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

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Abstract
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 Ca-free solution. The magnitude and duration of the contractile response is enhanced if the muscle is preincubated in a high Ca medium (36 mM). The length of time that the muscle is preincubated in the high Ca medium influences the magnitude of the response as well as the amount of intracellular Ca accumulated by the fibers. The mechanical changes initiated by removing extracellular Ca ions are accompanied by a pronounced increase in the unidirectional efflux of Ca ions. The addition of ace-tylcholine or a high K medium enhances the tone of a muscle that is partially contracted in a Ca-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 K medium it enhances muscle tone even more. Acetylcholine, a high K medium and a Ca-free medium accelerate the migration of Ca ions from an intracellular depot to the cytoplasm by increasing membrane permeability. They also accelerate the movement of Ca ions between the external solution and the cytoplasm of the muscle fiber.