Waning cardiovascular responses to adrenergic drugs in conscious ageing rats.

Blood pressure and heart rate responses to various drugs were recorded in three groups of conscious rats at ages 3-, 12- and 26-months to determine whether cardiovascular responsiveness changes selectively with age. Basal mean pressures were higher while heart rates were lower in 26-month-old rats than in others. Phenylephrine, as an alpha-adrenergic agonist, produced significantly smaller pressor and bradycardic responses in 26-month-old than in younger rats. By contrast, pressor and bradycardic responses to angiotensin did not differ between age groups. Depressor responses produced by isoproterenol, as a beta-adrenergic agonist, were unaffected by age, but the accompanying tachycardia was significantly weaker in 26-month-old than in younger rats. On the other hand, combined alpha- and beta-adrenergic stimulation with epinephrine elicited pressor and bradycardic responses that were significantly smaller in 12- and 26-month-old rats than in 3-month-old rats. Thus, our results show that while vascular alpha-adrenergic and myocardial beta-adrenergic responses diminished with age, cardiovascular responses to angiotensin were essentially unaltered. Considered collectively these results suggest that ageing impairs responsiveness to alpha- and beta-adrenergic stimuli selectively without affecting that to other vasoactive drugs probably because as endogenous catecholamines increase with age, receptor occupancy also increases and the respective vascular and myocardial receptors become saturated.