Thyroid hormone exerts marked effects on cardiovascular function. Expression of cardiac alpha- and
beta-myosin heavy chain (MHC)
isoforms can be altered in response to
thyroid hormone as well as by hemodynamic changes imposed on the heart. The molecular mechanisms that mediate these changes are not completely known. We studied the contractile and
thyroid hormone responsiveness of the betaMHC promoter in both cultured cardiac myocytes and in vivo by direct
DNA transfer. Using transient transfection of neonatal rat cardiomyocytes, the activities of recombinant reporter plasmids containing betaMHC 5'-flanking sequences terminating at positions -2250, -1145, -670, and -354 were decreased significantly in cultures containing L-T3 (50 nM). Similar deletion analysis showed that 5'-flanking regions terminating within -2250 to -151 bp were contractility responsive; however, deletion to position -126 attenuated this response. In vivo betaMHC promoter activity, determined by injecting the recombinant plasmid into the myocardium, was significantly higher by 2-fold in
hyperthyroid than in euthyroid ventricles (2.47 +/- 0.41 vs. 1.33 +/- 0.25
luciferase/
chloramphenicol acetyltransferase; P<0.05). Increased ventricular workload, produced by
aortic coarctation for 5 days, resulted in ventricular
hypertrophy (heart/
body weight, 4.05 +/- 0.19 vs. 3.42 +/- 0.16 mg/g; P < 0.02) and a 3.4-fold increase in betaMHC
messenger RNA content. However, betaMHC promoter activity in vivo was not significantly different between rats experiencing
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 betaMHC promoter activity is T3 responsive in cultured myocytes and in vivo, but that the increase in betaMHC
messenger RNA observed in the in vivo pressure overloaded myocardium cannot be explained entirely by transcription control mechanisms.