A1 and A2 adenosine receptor modulation of contractility in the cauda epididymis of the guinea‐pig
Authors: Haynes John M , Alexander S P H , Hill Stephen J 
Published: January 2005

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).
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 pKB 8.75±0.88) and the maximal response to NECA was reduced.
In the presence of DPCPX (100 nM) the adenosine agonist NECA and the A2A 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 A2A 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).
(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 pKB 6.88±0.88) and also by the A2A adenosine receptor antagonist, ZM 241385 (apparent pKB 8.60± 0.76).
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 A2A subtype, which both inhibit contractility and also stimulate adenylyl cyclase.

British Journal of Pharmacology (1998) 125, 570–576; doi:10.1038/sj.bjp.0702095