Abstract
We have compared the vasopressor response of alpha-adrenoceptor agonists in young (2 month-old) and older (12 month-old) pithed rats both in the presence and absence of triiodo-L-thyronine (T3)-induced hyperthyroidism. There was no age-related changes in the pressor response to either the alpha1-selective agonist phenylephrine or the alpha2-selective agonists clonidine and UK-14, 304. However, T3-induced hyperthyroidism caused a selective reduction in postjunctional alpha2-adrenoceptor-mediated pressor responses but not alpha1-adrenoceptor-mediated ones in both age groups. In contrast to vascular alpha-adrenoceptors, there was an age-related decrease in the number of myocardial alpha1-adrenoceptor sites measured by specific binding of [3H]prazosin. With thyroid hormone treatment, the density of ventricular alpha, receptors was found to be further reduced even in the older rats. These results suggest that thyroid hormone and aging have different influences on the cardiovascular alpha-adrenoceptor system and that these two physiological variables are independently interrelated.
