ISOFORM-SPECIFIC SENSITIZATION OF ADENYLYL-CYCLASE ACTIVITY BY PRIOR ACTIVATION OF INHIBITORY RECEPTORS - ROLE OF BETA-GAMMA-SUBUNITS IN TRANSDUCING ENHANCED ACTIVITY OF THE TYPE-VI ISOFORM

Many different types of cells develop increased adenylyl cyclase activ ity (sensitization) on prior treatment with drugs such as opiates that acutely inhibit the enzyme. We found that human embryonic kidney (HEK ) 293 m2 cells, which express the inhibitory m2 muscarinic cholinergic receptor, exhibit a large increase in forskolin-stimulated cAMP synth esis when the cells are preincubated with the muscarinic agonist carba chol for greater than or equal to 5 min and forskolin stimulation is p erformed in the presence of the muscarinic antagonist atropine. To det ermine whether a specific isoform of adenylyl cyclase is susceptible t o the adaptation induced by prior activation of inhibitory receptors, cells were transfected with expression vectors encoding adenylyl cycla se types I, II, and VI, representing three major groups of the adenyly l cyclase family. Preincubation of the cells with carbachol for 30 min resulted in a significant increase in prostaglandin E(1)-stimulated c AMP accumulation in cells expressing type VI, but not type I or type I I, adenylyl cyclase. A similar selective increase in activity from typ e VI adenylyl cyclase was observed for prior treatment with the D-2 do pamine agonist quinpirole and stimulation of cAMP synthesis with human chorionic gonadotropin in cells transfected with expression vectors c oding for the cognate receptors. We next investigated whether beta gam ma subunits play a role in the sensitization of type VI adenylyl cycla se activity; using expression of alpha tau to inhibit beta gamma-media ted effects, we found that the quinpirole-induced sensitization of typ e VI adenylyl cyclase was abolished. However, beta gamma subunits do n ot seem to directly activate type VI adenylyl cyclase, in contrast wit h their ability to directly activate the type II enzyme. Therefore, be ta gamma subunits liberated after activation of inhibitory receptors s eem to indirectly cause an increase in activity of type VI adenylyl cy clase. Indirect activation of the type VI enzyme by beta gamma subunit s is a novel mechanism contributing to the sensitization of adenylyl c yclase.