DEVELOPMENT OF (Na⁺‐K⁺)‐ATPase IN RAT HINDBRAIN: INCREMENTS IN PARALLEL WITH Na⁺‐DEPENDENT PHOSPHORYLATION AND K⁺‐pNITROPHENYLPHOSPHATASE

Rat hindbrain NaI‐enriched microsomal (Na⁺‐K⁺)‐ATPase activity, K⁺‐pNPPase activity, and Na⁺‐dependent steady‐state phosphorylation levels all increase approx 10‐fold relative to microsomal protein between 5 days prenatally and 60 days postnatally. These activities, as well as the mean wet weight of the hindbrain, are at half of their 60 day values shortly after the 10th postnatal day. For all ages, these hindbrain activities average over twice those found in the forebrain in a companion paper (Bertoni & Siegel, 1978). Increases during development in the amount of the related phosphorylatable polypeptide, estimated by densitometry of stained polyacrylamide gels containing fixed amounts of microsomal protein dissolved in SDS, are in agreement with increases in steady state levels of Na⁺‐dependent phosphorylation. The fraction of total phosphorylation that is Na⁺‐dependent rises steadily during development consistent with, but not obligatorily due to, a conversion of some of the previously Na⁺‐independent portion. Mg2⁺‐ATPase and Mg²⁺ ‐pNPPase activities and steady‐state Na⁺,‐independent phosphorylation levels do not increase in parallel during development. These observations add further support to the proposed partial reaction scheme for (Na⁺‐ K⁺)‐ATPase. The major increments in (Na ⁺‐K⁺)‐ATPase occur simultaneously with the deposition of specialized plasma membranes, particularly in the molecular layer of the cerebellum, as described in previous studies of rat hind brain.