IDENTIFICATION AND FUNCTIONAL-CHARACTERIZATION OF A(1) AND A(2) ADENOSINE RECEPTORS IN THE RAT VAS-DEFERENS - A COMPARISON WITH A(1) RECEPTORS IN GUINEA-PIG LEFT ATRIUM AND A(2) RECEPTORS IN GUINEA-PIG AORTA

This study was undertaken to characterize the adenosine receptors in t he rat vas deferens. Because adenosine receptors have been well charac terized in the cardiovascular system of the guinea pig, antagonist dis sociation constants (pK(B) values) in the rat vas deferens were compar ed with those from the left atrium (A(1)) and the aorta (A(2)) Of the guinea pig. The A(1)-selective agonists (+/-)-N-6-endonorbornan-2-yl-5 '-N-hydroxy ethylcarboxamidoadenosine (N-0723) and N-6-cyclohexyladeno sine (CHA) and the nonselective agonist 5'-N-ethylcarboxamidoadenosine (NECA) inhibited the electrically evoked contractions of both the vas deferens and left atrium with a potency order of N-0723 > NECA = CHA. The A(2a)-selective agonist hyl)-phenethylamino]5'-N-ethylcarboxamido adenosine (CGS21680) was equipotent to NECA in the vas deferens but wa s 500-fold less potent than NECA in the left atrium. In the aorta only NECA was a potent agonist. The nonselective adenosine receptor antago nist 8-phenyltheophylline antagonized the responses in all three tissu es with approximately equal potency (pK(B) approximate to 6.6). In the rat vas deferens, the Al-selective antagonists (+/-)-N-6-endonorboman -2-yl-9-methyladenine (N-0861) and 1,3-dipropyl-8-cyclopentylxanthine (DPCPX) were more potent at antagonizing the responses to Al-selective agonists (pK(B) approximate to 8.8 and 6.4, respectively) than they w ere at antagonizing the responses to NECA and CGS21680 (pK(B) = 6.3 an d <5, respectively). However, in the left atrium, N-0861 (pK(B) = 6.2) and DPCPX (pK(B) = 8.9) were no more potent in antagonizing responses to the Al-selective agonists than they were in antagonizing responses to NECA. In the aorta, neither DPCPX (pK(B) = 6.5) nor N-0861 (pK(B) < 4.5) was a potent antagonist of NECA-induced relaxations. The result s suggest that the guinea pig left atrium contains a homogeneous popul ation of Al receptors and that the guinea pig aorta contains a homogen eous population of A(2b) receptors, whereas the rat vas deferens conta ins a mixed population of both A(1) and A(2a) adenosine receptors.