Hormone-evoked calcium release from intracellular stores is a quantal process.
Ca2+ mobilization by hormones, ionomycin, and inositol 1,4,5-trisphosphate (Ins-1,4,5-P3) were studied to determine whether Ca2+ release is a continuous or a quantal process. Hormone-mediated Ca2+ release occurs only during the first 2-4 s of stimulation. Stimulation of acini with a maximal hormone concentration following stimulation with a submaximal concentration resulted in free cytosolic Ca2+ concentration ([Ca2+]i) increase and 45Ca efflux. The peak [Ca2+]i increase induced by a maximal concentration of agonist was nearly constant when cells were prestimulated with a submaximal dose for 1-15 min. Submaximal hormone concentrations release only a fraction of intracellular 45Ca2+, after which intracellular Ca2+ content remains constant. The partially released stores remain depleted until cell stimulation is terminated, at which time the stores reload with Ca2+. For comparison, increasing concentrations of ionomycin resulted in increasing rates of Ca2+ release. Each ionomycin concentration released all the Ca2+ from intracellular stores. We therefore conclude that hormone-evoked Ca2+ release is a quantal rather than a continuous process. In permeabilized cells, increasing concentrations of Ins-1,4,5-P3 resulted in an increased fraction of Ca2+ release. No submaximal Ins-1,4,5-P3 concentration was capable of releasing all the Ins-1,4,5-P3-mobilizable Ca2+. Therefore, it appears that the quantal properties of hormone-evoked Ca2+ release reflect the quantal properties of Ins-1,4,5-P3-mediated Ca2+ release from intracellular stores.
Muallem
S
S
Laboratory of Membrane Biology, Cedars-Sinai Medical Center, Los Angeles, California 90048.
Pandol
S J
SJ
Beeker
T G
TG
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology*; D004987:Ethers / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate; D007295:Inositol Phosphates / Q000494:pharmacology; D015759:Ionomycin; D007700:Kinetics; D010179:Pancreas / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D012844:Sincalide / Q000494:pharmacology*
D004987:Ethers; D007295:Inositol Phosphates; D015759:Ionomycin; D001285:Atropine; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D012844:Sincalide; D002118:Calcium