The rat distal colon shows rapid changes in both structure and function during adaptation to certain physiological and pathological conditions. Adaptive responses involve regulation of membrane channel and transporter expression as well as of pericryptal cell growth and epithelial permeability. This study examines the individual contributions of aldosterone and vasopressin to the Na⁺ concentration and trophic changes in rat distal colon mucosa. Experiments were conducted in rats infused with aldosterone and vasopressin using osmotic pumps. Mucosal Na⁺ concentration was determined by confocal microscopy using a low-affinity Na⁺-sensitive fluorescent die; crypt permeability was monitored with FITC-dextran, and a-SMA, claudin 4, ENaC and AQP2 expression were determined by immunolocalization. The results show that the distal colon responds to both aldosterone and vasopressin with reduced permeability and increased pericryptal myofibroblast growth; both enabling the accumulation of Na⁺ in the pericryptal space. However, aldosterone up-regulated ENaC without affecting AQP2 expression, consistent with its role in Na⁺ recovery; whilst vasopressin stimulated AQP2 expression in the surface epithelium without affecting crypt ENaC, consistent with its antidiuretic role. The common effects of aldosterone and vasopressin in maintaining hypertonic NaCl concentration in the pericryptal space indicate that both individually may increase the rate of fluid absorption from the faeces.

Supported by grants BFI2003-05124 and BFU2006-08410/BFI (Ministerio de Ciencia y Tecnología, Spain).