Functional interaction of a β-adrenergic agonist and cyclic GMP phosphodiesterase inhibitor in control and hypertrophic cardiomyocytes

This study tested the hypothesis that the positive inotropic effect of β-adrenoceptor stimulation would be inhibited by increases in cyclic GMP in control cardiomyocytes and that this response would be modified in hypertrophic cardiomyocytes. Cell functional data as well as GMP and cyclic AMP data were collected from 7 control and 7 1K1C (one-kidney-one-clip) renal hypertensive hypertrophic rabbits. Using isolated control and IKIC ventricular myocytes, data were obtained at baseline and after treatment with the β-adrenoceptor agonist isoproterenol (10^{-8, -6} mol/l) or the cyclic GMP-phosphodiesterase inhibitor zaprinast (10^-5 mol/l) followed by isoproterenol (10^{-8, -6} mol/l). We found that in control rabbits, isoproterenol (10^-6 mol/l) increased percent shortening (4.8 ± 0.2 to 6.4 ± 0.3%) and cyclic AMP (2.3 ± 0.3 to 5.0 ± 0.7 pmol/10⁵ cells). Zaprinast 10^-5 mol/l increased cyclic GMP (150 ± 20 to 209 ± 14 fmol/10⁵ cells) and decreased percent shortening (6.2 ± 0.4 to 5.2 ± 0.3). Zaprinast 10 ^-5 mol/l prevented the functional response to isoproterenol in control (5.2 ± 0.3 to 4.7 ± 0.3), without changing cyclic AMP levels. In 1K1C rabbits, isoproterenol (10^-6 mol/l) increased cyclic AMP (4.9 ± 0.8 to 7.6 ± 1.4 pmol/10⁵ cells) without changing function. Zaprinast 10^-5 mol/l increased cyclic GMP (182 ± 23 to 233 ± 24 fmol/10⁵ cells) and decreased percent shortening (6.6 ± 0.9 to 4.7 ± 0.5), but did not alter the lack of effect of isoproterenol in 1K1C. In control cardiomyocytes, cyclic GMP blunted the isoproterenol contraction response without changing cyclic AMP levels, but isoproterenol's functional effect was not seen in 1K1C cardiomyocytes.