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 Ca(2+)-releasing actions of two inhibitors of endoplasmic-reticulum (ER) Ca(2+)-ATPase, thapsigargin and 2,5-di-(t-butyl)-1,4-benzohydroquinone (tBuBHQ), in electropermeabilized insulin-secreting RINm5F cells. Ambient free calcium concentration ([Ca2+]) was monitored by Ca(2+)-selective mini-electrodes. After ATP-dependent Ca2+ uptake, thapsigargin and tBuBHQ released Ca2+ with and EC50 of approximately 37 nM and approximately 2 microM 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 [Ca2+] 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 Ca(2+)-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.
Islam
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Rolf Luft Center for Diabetes Research, Department of Edocrinology, Karolinska Institute, Karolinska Hospital, Stockholm, Sweden.
Berggren
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D000818:Animals; D000975:Antioxidants / Q000494:pharmacology*; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002451:Cell Compartmentation; D002463:Cell Membrane Permeability; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism; D006873:Hydroquinones / Q000494:pharmacology*; D015544:Inositol 1,4,5-Trisphosphate / Q000378:metabolism*; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D051381:Rats; D013729:Terpenes / Q000494:pharmacology*; D019284:Thapsigargin; D014407:Tumor Cells, Cultured
D000975:Antioxidants; D006873:Hydroquinones; D007328:Insulin; D013729:Terpenes; C019359:2,5-di-tert-butylhydroquinone; D019284:Thapsigargin; D015544:Inositol 1,4,5-Trisphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium