INACTIVATION OF G(I-ALPHA) PROTEINS INCREASES ARRHYTHMOGENIC EFFECTS OF BETA-ADRENERGIC STIMULATION IN THE HEART

Chronic treatment of rats with carbachol downregulates M-cholinoceptor s and inhibitory, pertussis toxin (PTX)-sensitive G protein alpha-subu nits (G(i alpha)) and sensitizes the heart to arrhythmogenic effects o f isoprenaline (ISO), suggesting a causal relationship. To test this h ypothesis by a more direct and quantitative approach, nine groups of r ats 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 biochemica lly by P-32-ADP-ribosylation in vitro and functionally by measuring co ntractile effects of carbachol. Effects of ISO were studied in spontan eously beating right atria (RA) and isolated papillary muscles (PM; pa ced 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) clo sely correlated with an increased efficacy of ISO to induce spontaneou s 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, I n contrast, pretreatment with PTX only modestly and not clearly dose-d ependently increased the inotropic potency of ISO (PTX 100 mu g/kg: EC 50 28 v 81 nmol/l in control) and did not affect the chronotropic effe ct of ISO. The disparity of the functional consequences of PTX treatme nt suggest that under physiological conditions, G(i alpha) serve mainl y to suppress arrhythmogenic, but not or to a minor extent, positive c hronotropic or inotropic effects of beta-adrenoceptor activation. (C) 1998 Academic Press.