Maximum and submaximum Ca-activated tension in mechanically disrupted rat ventricular fibres was examined in solutions containing 30 μM, 100 μM and 4 mM MgATP and either 50 μM or 1 mM ionized Mg. In the absence of added Ca, significant amounts of base line tension (up to 50% of maximum) develop in solutions containing less than 30 μM MgATP. This effect is Mg dependent; more tension is produced with 50 μM Mg than with 1 mM. Increasing the MgATP concentration shifts the pCa-% maximum tension relationship in the direction of increasing Ca required for activation. At 50 μM Mg the pCa which produces 50% maximum tension is 5.8, 5.3 and 5.5 for the 30 μM, 100 μM and 4 mM MgATP solutions. The effect of MgATP on position is relatively independent of the Mg concentration. The steepness of the pCa-% maximum tension curve increases as MgATP is elevated to the millimolar range. The Hill coefficients for the different MgATP curves at 50 μM Mg are 1.1, 1.3 and 3.0. This change in steepness accounts for the slightly lower Ca concentration needed for half maximum tension as the MgATP concentration is increased to millimolar levels. Raising the Mg concentration to 1 mM greatly diminishes the effect of MgATP on the slope of the pCa tension relationship. The maximum tension a fibre bundle can produce decreases as the amount of MgATP is raised from micromolar to millimolar levels. For 50 μM Mg, maximum tension drops about 35% as MgATP is raised from 30 μM to 4 mM. For any concentration of MgATP, maximum tension is higher at 1 mM Mg than at 50 μM Mg. Existing theories of interaction between myosin heads and the thin filament are sufficient to account for the effects of MgATP on the position of the pCa tension curves and on maximum tension. The effects on slope are less satisfactorily explained.
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