MOBILIZATION OF CA2+ BY THAPSIGARGIN AND 2,5-DI-(T-BUTYL)-1,4-BENZOHYDROQUINONE IN PERMEABILIZED INSULIN-SECRETING RINM5F CELLS - EVIDENCE FOR SEPARATE UPTAKE AND RELEASE COMPARTMENTS IN INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE CA2+ POOL

We characterized and directly compared the Ca2+-releasing actions of two inhibitors of endoplasmic-reticulum (ER) Ca2+ATPase, thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydro-quinone (tBuBHQ), in electropermeabilized insulin-secreting RINm5F cells. Ambient free calcium concentration ([Ca2+]) was monitored by Ca2+-selective mini-electrodes. After ATP-dependent Ca2+ uptake, thapsigargin and tBuBHQ released Ca2+ with an EC50 of approximately 37 nM and approximately 2 muM respectively. Both agents mobilized Ca2+ predominantly from the Ins(1,4,5)P3-sensitive Ca2+ pool, and in this respect thapsigargin was more specific than tBuBHQ. The total increase in [Ca2+] obtained with thapsigargin and Ins(1,4,5)P3 was, on the average, only 7 % greater than that with Ins(1,4,5)P3 alone. In contrast, the total increase in [Ca 2+1 obtained with tBuBHQ and Ins(1,4,5)P3 was 33 % greater than that obtained with only InsP3 (P < 0.05). Although Ca2+ was rapidly mobilized by thapsigargin and tBuBHQ, complete depletion of the Ins(1,4,5)P3-sensitive Ca2+ pool was difficult to achieve. After the release by thapsigargin or tBuBHQ, Ins(1,4,5)P3 induced additional Ca2+ release. The additional Ins(1,4,5)P3-induced Ca2+ release was not altered by supramaximal concentrations of thapsigargin and tBuBHQ, or by Bafilomycin A1, an inhibitor of V-type ATPases, but was decreased by prolonged treatment with the ER Ca2+-ATPase inhibitors. These results suggest the existence of distinct uptake and release compartments within the Ins(1,4,5)P3-sensitive Ca2+ pool. When treated with the inhibitors, the two compartments became distinguishable on the basis of their Ca2+ permeability. Apparently, thapsigargin and tBuBHQ readily mobilized Ca2+ from the uptake compartment, whereas Ca2+ from the release compartment could be mobilized only very slowly, in the absence of Ins(1,4,5)P3.