The action of Ca2+ , Mg2+ and H+ on the contraction threshold of frog skeletal muscle: Evidence for surface charges controlling electro-mechanical coupling.

The dependence of the threshold potential for contraction of pH and the concentration of Ca2+ and Mg2+ in the bathing solution was measured in frog skeletal muscle. Decreasing the pH from 10.3 to 4.65 resulted in a threshold shift to more positive potentials. Between pH 6.5 and 8.5 the concentration threshold was almost pH -independent. Increasing [Ca2+]o (in the concentration range 0.5-50 mM) shifted the curves relating contraction threshold to pH to less negative potentials and diminished the overall pH-dependence. The contraction threshold exhibited a similar dependence on [Ca2+]o and [Mg2+]o, the two curves running parallel in the concentration range of 5-50 mM, but Mg2+ was only c. 0.6 as effective as Ca2+. To explain these results a surface charge model is proposed which assumes that two acidic groups, sigma1 and sigma2, and one basic group, sigma3, reside at the outer surface of the membrane of the T-system. Alterations in the extracellular medium exert their influence on the electro-mechanical coupling process by changing the surface potential. The groups will be titrated by protons and their charges screened off by the divalent cations. In addition, Ca2+ was supposed to bind with a weak dissociation constant (23 M) to the two acidic groups. The chosen charge densities are: sigma1 = -0.0085/A2 [= -1e/(10.8 A)2], sigma2 = -0.0037/A2 [= -1e/(16.4 A)2], sigma3 = 0.0028/A2 [= + 1e/(18.9 A)2] with intrinsic dissociation constants KH1 = 10(-2.0)M, KH2 = 10(-4.1)M, and KH3 = 10(-8.5) M. The measured threshold values are satisfactorily described by this model except at extreme alkaline and acid pH values.