The ability of acetylcholine (ACh) to inhibit beta-agonist stimulated calcium current was compared to its ability to activate the inwardly rectifying potassium current IK(ACh) in frog atrial myocytes. As suggested by previous studies, ACh inhibited the calcium current at concentrations (EC50 = 8 nM) significantly lower than those required for the activation of IK(ACh) (EC50 = 101 nM). The pharmacological profiles of the two responses suggest that despite the differences in agonist sensitivity, both are mediated by the same (m2) type of muscarinic receptors. Intracellular application of GDP beta S, an inhibitor of G protein function, completely abolished both responses, implying that both actions of ACh are coupled to effectors by G proteins. In contrast, intracellular application of pertussis toxin (PTX) shifted to higher concentrations (EC50 = 170 nM) but did not abolish inhibition of the calcium current by ACh even though the block of the IK(ACh) response was complete. Increasingly large PTX concentrations and/or prolonged PTX treatments revealed a limiting, PTX-resistant inhibitory component that appears to be mediated by a PTX-insensitive G protein distinct from that mediating IK(ACh). For the PTX-sensitive components, the different agonist dependencies of IK(ACh) activation and calcium current inhibition may imply that different G proteins mediate each response although alternate possibilities involving the same G protein either functionally sequestered and/or differentially affected by interactions with effectors, can not be ruled out.

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