CARDIAC SARCOPLASMIC-RETICULUM CALCIUM-TRANSPORT ACTIVITY OF THYROIDECTOMIZED RATS - THE ROLE OF ENDOGENOUS MYOCARDIAL ACYLCARNITINES AND CALCIUM-PUMP PROTEIN

The effect of hypothyroidism on isolated rat cardiac sarcoplasmic reticulum (SR) calcium transport activity was determined. Cardiac SR was studied 2, 4 and 6 weeks following surgical removal of the thyroid gland. Thyroidectomized rats had reduced body weight and left ventricular weight 4 and 6 weeks after thyroidectomy. The rate of SR calcium transport activity was not affected 2 weeks after thyroidectomy, but was reduced 4 and 6 weeks after thyroidectomy. To elucidate the mechanism responsible for altered calcium transport activity, the roles of endogenous SR acylcarnitine and SR calcium pump protein were determined. Thyroidectomy did not affect the level of endogenous acylcarnitine associated with the SR membranes isolated at the time points studied. The level of acylphosphoprotein, putatively the SR calcium pump protein. was not affected 2 weeks following thyroidectomy, but was significantly reduced in SR 4 weeks postthyroidectomy. These studies suggest that the quantity of SR calcium pump sites is reduced in hypothyroidism and that this reduction may explain the altered SR calcium transport activity observed.