Double-hyperbolic force-velocity relation in frog muscle fibres.

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1. The relationship between force and velocity of shortening was studied at 2.10 micron sarcomere length during fused tetani (1-3 degrees C) in single fibres isolated from the anterior tibialis muscle of Rana temporaria. The speed of shortening was recorded from the whole fibre and, in some experiments, simultaneously from a short (ca. 0.6 mm) segment, while the preparation was released to shorten isotonically at selected force levels ('load-clamp' recording). The segment was defined by opaque markers of hair that were placed on the fibre surface. The distance between the markers was recorded by means of a photo-electric detector system. 2. The force-velocity relation had two distinct regions, each one exhibiting an upwards concave shape, that were located within the ranges 0-78 and 78-100% of the measured isometric force (P0), respectively. The two portions of the force-velocity relation could be fitted well by hyperbolic functions or by single-exponential functions. The curvature was more pronounced in the high-force region than at low-intermediate loads. The transition between the two portions of the force-velocity relation (the 'break point' of the force-velocity curve) occurred at 78.4 +/- 0.4% of P0 (mean +/- S.E. of mean, n = 12) corresponding to 10.9 +/- 0.4% of maximum velocity of shortening (Vmax). The general shape of the force-velocity curve, and the appearance of a break point near 78% of P0, was the same when measurements were made from the whole fibre and from a short segment along the same fibre. 3. The 'negative' branch of the force-velocity relation was delineated for loads ranging from P0 to 1.6-1.8 P0 in five experiments. The negative branch formed a smooth continuation of the force-velocity relation recorded between 0.78 P0 and P0. The force-velocity relation was nearly flat between 0.90 P0 and 1.20 P0, the difference in speed of shortening or elongation being 1.8 +/- 0.3% (mean +/- S.E. of mean, n = 5) of Vmax over this range. 4. An increase in sarcomere length from 1.85 to 2.60 micron did not affect Vmax but caused a steady decrease in curvature of the force-velocity relation, both at low-intermediate loads and in the high-force range. Similar changes in shape of the force-velocity relation were produced by osmotic compression of the fibre in a Ringer solution made hypertonic by addition of 98 mM-sucrose.(ABSTRACT TRUNCATED AT 400 WORDS)

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