EVIDENCE FOR PRESENCE AND HORMONAL-REGULATION OF PROTEIN PHOSPHATASE INHIBITOR-1 IN VENTRICULAR CARDIOMYOCYTE

Protein phosphatase inhibitor-1 (PPI-1) has been shown to be present i n heart tissue and smooth muscle. Whether PPT-1 is present in cardiomy ocytes is not known. The purpose of this study was to determine whethe r PPI-1 is present and is hormonally regulated in cardiomyocytes. A tr ichloroacetic acid (TCA) extract enriched in PPI-1 was isolated from g uinea pig ventricular cardiomyocytes. The TCA extract inhibited the ac tivity of type 1 protein phosphatase by 20 +/- 4% (n = 3 expts). On ph osphorylation by the catalytic subunit of adenosine 3',5'-cyclic monop hosphate-dependent protein kinase, the extent of this inhibition was a ugmented to 4.5-fold. Dephosphorylation of the phosphorylated TCA extr act by type 2 protein phosphatase reduced inhibition to 2 +/- 0.2% (n = 3 expts). To determine whether isoproterenol increases phosphorylati on of PPI-1 in cardiomyocytes, the TCA extracts were prepared from car diomyocytes treated with 1 mu M isoproterenol and from untreated cardi omyocytes. The inhibitory activity of the TCA extract in untreated car diomyocytes was 25 +/- 3% (n = 3 expts) and increased to 75 +/- 2% (n = 3 expts) in isoproterenol-treated cardiomyocytes. With the use of a rabbit skeletal muscle PPI-1 antibody, immunoblots of the TCA extract of cardiomyocytes identified a 28-kDa protein. A 28-kDa protein was al so immunoprecipitated from a TCA extract isolated from isoproterenol-t reated P-32-labeled cardiomyocytes. The immunoprecipitation was blocke d by the addition of excess amounts of purified rabbit skeletal muscle PPI-1. Isoproterenol-treated cardiomyocytes increased the phosphoryla tion of the 28-kDa protein by 232 +/- 20% (n = 3 expts) compared with untreated cardiomyocytes. We conclude that 1) the 28-kDa protein is PP I-1, 2) PPI-1 is present in ventricular cardiomyocytes, and 3) PPI-1 i s hormonally regulated. A decrease in type 1 protein phosphatase activ ity through phosphorylation of PPI-1 may be an important pathway for a ugmenting cardiac contractility.