Alpha-adrenoceptor responsiveness in the aged rat.

The effects of ageing on vascular alpha 1- and alpha 2-adrenoceptors were examined using anaesthetised and pithed young (3-7 months) and old (21-24 months) Sprague-Dawley rats. In pithed animals, the pressor and cardioinhibitory effects of the alpha 2-adrenoceptor agonist xylazine were significantly reduced in old animals (8- and 6-fold shift), but the pressor effects of the alpha 1-adrenoceptor agonist amidephrine were not significantly altered by ageing. In anaesthetised rats, the pressor response to the mixed alpha-adrenoceptor agonist noradrenaline (NA) was not significantly altered, and the pressor potency of the alpha 1-adrenoceptor agonist amidephrine was only slightly reduced (1.7-fold shift) in old animals. In the presence of cocaine (1 mg kg-1) the pressor potency of NA was markedly reduced (13-fold shift) in old animals. In the presence of prazosin (1 mg kg-1) to eliminate alpha 1-adrenoceptor-mediated responses, the pressor potency of NA was markedly reduced in old as animals as compared to young animals (16-fold shift). The neuronal uptake blocker cocaine (1 mg kg-1) significantly potentiated the pressor response to NA only in young. In summary, we have found a reduced alpha 2-adrenoceptor-mediated pressor and cardioinhibitory responsiveness and a reduced neuronal uptake of NA in old animals with little change in alpha 1-adrenoceptor-mediated vascular responsiveness.