Short-term exposure of intact ventricular myocytes to isoproterenol results in desensitization of adenylate cyclase

Abstract

We have examined the effects of short-term exposure to low (50 nM) levels of the β-adrenoceptor agonist isoproterenol on the ability of intact 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, dose-dependent increases in cAMP formation; 5-min exposure raised intracellular cAMP content to 6/5 + 0.7 pmol/mg protein and 5.4 ± 0.9 pmol/mg protein respectively, in the absence and presence of the phosphodiesterase inhibitor 4-(3-butoxy-4-methoxybenzyl)imiazolidin-2-one (RO 20-1724). In myocytes incubated for 5–20 min with isoproterenol, washed twice to remove the agonist, and then rechallenged for 20 min with the agonist in the presence of RO 20-1724, a 10-min preexposure to the agonist resulted in a significant reduction (≈ 20%; P < 0.05) in the receptor's responsiveness to the rechallenge, and increasing the preexposure time to 20 min resulted in the maximum attenuation in responsiveness (≈ 30%; P < 0.001). Myocytes preincubated with forskolin (1 μM), which directly activates adenylate cyclase contained 9.0 ± 0.7 pmol cAMP/mg protein after 5 min exposure, and up to 20 min preexposure to forskolin did not alter the cells' responsiveness to subsequent challenge with isoproterenol. The results of this study indicate that exposing freshly isolated rat ventricular myocytes to nanomolar levels of isoproterenol for as little as 10 min desensitizes them to subsequent challenge with agonist. The process appears to be homologous, since it was not observed when forskolin was used to elevate cAMP levels in the initial incubation.