BETA-ADRENOCEPTOR-COUPLED G(S) PROTEIN FACILITATES THE ACTIVATION OF CAMP-DEPENDENT CARDIAC CL- CURRENT

Here a comparison is made between adenosine 3',5'-cyclic monophosphate (cAMP)-activated Cl- current (I-Cl) density and activation time cours e in response to beta-adrenoceptor stimulation with isoproterenol and adenylyl cyclase activation with forskolin. Saturating concentrations of isoproterenol and forskolin failed to activate an I-Cl in guinea pi g atrial as well as in rat and frog ventricular cardiomyocytes. In gui nea pig ventricular cardiomyocytes, step application of 1 mu M isoprot erenol induced an I-Cl of -0.89 +/- 0.32 pA/pF (holding potential -40 mV, temperature 22 +/- 1 degrees C). I-Cl activation started after 3 /- 1 s, was complete within 44 +/- 9 s, and was abolished after cell d ialysis with the Rp diastereomer of adenosine 3',5'-cyclic monophospho thioate. Stimulation with increasing concentrations of forskolin (0.01 -10 mu M) increased I-Cl density and accelerated Icl activation. With 1 mu M forskolin, I-Cl density was maximal (-0.57 +/- 0.30 pA/pF) but significantly smaller than that achieved with 1 mu M isoproterenol. Al though I-Cl density could not be further augmented by forskolin >1 mu M, current activation (latency 28 +/- 8 s, full activation after 112 /- 8 s with 1 mu M forskolin) was further accelerated by 3 and 10 mu M forskolin. However, I-Cl activation with 10 mu M forskolin was still slower than that with 1 mu M isoproterenol. A low isoproterenol concen tration (1 nM), which did not activate I-Cl by itself, accelerated the 1 mu M forskolin-induced activation of I-Cl by 35%; this speeding up was abolished after cell dialysis with guanosine 5'-O-(2-thiodiphospha te). I-Cl deactivation after the washout of 1 mu M forskolin or 1 mu M isoproterenol followed a similar time course. After stimulation with 10 mu M forskolin or 1 mu M forskolin + 1 mu M isoproterenol, but not with 1 mu M forskolin + 1 nM isoproterenol, the decay of I-Cl was sign ificantly delayed. These results indicate that both cAMP-dependent and cAMP-independent G protein pathways contribute to the regulation of g uinea pig ventricular I-Cl.