The Rat Testicular Ai Adenosine Receptor-Adenylate Cyclase System

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Abstract
When adenosine interacts with membrane-bound A1 receptors, it is capable of inhibiting the enzyme adenylate cyclase in brain and fat tissue. In this paper we characterized the A1 adenosine receptor-adenylate cyclase system of the rat tests. The agonist radioligand (-)-N-[3-[125I]iodo-4-hydroxyphenyl-(isoproyl)]adenosine binds with high affinity (Kd, .simeq. 1 nM) in a saturable manner (maximum binding, .simeq. 600 fmol/mg protein). The A1 adenosine receptor binding displays the appropriate pharmacology, stereospecificity, and sensitivity to guanine nucleotides. The testicular A1 receptor is also coupled in an inhibitory manner to the enzyme adenylase cyclase, as demonsrated by the ability of N6-R-phenyl-2-propyladenosine to inhibit isoproterenol- and forskolin-stimulated cAMP accumulation. The testicular A1 receptor can be solubilized in high yield and in an active form with the detergent digitonin. The solubilized receptor retains all of the pharmacological properties of the membrane-bound receptor. Although there are many similarities among the A1 receptor from testes, brain, and fat tissue, the testicualr A1 receptor displays a larger apparent mol wt. By photoaffinity labeling, the A1 adenosine receptor-binding subunit of fat and brain are 38,000 mol wt proteins, while the testicular A1 receptor binding subunits is a 42,000 mol wt protein.