Intestinal 5'deiodinase activity of developing and adult chickens selected for high and low body weight.

Intestinal 5'deiodinase activity (5'D) in adults from lines of chickens selected for high (HW) and low (LW) juvenile body weight was studied in vitro using reverse T3 substrate. This avian intestinal 5'D activity is similar in its biochemical characteristics to the hepatic 5'D activity of mammals and birds. Adult 5'D activity was higher in line HW than LW; a pattern opposite to that prior to hatching. Previous studies have shown that thyroid hormones are important in triggering differentiation of the intestine during the latter part of embryonic life. Our studies indicate that the intestinal tissue is capable of producing T3 that may be important in stimulating differentiation of the intestine itself, in preparation for posthatching food assimilation. The intestines of LW embryos, which hatch earlier than HW embryos, show both more rapid maturation, as indicated by intestinal alkaline phosphatase activity (a marker of differentiation), and higher 5'D activity. Increases in intestinal 5'D in the HW line at hatch may be important in the differential intestinal growth that leads to higher relative intestinal weights in HW than LW chicks after hatch. The magnitude of 5'D activity and the relative size of the intestine suggest that this organ could be an important contributor to plasma T3.