Effects of ageing and adrenergic stimulation on alpha 1- and beta-adrenoceptors and phospholipid fatty acids in rat heart.

The purpose of this study was to examine the influence of ageing on the alterations in binding characteristics of adrenoceptors and membrane phospholipid fatty acids in rat heart following repeated administration of epinephrine. The maximal number of binding sites (Bmax) and dissociation constant (Kd) of [3H]prazosin and [3H]dihydroalprenolol binding to alpha 1- and beta-adrenoceptors, respectively, changed significantly during ageing. The downregulation of alpha 1- and beta-adrenoceptors after repeated epinephrine administration for one week, did not differ with age, but the response of the affinity (1/Kd) of both alpha 1- and beta-adrenoceptors to epinephrine treatment was age dependent. In 3-month-old rats the affinity of alpha 1-adrenoceptors was decreased after epinephrine treatment but the affinity of beta-adrenoceptors was unchanged. In 10- and 23-month-old rats the affinity of beta-adrenoceptors decreased after epinephrine treatment but the affinity of alpha 1-adrenoceptors did not change. During ageing the linoleic acid (18:2(n-6)) level decreased in phosphatidylcholine and the arachidonic acid (20:4(n-6)) level increased in phosphatidylcholine and phosphatidylethanolamine. After epinephrine administration the 18:2(n-6) level decreased and the docosahexaenoic acid (22:6(n-3)) level increased in phosphatidylcholine and phosphatidylethanolamine and those changes were not age dependent. The 20:4(n-6) level increased in phosphatidylcholine after epinephrine administration, but that increase was smaller in old than in young rats. The results show that both ageing and epinephrine administration simultaneously modify the fatty acid composition of membrane phospholipids and the binding properties of alpha 1- and beta-adrenoceptors in rat heart.