Ca2+ activated isometric tension in mechanically skinned rat ventricular fibers were measured in solutions containing 3X10-5 M, 1X10-4 M, 1X10-3 M, and 2X10-3 M free MgATP2-. In addition all solutions contained 7 mM EGTA, 70 mM K+, Cl-, 50 μM Mg2+, 15 mM creatine phosphate, creatine phosphokinase, imidazole and varying amounts of Ca2+ (pH = 7.0, ionic strength = 0.15 M, T = 21°C). For each [MgATP2-] the mean percent of maximum tension was plotted vs pCa and sigmoid curves were fit using the Hill equation. Increasing [MgATP2-] shifted the pCa vs percent maximum tension relationship in the direction of increasing Ca2+ required for activation. The pCa which produced 50% of maximum tension was 5.8, 5.3, 5.4, 5.5 for 3X10-5 M, 1X10-4 M, 1X10-3 M, and 2X10-3 M MgATP2- solutions. The Hill coefficients (1.19, 1.24, 2.20, 2.99) and the steepness of the curves increased as the [MgATP2-] was elevated. This change in steepness accounts for the slightly lower [Ca2+] needed for half maximum tension as the [MgATP2-] is increased to millimolar levels. The shift in the pCa vs percent tension relationship as [MgATP2-] is changed from 3X10-5 M to 1X10-4 M is consistent with the hypothesis of Weber and co workers that a critical number of rigor bridges can affect the requlatory properties of troponin. The increases in slope at higher MgATP2- levels suggest that even in the absence of rigor bridges the Ca2+ sensitivity may be affected by the formation of a critical number of force generating bridges.
|Original language||English (US)|
|Issue number||2 II|
|State||Published - Jan 1 1975|
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