The Appearance of a Highly Digitalis-Sensitive Isoform of Na⁺, K⁺-ATPase during Maturation In Vitro of Primary Cultured Rat Cerebral Neurons¹

Immature neurons from the cerebra of 17-day rat fetuses were cultured, and changes in Na⁺, K-ATPase activity of the cells were investigated during maturation in culture. The Na⁺, K⁺-ATPase activity of the particulate fraction from the cells increased during the course of culture, up to 0.38±0.01 (µmol/min/mg protein) (=8.25±0.85 (µmol/min/mg DNA)) by day 13 in culture. The values were more than 3 and 19 times those of the 17-day fetal cerebrum on the bases of protein and DNA, respectively. The enzyme in the immature neurons was mainly a weakly digitalis-sensitive form in terms of the inhibition pattern of the Na⁺, K⁺-ATPase activity by strophanthidin. Along with the maturation in vitro of the neurons, a highly digitalis-sensitive form of the enzyme was shown to appear and increase by the biphasic inhibition patterns of Na⁺, K⁺-ATPase activity by strophanthidin and of K⁺ uptake activity by ouabain. The enzyme activity of the highly sensitive form overwhelmed that of the weakly sensitive form by day 13 in culture. In cultured rat cerebral astrocytes, the enzyme was judged to be only the weakly digitalis-sensitive form from the simple inhibition patterns of the Na⁺, K⁺-ATPase and K⁺ uptake activities by digitalis.