1. Calcium currents (ICa) were measured in single cells isolated from frog ventricle using the whole-cell patch-clamp technique and a perfused pipette. The dose-dependent stimulatory effects of isoprenaline (Iso, 0.1-100 microM) and forskolin (Fo. 0.1-50 microM) on ICa were determined in the presence and absence of acetylcholine (ACh, 10 microM) and/or threshold concentrations of Fo (0.2 microM) and Iso (0.05 microM), respectively. EC50 (i.e. concentration of Iso or Fo at which the response was 50% of the maximum) and Emax (i.e. maximal stimulation of Ica expressed as percentage increase in ICa with respect to control) were measured under each condition. 2. ACh increased EC50 for the stimulatory action of Iso on ICa from 0.84 to 3.72 microM while it reduced Emax from 658 to 185%. Thus, ACh mainly reduced the efficacy of Iso to stimulate ICa. 3. ACh increased EC50 for the stimulatory action of Fo on ICa from 2.06 to 10.26 microM but only slightly reduced Emax from 893 to 778%. Thus, ACh mainly reduced the potency of Fo to stimulate ICa. 4. Intracellular perfusion with 100 microM of hydrolysis-resistant GTP analogues, GTP-gamma-S [guanosine-5'-O-(3-thiotriphosphate)] and Gpp (NH)p (5'-guanylylimido-diphosphate), had no effect on basal ICa but reduced by greater than 50% the stimulatory effect of 2 microM-Iso on ICa. 5. In the presence of Gpp(NH)p or GTP-gamma-S, Fo (3 microM) reversibly increased ICa by 490%, as compared to a 717% increase in control (GTP) intracellular solution. Although ACh could still inhibit Fo-stimulated ICa, the degree of inhibition was significantly smaller than in the presence of GTP. 6. Extracellular perfusion with low concentrations of a combination of Iso (33 nM) and Fo (330 nM) enhanced ICa to a much greater extent than did either agent alone at 3 times higher concentrations. Thus, low concentrations of Iso and Fo appear to increase ICa in a synergistic fashion. 7. ICa stimulated by a combination of Iso and Fo appeared to be more resistant to inhibition by ACh than when stimulated by either alone. It was the efficacy, rather than the potency, of ACh to inhibit ICa that was reduced upon dual stimulation of ICa. 8. In the presence of 0.2 microM-Fo, EC50 and Emax for the effects of Iso on ICa were 0.27 microM and 619%, respectively. By comparison with the effects of Iso alone, Fo reduced EC50 approximately 3 times with no significant change in maximal stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)