Effects of external calcium concentration and pH on charge movement in frog skeletal muscle

1. The effects of both external Ca2+ (1.8, 25, 50 and 100 mM) and external pH (pH 5.5, 7.15, and 9.0) on the voltage-dependence of charge movement in frog skeletal muscle were examined using the three intracellular micro-electrode voltage-clamp technique. 2. The two-state model of Schneider & Chandler (1973) was used to describe the voltage distribution of membrane charge. The parameters of this model are: Qmax, the maximum quantity of charge; V, the potential of equal distribution of charge; and k, a constant relating to the steepness of the charge vs. voltage relationship. 3. In 1.8 mM external Ca2+, alterations, in external pH shifted the transition potential, V, from a mean +/- S.E. of mean of -36.5 +/- 0.9 mV at pH 7.15 to -25.8 +/- 1.3 mV at pH 5.5 and to -42.5 +/- 1.8 mV at pH 9.0. These shifts are consistent with surface charge theory. No significant changes in Qmax or k were observed over the range of pH 5.5--9.0. 4. A reasonable fit of surface charge theory to the shifts in V over the range pH 5.5--9.0 could be obtained with surface charge densities and binding constants: sigma 1 = -1 e/165 A2, pK1 = 3.9 and sigma 2 = -1 e/400 A2, pK2 = 8. 5. However, at pH 7.15, both V and k changed with increasing external Ca2+ concentration. V shifted from -34.9 +/- 3.7 mV in 1.8 mM-Ca2+ to -13.8 +/- 5.1 mV, -19.3 +/- 3.6 mV and 3.3 +/- 9.3 mV in 25, 50 and 100 mM-Ca2+ respectively. k increased from 8.3 +/- 0.6 mV in 1.8 mM-Ca2+ to 15.3 +/- 1.4 mV, 14.6 +/- 1.6 mV and 20.0 +/- 2.9 mV in 25, 50 and 100 mM-Ca2+. Changes in k reflect decreases in the apparent charged particle valence from approximately 3 in 1.8 mM-Ca2+ to approximately 1.2 in 100 mM-Ca2+. As the external Ca2+ concentration was raised, Qmax was at least as large as that measured in 1.8 mM-Ca2+. The 43% decrease in the apparent valence of the charged groups cannot be explained by simple surface charge theory and may reflect a specific interaction between external Ca2+ and the charged groups. 6. Shifts in V with alterations in external pH and Ca2+ concentration are consistent with the effects of these agents on the contraction threshold of muscle fibres. This observation lends further support to the hypothesis that the charge movement is involved in gating muscle contraction and that the charged particles respond to changes in the electric field across the muscle cell membrane. 7. No difference was observed in the charge movement parameters of fibres from both room-temperature and cold-adapted frog tested at 2--5 degrees C in 1.8 mM-Ca2+ at pH 7.15.