SLOW SKELETAL TROPONIN-I GENE-TRANSFER, EXPRESSION, AND MYOFILAMENT INCORPORATION ENHANCES ADULT CARDIAC MYOCYTE CONTRACTILE FUNCTION

The functional significance of the developmental transition from slow skeletal troponin I (ssTnI) to cardiac TnI (cTnI) isoform expression i n cardiac myocytes remains unclear, We show here the effects of adenov irus-mediated ssTnI gene transfer on myofilament structure and functio n in adult cardiac myocytes in primary culture, Gene transfer resulted in the rapid, uniform, and nearly complete replacement of endogenous cTnI with the ssTnI isoform with no detected changes in sarcomeric ult rastructure, or in the isoforms and stoichiometry of other myofilament proteins compared with control myocytes over 7 days in primary cultur e, In functional studies on permeabilized single cardiac myocytes, the threshold for Ca2+-activated contraction was significantly lowered in adult cardiac myocytes expressing ssTnI relative to control values, T he tension-Ca2+ relationship was unchanged from controls in primary cu ltures of cardiac myocytes treated with adenovirus containing the adul t cardiac troponin T (TnT) or cTnI cDNAs. These results indicate that changes in Ca2+ activation of tension in ssTnI-expressing cardiac myoc ytes were isoform-specific, and not due to nonspecific functional chan ges resulting from overexpression of a myofilament protein, Further, C a2+-activated tension development was enhanced in cardiac myocytes exp ressing ssTnI compared with control values under conditions mimicking the acidosis found during myocardial ischemia, These results show that ssTnI enhances contractile sensitivity to Ca2+ activation under physi ological and acidic pH conditions in adult rat cardiac myocytes, and d emonstrate the utility of adenovirus vectors for rapid and efficient g enetic modification of the cardiac myofilament for structure/function studies in cardiac myocytes.