Amiloride inhibition of guinea pig contractile responses to acetylcholine.

1. Amiloride inhibited the tonic component of the responses of the isolated guinea pig ileum to high (1 microM, which is above ED50), but not to low (44 nM) concentrations of acetylcholine. 2. The inhibition was concentration-dependent in the range 0.1 microM-10 microM and was attenuated in low-Na+ or high-Ca2+ media. 3. Amiloride (10 microM) inhibited the increase of 45Ca2+ uptake induced by 1 microM acetylcholine in the longitudinal muscle of the guinea pig ileum. 4. These results suggest that amiloride inhibits agonist-induced Na+ influx and indirectly Ca2+ influx through the Na+/Ca2+ exchange mechanism.