Dependence of tonic tension on extracellular calcium in rat extraocular muscle

Ca2+-free saline containing 3.0 mM Mg2+ virtually abolishes the tonic tension evoked by depolarization with a high K+ concentration of the tonic multiply innervated fibers of rat extraocular muscles. The tonic tension abolished by Ca2+ withdrawal is restored when Ca2+ is substituted by Sr2+ but not by Ni2+. The increase of Mg2+ reduces the tonic tension and displaces the tension-log K+ relationship to the right. Cd2+ significantly reduces the tension amplitude but does not shift the tension-log K+ relationship. The organic blocker of Ca2+ channels, nifedipine (1-10 microM), has no effect on the tonic tension. In contrast, diltiazem (20 microM) reduces the amplitude of the responses without changing the tension-log K+ relationship. Both foreign anions NO3- and SCN- potentiate tonic tension without changing the tension-log K+ relationship. SCN- increases the resting tension of the muscle; this effect depends on Ca2+. In conclusion, the disappearance of tonic tension after Ca2+ withdrawal is not due to depolarization of the fibers or inactivation of the contractile responses. It is suggested that entry of extracellular Ca2+, via a voltage-dependent Ca2+ conductance, or specific interactions of Ca2+ with membrane sites involved in the regulation of excitation-contraction coupling play a role in evoking tension in tonic fibers.