ONTOGENY OF K+ TRANSPORT IN RAT DISTAL COLON

Infants need to retain more K+ than adults to avoid growth retardation . Since the K+ requirements are different in infants (I) and in adults (A), the mechanisms regulating K+ homeostasis should also be differen t. The colon plays an important role for the regulation of K+ homeosta sis. Colonic K+ transport is bidirectional. In this study we have exam ined the development of colonic K+ transport with special reference to the contribution of different K+-transporting pathways. The net colon ic K+ uptake, as determined by in vivo perfusion studies and by Rb-86 uptake, was significantly higher in I than in A rats. In both I and A colon, similar to 80% of total Rb-86 uptake was dependent on vanadate- sensitive P-type adenosinetriphosphatases (ATPases), but the contribut ion of these different ATPases changes during development. The activit y of colonic Na+-K+-ATPase, measured as ouabain-sensitive Na+-dependen t ATP hydrolysis and as Rb-86 uptake, was lower in I than in A rats. I n contrast, the activity of K+-ATPases located in apical membrane and measured as ouabain insensitive and SCH-28080 sensitive, as ouabain-se nsitive Na+-independent ATP hydrolysis, and as Rb-86 uptake was signif icantly higher in I than in A rats. The ratio between apically located K+-ATPases and basolateral Na+-K+-ATPase activities was almost 3.2-fo ld higher in I than in A colon. We identified with Northern blot the e xpression of the colonic H+-K+-ATPase and the Na+-K+-ATPase a-subunits . The alpha-mRNA expression of both ATPases was significantly higher i n I than in A rats. The pH and K+ sensitivity of the ouabain-insensiti ve, SCH-28080-sensitive K+-ATPase was the same in I and A colons. In c onclusion, the relative activity of apical K+ absorbing ATPases is hig her in the I than in the A colon, which should aid infants in retainin g K+.