Developmental and thyroid hormone regulation of two molecular forms of Na+-K+-ATPase in brain

In the brain there are two isozymes of Na+-K+-ATPase differing in their catalytic subunits: alpha, indistinguishable from the kidney form of alpha, and alpha +, found in axolemma. The time course of the increase in each alpha during development was described by quantitating the abundance of each form, studied in unpurified membranes resolved by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, with specific antibodies and with fluorescein 5'-isothiocyanate. Both the alpha and alpha + subunits, quantitated with antibodies, increased 10-fold in abundance from 18 days gestation to 20 days of age, with alpha + increasing more rapidly than alpha early in development. A 10-fold increase in enzyme activity was also observed during this period. Using fluorescein 5'-isothiocyanate to quantitate the two alpha subunits, a similar increase in alpha + was observed with less of an increase in alpha. The ratio of alpha + to alpha increased from 0.75 at 18 days gestation to 3 at 3 days of age remaining at this ratio to 20 days of age. The possibility that thyroid hormone, a known regulator of brain Na+-K+-ATPase during development, differentially regulated the two forms was tested using 15-day-old hypothyroid rats. The abundance of both forms of alpha was similarly decreased: alpha + to 69% and alpha to 48% of control values. Na+-K+-ATPase activity was 70% of control. We conclude that both alpha and alpha + abundance increase in the brain during pre-and neonatal development and that the increase in both alpha subunits is regulated, directly or indirectly, by thyroid hormones.