Characterization of "depolarization"-induced calcium release from sarcoplasmic reticulum in vitro with the use of membrane potential probe

In the triad, the complex of transverse (T) tubule and sarcoplasmic reticulum (SR) Ca²⁺ release is induced from SR by mediation of the T-tubule. We report here evidence that this Ca²⁺ release is produced by depolarization of the T-tubule moiety. Thus, we found that the amount of [¹⁴C]SCN^- taken up by T-tubules and triads (but not that by SR) increased upon incubation with (K, Na) gluconate, Mg ATP, indicating that the T-tubule was polarized making the lumenal side (equivalent to the extracellular side of an intact muscle fiber) more positive. Upon mixing with choline chloride, the procedure to induce Ca²⁺ release, [¹⁴C]SCN^- uptake decreased, indicating that the T-tubule became depolarized. Activation of the T-tubule polarization by Na⁺ and prevention of it by digoxin [inhibitor of the (Na⁺, K⁺) pump], respectively, led to activation and inhibition of choline chloride-induced SR Ca²⁺ release.