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

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Abstract
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)