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1 Department of Biological Sciences, Marquette University, Milwaukee, WI, USA
* To whom correspondence should be addressed. E-mail: ndebold{at}physiology.med.uvm.edu.
Elevated levels of Pi are believed to cause a substantial proportion of the loss in muscular force and power output during fatigue from intense contractile activity. However, support for this hypothesis is based, in part, on data from skinned single fibers obtained at low temperatures (
15°C). The effect of high Pi (30 mM) on the contractile function of chemically skinned single fibers was examined at both low (15°C) and high (30°C) temperatures using fibers isolated from rat soleus (type I fibers) and gastrocnemius (type II fibers) muscles. Elevating Pi from 0 to 30 mM, at saturating free Ca++ levels, depressed maximal isometric force (Po) 54% at 15°C and 19% at 30°C, (significant interaction, p <0.05) in type I fibers. Similarly, the Po of type II fibers was also significantly more sensitive to high levels of Pi at the lower (50% decrease) vs. higher temperature (5% decrease). The maximal shortening velocity of both type I and type II fibers was not significantly affected by elevated Pi at either temperature. However, peak fiber power was depressed 49% at 15°C but only 16% at 30°C in type I fibers. Similarly, in type II fibers peak power was depressed 40% and 18% at 15 and 30°C respectively. These data suggest that near physiological temperatures and at saturating levels of intracellular Ca++, elevated levels of Pi contribute less to fatigue than might be inferred from data obtained at lower temperatures.
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