SURAMIN INCREASES THE EFFICACY OF ATP TO ACTIVATE AN INWARD CURRENT IN MYENTERIC NEURONS FROM GUINEA-PIG ILEUM

Previous reports have indicated that suramin antagonizes, in a competitive manner, the actions of adenosine 5'-triphosphate (ATP) mediated by P2 purinoceptors. In neurons from the myenteric plexus, however, suramin has different effects on an ATP-activated conductance. During whole-cell recordings, ATP (3-1000 muM) evoked a rapidly desensitizing inward current in a concentration-dependent manner. The amplitude of the ATP-activated current was increased, in a reversible manner, when suramin was applied previous to or during ATP stimulation. This effect was increased by raising the suramin concentration. Higher concentrations of suramin were required for higher concentrations of ATP; the EC50 values of suramin were 5 +/- 1.1 muM and 143 +/- 48 nM when ATP concentrations of 30 and 10 muM were used. No change in the holding current was observed with suramin alone. These observations suggest that a different subtype of P2 purinoceptors could be expressed in the myenteric neurons.