Depolarization-induced Ca2+ release in ischemic spinal cord white matter involves L-type Ca2+ channel activation of ryanodine receptors

The mechanisms of Ca2+ release from intracellular stores in CNS white matter remain undefined. In rat dorsal columns, electrophysiological recordings showed that in vitro ischemia caused severe injury, which persisted after removal of extracellular Ca2+; Ca2+ imaging confirmed that an axoplasmic Ca2+ rise persisted in Ca2+-free perfusate. However, depletion of Ca2+ stores or reduction of ischemic depolarization (low Na+, TTX) were protective, but only in Ca2+-free bath. Ryanodine or blockers of L-type Ca2+ channel voltage sensors (nimodipine, diltiazem, but not Cd2+) were also protective in zero Ca2+, but their effects were not additive with ryanodine. Immunoprecipitation revealed an association between L-type Ca2+ channels and RyRs, and immunohistochemistry confirmed colocalization of Ca2+ channels and RyR clusters on axons. Similar to "excitation-contraction coupling" in skeletal muscle, these results indicate a functional coupling whereby depolarization sensed by L-type Ca2+ channels activates RyRs, thus releasing damaging amounts of Ca2+ under pathological conditions in white matter.