Regional expression of protein phosphatase type 1 and 2A catalytic subunitisoforms in the human heart

In mammalian species, including man, the duration of myocardial contraction is shorter in atria than ventricles. Total contraction time depends at lea st in part on phosphorylation and dephosphorylation of cardiac regulatory p roteins. Dephosphorylation reactions are mediated by protein phosphatases. In the mammalian heart more than 90% of the protein phosphatase (PP) activi ty consists of PP1 and PP2A. Therefore, the aim of this study was to invest igate which isoforms of PP1 and PP2A are present in human myocardium and wh ether their expression is regionally different. RT-PCR and Northern blottin g revealed that all isoforms of PP1 and PP2A presently known are expressed in the human heart. Expression levels of PP1 alpha, delta, and gamma as wel l as 2A alpha were higher in right ventricles than in right atria. However, there was no such difference for PP2A beta. At the protein level PP1 alpha was unchanged, whereas PP2A, was by 56% higher in right ventricles compare d to atria. The phosphorylation state of TnI was lower in right ventricle t han in right atrium. Thus, lower protein expression of PP2A in atrium could contribute to the faster relaxation by increasing the phosphorylation stat e of TnI. We conclude that expression of PP1 and PP2A isoforms is regionall y regulated in the human heart. (C) 2000 Academic Press.