Pertussis toxin suppresses carbachol-evoked cardiodepression but does not modify cardiostimulation mediated through beta(1)- and putative beta(4)-adrenoceptors in mouse left atria: no evidence for beta(2)- and beta(3)-adrenoceptor function

Activation of beta(1)-, beta(2)-, beta(3)- and putative beta(4)-adrenocepto rs modifies cardiac function. These receptors are usually coupled to G(s) p rotein, but beta(2)- and beta(3)-adrenoceptors could also couple to G(i/o) proteins. The mouse heart is used increasingly for studies of genetically d isrupted or overexpressed proteins, including beta-adrenoceptor subtypes. W e therefore investigated in contracting mouse left atria (2 Hz, 37 degrees C) if inactivation of G(i/o) proteins with pertussis toxin modifies or unco vers effects mediated through beta-adrenoceptor subtypes. The negative inot ropic effects of carbachol in atria exposed to catecholamine or high calciu m (6.8 mmol/l) were assumed to be mediated through activation of muscarinic receptors coupled to G(i/o). We report conditions under which incubation o f left atria with 200 ng/ml pertussis toxin for 24 h nearly abolished the c arbachol responses. Although it has been reported that muscarinic receptor- mediated cardiodepression has an obligatory contribution of nitric oxide, t he nitric oxide synthase inhibitor N-G-monomethyl-L-arginine (0.1-1 mmol/l) did not modify the negative inotropic effects of carbachol, inconsistent w ith an involvement of nitric oxide. The positive inotropic effects of (-)-n oradrenaline and (-)-adrenaline, mediated through beta(1)-adrenoceptors, we re not affected by pertussis toxin. (-)-Adrenaline did not cause positive i notropic effects attributable to beta(2)-adrenoceptor-mediation, in the pre sence of CGP 20712A (300 nmol/l) to block beta(1)-adrenoceptors, in control atria or atria pretreated with pertussis toxin. The positive inotropic eff ects of (-)-CGP 12177 (1 mu mol/l), a compound with agonist activity at the putative beta(4)-adrenoceptor, were unaffected by pertussis toxin. The bet a(3)-adrenoceptor-selective agonist BRL 37344 (1 mu mol/l), in the presence of (-)-propranolol (200 nmol/l), did not cause positive or negative inotro pic effects in control and pertussis toxin-treated atria. In left atria obt ained from mice injected with 150 mu g/kg i.p. pertussis toxin which abolis hed carbachol-evoked cardiodepression, the positive inotropic effects of (- )-adrenaline were antagonised by CGP 20712A. The beta(2)-adrenoceptor-selec tive antagonist ICI 118551 (50 nmol/l) did not cause additional blockade of the effects of high (-)-adrenaline concentrations in the presence of CGP 2 0712A, ruling out the involvement of beta(2)-adrenoceptors. The results wit h intraparenteral PTX validate our in vitro PTX method. We conclude that in hibition of murine G(i/o) proteins does not alter atrial positive inotropic effects mediated through beta(1)- and putative beta(4)-adrenoceptors and d oes not reveal functional beta(2)- and beta(3)-adrenoceptors.