Antagonists inactivate the inositol 1,4,5-trisphosphate (Ins-1,4,5-P3)-dependent Ca2+ channel independent of Ins-1,4,5-P3 metabolism.
Streptolysin O-permeable pancreatic acini, which retain intact signaling systems, were used to study the regulation of the inositol 1,4,5-trisphosphate (Ins-1,4,5-P3)-activated Ca2+ channel during agonist stimulation and antagonist inhibition. Stimulation of permeable cells with carbachol induced rapid Ca2+ release from internal stores. Addition of heparin prior to or after agonist stimulation inhibited the release, indicating the activation of the Ins-1,4,5-P3-dependent Ca2+ channels by the agonist. Termination of cell stimulation with the specific antagonist atropine rapidly inactivated the release channels. Channel inactivation by the antagonist was independent of Ins-1,4,5-P3 levels since (a) addition of atropine to carbachol-stimulated cells resulted in a slow hydrolysis of Ins-1,4,5-P3, (b) addition of 10-fold excess Ins-1,4,5-P3 together with the agonist did not prevent channel inactivation by the antagonist, and (c) the antagonist inactivated Ca2+ release in the presence of saturating concentration of the nonhydrolyzable Ins-2,4,5-P3. Hence, the antagonist appears to stabilize the Ins-1,4,5-P3-activated Ca2+ channel in a state refractory to Ins-1,4,5-P3. These findings are the first direct evidence that the channel can exist in such a refractory state.
Zhang
B X
BX
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235-9040.
Tortorici
G
G
Xu
X
X
Muallem
S
S
D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers; D002217:Carbachol / Q000494:pharmacology; D006493:Heparin / Q000494:pharmacology; D066298:In Vitro Techniques; D015544:Inositol 1,4,5-Trisphosphate / Q000502:physiology*; D015640:Ion Channel Gating / Q000187:drug effects; D010179:Pancreas / Q000378:metabolism; D051381:Rats; D015398:Signal Transduction / Q000187:drug effects
D002121:Calcium Channel Blockers; D001285:Atropine; D015544:Inositol 1,4,5-Trisphosphate; D002217:Carbachol; D006493:Heparin; D002118:Calcium