On the mechanism of the reduction by thyroid hormone of beta-adrenergic relaxation rate stimulation in rat heart.

The effects of beta-adrenergic stimulation on the relaxation rate and the Ca2+-transport rate in sarcoplasmic reticulum of hypothyroid, euthyroid and hyperthyroid rat hearts were studied. Administration of isoproterenol (0.1 microM) to perfused, electrically stimulated hearts (5 Hz) caused a decrease in the half-time of relaxation (RT 1/2) the extent of which depended on the thyroid status, i.e. hypothyroid (-24%), euthyroid (-19%) or hyperthyroid (-8%). A similar decreasing effect was found for the stimulation of Ca2+ transport in isolated SR by cyclic AMP and protein kinase, i.e. hypothyroid (75%), euthyroid (37%) and hyperthyroid (20%). These alterations were not due to differences in endogenous protein kinase activity or cyclic AMP production. Estimations of Ca2+-ATPase and phospholamban (PL) content of the sarcoplasmic reticulum were obtained by measurement of the phosphorylated forms of Ca2+-ATPase (E-P) and phospholamban (PL-P) followed by electrophoresis and autoradiography. A 3-fold decrease of PL-P, accompanied by a 2-fold increase of E-P per mg of protein was observed in sarcoplasmic reticulum preparations in the direction hypothyroid----hyperthyroid. Consequently the E-P/PL-P ratio increased from 0.32 (hypothyroid), through 0.81 (euthyroid) to 1.69 (hyperthyroid). In spite of certain limitations inherent to quantification of Ca2+-ATPase and phospholamban by their phosphorylated products, these data provide strong evidence that during thyroid-hormone mediated cardiac hypertrophy, with concomitant proliferation of the sarcoplasmic reticulum, the relative amount of phospholamban decreases with respect to Ca2+-ATPase. This could provide an explanation for the observed gradual diminishment of the beta-adrenergic effect on the relaxation rate when cardiac tissue is exposed to increasing amounts of thyroid hormone.