CARDIAC RESPONSIVENESS TO ALPHA AND BETA-ADRENERGIC AMINES - EFFECTS OF CARBACHOL AND HYPOTHYROIDISM

  • 1 January 1981
    • journal article
    • research article
    • Vol. 219  (1) , 231-234
Abstract
Previous reports of cardiac .beta.- to .alpha.-adrenoceptor interconversion secondary to hypothyroidism left open the alternative possibility of a functional influence by hypothyroidism on the inotropic and chronotropic effects of adrenergic amines through a different mechanism. To test this possibility, the effects of hypothyroidism (thyroidectomy) were compared with those of acute carbachol pretreatment on the responses of isolated rat atria to the selective .beta.- and .alpha.-adrenoceptor agonists isoproterenol and methoxamine. Both hypothyroidism and acute carbachol pretreatment (3 .times. 10-7-10-6 M) reduced basal right atrial rates and left atrial tensions, caused an apparent decrease in the inotropic and chronotropic potencies of isoproterenol, reduced the degree of antagonism by propranolol of the responses to isoproterenol, increased the maximum inotropic response of left atria to methoxamine and converted a lack of response to a positive chronotropic response of right atria to methoxamine. Equivalent reductions of basal rates by hypothermia, or of basal tensions by lowered Ca ion concentrations, did not affect the responses to isoproterenol or methoxamine. Apparently, both carbachol pretreatment and hypothyroidism functionally antagonize the responses to isoproterenol and enhance the responses to methoxamine by means other than adrenoceptor interconversion.