Rc. Gupta et al., EVIDENCE FOR PRESENCE AND HORMONAL-REGULATION OF PROTEIN PHOSPHATASE INHIBITOR-1 IN VENTRICULAR CARDIOMYOCYTE, American journal of physiology. Heart and circulatory physiology, 39(4), 1996, pp. 1159-1164
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.