Tension in mechanically disrupted mammalian cardiac cells: effects of magnesium adenosine triphosphate.

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