Pharmacological characterization of protein phosphatase activities in preparations from failing human hearts

P-Adrenoceptor stimulation acts in the heart in part by increasing the phos phorylation state of phospholamban and phospholemman. There is evidence tha t the beta-adrenoceptor-mediated increase in phospholamban phosphorylation is in part due to inhibition of type 1 phosphatases. The aim of the present study was to elucidate which phosphatases dephosphorylate phospholamban an d phospholemman in the human heart. In the past, cardiac serine/threonine p hosphatases have been studied using phosphorylase a as substrate. Here, typ e 1 and type 2A phosphatase activities were studied in preparations from fa iling human hearts using phosphorylated phospholamban and phospholemman as substrates. Phospholamban and phospholemman phosphatase activity was detect able in human cardiac homogenates. Moreover, using a heparin-Sepharose colu mn, the catalytic subunits of type 1 and type 2A phosphatases could be sepa rated from human ventricles. Okadaic acid and cantharidin inhibited phospha tase activities dephosphorylating phospholamban, phospholemman, and phospho rylase a in homogenates in a concentration-dependent manner. However, okada ic acid was more potent. Cantharidin inhibited type 2A and type 1 activitie s against all substrates studied with IC50 values <15 nM and >290 nM, respe ctively. Okadaic acid inhibited type 1 and type 2A phosphatase activities a s effectively but 10-30 times more potently than cantharidin. This work pro vides evidence that in the human heart, type 1 and 2A phosphatases are invo lved in the dephosphorylation of phospholamban and phospholemman and could play a role in the effects of beta-adrenergic stimulation in the heart.