CHARACTERIZATION OF TENSION DECLINE IN DIFFERENT TYPES OF FATIGUE-RESISTANT SKELETAL-MUSCLE FIBERS OF THE FROG - LOW EXTRACELLULAR CALCIUM EFFECTS

Twitch and tetanic tension have been measured in single skeletal muscl e 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, twit ch fibers were subdivided in three groups (resembling those as describ ed by Westerblad and Lannergren 1986 in Xenopus), that is fatigue-resi stant (FR), moderately fatigued (MF) and easily fatiguing (EF). It was found futher that the fibres differ in tetanic tension decline resist ance i.e. fatiguability relating to some basic contractile parameters including the amplitude, the rates parameters of twitch and tetanus te nsion as well as the tetanus/twitch tension ratio. The main difference s observed concern: 1) The inability to maintain the maximum tetanic t ension plateau (IMT) during single tetanus. IMT was 18 times higher in EF fibres and 4 times higher in MF fibres, respectively, in compariso n with FR fibres. IMT is the first parameter to change significantly d uring repetitive tetanic stimulation. 2) The different fibre types sho w pronounced differences in twitch contraction and tetanus tension dur ing repetitive tetanic stimulation. There is a conspicuous facilitatio n of twitch tension during and after cessation of repetitive stimulati on in FR fibres; the MF and EF fibres show, on the contrary, a depress ion of twitches. 3) Recovery to original (prefatigue) values is rapid in FR fibres, but slow, however, in EF fibres. 4) Removal of extracell ular Ca2+ intensified the inability to maintain the maximum tetanic te nsion (IMT) and the tetanic tension decline, especially in fibres with an initial high fatigue-resistance.We assume that the results might b e 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 c alcium ions. A possible cause of the failure may be an intensification of the inactivation process.