Effects of Muscle Length on Intracellular Ca²⁺ During Isometric Contraction of Tracheal Smooth Muscle

Abstract

In both smooth and striated muscles, the active force a muscle can develop is related to its length. In striated muscles, the decrease in force that occurs as muscle length is decreased below the length of maximal active tension (L_O) may result from a number of factors: length-dependent changes in the Ca²⁺-sensitivity of contractile proteins, length-dependent changes in the amount of activator Ca²⁺ supplied to contractile proteins, and mechanical factors affecting filament overlap (Gordon et al., 1966; Taylor and Rudel, 1970; Endo, 1973; Allen and Kurihara, 1982; Stephenson and Wendt, 1984; ). Studies of striated muscles, particularly cardiac muscle, suggest that as muscle length is decreased below L_O there is a decrease in the activation of the myofilaments (Jewell,1977; Allen and Kurihara,1982; Stephenson and Wendt,1984). Also, when a striated muscle is allowed to shorten during active contraction, force redevelopment immediately after the shortening is reduced. This phenomenon, termed “shortening deactivation,” is dependent on Ca²⁺ and can be decreased or abolished by increasing the activation of the muscle ( Edman and Kiessling, 1971; Briden and Alpert, 1972; Bodem and Sonnenblick, 1974; Ekelund and Edman, 1982).