Zh. Yucai,1 Z. Sixiang,2 Zh. Guanghui,1 Ch. Weihua,2 L. Wenjing,1 P. Xianwen,1 M. Yongjiang1 and W. Yong1
1. Department of Animal Science, Southwest Nationalities College, Chengdu 610041, Sichuan, P.R. China
2. College of Animal Medicine, Nanjing Agricultural University, Nanjing 210095, Jiangsu, P.R. China
Insulin, prolactin and growth hormone concentrations in yak colostrum and milk were analysed by radio-immunoassay (RIA) method. Colostrum from day 1 of lactation contained the highest concentrations of the three hormones compared with the colostrum from day 2 to 7 of lactation and the milk from early lactation. Except growth hormone, concentrations of both insulin and prolactin in colostrum were significantly higher than concentrations in milk of early lactation. There were significant differences among individuals in the concentrations of the three hormones in colostrum.
Keywords: Colostrum, growth hormone, insulin, prolactin, yak
Hormones in milk are related functionally to the development, growth and secretion of mammary gland (Wang and Zou 1995). Most hormones in milk are uptakes from plasma by mammary gland. Colostrum usually contains higher levels of hormones than does plasma and milk and the intakes of these hormones via colostrum have been proven to play an important role in metabolism and growth of newborn offspring (Lorraine et al. 1986). However, there is limited information on hormones in yak milk. What is available is confined only to reproductive hormones in colostrum and milk (Wei et al. 1990; Yu et al. 1993). The objective of this study was to investigate the dynamic changes of insulin, prolactin, and growth hormone in colostrum from the first week after parturition.
All samples were obtained from Longri breeding stock farm in Sichuan Province, P.R. China. Colostrum samples were collected daily from six Maiwa yak in the first week after parturition. Approximately 50 mL of colostrum were obtained from each yak by manual milking in the morning. A total of 28 milk samples from early lactation (from 20 to 40 days after parturition) were collected as control. Five samples of plasma were collected from lactating yak cows at the same stage of lactation for prolactin analysis. All samples were stored at –20°C.
Skimmed milk was obtained by separating fat from whole milk under centrifugation for 20 min at 4°C. Insulin, prolactin and growth hormone concentrations in the skimmed colostrum and milk were assayed by radio-immunoassay (RIA) method. RIA kits for human insulin and growth hormone were obtained from Shanghai Biological Product Institute, and RIA kits for human prolactin were obtained from Tianjin Depu Biotechnique and Medical Product Company Ltd. Bovine growth hormone concentration (Y) was calculated as: Y = (X - 0.2926)/0.019, where X was the value detected with RIA kit for human growth hormone.
The highest levels of insulin, prolactin and growth hormone were observed in colostrum from day 1 of lactation, with big variations among individuals (Table 1). Insulin concentration in day 1 colostrum ranged from 86.49 mu/mL to 1335.84 mu/mL, prolactin from 4.80 ng/mL to 183.23 ng/mL, and growth hormone from 66.71 ng/mL to 2366.71 ng/mL. These hormones declined sharply within one or two days after parturition. Concentrations of both insulin and prolactin in colostrum were consistently significantly higher than that in the milk from early lactation, while the growth hormone was significantly higher only in colostrum from day 1 of lactation relative to that in the milk from early lactation. Milk prolactin level was similar to that of plasma of lactating yak (0.49 ± 0.43, n = 5).
Table 1. Hormone concentrations in yak colostrum and milk.
Samples |
Days of lactation |
Insulin |
Prolactin |
Growth hormone |
Colostrum |
1 |
457.26 ± 412.97** |
43.39 ± 63.97* |
456.88 ± 854.49* |
2 |
446.96 ± 549.23** |
2.05 ± 0.66** |
53.11 ± 8.84 | |
3 |
85.09 ± 05.35** |
1.73 ± 0.60** |
54.77 ± 7.08 | |
4 |
58.27 ± 7.58** |
2.14 ± 0.93** |
51.53 ± 9.80 | |
5 |
40.22 ± 14.74** |
1.41 ± 0.76** |
49.95 ± 12.53 | |
6 |
37.42 ± 9.87* |
1.13 ± 0.33** |
56.33 ± 9.00 | |
7 |
27.65 ± 5.91 |
1.15 ± 0.46* |
52.58 ± 13.57 | |
Milk |
20–40 |
25.58 ± 9.80 |
0.31 ± 0.23 |
46.52 ± 13.15 |
More than 50 hormones and growth factors have been detected in milk (Koldovsky 1980; Wang and Zou 1995). They include insulin, prolactin, growth hormone, estradiol, progesterone, insulin-like growth factor I (IGF-I), and epidermal growth factor (EGF). These hormones or growth factors can be absorbed in intact forms by newborn animals such as calves and piglets through colostrum within 24–48 hours after parturition, and hence exert effects on the metabolism and growth of the newborn offspring (Lorraine et al. 1986). Some hormones and growth factors have direct influences on the intestinal development and growth of the suckling offspring (Buts et al. 1988).
Insulin is one of the most studied hormones in milk involved in the early intestinal development and maturity of newborn offspring. Its dynamic change pattern in yak' colostrum was similar to that of other species. The sustained high levels of insulin and prolactin in yak' colostrum might be beneficial to the sucking calves, even after the gut closure, by mechanisms such as the binding of insulin to IGF-I receptor or insulin receptor located in intestinal tissue. The physiological significance of yak' colostrum could be its capacity to provide metabolic signals (hormones and growth factors) and passive immunity to the newborn calves (Zheng and Han 1999).
This work was supported with funds from the National Natural Science Foundation of China, and Animal Physiology and Biochemistry Laboratory of Nanjing Agricultural University.
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