SHORT EXPOSURE OF INTACT VENTRICULAR MYOCYTES TO ISOPROTERENOL RESULTS IN DESENSITIZATION OF ADENYLATE-CYCLASE

We have examined the effects of short-term exposure to low (50 nM) lev els of the beta-adrenoceptor agonist isoproterenol on the ability of i ntact rat ventricular myocytes to increase their intracellular levels of cyclic AMP (cAMP) in response to subsequent isoproterenol exposure. Exposing freshly isolated, nonstimulated myocytes (which contained 3. 5 +/- 0.3 pmol cAMP/ mg protein) to isoproterenol resulted in rapid, d ose-dependent increases in cAMP formation; 5-min exposure raised intra cellular cAMP content to 6.5 +/- 0.7 pmol/mg protein and 54.6 +/- 0.9 pmol/mg protein, respectively, in the absence and presence of the phos phodiesterase inhibitor 4-(3-butoxy-4-methoxybenzyl)imiazolidin- (RO 2 0-1724). In myocytes incubated for 5-20 min with isoproterenol, washed twice to remove the agonist, and then rechallenged for 20 min with th e agonist in the presence of Ro 20-1724, a 10-min preexposure to the a gonist resulted in a significant reduction (approximate to 20%; P < 0. 05) in the receptors' responsiveness to the rechallenge, and increasin g the preexposure time to 20 min resulted in the maximum attenuation i n responsiveness (approximate to 30%; P < 0.001). Myocytes preincubate d with forskolin (1 mu M), which directly activates adenylate cyclase contained 9.0 +/- 0.7 pmol cAMP/mg protein after 5 min exposure, and u p to 20 min preexposure to forskolin did not alter the cells' responsi veness to subsequent challenge with isoproterenol. The results of this study indicate that exposing freshly isolated rat ventricular myocyte s to nanomolar levels of isoproterenol for as little as 10 min desensi tizes them to subsequent challenge with agonist. The process appears t o be homologous, since it was not observed when forskolin was used to elevate cAMP levels in the initial incubation.