Characterization of tension decline in different types of fatigue-resistant skeletal muscle fibres of the frog. Low extracellular calcium effects.

Twitch and tetanic tension have been measured in single skeletal muscle fibres (m.ileofibularis, Rana esculenta and Rana temporaria). On the basis of resistance to fatigue produced by repetitive tetanic stable frequency stimulation with various numbers of stimulation trains, twitch fibers were subdivided in three groups (resembling those as described by Westerblad and Lännergren 1986 in Xenopus), that is fatigue-resistant (FR), moderately fatigued (MF) and easily fatiguing (EF). It was found further that the fibres differ in tetanic tension decline resistance i.e. fatiguability relating to some basic contractile parameters including the amplitude, the rates parameters of twitch and tetanus tension as well as the tetanus/twitch tension ratio. The main differences observed concern: 1) The inability to maintain the maximum tetanic tension plateau (IMT) during single tetanus. IMT was 18 times higher in EF fibres and 4 times higher in MF fibres, respectively, in comparison with FR fibres. IMT is the first parameter to change significantly during repetitive tetanic stimulation. 2) The different fibre types show pronounced differences in twitch contraction and tetanus tension during repetitive tetanic stimulation. There is a conspicuous facilitation of twitch tension during and after cessation of repetitive stimulation in FR fibres; the MF and EF fibres show, on the contrary, a depression of twitches. 3) Recovery to original (prefatigue) values is rapid in FR fibres, but slow, however, in EF fibres. 4) Removal of extracellular Ca2+ intensified the inability to maintain the maximum tetanic tension (IMT) and the tetanic tension decline, especially in fibres with an initial high fatigue-resistance. We assume that the results might be explained by a different refractoriness of transmission between the T-tubules and the sarcoplasmic reticulum in examined fibres and/or by a different dependency of the T-SR transmission on the extracellular calcium ions. A possible cause of the failure may be an intensification of the inactivation process.