THE EFFECT OF ANGIOTENSIN-II ON MYOSIN HEAVY-CHAIN EXPRESSION IN CULTURED MYOCARDIAL-CELLS

Angiotensin II (AII), the principal mediator of the renin-angiotensin system, is an important regulator of vascular and cardiac homeostasis. AII has also been shown to be a regulator of cardiac hypertrophy and of the corresponding changes in amount and composition of certain tiss ue proteins. We examined the trophic effects of AII on cultured myocyt es derived from neonatal rat ventricles and followed, by Northern blot analysis and polyacrylamide gel electrophoresis, the expression of al pha- and beta-myosin heavy chain iso-mRNAs and isoproteins. Our findin gs show that a single administration of AII is sufficient to induce a trophic response in cultured beating myocytes and to enhance the expre ssion of P-myosin heavy chain iso-mRNA and isoprotein, having no effec t on alpha-myosin heavy chain. Induction of alpha-myosin heavy chain e xpression by thyroid hormone before AII was administered showed that A II could not potentiate a shift from alpha- to beta-myosin heavy chain predominance. We suggest that the potency of AII to regulate the expr ession of myosin heavy chain isogenes is restricted to the beta isofor m and is overridden by thyroid hormone.