1984 Volume 25 Issue 1 Pages 113-125
This study examined changes in cardiac myofibrillar ATPase activity during the development of hyperthyroidism and the relationships between changes in myofibrillar ATPase activity and hemodynamic parameters in rabbits. Calcium-stimulated, magnesium-dependent ATPase (Ca-ATPase) activity of cardiac myofibrils exhibited about a 30% increase after the first week of treatment with excess desiccated thyroid tissue powder (thyroid-treatment) and did not show further increase during thyroid-treatment. Development of cardiac hypertrophy and increases in systolic and diastolic blood pressure were dependent upon prolonged periods of thyroid-treatment lasting from 3 days to 4 weeks, whereas the maximal increase (about 55%) in heart rate was seen after the first week of thyroid-treatment. The elevated level lasted throughout subsequent thyroid-treatment. Lineweaver-Burk plots showed an increase in Km and Vmax values for Ca-ATPase activity of my-ofibrillar preparation from the thyrotoxic hearts of treated animals. Increased Ca-ATPase activity was correlated with thyroid-induced changes in heart rate. Propranolol partially reduced the thyroidinduced increase in Ca-ATPase activity and heart rate, suggesting participation of beta-adrenoceptors in the thyroid-induced increases of Ca-ATPase activity and heart rate.