Inhibition of type 1 protein phosphatase activity by activation of β-adrenoceptors in ventricular myocardium

The regulation of protein phosphatase (PP) activity by cardiac β-adrenergic receptor stimulation with isoproterenol (ISO) was studied in four groups of guinea pigs consisting of seven animals each. Group 1 received the vehicle solution only intraperitoneally; group 2, 6 μg/kg of ISO; group 3, 60 μg/kg of ISO; and group 4, 600 μg/kg of ISO. Total PP activity (consisting of both type 1 and type 2A PP), activity of each PP subtype, the cAMP-dependent protein kinase activity ratio (−cAMP/+cAMP), the phosphorylation of PP inhibitor 1, and the phosphorylation of phospholamban were measured in ventricular tissue. PP activity was also studied in ventricular cardiomyocytes isolated from guinea pigs treated with and without 1 μM ISO or 1 μM ISO plus 10 μM propranolol, an antagonist of the β-adrenoceptor. PP activity decreased significantly in membrane vesicles, but not in cytosolic fractions, of guinea pigs treated with 60 and 600 μg/kg of ISO compared with untreated animals. The PKA activity ratio, PLB phosphorylation, and PP inhibitor 1 phosphorylation increased in ventricles of guinea pigs treated with 60 and 600 μg/kg of ISO compared with vehicle-treated animals. The decrease in overall PP activity was due primarily to a reduction in type 1 but not type 2A PP activity. In isolated ventricular cardiomyocytes, PP activity was decreased significantly after treatment with 1 μM ISO, and this inhibition was reversed by treatment with 10 μM propranolol. The membrane vesicles of group 1 animals did not release any catalytic subunit of type 1 PP upon phosphorylation by exogenous PKA. These results indicate that activation of cardiac β-adrenoceptors inhibits type 1 PP activity via phosphorylation of PP inhibitor 1 in the ventricles. This effect is associated with the well-known effect of ISO on increases in the PKA activity ratio and PLB phosphorylation. Inhibition of type 1 PP activity could be one possible mechanism, in addition to activation of adenylate cyclase, by which ISO mediates enhanced contractility of the heart.