Morphometric and functional studies of proximal and distal colon in young and adult rats.

Net sodium and fluid absorption in the colon is higher in young than in adult rats. The mechanism behind the high absorption in young rats is not well understood. In this study, we investigated the structural and functional properties of the proximal and the distal colon in 20- and 40-day-old rats. The surface area of the mucosa along the luminal surface of the gut in relation to the mucosal volume (surface density) was determined by morphometry. Net sodium and water absorption was determined by in vivo perfusion experiments. The surface density of the mucosa was greater in the proximal than in the distal colon in both 20- and 40-day-old rats. No developmental change in the surface density of the mucosa occurred between 20 and 40 days of age, however, in either the proximal or distal colon. Net sodium and fluid absorption per unit of weight were higher in both segments in 20- than in 40-day-old rats, and higher in the distal than in the proximal colon in 40-day-old rats. We conclude that other factors than the absorptive area are responsible for the decrease in the net colonic sodium and fluid absorption observed during late postnatal development and for the differences in sodium net absorption between the proximal and distal colon in 40-day-old rats.