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

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 (Bmax), the dissociation constant (Kd) 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, Bmax and Kd did not differ significantly between the two groups; Bmax; l, 14.7 ± 1.3 v C, 13.4 ± 0.9 f mol/mg protein; Kd: 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 MnCl2-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; MnCl2 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.