PURINERGIC MODULATION OF SPONTANEOUS ACTIVITY AND OF RESPONSES TO HIGH POTASSIUM AND ACETYLCHOLINE IN RAT ILEAL SMOOTH-MUSCLE

1. In rat ileal smooth muscle both adenosine and ATP at 10(-4) M signi ficantly enhanced spontaneous mechanical activity. The excitatory acti ons of adenosine were blocked by the P1 receptor antagonist 8-phenylth eophylline and the excitatory effects of ATP were significantly reduce d by the P2 receptor antagonist quinidine. 2. The P2 receptor desensit izer alpha,beta-methylene-ATP was without effect on ACh responses nor did the stable analogue beta,gamma-methylene-ATP exert any effect on s pontaneous mechanical activity. 3. Pretreatment with adenosine caused a dose-dependent enhancement of K-induced contractures in the ileum. L ow adenosine concentrations slightly inhibited and high concentrations slightly enhanced ACh-induced contractures in the ileum. 4. ATP poten tiated the phasic component of the ileal K-induced contracture but str ongly inhibited tonic force at high concentrations. This agent slightl y inhibited the phasic component of the ACh-induced contracture while strongly inhibiting ACh-induced tonic force. 5. Alpha,beta-methylene-A TP inhibited ileal muscle ACh induced contractures while it potentiate d both phasic and tonic K-induced contractures. Beta,gamma-methylene A TP inhibited ACh-induced contractures but it enhanced K-induced phasic contractures while inhibiting K-induced tonic force. 6. The results o f this study suggest that rat ileum may contain the A1 subtype of the P1 receptor but the evidence for a P2 receptor subtype is conflicting despite the inhibition of ATP actions by quinidine. 7. The inhibition of K- and ACh-induced tonic force suggests that adenosine and ATP inte ractions with ileal smooth muscle may inactivate slow voltage-dependen t calcium channels leading to EC uncoupling.