ANTAGONISTS INACTIVATE THE INOSITOL 1,4,5-TRISPHOSPHATE (INS-1,4,5-P-3)-DEPENDENT CA2-1,4,5-P(3)P METABOLISM( CHANNEL INDEPENDENT OF INS)

Streptolysin O-permeable pancreatic acini, which retain intact signali ng systems, were used to study the regulation of the inositol 1,4,5-tr isphosphate (Ins-1,4,5-P-3)-activated Ca2+ channel during agonist stim ulation and antagonist inhibition. Stimulation of permeable cells with carbachol induced rapid Ca2+ release from internal stores. Addition o f heparin prior to or after agonist stimulation inhibited the release, indicating the activation of the Ins-1,4,5-P-3-dependent Ca2+ channel s by the agonist. Termination of cell stimulation with the specific an tagonist atropine rapidly inactivated the release channels. Channel in activation by the antagonist was independent of Ins-1,4,5-P-3 levels s ince (a) addition of atropine to carbachol-stimulated cells resulted i n a slow hydrolysis of Ins-1,4,5-P-3, (b) addition of 10-fold excess I ns-1,4,5-P-3 together with the agonist did not prevent channel inactiv ation by the antagonist, and (c) the antagonist inactivated Ca2+ relea se in the presence of saturating concentration of the nonhydrolyzable Ins-2,4,5-P-3. Hence, the antagonist appears to stabilize the Ins 1,4, 5-P-3-activated Ca2+ channel in a state refractory to Ins-1,4,5-P-3. T hese findings are the first direct evidence that the channel can exist in such a refractory state.