The action of the ionophore ionomycin in guinea-pig intestinal smooth muscle

The ionophore ionomycin produced concentration-dependent (5 x 10^-9 to 5 x 10^-6 M) contractions in guinea-pig ileal longitudinal smooth muscle. Responses were dependent on extracellular Ca²⁺, consistent with the known role of this Ca²⁺ source in supporting excitation-contraction coupling to a variety of stimulants in this tissue. Responses were insensitive to atropine (10^-6 M) but were dependent upon extracellular Na⁺ and were completely blocked by low concentrations of the Ca²⁺-channel antagonists nicardipine, YC-93 (5 x 10^-7 M), and D-600 (5 x 10^-6 M). The behaviour of ionomycin is very similar to that shown by A 23187 in this tissue. Ionomycin, like A 23187, can apparently activate D-600 sensitive Ca²⁺ channels in the guinea-pig intestinal smooth muscle rather than simply translocating Ca²⁺ EXT.