THAPSIGARGIN-SENSITIVE CA2-ATPASES ACCOUNT FOR CA2+ UPTAKE TO INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE AND CAFFEINE-SENSITIVE CA2+ STORES IN ADRENAL CHROMAFFIN CELLS()

In chromaffin cells of adrenal medulla, heterogeneity of Ca2+ stores h as been suggested with respect to the mechanisms of Ca2+ uptake and re lease. We have examined Ca2+-ATPases responsible for loading of Ca2+ s tores in these cells for their sensitivity to thapsigargin, a highly s elective inhibitor of the SERCA [sarco(endo)plasmic reticulum calcium ATPase] family of intracellular Ca2+ pumps. Using immunostaining, we s tudied the distribution of Ca2+-ATPases, and of receptors for inositol 1,4,5-trisphosphate (InsP(3)) and ryanodine, in the density-gradient fractions of microsomes from bovine adrenal medulla. In parallel, we e xamined distribution profiles of ATP-dependent Ca2+ uptake in the same fractions, along with subcellular markers for plasma membranes and en doplasmic reticulum (ER). Two Ca2+-ATPase-like proteins (116 and 100 k Da) were detected, consistent with the presence of SERCA 2b and SERCA 3 isoenzymes of Ca2+ pumps. The distribution of these putative Ca2+-AT Pase isoenzymes paralleled that of InsP(3) and ryanodine receptors. Th is distribution of ER Ca2+-ATPases, as determined immunologically, was consistent with that of thapsigargin-sensitive, but not of thapsigarg in-insensitive, ATP-dependent Ca2+ uptake. In contrast, the distributi on profile of the thapsigargin-insensitive Ca2+ uptake was strongly co rrelated to that of plasma membranes, and co-distributed with plasma m embrane Ca2+-ATPase detected immunologically. In isolated, permeabiliz ed chromaffin cells, InsP(3) and caffeine induced Ca2+ release followi ng an ATP-dependent Ca2+ accumulation to the stores. This accumulation was abolished by thapsigargin. Together, these data strongly indicate that the thapsigargin-sensitive, presumably SERCA-type Ca2+-ATPases a ccount for Ca2+ uptake to InsP(3)-sensitive, as well as to caffeine-se nsitive, Ca2+ stores in bovine adrenal chromaffin cells.