A2A adenosine receptor mediated potassium channel activation in rat epididymal smooth muscle

1 The effects of A(2) adenosine receptor agonists upon phenylephrine-stimulated contractility in preparations of rat epididymis were investigated. 2 Preparations responded to phenylephrine (3 mu M) With submaximal contractions. Adenosine and the stable agonists 5'-N-ethylcarboxamido-adenosine (NECA) and 2-p-(2-carboxyethyl) phenethylamino-N-ethylcarboxamide adenosine (CGS 21680) inhibited phenylephrine-induced contractions (potency order, NECA > CGS 21680 > adenosine). The A(2A) receptor-selective antagonist, 4-(2-[7-amino-2-(2-furyl)[1,2,4]-triazolo-[2,3-a (ZM 241385, 30 mu M) blocked the response to NECA. 3 The A(2A) adenosine receptor-mediated inhibitory responses to NECA were reduced by the K-ATP channel blocker, glibenclamide (3 mu M) and abolished by charybdotoxin (100 nM). 4 The diterpene forskolin elicited a concentration-dependent inhibition of phenylephrine (3 nM)stimulated contractility (by 62 +/- 8% of control at 100 mu M). Charybdotoxin (100 nM), but not glibenclamide (3 mu M) blocked the forskolin (10 mu M) inhibition of phenylephrine-stimulated contractility. 5 NECA elicited concentration-dependent increases in both cyclic AMP and cyclic GMP accumulation which were antagonized by ZM 241385 (30 nM). 6 The protein kinase G activator, APT-cyclic GMP (8-(-Aminophenylthio) guanosine-3',5'-cyclic monophosphate) and the protein kinase A activator (Sp)-8-bromoadenosine-3',5'-cyclic monophosphorothioate (Sp-8-Br-cyclic AMPs), inhibited phenylephrine (3 mu M) induced contractions of rat epididymis. Glibenclamide (3 mu M), but not charybdotoxin (100 nM), inhibited ATP-cyclic GMP responses. Charybdotoxin (100 nM), but not glibenclamide (3 mu M) reduced the effect of Sp-8-Br-cyclic AMPs. 7 This study shows that the A(2A) adenosine receptor inhibition of epididymal contractility may be mediated through the activation of charybdotoxin- and glibenclamide-sensitive potassium channels and may involve the activation of both protein kinases A and G.