β-Adrenoceptor-G protein-Adenylate Cyclase Complex in Rat Hearts with Ischemic Heart Failure Produced by Coronary Artery Ligation

Abstract

We examined the role of the myocardial β-adrenoceptor-G protein-adenylate cyclase complex in 10-week-old Wistar rats with ischemic heart failure produced by ligating the left coronary artery (l) and in sham-operated control rats (C). We determined the number of β-adrenoceptors (B max), the dissociation constant (K d) using a binding assay and adenylate cyclase activity. Levels of mRNA encoding for the α subunit of the stimulatory guanine nucleotide-binding protein (Gsα) and the α subunit of the inhibitory guanine nucleotide-binding protein (Giα) were measured by Northern blot analysis. The amounts of Gsα and Giα were measured by Western blot analysis, B max and K d did not differ significantly between the two groups; B max; l, 14.7 ± 1.3 v C, 13.4 ± 0.9 f mol/mg protein; K d: l, 345 ± 31 v C, 340 ± 28 pM (mean ± standard error, S.E.). There were no significant differences in Gsα and Giα concentrations between the two groups as measured by Northern blot analysis (Gsα: l, 91.6 ± 4.5 v C, 96.5 ± 2.3%; Giα l, 95.4 ± 3.6 v C. 90.0 ± 3.0%) or by Western blot analysis (Gsα: l, 95.2 ± 2.0 v C. 94.5 ± 2.6%; Giα l, 91.5 ± 3.0 v C. 95.1 ± 2.9%). Activity of basal and MnCl 2-stimulated adenylate cyclase did not differ significantly in the two groups; basal: l, 7.5 ± 0.7 v C, 8.1 ± 0.5 pmol cAMP/mg protein/min; MnCl 2 l, 80.8 ± 5.8 v C, 86.4 ± 6.7 pmol cAMP/mg protein/min. Sodium fluoride and forskolin-stimulated adenylate cyclase activity were significantly lower in the hearts with ischemic failure compared with controls (sodium fluoride: l, 68.5 ± 5.6 v C. 103 ± 4.8 pmol cAMP/mg protein/min: forskolin; l, 84.6 ± 6.5 v C, 117.1 ± 5.6 pmol cAMP/mg protein/min). These data suggest the presence of myocardial Gsα dysfunction in ischemic heart failure. We conclude that such a dysfunction in Gsα may contribute to the contractile abnormalities in ischemic heart failure.