Effects of triiodo-L-thyronine (T3)-induced hyperthyroidism on the cardiovascular alpha-adrenoceptor system in young and older rats.

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 alpha 1-selective agonist phenylephrine or the alpha 2-selective agonists clonidine and UK-14,304. However, T3-induced hyperthyroidism caused a selective reduction in postjunctional alpha 2-adrenoceptor-mediated pressor responses but not alpha 1-adrenoceptor-mediated ones in both age groups. In contrast to vascular alpha-adrenoceptors, there was an age-related decrease in the number of myocardial alpha 1-adrenoceptor sites measured by specific binding of [3H]prazosin. With thyroid hormone treatment, the density of ventricular alpha 1 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.