Pertussis toxin suppresses carbachol-evoked cardiodepression but does not modify cardiostimulation mediated through β₁- and putative β₄-adrenoceptors in mouse left atria: no evidence for β₂- and β₃-adrenoceptor function

Abstract.

Activation of β₁-, β₂-, β₃- and putative β₄-adrenoceptors modifies cardiac function. These receptors are usually coupled to G_s protein, but β₂- and β₃-adrenoceptors could also couple to G_i/o proteins. The mouse heart is used increasingly for studies of genetically disrupted or overexpressed proteins, including β-adrenoceptor subtypes. We therefore investigated in contracting mouse left atria (2 Hz, 37°C) if inactivation of G_i/o proteins with pertussis toxin modifies or uncovers effects mediated through β-adrenoceptor subtypes. The negative inotropic effects of carbachol in atria exposed to catecholamine or high calcium (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 of left atria with 200 ng/ml pertussis toxin for 24 h nearly abolished the carbachol responses. Although it has been reported that muscarinic receptor-mediated cardiodepression has an obligatory contribution of nitric oxide, the nitric oxide synthase inhibitor N ^G-monomethyl-l-arginine (0.1–1 mmol/l) did not modify the negative inotropic effects of carbachol, inconsistent with an involvement of nitric oxide. The positive inotropic effects of (–)-noradrenaline and (–)-adrenaline, mediated through β₁-adrenoceptors, were not affected by pertussis toxin. (–)-Adrenaline did not cause positive inotropic effects attributable to β₂-adrenoceptor-mediation, in the presence of CGP 20712A (300 nmol/l) to block β₁-adrenoceptors, in control atria or atria pretreated with pertussis toxin. The positive inotropic effects of (–)-CGP 12177 (1 µmol/l), a compound with agonist activity at the putative β₄-adrenoceptor, were unaffected by pertussis toxin. The β₃-adrenoceptor-selective agonist BRL 37344 (1 µmol/l), in the presence of (–)-propranolol (200 nmol/l), did not cause positive or negative inotropic effects in control and pertussis toxin-treated atria. In left atria obtained from mice injected with 150 µg/kg i.p. pertussis toxin which abolished carbachol-evoked cardiodepression, the positive inotropic effects of (–)-adrenaline were antagonised by CGP 20712A. The β₂-adrenoceptor-selective antagonist ICI 118551 (50 nmol/l) did not cause additional blockade of the effects of high (–)-adrenaline concentrations in the presence of CGP 20712A, ruling out the involvement of β₂-adrenoceptors. The results with intraparenteral PTX validate our in vitro PTX method. We conclude that inhibition of murine G_i/o proteins does not alter atrial positive inotropic effects mediated through β₁- and putative β₄-adrenoceptors and does not reveal functional β₂- and β₃-adrenoceptors.