Development and regional distribution of two molecular forms of the catalytic subunit of the Na,K-ATPase in rat brain.

Two molecular forms of the catalytic subunit of the Na,K-ATPase can be isolated from brain (1). Only the alpha form was detected in rat embryo brain at 13 days of gestation (E13). The alpha (+) form, which is characteristic of myelinated axons, appeared at E15 before myelination begins. Hence its expression is not dependent on prior myelination. Axonal transport of the alpha (+) form was demonstrated in 4 day-old rats. The ratio of alpha (+): alpha was 1:1 in adult retina, cortex and cerebellum and 10:1 in brain stem. Although alpha (+) is characteristic of myelinated axons, this regional difference was present not only in enzyme extracted from crude microsomes, that contain myelinated axon fragments, but also in enzyme from isolated synaptosomes. Hence, the alpha (+): alpha ratio is an inherent characteristic of the neuron and does not depend on regional differences in myelination.