REDUCED ADENOSINE A(1) RECEPTOR AND G(ALPHA) PROTEIN COUPLING IN RAT VENTRICULAR MYOCARDIUM DURING AGING

Adenosine A(1) receptor (A(1)-AdoR) function in rat ventricles has pre viously been shown to decrease with age. In the present study, using t he ligand [H-3]8-cyclopentyl-1,3-dipropylxanthine ([H-3]DPCPX) and coi mmunoprecipitation of A(1)-AdoRs with their associated G proteins, we determined the specific binding of A(1)-AdoR and A(1)-AdoR/G protein c oupling in ventricular myocardium of 6- to 24-month-old Fischer 344 ra ts. The densities (B-max) of A(1)-AdoRs were 5.8+/-0.8 fmol/mg protein in 6-month-old rats and 6.1+/-1.4 fmol/mg protein in 24-month-old rat s, and the dissociation constants (K-d) were 0.32+/-0.04 nmol/L in 6-m onth-old rats and 0.34+/-0.05 nmol/L in 24-month-old rats (P>.05). Ana lysis of the dose-dependent displacement of [H-3]DPCPX binding by the selective A(1)-receptor agonist, N-6-p-sulfophenyladenosine (SPA), yie lded two affinity binding sites in both 6- and 24-month-old rats. Howe ver, the proportion of high-affinity A(1)-AdoRs was significantly lowe r in 24-month-old rats (23.5%) compared with 6-month-old rats (54.9%) (P<.05). In solubilized ventricular membranes, specific [H-3]DPCPX bin ding sites were detected in immunoprecipitates of G(alpha i3) and G(al pha 0) antisera but not with antibodies for other G(alpha) proteins. T he basal coimmunoprecipitation of A(1)-AdoR with G(alpha i3) and G(alp ha 0) proteins decreased by 22% and 21%, respectively, in ventricular membranes of 24-month-old rats compared with that in 6-month-old anima ls. A(1)-AdoR stimulation with SPA increased the coprecipitation of A( 1)-AdoR with G(alpha i3) and G(alpha 0) proteins by 287% and 245%, res pectively, in 6-month-old rats but only by 129% and 140%, respectively , in 24-month-old rats (P<.01). In the absence of changes in A(1)-AdoR density and G(alpha) protein levels, an age-related decline in high-a ffinity A(1)-AdoR binding sites and a reduction in the association of A(1)-AdoR with G(alpha) proteins suggest that the age-related decrease in ventricular A(1)-AdoR-mediated response is related to a reduction in the coupling between A(1)-AdoR and their G proteins.