Fig. 1. The effects of removing external Ca²⁺ or addition of the InsP₃ receptor blocker 2-APB on the contractile response of the guinea-pig colon to carbachol (CCh). (A) The contraction produced by CCh (0.5 µM) alone comprised an initial fast phasic component then declined to a lower maintained level (tonic component). Ca²⁺ withdrawal for 3 minutes significantly reduced (P<0.05, n=17) the amplitude of the tonic component. (B) In the presence of 2-APB (100 µM, 15 minutes) both phasic and tonic components of the CCh response were reduced significantly. The contractions were largely restored on washout of 2-APB.