CALCIUM POOLS IN EHRLICH CARCINOMA-CELLS - A MAJOR, HIGH-AFFINITY CA2-TRISPHOSPHATE AND THAPSIGARGIN( POOL IS SENSITIVE TO BOTH INOSITOL 1,4,5)

To investigate the presence and the size of different non-mitochondria l Ca2+ pools of Ehrlich ascites tumor cells (EATCs), digitonin-permeab ilized cells were allowed to accumulate Ca2+ in the presence of mitoch ondrial inhibitors and treated with the reticular Ca2+-ATPase inhibito r thapsigargin, IP3 and the Ca2+ ionophore A23187. Emptying of thapsig argin-sensitive Ca2+ stores prevented any Ca2+ release by IP3, and, af ter IP3 addition, little or no Ca2+ was released by thapsigargin. In b oth instances, a further Ca2+ release was accomplished by A23187, The IP3-thapsigargin-sensitive pool and the residual A23187-sensitive one corresponded to approximately 60 and 37% of non-mitochondrial stored C a2+, respectively. In intact EATCs, IP3-dependent agonists and thapsig argin discharged Ca2+ pools almost completely overlapping, and A32187 released a minor residual Ca2+ pool. The IP3-insensitive pool appeared to have a relatively low affinity for Ca2+ (below 600 nM). The high a ffinity, IP3-sensitive Ca2+ pool was discharged in a 'quantal' manner following step additions of sub maximal [IP3], and the IP3-induced fra ctional Ca2+ release was more marked at higher concentrations of stare d (luminal) Ca2+, The IP3-sensitive Ca2+ pool appeared to be devoid of the Ca2+-activated Ca2+ release channel since caffeine did not releas ed and Ca2+ in intact and permeabilized EATCs, and Western blot analys es of EATC microsomal membranes failed to detect any known ryanodine r eceptor isoform.