title: T-tubule depolarization-induced SR Ca2+ release is controlled by dihydropyridine receptor- and Ca(2+)-dependent mechanisms in cell homogenates from rabbit skeletal muscle.
abstract: In vertebrate skeletal muscle, the voltage-dependent mechanism of rapid sarcoplasmic reticulum (SR) Ca2+ 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 45Ca2+ 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, approximately 20% of the total sequestered Ca2+ (approximately 30 nmol 45Ca2+/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 > or = 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 Ca(2+)-dependent mechanisms.
authors.affiliation: Department of Biochemistry and Biophysics, University of North Carolina, Chapel Hill 27599-7260, USA.
authors.forename: K
authors.initials: K
authors.lastname: Anderson
authors.forename: G
authors.initials: G
authors.lastname: Meissner
mesh_terms: D000818:Animals; D002118:Calcium / Q000502:physiology*; D020746:Calcium Channels, L-Type; D004533:Egtazic Acid / Q000031:analogs & derivatives / Q000494:pharmacology; D004594:Electrophysiology; D017275:Isradipine / Q000494:pharmacology; D008870:Microtubules / Q000502:physiology*; D009124:Muscle Proteins / Q000037:antagonists & inhibitors; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism*; D009543:Nifedipine / Q000494:pharmacology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*; D014700:Verapamil / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D020746:Calcium Channels, L-Type; D009124:Muscle Proteins; D004533:Egtazic Acid; C043184:4-desmethoxyverapamil; D014700:Verapamil; D009543:Nifedipine; C025603:1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid; D002118:Calcium; D017275:Isradipine