Somatotropin is a protein hormone derived from anterior pituitary gland. Its secretion is regulated by two hypothalamic peptides that are growth hormone releasing factor for stimulating activities of growth hormone or somatostatin for inhibiting the release of growth hormone (Tuggle et al., 1996). Somatotropin in bovine somatotropin (bST) and porcine somatotropin (pST) is similar up to 90% in the amino acid sequence (about 191 amino acids) (Etherton et al., 1993; Bauman et al., 1993). However, human somatotropin (hST) differ about structure of amino acid sequence of bST and pST up to 35% and both of them are not biologically active in human (Carr et al., 1976; Lesniak et al., 1977; Moore et al., 1985).
Somatotropin is a homeorhetic control that affects many target tissues by shifting the nutrient partitioning among these tissues and as a result more nutrients are used for milk synthesis whereas homeostasis control has a connection with the maintenance of physiological balance within the animal body. Thus, rbST administration in lactation goats was higher milk production in compare to without rbST in animal. The biological influences of somatotropin can be divided by two physiological processes which are somatogenic and metabolic. The somatogenic effect of ST mainly excites cell proliferation by indirect via IGF-I (Rechler et al., 1990). Smaller parts may be secondary factor influences resulting from the increased synthesis activity in the mammary gland or change in the energy status of cows (Bauman et al., 1986). The metabolic effects are direct action of ST that involves a variety of tissues and the metabolism of all nutrients: carbohydrate, lipid, protein, minerals. According to Bauman (1986) found that the metabolic adaptations in glucose turnover and oxidation accommodated the extra glucose required for increased lactose synthesis during ST administration and the reduction of glucose oxidation could contribute about 30% of the extra glucose required. In addition, somatotropin administration in goats increased blood flow which means that somatotropin also increased metabolic activity in the mammary gland. These could lead more available nutrients for milk production (Mepharn et al., 1984).
Recombinant bovine growth hormone (rbGH) or recombinant bovine somatotropin (rbST) refers to bovine growth hormone that is manufactured in a laboratory using genetic technology. This synthetic hormone is marketed to dairy farmer to increase milk production. The Monsanto Corporation has developed and markets bST. Greater than eight amino acids added to the terminal amino for producing recombinant bovine somatotropin. There are four types of rbST product, Somagrebove (American Home Products), Somavubove (Pharmacia and Upjohn), Sometribove (Monsanto) and Somidibove (Elanco) that are popularly used by farmer. bST can be injected daily or more commonly every two weeks using a prolonged release formulation.
Effects of rbST on IGF-I concentration:
IGF-1, along with IGF-2, belongs to a family of insulin-like growth factors (IGFs) that share close structural homology to the precursor form of insulin (pro-insulin) (Leroith et al., 1993). In the circulation, IGF-1 primarily exists in a ternary complex along with the IGF binding protein-3 or -5 (IGFBP-3 or -5) and the acid-labile sub-unit (ALS), while it can exist in a binary complex with the other IGFBPs (IGFBP-1, -2, -4, -6) in the circulation as well as the peripheral tissues. These binary and ternary complexes modulate the bioavailability of circulating IGFs (Leroith et al., 1996). However, a small fraction (less than 5%) of circulating IGF-1 may also exist as free IGF-1. The somatomedin hypothesis, in its original form, stated that GH promotes somatic growth indirectly via the production of a secreted factor called somatomedin-C (IGF-1) (Salmon et al., 1957). It was believed that the liver is the primary source of IGF-1. However, since then this hypothesis has been revised to accommodate data demonstrating that the liver is not the only source of IGF-1. In fact, IGF-1 synthesized by extra-hepatic tissues can exert GH-independent autocrine/paracrine effects in the local environment. GH is also known to have IGF-1-independent effects (Leroith et al., 2001). Apart from its effects on growth and development, IGF-1 also has insulin like effects on metabolism (Pennisi et al., 2006; Clemmons et al., 2005). Furthermore, IGF-1 negatively regulates GH secretion through feedback mechanisms (Yamashita et al., 1987).
In cattle, treatment with bST increases IGF-1 concentration in plasma (Bilby et al., 2004; Wilaiporn et al., 2009) and milk (Prosser et al., 1989). These results are similar to response in dairy goat with bST (Disenhaus et al., 1995; Polratana et al., 2004). Concentration of IGF-I in plasma rises about 6 - 12h after bST injection and reaches maximum level in 48h later. Cows with lower nutritional state have a lower basal level of IGF-1 (Hodgkinson et al., 1991) or a negative energy balance reduced hepatic IGF-1 production (Weller et al., 1994). However, an increase in dry matter intake could contribute to be an increase of nutrients for stimulating IGF-I synthesis in response to bST in dairy cows (Dolrudee et al., 2009).
Effect of bovine somatotropin on feed intake and nutrient digestibility:
Sallam et al. (2005) found that dry matter intake of ewes did not differ significantly between control and 100mg rbST treatment (DMI: 2.174 ± 0.029 vs. 2.262 ± 0.27 kg/day). The unchanged total DM intake was consistent with data for cows (Chilliard, 1988a) and goats (Disenhaus et al., 1995; Chadio et al., 2000). In addition, Polratana et al (2004) found that dry matter intake of concentrate in goat decreased significantly in the control group compare to rbST group at first and second injection of experiment. Total DMI as a percent body weight were not significantly different between control and experimental group in all periods of experiment and between periods in the same group. However, the experiment of Wilaiporn et al. (2009) and Dolrudee et al. (2009) show that there were higher DMI in cows with treated rbST compare to control in all stages of lactation. The increase feed intake may be dependent on the increase in milk production, body condition shift, environmental condition and the nutrients of diet (particularly energy density of diet). Overall, cows supplemented with bST adjust their voluntary feed intake in relation to the additional nutrient required for increased milk yield.
rbST administration in dairy cows was not affected on nutrient digestibility when compare to control in entire lactation cycle and also not significantly different between cooled cow and non-cooled cow (Wilaiporn et al., 2009) in agreement with other studies that carried out on lactating buffaloes (Khattab et al., 2008). Thus, the nutrient requirement for maintenance and per unit of milk remains available when dairy cows are injected with bST.
Effects of bovine somatotropin on milk production:
Milk yield immediately increases after bST administration and reaches a maximum during the first week. If treatment is terminated, milk yield gradually returns to pretreatment levels over a similar time period. However, when treatment is continued, the increased milk yield is maintained (Peel and Bauman, 1987). Thus, bST results in a greater peak milk yield and an increased persistency in yield over the lactation cycle.
Milk yield increases after bST treatment is observed in cows of all parities, but the magnitude of the increase in milk yield varies according to stage of lactation. In general, response has been small when bST is administered in early lactation prior to peak yield. In addition, bST increases milk yield by 10% when administered in early to mid-lactation, and by 40% in late lactation (Bauman and Vernon, 1993). Therefore, possible commercial use would probably be over the last two thirds or three fourths of the lactation cycle.
In studies of Wilaipon et al. (2009) and Dolrudee (2009) found that the milk yield of cooled cows treated with rbST were slightly higher than non-cooled cows. Moreover, the mean value of respiratory rate and the rectal temperature of cows under misty-fan cooling system (MF) were slower than under normal shade (NS) with or without treatment of rbST. Similarly, Chadio et al. (2000) found that milk yield rose significantly to 12.6% over the entire experimental time when lactating goats supplemented with rbST. They also noticed that the bST treatment (injection of 160 mg rbST at 14-day interval) boomed milk yield by 40% at 12 - 15th week of lactation. In another study, the animal were injected lower dose (90 mg rbST at 4-week interval) at the same lactation period and the improvement of milk yield was only 13.9% (Gallo et al., 1997). This is similar to dairy cows when they receive rbST (table 1).
Table 1. Milk yields [3.5 % fat corrected (FCM)] during short-term treatments of ST at different stages of lactation, at different dose
Variable
Treatment
3.5% FCM
Increase during treatment
Stage of lactation
(Kg/day)
Wk 41
Control
Somatotropin
37.9
44.2
6.3
Wk 101
Control
Somatotropin
34.1
40.5
6.4
Wk 122
Control
Somatotropin
28.2
32.9
4.7
Wk 352
Control
Somatotropin
14.5
19.3
4.8
Dose response
Control3
Somatotropin
5 IU/d
10 IU/d
25 IU/d
50 IU/d
100 IU/d
27.5
29.1
28.6
31.9
35.6
37.6
1.6
1.1
4.4
8.1
10.1
1Richard et al. (1985)
2Peel et al. (1983)
3Eppard et al. (1985)
Effects of rbST on milk composition:
Bovine somatotropin does not change the composition of milk in any significant way. The fat and protein level in milk differs due to genetic, stage of lactation, age, diet composition, nutrition status. The milk composition from bST supplemented cows is influenced by these factors (Bauman et al., 1999) and the same result was found in lactating ewes (Sallam et al., 2005). However, there was light increase of proportion of long chain fatty acids in milk during first and second week of bST supplemented dairy goats, indicating higher lipolysis for bST goats (Disenhaus et al., 1995), as for cows during short-term experiment (McDowell, 1991). Other studies found that milk fat concentration rose to throughout the experiment when goats administrated with rbST (3.26 ± 0.09% vs. 2.9 ± 0.08%) and also an enhancement in short chain fatty acid in this experiment (Chadio et al., 2000). Thus, rbST administration in lactating animal directly affected on adipose tissue by inducing either lypogenesis or lypolysis with relation to energy balance. When bST supplemented cows are in positive energy balance, the adipose tissue would reduce lipogenesis; conversely, proportion of lipolysis are increased if bST cows are in negative energy balance (Bauman et al., 1999).
Role of leptin in energy balance:
Leptin is secreted into the circulatory system by the adipocyte as a function of the energy stores (Frederich et al. 1995; Weigle et al. 1997). Schwartz et al., (1996) showed that serum and plasma leptin levels are higher in subjects with a higher in BMI and a higher per cent total body fat. After secretion, leptin signals to the hypothalamus and gives information about the status of the body energy stores. Leptin had influence on various biological mechanisms including reproduction, the immune and inflammatory response, angiogenesis (Mantzoros et al. 1997; Takeda et al. 2002). Most interestingly, leptin function signals to key regulatory centre in the brain to decrease food intake and to regulate body weight and energy homeostasis (Pelleymounter et al. 1995; Halaas et al. 1995).
Leptin treatment results in decreased appetite, weight loss, increased physical activity, changes in endocrine function and metabolism, and beneficial effects on ingestive and noningestive behaviour in leptin-deficient patients (Farooqi et al., 2001; Jeon et al., 2003). The satiety effects of leptin have been observed ewes with administration of recombinant human leptin for 3 days. This treatment causes a decrease in voluntary dry matter intake by approximately one third of normal intake (Henry et al., 1999). However,The anorexigenic effects of leptin were lost when growing and adult sheep were underfed (Morrison et al., 2001; Henry et al., 2001). Wilaiporn et al. 2009 found that cows with rbST had greater DMI both in non-cooled and cooled cows when compared with pre-supplemental period. These changes were accompanied with the reduction of the plasma leptin concentration. These results indicate that the level of leptin hormone influenced by feed intake in ruminants and also regulated by the effect of exogenous rbST. There are some reports have shown that the leptin level was also affected by environment and/or daylight as in summer month (Halaas et al., 1995; Morrison et al., 2001).
Overall, when there is a lot of adipose tissue, production of leptin increases to activate the satiety centre in the hypothalamus and reduces food intake. Conversely, when adipose tissue reserves decrease due to limited availability of food, leptin levels decrease and appetite increases.
