DIVERGENCE OF BETA-MYOSIN HEAVY-CHAIN (BETA-MHC) EXPRESSION IN FETAL-RAT CARDIOMYOCYTES IN-VITRO AND ADULT-RAT HEART IN-VIVO

The myosin heavy chain gene products are an important determinant of m yocardial functional properties. Although a strong positive element (b eta f1) within the beta MHC promoter region has previously been identi fied, to date no species comparisons in promoter strength have been ma de. To examine this question, we have used beta MHC deletion construct s, containing the rat or human beta f1 enhancer region, to determine e xpression both in vitro using rat fetal cardiomyocytes and in vivo by direct injection into adult rat heart. When reporter constructs were t ransfected into cultured fetal rat cardiomyocytes, the human beta repo rter was expressed more than 3 fold above the equivalent rat construct . Exchange of the beta f1 enhancer indicated that the human sequence o f the beta f1 enhancer was largely responsible. However, these finding s were not replicated when the reporters were injected into the adult rat heart. In the adult myocardium the levels of reporter expression w ere similar for the beta MHC promoter reporters studied. These finding s demonstrate a divergence between primary cardiomyocytes maintained i n culture and the cardiomyocytes found within the intact adult heart. (C) 1997 Academic Press