Effects of ramp shortening during linear phase of relaxation on [Ca2+](i) in intact skeletal muscle fibers

The effects of shortening distance at V-u, the unloaded shortening speed, a nd filament overlap on the amount of extra Ca2+ released during relaxation in muscle, as indicated by the bump area, were studied. Single, intact frog skeletal muscle fibers at 3 degrees C were used. The myoplasmic free Ca2concentration ([Ca2+](i)) was estimated by using fura 2 salt injected into the myoplasm. Ramps were applied, either at full overlap with different siz es or at varying overlaps with a fixed size, in the linear phase of relaxat ion. At full overlap, a plot of bump area vs. ramp size was fit by using a sigmoidal curve with one-half of the bump area equal to 25.9 nm. With a fix ed ramp size of 100 nm/half-sarcomere, the plot of bump area vs. mean sarco mere length (SLm) was fit by a straight Line intersecting the SLm axis at s imilar to 3.5 mu m, close to just no overlap. The results suggest that the transition in the distribution of attached cross bridges from the isometric case to one appropriate for unloaded shortening at V-u is completed within 50 nm/half-sarcomere and support the view that attached cross bridges in t he overlap zone influence the affinity of Ca2+ for troponin C in the thin f ilament.