Adrenoceptor-mediated cardiac and vascular responses in hypothyroid rats

This study has investigated adrenoceptor-mediated responses and β-adrenoceptors in neonatal-onset hypothyroidism in rats. Four groups of adult rats were studied: controls, neonatal-onset uncorrected hypothyroidism (continuous oral methimazole treatment) and after chronic triiodothyronine (T) replacement of these rats at either 25 or 100 μg kg/day for 8 weeks beginning at 12 weeks of age. Hypothyroid rats were 61% smaller with an 18% decrease in heart rate; food and water intake were reduced to 43% and 52%, respectively; O consumption was reduced to 20% and rectal temperature was 2.9 lower. T administration increased body weight to 60-62% of controls; metabolic changes were reversed; but tachycardia and cardiac hypertrophy (60-120% increases) resulted. The positive inotropic responses to the selective (α-adrenoceptor agonist, phenylephrine, in left ventricular papillary muscles were abolished; the β-adrenoceptor agonist, noradrenaline, was significantly less potent as an inotropic compound in isolated cardiac tissues from hypothyroid rats. The potency of phenylephrine to contract thoracic aortic rings was reduced in hypothyroid rats. These changes in α- and β-adrenoceptor mediated responses were reversed by T administration. Both β- and β-adrenoceptor densities were increased in the hypothyroid left ventricle; T administration further increased β-adrenoceptor density. We conclude that neonatal hypothyroidism produces pronounced physiological responses, changes in adrenoceptor-mediated responses and an increased ventricular β-adrenoceptor density. T replacement reversed the changes in cardiac responses and metabolic parameters, except body weight, but produced cardiac symptoms of hyperthyroidism (tachycardia, hypertrophy as well as an increased β-adrenoceptor density)