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

1. Maximum and submaximum Ca-activated tension in mechanically disrupted rat ventricular fibres was examined in solutions containing 30 micron, 100 micron and 4 mM-MgATP and either 50 micron or 1 mM ionized Mg. 2. In the absence of added Ca, significant amounts of base-line tension (up to 50% of maximum) develop in solutions containing less than 30 micron-MgATP. This effect is Mg-dependent; more tension is produced with 50 micron-Mg than with 1 mM. 3. Increasing the MgATP concentration shifts the pCa-% maximum tension relationship in the direction of increasing Ca required for activation. At 50 micron-Mg the pCa which produces 50% maximum tension is 5-8, 5-3 and 5-5 for the 30 micron, 100 micron and 4 mM-MgATP solutions. The effect of MgATP on position is relatively independent of the Mg concentration. 4. 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 micron-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. 5. The maximum tnesion a fibre bundle can produce decreases as the amount of MgATP is raised from micromolar to millimolar levels. For 50 muM-Mg, maximum tension drops about 35% as MgATP is raised from 30 micronM to 4 mM. For any concentraiton of MgATP, maximum tension is higher at 1 mM-Mg than at 50 micron-Mg. 6. 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.