A comparison of the actions of acetylcholine and bradykinin on the guinea pig ileum.

Procaine (8.5 x 10(-5) M) inhibited the action of acetylcholine but not that of bradykinin on the guinea pig ileum. Attempts were made to identify the difference in stimulus-response coupling which would explain this difference in sensitivity. The responses to both agonists were dependent upon the presence of Ca2+ in the extracellular fluid, were inhibited to approximately the same extent by the addition of 0.25 mM Mn2+ to the bathing fluid and were equally sensitivity to reduction in Na+ concentration of the extracellular fluid. Neither acetylcholine nor bradykinin could provoke ileal contraction when Sr2+ was substituted for Ca2+. Thus there was no indication of the difference in the stimulus-response pathways for the two agonists which must exist to explain the differential sensitivity to procaine.