The Effect of Hyperthyroidism and Hypothyroidism on Alpha 1- and Alpha 2-adrenergic Responsiveness in Rat Aortic Smooth Muscle

We report the role of thyroid hormones on in vitro responsiveness of rat aortic smooth muscle to alpha-adrenergic stimulation. Four groups of rats: hypothyroid, hyperthyroid, thyroxine (0.1 mg/kg) treated hypothyroid and controls were employed. Response of alpha 1- and alpha 2-adrenoceptors was evoked with 6 incremental doses (10(-9) to 10(-4) M) of preferential alpha 1-agonist, phenylephrine and alpha 2-agonist, clonidine respectively. alpha 1-Adrenoceptors were also evoked by phenylephrine after blockade of alpha 2-adrenoceptors with 10(-7) M yohimbine. Similarly, alpha 2-adrenoceptors were stimulated with clonidine after blocking alpha 1-adrenoceptors with selective antagonists prazosin (10(-7) M). Aortic responsiveness to alpha-agonist norepinephrine was compared between the aortae of hypothyroid and euthyroid rats after blockade of alpha 2-adrenoceptors with 10(-4) M corynanthine. We report that in hypothyroid aortae, alpha 1-adrenergic response was significantly decreased, the dose response curve shifted to the right and the maximal response was 30% less than the normal; alpha 2-adrenergic response was completely inhibited in hypothyroid state; also, IP injections of 0.1 mg/kg thyroxine twice in 48 h to thyroidectomized rats reversed the effects of hypothyroidism on both alpha 1- and alpha 2-adrenergic response. Hyperthyroidism did not alter alpha 1- and alpha 2-adrenergic response. These results signify the role of thyroid hormones in the regulation of alpha-adrenergic response in rat aortae.