Recovery of force during postcontractile depression in single Xenopus muscle fibers

This study examined the relationship between force and cytosolic free calci um concentration ([Ca2+](c)) in different fiber types from Xenopus before, during, and after cells underwent post-contractile depression (PCD). During a standardized fatigue run, force in the two fast fatiguing (FF) fiber typ es (types 1 and 2, n = 10) fell more quickly (5.8 vs. 8.1 min) and to a gre ater degree [0.36 vs. 0.51 of initial (P-o)] than in the slow fatiguing (SF ) fiber type (type 3, n = 11). After the initial fatigue run, both FF and S F experienced a drop in force to <15% P-o (PCD) at a similar time (20.6 vs. 21.4 min). A second stimulation period, undertaken during PCD, produced si gnificant recovery of force in both groups, but significantly more so in SF than FF (64 <plus/minus> 7 vs. 29 +/- 2% P-o). This force recovery during PCD was accompanied by a significant increase in peak [Ca2+](c), particular ly in SF. However, despite the significant recovery of force during stimula tion while in PCD, the amount of force produced for a given peak [Ca2+](c) was significantly lower in both groups during PCD than at any other point i n the experiment. A final stimulation period, initiated when all fibers had recovered from PCD, demonstrated a recovery of both force and peak [Ca2+]( c) in both groups, but this recovery was significantly greater in SF vs. FF . These data demonstrate that with continuous electrical stimulation, it is possible to produce a significant recovery of force production during the normally quiescent period of PCD, but that it occurs with a decreased muscl e force production for a given peak [Ca2+](c). This suggests that factors o ther than structural alterations of the sarcoplasmic reticulum are likely t he cause of PCD in these fibers.