Effects of aging and food restriction on α₁-adrenergic receptors and G-protein in the rat parotid gland

The aim of this study is to investigate age-related changes in α₁- adrenergic receptor and G-protein in rat parotid gland, and to examine whether the age-related changes are modulated by food restriction. The binding characteristics of α₁-adrenergic receptor were measured in parotid cell membranes from 3-, 6-, 12-, 18-, and 24-month-old ad libitum-fed (AL) and food-restricted (FR) rats. The number of α₁-adrenergic receptors (B(max)) increased between 3 and 6 months by 24.5% and 16.2% in AL and FR rats, respectively, with no further significant change up to 24 months of age. The K(d) values of [³H]prazosin binding increased by 53.6% in AL rats, but decreased by 16.1% in FR rats between 3 and 6 months, and the changes were maintained up to 24 months of age. The binding activity of G-protein with [³⁵S]guanosine 5'-[γ-Thio]triphosphate (GTP[γS]) in parotid cell membranes was measured in 6- and 24-month-old AL and FR rats. Basal GTP binding capacity decreased by 22.3% and 26.0% in AL and FR rats, respectively, between 6 and 24 months. Epinephrine-stimulated increase in GTP binding capacity decreased by 19.2% between 6 and 24 months in AL rats, but not in FR rats. The K(d) values of [³⁵S]GTP[γS] binding were not affected by aging or food restriction. In summary: (1) FR decreased maturation- related increase in the number of α₁-adrenergic receptors and prevented maturation-related decrease in receptor binding affinity between 3 and 6 months of age. However, between 6 and 24 months of age FR had no further significant effect on the number of α₁-adrenergic receptors and on receptor binding affinity. (2) FR prevented age-related decrease in α₁-adrenergic agonist-stimulated increase in GTP binding capacity. We conclude that FR may have a modulatory effect on age-related impairment in α₁-adrenergic receptor function resulting from altered G-protein binding activity in the rat parotid gland.