Evidence against the hypothesis that thyroidectomy results in cardiac adrenoceptor interconversion.

It has been suggested that hypothyroidism causes an interconversion of cardiac atrial beta to alpha adrenoceptors. However, hypothyroidism can also decrease basal developed tensions and contraction frequencies of isolated atria, and these effects complicate the interpretation of the results. Therefore, we compared the effects of hypothyroidism on atrial responses to adrenergic amines with those of other interventions that equivalently reduced basal tensions and contraction frequencies. The results showed that hypothyroidism and acute carbachol pretreatment each enhanced the maximum responses of rat atria to methoxamine and reduced the potency (pD2) of dl-isoproterenol. In contrast, neither low calcium (left atria) nor low temperature (right atria) altered the responses to either agonist other than to reduce the initial tensions or rates of contraction. The results suggest that the effects of hypothyroidism are the result of neither altered baselines nor adrenoceptor interconversion. Instead, both hypothyroidism and acute carbachol pretreatment may functionally antagonize the effects of beta- and enhance the effects of alpha-adrenoceptor agonists.