Enhancement of the cardiac phosphorylase activating effect of dibutyryl cyclic AMP by thyroid hormone.

The injection of dibutyryl cyclic AMP into the perfused rat heart resulted in dose-dependent increases in contractile force and phosphorylase a activity. In hearts from hyperthyroid animals both responses were markedly enhanced. The data indicate that the hyperthyroid state may produce changes in the heart making it more responsive to cyclic AMP. Such a change could occur at the level of protein kinase. The data may also indicate that, in hyperthyroidism, the heart is more capable of taking up dibutyryl cyclic AMP or of converting it to an active product.