### Abstract

Ca^{2+} 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 MgATP^{2-}. In addition all solutions contained 7 mM EGTA, 70 mM K^{+}, Cl^{-}, 50 μM Mg^{2+}, 15 mM creatine phosphate, creatine phosphokinase, imidazole and varying amounts of Ca^{2+} (pH = 7.0, ionic strength = 0.15 M, T = 21°C). For each [MgATP^{2-}] the mean percent of maximum tension was plotted vs pCa and sigmoid curves were fit using the Hill equation. Increasing [MgATP^{2-}] shifted the pCa vs percent maximum tension relationship in the direction of increasing Ca^{2+} 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 MgATP^{2-} solutions. The Hill coefficients (1.19, 1.24, 2.20, 2.99) and the steepness of the curves increased as the [MgATP^{2-}] was elevated. This change in steepness accounts for the slightly lower [Ca^{2+}] needed for half maximum tension as the [MgATP^{2-}] is increased to millimolar levels. The shift in the pCa vs percent tension relationship as [MgATP^{2-}] 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 MgATP^{2-} levels suggest that even in the absence of rigor bridges the Ca^{2+} sensitivity may be affected by the formation of a critical number of force generating bridges.

Original language | English (US) |
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Pages (from-to) | 153A |

Journal | Biophysical journal |

Volume | 15 |

Issue number | 2 II |

State | Published - Jan 1 1975 |

Externally published | Yes |

### ASJC Scopus subject areas

- Biophysics

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## Cite this

*Biophysical journal*,

*15*(2 II), 153A.