The developmental switch in ventricular myosin expression in vivo is not triggered by an increase in cyclic AMP

Ventricular myosin heavy chain (HC) expression undergoes a rapid change from the beta to the alpha isoform shortly after birth. Thyroid hormone is required for this transition to occur, but the time course of developmental changes in circulating thyroid hormone levels suggests that it cannot be the trigger for this event. In this study, the possibility was examined that cyclic AMP (cAMP), either acting separately or as a mediator of the permissive actions of thyroid hormone, triggers the developmental transition in ventricular myosin HC expression. One-day-old euthyroid or propylthiouracil-hypothyroid rat pups were treated with a membrane-permeable cAMP analogue, 8-bromo-cAMP (8-Br-cAMP) or triiodothyronine (T3). Two and four hours after acute treatment, the relative synthetic rates of alpha and beta myosin HC were measured using an in vivo pulse labelling technique. Myosin HC isoforms were separated electrophoretically and then quantitated by densitometry. Acute treatment in vivo with 8-Br-cAMP did not alter the relative rate of alpha myosin HC synthesis in euthyroid animals at either 2 or 4 h. Hypothyroidism significantly reduced the relative rate of alpha HC synthesis and obturated the transition from beta to alpha HC. The effect on synthesis was rapidly reversed by acute treatment with T3, but not with 8-Br-cAMP. Thus, an increase in cAMP does not appear to trigger the developmental change in myosin isoform expression, nor does it appear to mediate the stimulatory effect of thyroid hormone on alpha myosin HC synthesis. Instead, thyroid hormone is likely to be acting directly on the alpha HC gene by binding to the thyroid response element. The identity of stimuli that mediate the increased responsiveness to thyroid hormone during development remains to be determined, but it is not due to an increase in the levels of cAMP in perinatal cardiocytes in vivo.