CHANGES OF CALMODULIN CONCENTRATION AND CYCLIC 3',5'-NUCLEOTIDE PHOSPHODIESTERASE ACTIVITIES IN CARDIAC-MUSCLE OF HYPERTHYROID AND HYPOTHYROID RATS

To investigate the effect of thyroid hormone on cardiac muscle dysfunc tion in hyper- and hypothyroid states, we evaluated cyclic 3',5'-nucle otide metabolism by measuring cyclic 3',5'-nucleotide phosphodiesteras e activity and calmodulin concentrations in the cardiac muscles of hyp er- and hypothyroid rats. Cyclic AMP (cAMP) concentration was signific antly high in the cardiac muscle of hyperthyroid rats and low in that from hypothyroid rats compared with control rats. Cyclic AMP and cycli c GMP phosphodiesterase activities were significantly decreased in the soluble fraction of cardiac muscle from hyperthyroid rats and markedl y increased in this fraction in hypothyroid rats compared with normal animals. Calmodulin concentration was high in hyperthyroid and low in hypothyroid rats. It was concluded from these findings that low cAMP-p hosphodiesterase activity might, in part, bring about the high concent ration of cAMP. Calmodulin was sigificantly high in the cardiac muscle of hyperthyroid rats and the reverse was the case in hypothyroid rats compared with normal rats. The implication is that, in hyper- and hyp othyroid states, these changes may play an important role in cardiac f unction via their effect on cyclic nucleotide and Ca2+ metabolism.