The isometric tension response of single "skinned' rabbit soleus muscle fibers to MgATP and McITP in the absence of calcium was studied. [MgATP] or [MgITP] was varied in solutions of ionic strength 0.30 and temperature 20 degrees C. Steady-state tension that developed in MgATP or MgITP solutions was a biphasic bell-shaped function of log [MgATP] or log [MgITP] which increased from zero to maximum tension and then declined again to zero. Analysis of the data showed that, under comparable ionic conditions, percent tension vs. log [MgATP] and percent tension vs. log [MgITP] curves are not parallel. Instead, the percent tension vs. log [MgITP] curve is much broader. Additionally, under comparable ionic conditions maximum tension in MgITP solutions was higher than in MgATP solutions. In addition, in MgATP solutions, pH, [K+], and excess ATP were varied. Raising pH from 7 to 8, [K+] from 46 mM to 200 mM, or decreasing excess ATP from 2 to 0.5 mM all increased maximum tension. None of these factors, however, influenced the shape or position of the percent tension vs. log [MgATP] curve.

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