Absorption and distribution of epidermal growth factor in the gastrointestinal tract of suckling rats.

Abstract

Suckling rats show a low level of synthesis of epidermal growth factor (EGF) and obtain this important polypeptide from maternal milk. Thus, this study was performed to characterize how and where in the gastrointestinal tract the absorption of rat-EGF (rEGF) occurs and to which organs it is delivered. Although radiolabeled EGF transferred to the liver by the ileum was significantly higher than that by the jejunum, the total amounts of immunoreactive and receptor-binding active ¹²⁵I-rEGF in the liver were significantly higher from jejunal than from ileal absorption. These results indicate that the ileum degrades most of the ¹²⁵I-rEGF during absorption, whereas the jejunum absorbs more intact ¹²⁵I-rEGF. Higher numbers of EGF receptors were found in the brush border membranes of jejunum than that of the ileum, suggesting that the larger amounts of intact EGF delivered by the jejunum to the liver may be due to a higher capacity of EGF receptors in the jejunum. Liver and intestine were the main organs taking up the radioactivity after intrafemoral vein ¹²⁵I-rEGF administration. The degradation rates of EGF in the liver, intestine and pancreas were significantly lower in suckling as opposed to adult rats. In suckling rats intact rEGF was secreted by the liver into the bile and by the enterocytes into the lumen of the intestine. The competitive effects of unlabeled rEGF were found at both sides of the small intestine and at the circulatory side of the liver, submandibular glands, pancreas, and other organs, indicating a receptor-related uptake of EGF from the lumen and circulation of these organs in suckling rats. The results indicate that after absorption maternal milk-EGF stays mainly in the digestive system during the suckling period. The amount of EGF in each organ may remain relatively stable on a daily basis since EGF is absorbed continuously from the jejunum in suckling rats and is distributed and circulated in the digestive system by hepatico-enteric, entero-enteric, and perhaps pancreatico-enteric circulations; in this way, EGF accelerates the growth of the digestive system.