THE EFFECT OF ADRENERGIC AGONISTS ON THE CYCLIC-AMP LEVELS IN ATRIA FROM EUTHYROID AND HYPOTHYROID RATS

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  • 1 January 1980
    • journal article
    • research article
    • Vol. 29  (3) , 487-497
Abstract
To examine the effects of thyroid state on the production of cAMP, 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 had no effect on cAMP production in either the euthyroid or hypothyroid left and right atria. These effects were qualitatively different from those observed using a .beta.-adrenoceptor agonist. Isoproterenol increased cAMP 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, e.g., only the .beta.-induced responses are cAMP dependent. The data do not support the hypothesis that rat heart adrenoceptors undergo a conversion from .beta.- and .alpha.- in the hypothyroid state in terms of cAMP production.