Mechanical, electrical, and morphological characteristics of skeletal muscle fibers from Xenopus and other species of frogs.

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Abstract
Mechanical, electrical and morphological properties of iliofibularis or semitendinosus of Xenopus laevis, Rana catesbeiana and R. nigromaculata were investigated in an attempt to find out the differences between them which will give the basic knowledge for the study of excitation-contraction coupling. With application of electrical stimulation, a single muscle fiber from Xenopus contracted at a faster rate of rise than did the other muscles tested. The maximum rate of rise (.ovrhdot.Tmax) of tension was in the order of Xenopus, R. catesbeiana and R. nigromaculata. Ca2+-sensitivity and .ovrhdot.Tmax of mechanically skinned fibers of Xenopus resembled those of R. catesbeniana. Xenopus muscle had a small cross-sectional area of T-tubule compared with that in other species and the action potential exhibited a small positive-going hump. The volume density of the terminal cisternae of sarcoplasmic reticulum (SC) to the myofibril was the largest in the Xenopus muscle with a statistically significant difference. Thus, the Xenopus muscle appears to be good material for investigation of mechanisms related to Ca2+ release from SR, as elicited by the excitation of T-tubules.