Regulation of ion transport in the gastrointestinal tract : aspects of early development

Abstract: Ion transport in the gastrointestinal tract is vital for cellular homeostasis and to ensure growth during ontogeny. We have therefore investigated the regulation and maturation of sodium, potassium and hydrogen transport in the gastrointestinal tract. Net jejunal sodium absorption is significantly higher in young as compared to adult rats. During high salt intake, net sodium absorption decreases in young but not in adult rats. When dopamine production was inhibited we did not observe decreased sodium absorption in young rats on high salt diet. The reduced sodium absorption during high salt intake in young rats requires endogenous dopamine action. Net colonic potassium absorption in young rats is five times higher than in adult ones. The majority of colonic net potassium absorption occurs in the distal part of the colon. Using different approaches this study shows that apically located transporters which reabsorb potassium are upregulated in the infant rat while secreting basolateral transporters are upregulated in adult rats. Infant rat H+, K+- ATPase and ouabainsensitive sodium independent ATPase were higher but Na+, K+-ATPase was lower than in the adult rat. Angiotensin II induces potassium secretion in rat distal colon. This effect is mediated through both receptors (AT1, and AT2) and by at least three different pathways (Na+, K+, 2CIcotransporter, the apical barium-sensitive potassium channels and the Na+, K+-ATPase). Rat gastric H+, K+- ATPase matures postnatally. The expression of H+, K+-ATPase increases from day 10. Gastric H+, K+- ATPase increased 2.5-fold when 10-day-old rats were treated with a single dose of glucocorticoids. A study of human gastric H+, K+- ATPase enzyme was performed on biopsies from neonates. The amount of H+, K- ATPase increases in relation to gestational and postnatal age. Boys have a significantly higher amount than girls. This is the first study on a large number of infants showing that gastric H+, K+- ATPase is present before term birth and that it increases during development. Conclusion: In summary, these studies of ion transport in the gastrointestinal tract show new mechanisms that ensure proper regulation of sodium, potassium and hydrogen transport during development. We have found that glucorticoids and dopamine have specific actions on ion transport during early development. In addition we have found that angiotensin II regulates colonic potassium transport in a complex manner.