Effects of thyroid hormone on beta-adrenergic responsiveness of aging cardiovascular systems.

We have compared the effects of beta-adrenergic stimulation on the heart and peripheral vasculature of young (2-mo-old) and older (12-mo-old) rats both in the presence and absence of triiodothyronine (T3)-induced hyperthyroidism. The hemodynamic consequences of T3 treatment were less prominent in the aged hyperthyroid rats compared with young hyperthyroid rats (both in intact and pithed rats). There was a decrease in sensitivity of chronotropic responsiveness to isoproterenol in older pithed rats, which was apparently reversed by T3 treatment. The number and affinity of myocardial beta-adrenergic receptor sites measured by [125I]cyanopindolol were not significantly different in young and older control rats; also, beta-receptor density increased to a similar extent in both young and older T3-treated rats. The ability of isoproterenol to relax mesenteric arterial rings, markedly blunted in older rats, was partially restored by T3 treatment without their being any change in isoproterenol-mediated relaxation in the arterial preparations from young rats. The number and affinity of the beta-adrenergic receptors measured in the mesenteric arteries was unaffected by either aging or T3 treatment. The data suggest that effects of thyroid hormone and age-related alterations of cardiovascular responsiveness to beta-adrenergic stimulation are interrelated in a complex fashion with a net result that the hyperkinetic cardiovascular manifestations in hyperthyroidism are attenuated in the older animals.