A1 and A2 adenosine receptor modulation of contractility in the cauda epididymis of the guinea-pig

Abstract

1. The effects of adenosine receptor agonists upon phenylephrine-stimulated contractility and [3H]-cyclic adenosine monophosphate ([3H]-cyclic AMP) accumulation in the cauda epididymis of the guinea-pig were investigated. The α1-adrenoceptor agonist, phenylephrine elicited concentration dependent contractile responses from preparations of epididymis. In the absence or presence of the L-type Ca2+ channel blocker, nifedipine (10 μM) the non-selective adenosine receptor agonist, 5'-N-ethylcarboxamido-adenosine (NECA, 1 μM) shifted phenylephrine concentration-response curves to the left (4 and 5 fold respectively). Following the incubation of preparations with pertussis toxin (200 ng ml-1 24 h NECA shifted phenylephrine concentration-response curves to the right (5.7 ± 0.9 fold). 2. In the presence of phenylephrine (1 μM), NECA and the A1 adenosine receptor selective agonists, N6-cyclopentyladenosine (CPA) and (2S)-N6-[2-endo-norbornyl]adenosine ((S)-ENBA) elicited concentration-responses dependent contractions from preparations of epididymis (pEC50 values 8.18 ± 0.19, 7.79 ± 0.29 and 8.15 ± 0.43 respectively). The A3 adenosine receptor agonists N6-iodobenzyl-5'-N-methyl-carboxamido adenosine (IBMECA) and N6-2-(4-aminophenyl) ethyladenosine (APNEA) mimicked this effect (but only at concentrations greater than 10 μM). In the presence of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 30 nM) CPA concentration-response curves were shifted, in parallel to the right (apparent pK(B) 8.75 ± 0.88) and the maximal response to NECA was reduced. 3. In the presence of DPCPX (100 nM) the adenosine agonist NECA and the A(2A) adenosine receptor selective agonist, CGS 21680 (2-p-(2-carboxyethyl)-phenethylamino-N-ethylcarboxamido adenosine), but not CPA, inhibited phenylephrine (20 μM) stimulated contractions (pIC50 7.15 ± 0.48). This effect of NECA was blocked by xanthine amine congener (XAC, 1 μM) and the A(2A) adenosine receptor-selective antagonist 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5 -ylamino]ethyl)phenol (ZM 241385; 30 nM). 4. (S)-ENBA (in the absence and presence of ZM 241385, 100 nM), but not NECA or CPA inhibited the forskolin (30 μM)-stimulated accumulation of [3H]-cyclic AMP in preparations of the epididymis of the guinea-pig (by 17 ± 6% of control). In the presence of DPCPX (100 nM) NECA and CGS 21680, but not (S)-ENBA, increased the accumulation of [3H]-cyclic AMP in preparations of epididymis (pEC50 values 5.35 ± 0.35 and 6.42 ± 0.40 respectively), the NECA-induced elevation of [3H]-cyclic AMP was antagonised by XAC (apparent pK(B) 6.88 ± 0.88) and also by the A(2A) adenosine receptor antagonist, ZM 241385 (apparent pK(B) 8.60 ± 0.76). 5. These studies are consistent with the action of stable adenosine analogues at post-junctional A1 and A2 adenosine receptors in the epididymis of the guinea-pig. A1 Adenosine receptors potentiate α1-adrenoceptor contractility, an effect blocked by pertussis toxin, but which may not be dependent upon an inhibition of adenylyl cyclase. The epididymis of the guinea-pig also contains A2 adenosine receptors, possibly of the A(2A) subtype, which both inhibit contractility and also stimulate adenylyl cyclase.