcium Release Channel
D015220:Calcium Channels; D053496:Inositol 1,4,5-Trisphosphate Receptors; D009124:Muscle Proteins; D010710:Phosphates; D018160:Receptors, Cytoplasmic and Nuclear; D019837:Ryanodine Receptor Calcium Release Channel; D015759:Ionomycin; D000244:Adenosine Diphosphate; D015544:Inositol 1,4,5-Trisphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium