Mechanical Responses of Intestinal Smooth Muscle in a Calcium-Free Medium. ∗

Abstract

Summary

Longitudinal fibers from the guinea pig ileum undergo a transient increase in muscle tone when they are transferred from a physiological salt solution to a calcium-free solution, The magnitude and duration of the contractile response is enhanced if the muscle is preincubated in a high calcium medium (36 mM). The length of time that the muscle is preincubated in the high calcium medium influences the magnitude of the response as well as the amount of intracellular calcium accumulated by the fibers. The mechanical changes initiated by removing extracellular calcium ions are accompanied by a pronounced increase in the unidirectional efflux of calcium ions. The addition of acetylcholine or a high potassium medium enhances the tone of a muscle that is partially contracted in a calcium-free medium. The addition of 1.8 mM CaCl₂ depresses it. However, when the same concentration of CaCl₂ is added in the presence of acetylcholine or a high potassium medium it enhances muscle tone even more. We inferred from these data that acetylcholine, a high potassium medium, and a calcium-free medium, by increasing membrane permeability, accelerate the migration of calcium ions from an intracellular depot to the cytoplasm. The results suggest that they also accelerate the movement of calcium ions between the external solution and the cytoplasm of the muscle fiber.