Age-dependent reductions in A₁adenosine receptor expression in rat testes

Abstract

The rat testis expresses high levels of A 1 adenosine receptors (A 1 AR) that couple to the inhibition of adenylyl cyclase activity. However, the physiological role of these receptors in the testis is not clear. Previous studies have documented a number of changes in the testis associated with the aging process. The goal of this study was to assess whether alteration in the expression and function of the testicular A 1 AR occurs in aging, using the Fischer 344 rats as an aging model. Quantitation of A 1 AR expression by radioligand binding of [ 3 H]1,3-dipropyl-8-cyclopentylxanthine, an antagonist radioligand, indicates reductions in receptor number by 35 ± 13.3 and 53 ± 18.2% in 18- and 25-mo-old rats, respectively, compared with 3-mo-old rats. Similar reductions in A 1 AR expression were determined using Western blotting and receptor autoradiography. Quantitation of the G i proteins using selective antibodies indicate age-dependent reductions in the levels of α i-1,2 -, α i-3 - and β-subunits. Furthermore, the modulatory influences of guanosine 5′- O-(3-thiotriphosphate) on the binding of agonist and antagonist radioligands to the A 1 AR were substantially reduced. Northern blotting analysis of rat testicular poly(A) + RNA indicates both a 3.4-kb transcript and a 5.6-kb transcript that hybridized to the canine A 1 AR cDNA probe. The levels of the 5.6-kb transcript were decreased by 24 ± 18 and 52 ± 3% in the 18- and 25-mo-old rats, respectively, compared with the 3-mo-old rats. These results indicate age-dependent deficits in the A 1 AR signal transduction pathway in the testes and predict concomitant reductions in the action of adenosine.