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Effects of ramp shortening during linear phase of relaxation on [Ca 2+ ] i in intact skeletal muscle fibers

1999; American Physical Society; Volume: 276; Issue: 1 Linguagem: Inglês

10.1152/ajpcell.1999.276.1.c152

1522-1563

Yandong Jiang, Fred J. Julian,

Muscle Physiology and Disorders

The effects of shortening distance at V u , the unloaded shortening speed, and filament overlap on the amount of extra Ca 2+ released during relaxation in muscle, as indicated by the bump area, were studied. Single, intact frog skeletal muscle fibers at 3°C were used. The myoplasmic free Ca 2+ concentration ([Ca 2+ ] i ) was estimated by using fura 2 salt injected into the myoplasm. Ramps were applied, either at full overlap with different sizes or at varying overlaps with a fixed size, in the linear phase of relaxation. 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 fixed ramp size of 100 nm/half-sarcomere, the plot of bump area vs. mean sarcomere length (SL m ) was fit by a straight line intersecting the SL m axis at ∼3.5 μm, 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 the overlap zone influence the affinity of Ca 2+ for troponin C in the thin filament.