Evidence for the presence and hormonal regulation of protein
phosphatase inhibitor-1 in ventricular cardiomyocyte.
Gupta, Ramesh C., Joachim Neumann, August M. Watanabe, Michael Lesch,
Hani N. Sabbah.
Department of Medicine, Division of Cardiovascular Medicine Henry
Ford Heart and Vascular Institute, Henry Ford Hospital, Detroit,
Michigan 48202, Abteilung Allgemeine Pharmakologie, Universitats
-Krankenhaus Eppendorf, Martinistra e, 20246 Hamburg, FRG, Lilly
Research Laboratories, Eli Lilly and Company, Lilly Corporate Center,
Indianapolis, IN 46285
APStracts 2:0429H, 1995.
Protein phosphatase inhibitor-1 (PPI-1) has been shown to be present
in heart tissue and smooth muscle. Whether PPI-1 is present in
cardiomyocytes is not known. The purpose of this study was to
determine if PPI-1 is present and is hormonally regulated in
cardiomyocytes. A trichloroacetic acid (TCA) extract enriched in PPI
-1 was isolated from guinea pig ventricular cardiomyocytes. The TCA
extract inhibited the activity of type-1 protein phosphatase by 20
+/- 4% (n=3). Upon phosphorylation by the catalytic subunit of cAMP
-dependent protein kinase, the extent of this inhibition was augmented
to 4.5 fold. Dephosphorylation of the phosphorylated TCA extract by
type-2 protein phosphatase reduced inhibition to 2 +/- 0.2% (n=3). To
determine if isoproterenol increases phosphorylation of PPI-1 in
cardiomyocytes, the TCA extracts were prepared from cardiomyocytes
treated with and without 1 [mu]M isoproterenol. The inhibitory
activity of the TCA extract in untreated cardiomyocytes was 25 +/- 3%
(n=3) and increased to 75 +/- 2% (n=3) in isoproterenol-treated
cardiomyocytes. Using a rabbit skeletal muscle PPI-1 antibody,
immunoblots of the TCA extract of cardiomyocytes identified a 28 kDa
protein. A 28 kDa protein was also immunoprecipitated from a TCA
extract isolated from isoproterenol-treated 32P-labeled
cardiomyocytes. The immunoprecipitation was blocked by the addition
of excess amounts of purified rabbit skeletal muscle PPI-1.
Isoproterenol-treated cardiomyocytes increased the phosphorylation of
the 28 kDa protein by 232 +/- 20% (n=3) compared to untreated
cardiomyocytes. We conclude that 1) the 28 kDa protein is PPI-1, 2)
PPI-1 is present in ventricular cardiomyocyte and 3) PPI-1 is
hormonally regulated. A decrease in type-1 protein phosphatase
activity through phosphorylation of PPI-1 may be an important pathway
for augmenting cardiac contractility.
Received 10 April 1995; accepted in final form 15 August 1995.
APS Manuscript Number H347-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 31 October 95