Vesicular demyelination induced by raised intracellular calcium

Abstract

Incubation of nerve with high concentrations of the divalent cation ionophore A23187 produces myelin vesiculation (Schlaepfer 1977). This observation has now been extended using segments of rat ventral or dorsal root incubated with high (19 μM, 10 μg/ml) or low (1–1.5 μM) concentrations of A23187, or another divalent ionophore, ionomycin. Low concentrations of A23187 induced no vesiculation within a 2-h period. However, subsequent incubation of these roots in fresh, ionophore-free medium for 20 h, resulted in a prominent vesicular demyelination at the Schmidt-Lanterman incisures and paranodes of many fibres. At this time (22 h) the Schwann cells associated with some demyelinating internodes appeared vital upon ultrastructural examination: the cells also excluded the nuclear dye nigrosin. High concentrations of A23187 induced a similar vesicular demyelination in affected fibres within only 15–20 min. While the Schwann cells continued to exclude nigrosin for a further 4 h, their ultrastructural appearance indicated that they were probably in the early stages of necrosis. Incubation of moribund root with the ionophore produced no myelin vesiculation. At all ionophore concentrations, the myelin vesiculation was dependent upon the presence of extracellular Ca 2+, and could be modulated in severity by varying this concentration. Other divalent cations (Ba 2+, Co 2+, Mg 2+, Mn 2+, Ni 2+, Sr 2+) could not substitute for Ca 2+. The vesiculation induced by A23187 could be entirely prevented by the addition of Zn 2+ (⩾ 1 μM), Ni 2+ (⩾ 1–10 μM), Co 2+ (⩾ 100 μM) or Mn 2+ (⩾ 100 μM) to the bathing medium. A23187 applied to only part of an isolated internode resulted in a localization of the myelin disruption to that region. Ionomycin (⩾ 1 μM), an ionophore with a greater selectivity for Ca 2+ than A23187, also induced a prompt Ca 2+-dependent myelin vesiculation. We conclude that vesicular demyelination can be initiated in vital Schwann cells by a raised intracellular Ca 2+ concentration. Such demyelination does not necessarily lead to Schwann cell death. The possible relevance of the findings to vesicular demyelinating neuropathies is discussed, and a hypothesis regarding the mechanism of demyelination is advanced.