Quantification of Rat Cerebral Cortex Na + ,K + ‐ATPase: Effect of Age and Potassium Depletion

Na + ,K + ‐ATPase concentration in rat cerebral cortex was studied by vanadate‐facilitated [ 3 H]ouabain binding to intact samples and by K + ‐dependent 3‐ O ‐methylfluorescein phosphatase activity determinations in crude homogenates. Methodological errors of both methods were evaluated. [ 3 H]Ouabain binding to cerebral cortex obtained from 12‐week‐old rats measured incubating samples in buffer containing [ 3 H]ouabain, and ouabain at a final concentration of 1 × 10 –6 mol/L gave a value of 11,351 ± 177 (n = 5) pmol/g wet weight (mean ± SEM) without any significant variation between the lobes. Evaluation of affinity for ouabain was in agreement with a heterogeneous population of [ 3 H]ouabain binding sites. K + ‐dependent 3‐ O ‐methylfluorescein phosphatase activity in crude cerebral homogenates of age‐matched rats was 7.24 ± 0.14 (n = 5) μmol/min/g wet weight, corresponding to a Na + ,K + ‐ATPase concentration of 12,209 ± 236 pmol/g wet weight. It was concluded that the present methods were suitable for quantitative studies of cerebral cortex Na + ,K + ‐ATPase. The concentration of rat cerebral cortex Na + ,K + ‐ATPase showed ∼10‐fold increase within the first 4 weeks of life to reach a plateau of ∼11,000–12,000 pmol/g wet weight, indicating a larger synthesis of Na + ,K + pumps than tissue mass in rat cerebral cortex during the first 4 weeks of development. K + depletion induced by K + ‐deficient fodder for 2 weeks resulted in a slight tendency toward a reduction in K + content (6%, p > 0.5) and Na + ,K + ‐ATPase concentration (3%, p > 0.4) in cerebral cortex, whereas soleus muscle K + content and Na + ,K + ‐ATPase concentration were decreased by 30 ( p < 0.02) and 32% ( p < 0.001), respectively. Hence, during K + depletion, cerebral cortex can maintain almost normal K + homeostasis, whereas K + as well as Na + ,K + pumps are lost from skeletal muscles.