Growth factors in colostrum and milk Stability and absorption of milk-borne

R .J. Xu et al. Livestock Production Science 66 2000 95 –107 99 Table 3 postnatal pigs that feeding colostrum enhanced in- Concentrations of EGF, IGF-I, IGF-II, insulin and TGF-b in testinal macromolecule absorption and onset of gut porcine colostrum and milk closure Westrom et al., 1985; Sangild et al., 1999. a a Colostrum Milk Reference Factors affecting GI structure and function after weaning are complex and multiple, possibly includ- EGF ng ml | 1500 150–250 Jaeger et al., 1987 b 5 NR Vaughan et al., 1992 ing pathogenic bacterial interactions, psychological IGF-I ng ml 70–350 4–14 Simmen et al., 1988 stress, transient hypersensitivity to food components 72–136 10–27 Donovan et al., 1994 and withdrawal of maternal milk Pluske et al., c 541 NR Xu et al., 1996 1997. At the time of weaning, maternal milk is IGF-II ng ml 165–291 11–50 Donovan et al., 1994 b b withdrawn abruptly and supplementation of sows’ Insulin ng ml 12.4 1.6–3.3 Jaeger et al., 1987 b b 10.8–18.2 1.2–5.5 Westrom et al., 1987 milk or colostrum to piglets during the first week b c 16.5 NR Wang and Xu, 1996 after weaning can prevent villus atrophy, diarrhoea c TGFib ng ml 9–12 ND Xu et al., 1999 and growth retardation Mosenthin, 1998; Nabuurs, a Colostrum as the mammary secretion during the first 3 days 1998. after parturition and milk as the mammary secretion thereafter. b Converted to ng ml from the original published values using conversion factors of 7.175 pmol mU and 5743 g mol for insulin

5. Growth factors in colostrum and milk

and 6040 g mol for EGF. c NR, not reported; ND, not detected. From the above discussion, it is apparent that colostrum and milk have a profound effect on GI higher in colostrum than in milk, and often greater structure and function in neonatal animals. Such than the concentration in the maternal blood circula- effects may be partially attributable to various types tion Donovan et al., 1994. of growth factors in the milk. The topic of milk- borne growth factors has been discussed in a number of recent reviews Odle et al., 1996; Xu, 1996, 1998;

6. Stability and absorption of milk-borne

Zabielski et al., 1999. The knowledge in this area growth factors in the GI lumen has increased rapidly in recent years and new growth factors in maternal milk are reported each year. To Following oral ingestion, milk-borne growth fac- date, more than 20 types of hormones, growth factors tors may act directly on the GI tract or act on or regulatory peptides have been identified in milk of peripheral targets following absorption into the blood various species Xu, 1998. In porcine colostrum and circulation. A prerequisite for either of these actions, milk, epidermal growth factor EGF, insulin-like however, is survival in the GI lumen. Experimental growth factor-I IGF-I, IGF-II, insulin and trans- studies showed that the degradation rate of EGF was forming growth factor-b TGF-b have been de- about 5 and 15 after incubation for 20 min in the tected and the concentrations are presented in Table gastric contents of suckling and weaned pigs, respec- 3. From these data, it can be seen that there are large tively, while the degradation of IGF-I was negligible discrepancies among different reports. The dis- Fig. 2. The specific receptor binding ability of both crepancies may be due to different assay methods EGF and IGF-I did not change significantly after used by different researchers. Sample collection time incubation in gastric contents Shen and Xu, 1996, may also be an important factor affecting the con- 2000a. When incubated with intestinal fluids of centrations of growth factors in milk. For example, suckling and weaned pigs, the degradation of both IGF-I concentration in colostrum collected at the EGF and IGF-I was much greater in weaned than in time of parturition was about twice that in samples suckling pigs, and in suckling pigs the degradation collected 1 day after parturition and about five times was greater in the distal than in the proximal region that in samples collected 2 days after parturition Fig. 2. After oral administration of iodine-labeled Donovan et al., 1994. Despite the discrepancies EGF to newborn and 5-day-old suckling pigs, over among different reports, it is generally agreed that 60 of the radioactivity recovered from GI contents the concentrations of all the growth factors were 45 min later had similar chromatographic and acid 100 R .J. Xu et al. Livestock Production Science 66 2000 95 –107 proteolytic enzymes, was low in suckling pigs San- gild et al., 1991, and that the secretion capacity remained low by the age of weaning Cranwell, 1995. In neonatal pigs, the major proteases pro- duced by the stomach are pepsin B and chymosin, both having a strong milk-clotting activity but little proteolytic activity Cranwell, 1995. In an acidic gastric lumen with the presence of pepsin, EGF may undergo C-terminal truncation. Playford et al. 1995 reported that human EGF was truncated to EGF 1 – 49 and EGF forms in human gastric juice with a 1 – 46 pH of less than 4. Rao 1995 also reported that mouse EGF was truncated to EGF , EGF 1 – 52 1 – 48 and EGF forms in the GI lumen of mice. Both 1 – 47 studies, however, found that all these truncated forms of EGF possessed biological activity as demonstrated by their receptor binding ability. The high stability of growth factors in the intesti- nal lumen of suckling pigs may reflect a limited luminal digestive capacity in neonatal animals Cran- well, 1995. It has been shown that the activity of trypsin and chymotrypsin was low in the intestinal contents of suckling pigs when compared with that of weaned pigs Shen and Xu, 1996. In addition, the high stability of EGF and IGF-I in the intestinal fluid Fig. 2. Percentage degradation of EGF upper panel and IGF-I of suckling pigs may also be partially attributable to lower panel after incubation for 20 min in the contents of the stomach S, and the proximal P, mid M and distal D the presence of milk in the fluid. It has been reported sections of the intestine, of suckling and weaned pigs. Redrawn that porcine milk contains potent inhibitory activity from Shen and Xu 1996, 2000a. against trypsin and chymotrypsin, and prevents EGF and IGF-I hydrolysis in pig intestinal fluids Wes- precipitation characteristics to that of iodine-labeled trom et al., 1982; Shen and Xu, 1996, 2000a. These EGF Shen and Xu, 1998. The receptor binding findings suggest that colostrum, the natural carrier of ability of the iodine-labeled compounds recovered milk-borne growth factors, may protect the peptides from the gastric contents was almost 100 compar- from GI luminal digestion in the suckling young. able with iodine-labeled EGF. The receptor binding Whether milk-borne growth factors can be ab- ability remained high in the proximal small intestinal sorbed into the blood circulation in suckling pigs lumen 40–58 and it decreased to 15 in the following oral ingestion remains controversial. In distal small intestinal lumen in newborn pigs Shen newborn pigs, the gut epithelium is permeable to and Xu, 1998. macromolecule transmission, thus allowing absorp- The high survival rate of the growth factors in the tion of colostral immunoglobulins. The transmission gastric lumen of suckling and weaned pigs may be is apparently non-selective as virtually all macro- due to a limited secretion of gastric acid and protease molecules, such as bovine serum albumin and dex- in those animals. It was reported that the pH value of tran, are absorbed when they come in contact with gastric contents in both suckling and weaned pigs the intestinal mucosa Westrom et al., 1984. Cessa- was greater than 4 and no significant level of pepsin tion of such non-selective transmission gut closure A activity was detected Shen and Xu, 1996. It has occurs during the first 2 days after birth. However, also been reported that the secretion capacity of transmission of certain macromolecules may con- pepsin A and gastricsin, two major gastric tinue for some time after the onset of gut closure via R .J. Xu et al. Livestock Production Science 66 2000 95 –107 101 a receptor-mediated process. In our own laboratory, oral administration of iodine-labeled EGF to new- we observed that following oral administration of born and 5-day-old pigs, over 95 of the recovered iodinine-labeled IGF-I and fluorescent-labelled dex- radioactivity was found in the gastrointestinal tract, tran with porcine colostrum as the delivery vehicle, of which 78–86 was found in the luminal contents iodine-labelled IGF-I was detectable in the plasma of with the remaining found in the gastrointestinal wall both newborn and 3-day-old piglets while fluorescen- Shen and Xu, 1998. t-labelled dextran was detected in the plasma of only newborn pigs Xu and Wang, 1996. The finding suggests that milk-borne IGF-I can be absorbed in

7. Effects of milk-borne growth factors on GI