ISOPRENALINE-INDUCED INCREASE IN MESSENGER-RNA LEVELS OF INHIBITORY G-PROTEIN ALPHA-SUBUNITS IN RAT-HEART

Long-term beta-adrenergic stimulation has been shown to desensitize the beta-adrenoceptor/adenylyl cyclase signalling pathway at both the receptor and the G-protein level. To further elucidate the cellular mechanism of G-protein regulation we investigated the influence of prolonged infusion of isoprenaline (2.4 mg/kg.d) on myocardial mRNA levels of different G-protein alpha-subunits in rats. For comparison rats were treated with triiodothyronine (T3; 0.5 mg/kg.d) which induces cardiac hypertrophy like isoprenaline but has different effects on the adenylyl cyclase system. Isoprenaline- and T3-treated animals developed an increase in heart/body weight ratio of 41 +/- 3% and 27 +/- 4%, respectively (P < 0.05). Isoprenaline increased myocardial total RNA concentration by 39 +/- 6% (P < 0.05). Hybridization with P-32-labeled rat cDNAs demonstrated an expression rank order of G(s-alpha)-mRNA > G(i-alpha-2)-mRNA > G(i-alpha-3)-mRNA and no detectable expression of G(i-alpha-1)-mRNA in rat myocardium. mRNA levels of G(s-alpha), G(i-alpha-2) and G(i-alpha-3) were 36.9 +/- 1.28, 10.7 +/- 1.07 and 3.7 +/- 0.19 pg/mu-g total RNA, respectively. Isoprenaline increased G(i-alpha-2)- and G(i-alpha-3)-mRNA concentrations per mu-g total RNA by 49 +/- 18% and 27 +/- 7%, respectively (P < 0.05). This effect was abolished by simultaneously administered propranolol (9.9 mg/kg.d), indicating a beta-adrenoceptor-mediated mechanism. In contrast, T3-induced cardiac hypertrophy was not accompanied by changes in G(i-alpha)-mRNA expression. G(s-alpha)-mRNA levels were unaffected by either treatment. In conclusion, long-term stimulation with isoprenaline in vivo induces a beta-adrenoceptor-mediated increase in myocardial G(i-alpha-2)- and G(i-alpha-3)-mRNA without affecting G(s-alpha)-mRNA. These results suggest that similar increases in myocardial G(i-alpha-2)-mRNA in end-stage human heart failure may be at least partly explained by increased beta-adrenergic stimulation due to increased sympathetic activity.