Correlation of function and structure in developing rat distal colon.

The distal colon of suckling (14-day-old) and weanling (27-day-old) rats absorbs Na+ via channel-mediated, electrogenic amiloride-sensitive Na+ transport which disappears after weaning. This transport pathway is induced by aldosterone in superficial cells of colonic epithelium. The purpose of the present study was to correlate the changes in distal colon function with changes in the apical and basolateral cell membrane surface areas of superficial enterocytes. The basolateral but not apical membrane surface density (i.e. surface areas of the basolateral and apical membranes of superficial enterocytes per unit volume of superficial enterocytes) was found to increase between postnatal day 14 and 27 (i.e. during the weaning period). The plasma concentrations of aldosterone were very high during this period and electrogenic amiloride-sensitive Na+ transport and Na, K-ATPase activity were increased significantly. High dietary salt intake during the weaning period prevented the developmental increase of basolateral membrane surface density, inhibited electrogenic amiloride-sensitive Na+ transport and significantly depressed plasma aldosterone levels and Na, K-ATPase activity. Apical cell membrane surface density did not change significantly after a sodium load. Thus, high plasma concentrations of aldosterone and/or high colonic Na+ transport during weaning exert an important and selective effect on the basolateral membrane and are responsible for the developmental changes of this cell membrane surface during weaning. Concomitant changes in the morphology of superficial colonic enterocytes and in colonic Na+ transport suggest that the structure of the immature epithelium is related to its function.