The effect of adrenertgic agonists on the cyclic AMP levels in atria from euthyroid and hypothyroid rats.

To examine the effects of thyroid state on the production of cyclic AMP, left and right atria isolated from euthyroid and thyroidectomized male rats were exposed to the alpha adrenergic agonist methoxamine and the beta adrenergic agonist isoproterenol. Methoxamine increased force and rate in paced (1 Hz) left and spontaneously beating right atria, respectively, to a greater extent in the hypothyroid state than in the euthyroid state. Methoxamine was found to have no effect on cyclic AMP production in either the euthyroid or hypothyroid left and right atria. These effects were qualitatively different from those observed using a beta adrenoreceptor agonist. Isoproterenol increased cyclic AMP levels, force and rate in both thyroid states. The data of the present study support the hypothesis that the inotropic and chronotropic responses to alpha and beta adrenergic stimulation involves different mechanisms, such that only the beta-induced responses are cyclic AMP dependent. The data do not support the hypothesis that rat heart adrenoceptors undergo a conversion from beta to alpha in the hypothyroid state in terms of cyclic AMP production.