Postnatal changes in adrenal gluco- and mineralocorticoid secretion and colonic sodium and chloride transport were examined. New Zealand White rabbits, age 10-14, 18-22, and 25-30 days, and adult animals (6-10 wk) were studied. Serum cortisol, corticosterone, aldosterone, and mucosal Na(+)-K(+)-ATPase activities were measured in each age group. Transport studies were performed in vitro under short-circuited conditions in distal colon at all age groups and in proximal colon in days 10-14 and 18-22 and in adult animals. Serum glucocorticoids varied little until after day 30 when they rose to adult levels. On the other hand, serum aldosterone levels were two- to threefold higher in days 10-14 and 18-22 animals but fell to adult levels by day 25. In distal colon, amiloride-inhibitable electrogenic Na+ absorption was present at all ages but was significantly greater (P less than 0.01) in days 10-14 (3.8 +/- 0.5 mu eq.cm-2.h-1) and 18-22 (4.2 +/- 0.4) rabbits compared with adults (1.9 +/- 0.4) but not day 25-30 (2.8 +/- 0.5). In proximal colon, Na+ absorption was significantly higher (P less than 0.05) in day 10-14 (1.6 +/- 0.5 mu eq.cm-2.h-1) compared with day 18-22 (-0.2 +/- 0.5) and adults (0.06 +/- 0.5) and was amiloride insensitive. Neither chloride transport nor mucosal Na(+)-K(+)-ATPase demonstrated significant age-related changes in either region of colon. These results indicate that both proximal and distal colonic Na+ transport undergoes postnatal changes. In distal but not proximal colon these changes appear to be regulated by circulating aldosterone probably by increasing apical membrane permeability to Na+.
Brain Delivery of Thyrotropin-Releasing Hormone via a Novel Prodrug Approach
