G-PROTEIN EXPRESSION AND ADENYLATE-CYCLASE REGULATION IN VENTRICULAR CARDIOMYOCYTES FROM STZ-DIABETIC RATS

Isolated adult ventricular cardiomyocytes have been used to study the effects of insulin-deficient diabetes on the expression of cardiac G protein alpha-subunits. Immunoblot analysis of plasma membranes revealed the presence of three different G(s) proteins with molecular masses of 45, 47, and 52 kDa. Furthermore, cardiomyocytes were found to contain G(i-2) (41 kDa) and G(o) (39 kDa). Heart cells from streptozotocin-diabetic rats exhibited an unaltered expression of the G(s) proteins, whereas G(i-2) and G(o) were reduced by 58 +/- 2 and 27 +/- 11%, respectively. In cells from diabetic rats, adenosine 3',5'-cyclic monophosphate (cAMP) accumulation in response to isoproterenol decreased by similar to 30% at agonist concentrations of 10(-7) to 10(-5) M, with an unaltered maximum stimulation by forskolin. Treatment of cardiomyocytes with pertussis toxin resulted in an incremental increase of isoproterenol-stimulated cAMP formation, which was significantly lower in cardiac myocytes from streptozotocin-diabetic animals (19.2 +/- 1.7 vs. 11.5 +/- 2.4 pmol cAMP.5 x 10(4) cells(-1).10 min(-1)). The inhibition of the isoproterenol-induced cAMP accumulation by carbachol in the intact cell was not altered in streptozotocin-diabetes. In conclusion, our data show that insulin-deficient diabetes is associated with a reduced expression and concomitant functional loss of G(i) in ventricular cardiomyocytes. Receptor-mediated inhibition of adenylate cyclase remains unaffected by this process, whereas the beta-adrenergic stimulatory pathway involves an additional defect upstream of the adenylate cyclase/G protein system.