AJP - Cell Physiology, Vol 253, Issue 3 C375-C383, Copyright © 1987 by American Physiological Society
Dependence of tonic tension on extracellular calcium in rat extraocular muscle
D. J. Chiarandini and J. Jacoby
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.
