TRIIODOTHYRONINE RESTRICTS MYOFIBRILLAR GROWTH AND ENHANCES BEATING FREQUENCY IN CULTURED ADULT-RAT CARDIOMYOCYTES

Adult rat cardiomyocytes (ARC) isolated from ventricles follow a defin ed sequence of structural remodeling during culturing for 2-3 weeks. R od-shaped cells round up, attach to the substratum, and start growing out in all directions until they form contacts with one another and re sume rhythmic contractile activity. In general, myofibrils redevelop a long the actin scaffold into the periphery. IGF-I enhances this proces s while bFGF restricts the outgrowing of myofibrils to the central cel l area. Presence of T3 in the culture medium also restricts myofibrill ar growth like bFGF. At the same time, T3 increases spontaneous beatin g frequency in a dose-dependent manner. With 10 nM T3 beating frequenc y is increased three-fold versus control. Addition of isoproterenol or of epinephrine further increases the frequency at all T3 concentratio ns tested. Propranolol inhibits the fully stimulated beating frequency to about the same extent at all T3 concentrations. Therefore, T3 seem s to determine the beating frequency of ARC in culture directly and no t by changing the composition of the adrenoceptor population nor by ch anging their responsiveness.