Ca2+‐Independent Stimulation of Adenylate Cyclase Activity by Muscarinic Receptors in Rat Olfactory Bulb

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Abstract
In rat olfactory bulb homogenate, carbachol stimulated adenylate cyclase activity in a concentration-dependent manner (EC50 = 1.1 .mu.M). The carbachol stimulation occurred fully in membranes that had been prepared in the presence of 1 mM EGTA and incubated in a Ca2+-free enzyme reaction medium. Under these conditions, exogenous calmodulin (1 .mu.m) failed to stimulate adenylate cyclase activity. In miniprisms of olfactory bulb, carbachol (1 mM) increased accumulation of inositol phosphates, but this response was markedly reduced in a Ca2+-free medium. Moreover, the carbachol stimulation of adenylate cyclase activity was not affected by staurosporine at a concentration (1 .mu.M) that completely blocked the stimulatory effect of phorbol 12-myristate 13-acetate, an activator of Ca2+/phospholipid-dependent protein kinase. Quinacrine, a nonselective phospholipase A2 inhibitor, reduced the carbachol stimulation of adenylate cyclase activity, but this inhibition appeared to be competitive with a Ki of 0.2 .mu.M. Nordihydroguaiaretic acid and indomethacin, two inhibitors of arachidonic acid metabolism, failed to affect the carbachol response. These results indicate that in rat olfactory bulb, muscarinic receptors stimulate adenylate cyclase activity through a mechanism that is independent of Ca2+ and phospholipid hydrolysis.