Force during stretch and shortening of frog sartorius muscle: effects of intracellular acidification due to increased carbon dioxide.

The force-velocity relation of frog sartorius muscle was observed during slow stretch and during shortening in solutions with and without CO2 at extracellular pH (pHo) 6.9 and pHo 7.5 (5 degrees C). Less force was produced with CO2 than without CO2 during stretch, during shortening, and under isometric conditions. Compared with pHo 7.5, the effects were greater at pHo 6.9, where the concentration of CO2, a permeant acid, was greater and would cause a greater acidification of intracellular pH (pHi). The reduction of force caused by CO2 was smaller during stretch than during shortening or isometric contraction. This result indicates that the crossbridge states specific to stretch retain their ability to produce force better under acidic conditions than those characteristic of shortening and isometric conditions. This difference between stretch and shortening suggests that there may be compensating changes in the pattern of motor unit activity during fatigue in vivo.