Age-dependent reductions in A1 adenosine receptor expression in rat testes.

The rat testis expresses high levels of A1 adenosine receptors (A1 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 A1 AR occurs in aging, using the Fischer 344 rats as an aging model. Quantitation of A1 AR expression by radioligand binding of [3H]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 A1 AR expression were determined using Western blotting and receptor autoradiography. Quantitation of the Gi proteins using selective antibodies indicate age-dependent reductions in the levels of alpha i-1,2-, alpha i-3- and beta-subunits. Furthermore, the modulatory influences of guanosine 5'-O-(3-thiotriphosphate) on the binding of agonist and antagonist radioligands to the A1 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 A1 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 A1 AR signal transduction pathway in the testes and predict concomitant reductions in the action of adenosine.