[The effect of ionized calcium concentration on muscle contraction and on the effect of non-depolarizing...

The influence of serum ionized calcium (Ca2+) on the muscle contraction and on the effect of non-depolarizing neuromuscular blocking agent, d-tubocurarine (dTc), was investigated. The isolated phrenic nerve-diaphragma preparation of the rat was stimulated directly and indirectly at 0, 0.4, 0.7, 1.4, 2.5 mM of total calcium concentration (Ca2+: 0, 0.39, 0.52, 1.12, 2.32 mM, respectively). With indirect stimulation, no contraction was observed from 0 to 0.4 mM of calcium, but the twitch tension returned to 100 percent of control at over 0.7 mM of calcium. However, with direct stimulation, the twitch tension was obtained up to 100 percent of control independent of Ca2+ concentration. The ED50 of dTc was correlated well with the serum Ca2+. These findings suggest that the decrease of serum Ca2+ concentration interferes with the muscle contraction and potentiates the effect of dTc. We speculate that the site of its action may be primarily prejunctional.