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 different. The colon plays an important role for the regulation of K+ homeostasis. Colonic K+ transport is bidirectional. In this study we have examined the development of colonic K+ transport with special reference to the contribution of different K+-transporting pathways. The net colonic 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 contribution of these different ATPases changes during development. The activity of colonic Na+-K+-ATPase, measured as ouabain-sensitive Na+-dependent ATP hydrolysis and as Rb-86 uptake, was lower in I than in A rats. In contrast, the activity of K+-ATPases located in apical membrane and measured as ouabain insensitive and SCH-28080 sensitive, as ouabain-sensitive Na+-independent ATP hydrolysis, and as Rb-86 uptake was significantly higher in I than in A rats. The ratio between apically located K+-ATPases and basolateral Na+-K+-ATPase activities was almost 3.2-fold higher in I than in A colon. We identified with Northern blot the expression of the colonic H+-K+-ATPase and the Na+-K+-ATPase a-subunits. The alpha-mRNA expression of both ATPases was significantly higher in I than in A rats. The pH and K+ sensitivity of the ouabain-insensitive, SCH-28080-sensitive K+-ATPase was the same in I and A colons. In conclusion, the relative activity of apical K+ absorbing ATPases is higher in the I than in the A colon, which should aid infants in retaining K+.