Somatotropin has been shown to increase lactation performance in mammal ranging from laboratory animals to human Bauman and Vernon, 1993; Gunn et al., 1996. This effect has been investigated extensively in dairy cattle after available of recombinant bovine ST (rbST). ST causes increase milk yield with minor change in milk composition. It was demonstrated that the effect of ST to increase lactation performance is partly via IGF-I. The magnitude of rbST effect on milk yield depends on many factors, partly on the nutritional status. Cows with rbST adjust voluntary food intake related to their milk production. In addition, rbST have been demonstrated that to increase milk yield effectively in dairy goat (Disenhaus et al., 1995; Polratana et al., 2004).
In the tropical area, dairy cattle are suffered from high temperature and humidity. Lactation performance is attenuated partly by chronic heat stress. However, rbST treated dairy cows and goats have been shown to increase milk yield in the tropic (Chaiyabutr et al., 2005 and Polratana et al., 2004). Interestingly, these experiments revealed that food intake was also increased after rbST treatment in both dairy cow and goat in heat stress condition (Boonsanit et al., 2009; Polratana et al., 2004). The effect of rbST on feed intake in dairy animals is an interested view of this proposal.
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Food intake is a complex behavior that provides energy and necessary nutrients to the body. This behavior is controlled by intrinsic factors (sensory organ, gastrointestinal tract, adipose tissue and central integration etcâ€¦) and extrinsic factors (quantity and quality of feedstuff, feeding management etcâ€¦). Leptin (from adipose tissue) is an internal factor that produces anorectic effects on feed intake. The satiety effects of leptin are also observed in ruminants, administration of human leptin in ewes for 3 days decreased the voluntary dry matter intake to approximately a third of the preinfusion intake. Moreover, Wilaipon et al., (2009) found that administration bST in dairy cows decreased leptin concentration in all stages of lactation.
It is important to understand whether bST induced eating is direct effect from bST or indirectly via leptin or ghrelin. So, the aim of present study is to investigate the effect of bST on plasma leptin and ghrelin regarding to eating behavior.
2. Literature review:
Overview of somatotropin:
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. So, rbST administration in lactation goats was higher milk production in compare without rbST in animal. The biological effects of somatotropin can be broadly classified as either somatogenic or metabolic. The somatogenic effect of ST excites cell proliferation by indirect via IGF-I (Rechler et al., 1990). The metabolic effects are direct action of ST that involves a variety of tissues and the metabolism of all nutrients: carbohydrate, lipid, protein, minerals. These coordinated changes in tissue metabolism alter nutrient partitioning and thus play a key role in increasing growth performance or milk yield. 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. To make recombinant bovine somatotropin, the plasmid of a bacterium is cut by enzymes, and then combined with a cowâ€™s DNA. It is reintroduced to the bacterium, placed in a fermentation tank and allowed to multiply, then separated and purified before delivery to the farmer (Roush, 1991). This modern technology permitted the development of recombinant bovine somatotropin (rbST), which provided an unlimited source of ST for research and for commercial application.
Effect of bovine somatotropin on feed intake and nutrient digestibility:
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Sallam et al. (2005) found that body weight and dry matter intake of ewes were not significantly affected by rbST treatment. The unchanged total DM intake was consistent with data for cows (Chilliard, 1988a) and goats (Disenhaus et al., 1995). In addition, Kulapa et al (2004) found that dry matter intake of concentrate in goat decreased significantly in the control group. 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 and the nutrients of diet.
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).
Effects of heat stress and bovine somatotropin on milk production:
The improvement of milk yield that occurs in response to bST use in hot environments, potential exists for greater heat stress with the use of bST. It is probably due to increase metabolic activity and heat production associated with higher milk yield (Igono et al., 1985). During heat stress, administration of bovine somatotropin increased milk yield by 4% and heat production by 10%, although energy intake declined further than with heat alone. A study conducting during summer has been shown that temperature humidity index (THI) was relatively constant, rectal temperatures of cows administered bST were different from those of controls only on those days when relative humidity (RH) was evaluated (Sullivan et al., 1992). Thus, high milk yield cows exposure to direct sunlight with administration of bST they (the cow under sunlight) will have greater than heat stress and higher rectal temperatures than the un-shaded cowsâ€™ only (Elvinger et al., 1992). But, milk yield responses to bST administration show tend to increase during summer, it is demonstrated that the ability of cow maintain milk yield with bST in despite of high environmental temperature.
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. In addition, both cooled and non-cooled cows increased in respiratory rate and rectal temperature in all stage of lactation.
Effects of rbST on milk composition:
Bovine somatotropin does not change the composition of milk in any significant way. The concentration of fat and protein in milk differs due to genetic, stage of lactation, age, diet composition, nutrition status. These factors also affected the composition of milk from bST supplemented cows (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). 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).
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).
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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; Kulapa et al., 2004). 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).
Role of leptin in energy balance:
Leptin is released into the circulatory system by the adipose tissue 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 release by the adipose tissue, leptin signals to the brain, giving information about the status of the body energy stores. Leptin has been reported to have influence on various biological mechanism, including reproduction, the immune and inflammatory response, angiogenesis (Mantzoros et al. 1997; Takeda et al. 2002). Most interestingly, leptin function as a feedback mechanism that signals to key regulatory centre in the brain to inhibit food intake and to regulate body weight and energy homeostasis (Pelleymounter et al. 1995; Halaas et al. 1995).
Studies in mice and rats have demonstrated that the hypothalamus is the primary centre for regulation of food intake and body weight (Schawartz et al., 1996; Satoh et al., 1997). After leptin is released by the adipose tissue into the bloodstream, it crosses the BBB and binds to the hypothalamic leptin receptors, giving information about the status of the body energy stores (Golden et al., 1997; Meister et al., 2000; Sahu et al., 2004). By binding to its receptors, leptin influences the activity of various hypothalamic neurons and the expression of various orexigenic and anorexigenic neuropeptides. Orexigenic peptides, which levels are influenced by leptin, include neuropeptide Y (NPY), melanin concentrating hormone, agouti-related protein (AgRP), galanin, orexin and galanin-like peptide (GALP; Schwartz et al., 1996; Meister et al., 2000; Kumano et al., 2003). Anorexigenic peptides, which expressions seem to be modulated by leptin, include pro-opiomelanocortin (POMC), cocaine- and amphetamine-regulated transcript, neurotensin, corticotropin-releasing hormone (CRH) and brain-derived neurotrophic factor (Golden et al., 1997; Kristensen et al., 1998; Bariohay et al., 2005). The orexigenic and anorexigenic neurones, which are located in the various hypothalamic regions (arcuate nucleus, lateral hypothalamus, perifornical hypothalamus and paraventricular nucleus), interact with each other (Tritos et al., 1998; Cowley et al., 2003).
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). Furthermore, Weigle et al. (2003) showed that leptin seems to contribute to ongoing weight loss after 12 weeks of dietary fat restriction in healthy humans.
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).