THYROID-HORMONE AND HEMODYNAMIC REGULATION OF BETA-MYOSIN HEAVY-CHAINPROMOTER IN THE HEART

Thyroid hormone exerts marked effects on cardiovascular function. Expr ession of cardiac alpha- and beta-myosin heavy chain (MHC) isoforms ca n be altered in response to thyroid hormone as well as by hemodynamic changes imposed on the heart. The molecular mechanisms that mediate th ese changes are not completely known. We studied the contractile and t hyroid hormone responsiveness of the beta MHC promoter in both culture d cardiac myocytes and in vivo by direct DNA transfer. Using transient transfection of neonatal rat cardiomyocytes, the activities of recomb inant reporter plasmids containing beta MHC 5'-flanking sequences term inating at positions -2250, -1145, -670, and -354 were decreased signi ficantly in cultures containing L-T-3 (50 nM). Similar deletion analys is showed that 5'-flanking regions terminating within -2250 to -151 bp were contractily responsive; however, deletion to position -126 atten uated this response. In vivo beta MHC promoter activity, determined by injecting the recombinant plasmid into the myocardium, was significan tly higher by 2-fold in hypothyroid than in euthyroid ventricles (2.47 +/- 0.41 vs. 1.33 +/- 0.25 luciferase(chloramphenicol acetyltransfera se; P < 0.05). Increased ventricular workload, produced by aortic coar ctation for 5 days, resulted in ventricular hypertrophy (heart/body we ight, 4.05 +/- 0.19 us. 3.42 +/- 0.16 mg/g; P < 0.02) and a 3.4-fold i ncrease in beta MHC messenger RNA content. However, beta MHC promoter activity in vivo was not significantly different between rats experien cing aortic coarctation and sham-operated rats (1.49 +/- 0.41 vs. 0.96 +/- 0.27 luciferase/chloramphenicol acetyltransferase, respectively) and was similar to that in euthyroid animals. These results show that beta MHC promoter activity is T-3 responsive in cultured myocytes and in vivo, but that the increase in beta MHC messenger RNA observed in t he in vivo pressure-overloaded myocardium cannot be explained entirely by transcription control mechanisms.