Inactivation of Giα proteins increases arrhythmogenic effects of β-adrenergic stimulation in the heart

Chronic treatment of rats with carbachol downregulates M-cholinoceptors and inhibitory, pertussis toxin (PTX)-sensitive G protein alpha-subunits (G(i alpha)) and sensitizes the heart to arrhythmogenic effects of isoprenaline (ISO), suggesting a causal relationship. To test this hypothesis by a more direct and quantitative approach, nine groups of rats were treated for 24 h with increasing doses of PTX (1.25-200 mu g/kg i.v.). Inactivation of cardiac G(i alpha) was determined biochemically by P-32-ADP-ribosylation in vitro and functionally by measuring contractile effects of carbachol. Effects of ISO were studied in spontaneously beating right atria (RA) and isolated papillary muscles (PM; paced at 1 Hz), PTX increased heart rate in conscious animals (ECG) with a bell-shaped dose-dependency (maximal increase 120 beats/min at 7.5 mu g/kg). PTX dose-dependently inactivated 25-85% of total cardiac G(i alpha), which linearly correlated with a loss of the direct negative chronotropic effect of carbachol in atria, but not with a loss of its indirect negative inotropic effect in PM. The latter was resistant up to PTX 20 mu g/kg (= 70% inactivation), The decrease in G(i alpha) closely correlated with an increased efficacy of ISO to induce spontaneous contractile activity (automaticity) in PM. At 3 mu mol/l ISO, all PM from PTX 200 mu g/kg beat spontaneously compared to 10% in control, In contrast, pretreatment with PTX only modestly and not clearly dose-dependently increased the inotropic potency of ISO (PTX 100 mu g/kg: EC50 28 v 81 nmol/l in control) and did not affect the chronotropic effect of ISO. The disparity of the functional consequences of PTX treatment suggest that under physiological conditions, G(i alpha) serve mainly to suppress arrhythmogenic, but not or to a minor extent, positive chronotropic or inotropic effects of beta-adrenoceptor activation. (C) 1998 Academic Press.