Pituitary regulation of postnatal small intestinal ontogeny in the rat: differential regulation of digestive hydrolase maturation by thyroxine and growth hormone.

During the third week of postnatal life, dramatic ontogenic changes occur in the morphology and enzymology of the small intestine of the infant rat, enabling the animal to make the transition from milk to solid food. To investigate the roles of T4 and GH in regulation of these changes, infant rats were hypophysectomized on day 6 of life by the transauricular technique. Hypophysectomy resulted in diminution of somatic and intestinal growth as well as abnormal maturation of the disaccharidases lactase, sucrase, and maltase when measured on day 25. Administration of either T4 or GH to hypophysectomized animals resulted in moderately increased intestinal growth, while complete restoration of small intestinal growth resulted from administration of the combination of both hormones. Although T4, GH, or the combination of hormones reduced lactase activities, T4 alone produced normal maturation of sucrase and maltase. Neither hypophysectomy nor hormone replacement affected aminooligopeptidase. The molecular structure of lactase, analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis, was not altered to a major degree in hypophysectomized animals or animals that received hormone replacement, but minor alterations were evident in sucrase structure in hypophysectomy. These studies indicate that 1) T4 and GH actively participate in postnatal regulation of small intestinal ontogeny; 2) thyroid hormones act directly on developing intestinal tissues to independently produce the normal maturation of the disaccharidases by mechanisms that are not likely to involve alterations in processing of the enzyme-protein; and 3) maturation of aminooligopeptidase is not regulated by pituitary hormones, in distinct contrast to the disaccharidases.