AJP - Heart and Circulatory Physiology, Vol 234, Issue 4 426-H431, Copyright © 1978 by American Physiological Society
Enhanced phosphorylation of myocardial sarcoplasmic reticulum in experimental hyperthyroidism
C. J. Limas
Calcium transport by cardiac sarcoplasmic reticulum (SR) was compared in
hyperthyroid (HT) and euthyroid (ET) rats. Both Ca2+ uptake (97 +/- 3.1
nmol/mg per min in HT vs. 63 +/- 2.9 nmol/mg per min in ET, P less than
0.01) and CA2+ -stimulated ATPase activity (61 +/- 4.1 vs. 37 +/- 1.6 nmol
Pi/mg per min, P less than 0.01) were higher in the thyroxine-treated
animals. These changes were accompanied by enhanced cyclic AMP-dependent
phosphorylation of cardiac SR in hyperthyroid rats (180 +/- 4.3 pmol Pi/mg
per min vs. 117 +/- 4.2 pmol Pi/mg per min, P less than 0.01).
SDS-polyacrylamide gel electrophoresis of cardiac SR showed that
phosphorylation of a 22,000-dalton protein (phospholamban) primarily
accounted for the differences between the two groups. There was no
difference in the rate of SR dephosphorylation by endogenous phosphoprotein
phosphatase between HT and ET rats. Differences in cyclic AMP-dependent
phosphorylation between the two groups were blunted in the presence of
excess exogenous cyclic AMP-dependent protein kinase. These results suggest
that increased levels or activity of endogenous cyclic AMP-dependent
protein kinases may partially explain enhanced calcium transport by the
cardiac SR of hyperthyroid animals.
