T-TUBULE DEPOLARIZATION-INDUCED SR CA2+ RELEASE IS CONTROLLED BY DIHYDROPYRIDINE RECEPTOR-DEPENDENT AND CA2+-DEPENDENT MECHANISMS IN CELL HOMOGENATES FROM RABBIT SKELETAL-MUSCLE

In vertebrate skeletal muscle, the voltage-dependent mechanism of rapid sarcoplasmic reticulum (SR) CA(2+) release, commonly referred to as excitation-contraction (EC) coupling, is believed to be mediated by physical interaction between the transverse (T)-tubule voltage-sensing dihydropyridine receptor (DHPR) and the SR ryanodine receptor (RyR)/Ca2+ release channel. In this study, differential T-tubule and SR membrane monovalent ion permeabilities were exploited with the use of an ion-replacement protocol to study T-tubule depolarization-induced SR Ca-45(2+) release from rabbit skeletal muscle whole-cell homogenates. Specificity of Ca2+ release was ascertained with the use of the DHPR antagonists D888, nifedipine and PN200-110. In the presence of the ''slow'' complexing Ca2+ buffer EGTA, homogenates exhibited T-tubule depolarization-induced Ca2+ release comprised of an initial rapid phase followed by a slower release phase. During the rapid phase, similar to 20% of the total sequestered Ca2+ (similar to 30 nmol Ca-45(2+)/mg protein), corresponding to 100% of the caffeine-sensitive Ca2+ pool, was released within 50 ms. Rapid release could be inhibited fourfold by D888. Addition to release media of the ''fast'' complexing Ca2+ buffer BAPTA, at concentrations greater than or equal to 4 mM, nearly abolished rapid Ca2+ release, suggesting that most was Ca2+ dependent. Addition of millimolar concentrations of either Ca2+ or Mg2+ also greatly reduced rapid Ca2+ release. These results show that T-tubule depolarization-induced SR Ca2+ release from rabbit skeletal muscle homogenates is controlled by T-tubule membrane potential- and by Ca2+-dependent mechanisms.