Abstract
Summary
Thyroid hormone activates adenyl cyclase present in a particulate preparation of cat myocardium, an action that might contribute to the hyperdynamic circulatory state present in hyperthyroidism. Although the marked improvement in the cardiac manifestations of hyperthyroidism produced by reserpine is thought to be due to depletion of the norepinephrine stores of adrenergic nerve terminals, such improvement might be partly caused by an inhibitory effect of reserpine on the thyroid hormone-mediated activation of adenyl cyclase. We therefore examined the effects of reserpine on the capacity of thyroid hormone to activate adenyl cyclase in particulate preparations of cat myocardium. The results demonstrated that reserpine did not impair activation of adenyl cyclase by thyroxine and triiodothy-roxine when: (i) reserpine was present in the incubation mixture; and (ii) when cats had been pretreated with reserpine 24 hr prior to sacrifice. Half maximal activity in both the reserpine pretreated and normal cats was 4 × 10-7 M. Thus, it would appear that the salutary circulatory effects of reserpine in hyperthyroidism are not mediated by a direct effect on the activation of adenyl cyclase by thyroid hormone.
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