ONTOGENY OF GASTRIC H+-K+-ATPASE IN SUCKLING RABBITS

Gastric mucosal homogenates were prepared from resting and stimulated stomachs of rabbits, age 3-57 days postnatal, and fractionated by differential centrifugation. Total H+ -K+ -adenosinetriphosphatase (ATPase) (assayed as K+ -dependent ouabain-insensitive hydrolysis of p-nitro-phenyl phosphate) was low in the first 3 wk but rapidly accumulated between days 20 and 43. Specific activity rose eightfold from day 3 to a typically adult level of 2-mu-mol.mg-1.h-1 by day 43. The microsomal fraction (P3) was subfractionated on sucrose gradients (20,27, and 33% steps or 10-40% continuous gradient). H+ -K+ -ATPase from P3 of resting stomachs was distributed bimodally on the continuous gradients, with activity manly in the denser peak (or on the 33% sucrose step) before day 20, but accumulating mainly in the lighter peak (or in the lighter step-gradient fractions) after day 20. Throughout the age range tested, in vivo stimulation with histamine just before removal of the stomach caused a loss of most H+ -K+ -ATPase from P3 and an increase in H+ -K+ -ATPase in a lower-speed fraction P1. Thus, even in the 1st postnatal wk, when H+ -K+ -ATPase is low, most of the enzyme occurs in cells with histamine H-2 receptors and all the intracellular mechanisms for fusion of oxyntic cell tubulovesicles (enriched in P3) with the apical membrane (enriched in P1). These studies delineate a 3-wk period of sharply accelerated synthesis of H+ -K+ -ATPase before weaning. Age-related changes in distribution of H+ -K+ -ATPase among microsomal density subfractions suggest maturational changes either in the intracellular partitioning of the enzyme or in properties of the membranes containing the enzyme.