Regulation of Ca2+ transport by isolated organelles of a rat insulinoma. Studies with endoplasmic reticulum and secretory granules.
The regulation of extramicrosomal Ca2+ concentration maintained by suspensions of rat insulinoma microsomes was studied using Ca2+-selective minielectrodes. The Ca2+-transporting activity was MgATP dependent and correlated with the endoplasmic reticulum marker NADPH-cytochrome c reductase. When incubated in a high KCl medium containing Mg2+ and phosphate, the microsomes lowered [Ca2+] within less than 10 min to around 0.2 microM. They had a high Ca2+-sequestering activity since they were able to take up and retain several small Ca2+ additions. No evidence for a Na+/Ca2+ countertransport was obtained. The accumulated Ca2+ was released by the Ca2+ ionophore A23187 or upon transforming ATP into ADP using glucose plus hexokinase. The addition of ADP, at concentrations present in cells, resulted in a dose-dependent and reversible net Ca2+ efflux from the microsomes until a higher [Ca2+] steady state was reached. This was specific for ADP since GDP, UDP, CDP, IDP, and the nonhydrolyzable analogue methylene-ADP as well as AMP and cAMP did not reproduce the effect. Insulin secretory granules were unable to lower medium [Ca2+] or to take up a pulse addition of Ca2+. However, most of the large granular calcium content was released by A23187. The addition of Na+ and lowering or increasing medium pH by 0.2 pH unit did not induce Ca2+ uptake or efflux from the secretory granules. The results indicate that insulinoma endoplasmic reticulum but not insulin secretory granules may play a critical role in the regulation of cytosolic Ca2+. A variation in cellular ADP content following secretagogue addition might modulate Ca2+ fluxes across the endoplasmic reticulum and contribute in raising cytosolic Ca2+.
D007516:Adenoma, Islet Cell / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002458:Cell Fractionation / Q000379:methods; D002460:Cell Line; D003594:Cytoplasmic Granules / Q000378:metabolism*; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007340:Insulinoma / Q000378:metabolism*; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism; D009940:Organoids / Q000378:metabolism* / Q000648:ultrastructure; D010190:Pancreatic Neoplasms / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains
D007328:Insulin; D000001:Calcimycin; D000252:Calcium-Transporting ATPases; D002118:Calcium