DIFFERENTIAL REGULATION OF CARDIAC CA2+ AND K+ CURRENTS BY MUSCARINIC RECEPTORS

The ability of acetylcholine (ACh) to inhibit beta-agonist stimulated L-type calcium current (beta-I-Ca) was compared to its ability to activate the inwardly rectifying potassium current IK(ACh) in frog atrial myocytes. ACh inhibited beta-I-Ca at concentrations (EC(50) = 8 nM) significantly lower than those required for the activation of I-K(ACh) (EC(50) = 101 nM) However, pharmacological specificities suggest that both responses are mediated by the same (m(2)) type of muscarinic receptors (m-AChR). Intracellular application of GDP beta S completely abolished both responses, implying that both actions of ACh are coupled to effecters by G-proteins. In contrast, intracellular application of pertussis toxin (PTX) shifted to higher concentrations (EC(50) = 170 nM) but did not abolish inhibition of beta-I-Ca, even though the block of the I-K(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 I-K(ACh) Although atropine blocked the above actions of ACh, atropine was found to exert its own action in the absence of ACh. In frog ventricular cells, atropine induced a dose-dependent stimulation of beta-I-Ca (EC(50) = 0.6 nM; E(max) = 35%). However, atropine had no effect on basal I-Ca or on cAMP-stimulated I-Ca. This suggested that an increased adenylyl cyclase activity was required for atropine to produce its stimulatory effect on I-Ca. In the presence of Gpp(NH)p, atropine (1 mu M) slowed down by a factor of 2 the rate of spontaneous inhibition of beta-I-Ca. Similarly, atropine slowed down by a factor of 2-3 the rate of spontaneous activation of I-K(ACh) by Gpp(NH)p. Other m(2)-selective m-AChR antagonists mimicked these effects of atropine. It was concluded that unoccupied cardiac muscarinic receptors produce a significant spontaneous G-protein activation which is impaired upon antagonist binding, and strongly enhanced upon agonist binding.