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MUSCLE CELL BIOLOGY AND CELL MOTILITY

Effects of intracellular acidification and varied temperature on force, stiffness, and speed of shortening in frog muscle fibers

Department of Physiological Sciences, Biomedical Centre, University of Lund, S-221 84 Lund, Sweden

Submitted 31 October 2003 ; accepted in final form 25 February 2004

This study aimed to establish whether the temperature-dependent effect of acidification on maximum force observed in mammalian muscles also applies to frog muscle. Measurements of force, stiffness, and unloaded velocity of shortening in intact single muscle fibers from the anterior tibialis muscle of Rana temporaria were performed between 0 and 22°C during fused tetani in H₂CO₃-CO₂-buffered Ringer solution with pH adjusted to 7.0 and 6.3, respectively. The force-to-stiffness ratio increased as a rectilinear function of temperature between 0 and 20°C at pH 7.0. Lowering the pH to 6.3 reduced the tetanic force by 13.5 ± 1.2 and 11.5 ± 1.4% at 2.8 and 20.5°C, respectively, with only a minor reduction in fiber stiffness. The maximum speed of shortening was decreased by lowered pH by 12.9 ± 1.5 and 7.8 ± 1.1% at low and high temperature, respectively. Acidification increased the time to reach 70% of maximum force by 18.0% at 2°C; the same pH change performed at 20°C in the same fibers reduced the rise time by 24.1%. The same increase in the rate of rise of force at high temperature was also found at normal pH after the fibers were fatigued by frequent stimulation. It is concluded that, in frog muscle, the force-depressant effect of acidification does not vary significantly with temperature. By contrast, acidification affects the onset of activation in a manner that is critically dependent on temperature.

muscle contraction; pH

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