Lactation

Exogenous ST has been shown to enhance lactational performance in mammals ranging from laboratory animals to humans. The most extensive work is with lactating

Fig. 1 Relationship between porcine somatotropin (pST) dose and parameters of growth performance. (From Ref. 9.)

Fig. 1 Relationship between porcine somatotropin (pST) dose and parameters of growth performance. (From Ref. 9.)

cows, and bST has been approved for commercial use in 19 countries.1-10,11-1 Commercial use over the last decade has clearly demonstrated a consistent milk response, generally about 5 kg/d, and a marked improvement in productive efficiency while maintaining normal cow health and herd life.[11] There is generally little response to bST in early lactation before peak yield, so commercial use is during the declining phase of lactation (Fig. 2).

The composition of milk is unaltered by bST treatment. Therefore, the use of bST has no impact on nutrient requirements per unit of milk or the nutritional and manufacturing properties of milk.[9,10] Responses to bST have been observed for all dairy breeds, regardless of parity and genetic potential. Cows receiving bST increase intake in an amount that matches nutrient needs for the extra milk and allows for the normal replenishment of body reserves over the lactation cycle. Thus, offering a balanced diet in adequate amounts is important; if nutrition and management are inadequate or poor, the lactational response to bST will be attenuated or even abolished.[9,10,13]

MECHANISMS

Fat and Carbohydrate

The multitude of effects that have been ascribed to ST in the regulation of growth and lactation are outlined in Table 1. Depending on physiological state, ST coordinates metabolism to partition nutrients toward lean tissue and bone during growth or toward milk synthesis during lactation. Many effects of ST are direct and mediated through changing responses to homeostatic hormones such as insulin or catecholamines. Other effects are indirect and thought to be mediated by the insulin-like growth factor (IGF) system.

Basal lipogenesis in adipose tissue from growing pigs treated with pST is decreased by up to 85%.[14] In addition, the ability of insulin to stimulate lipogenesis and glucose transport is similarly reduced in adipose tissue obtained from ST-treated animals, and this is mainly due to a decreased sensitivity to insulin. Also, ST treatment reduces rates of glucose clearance in response to an insulin or glucose challenge, and there is an augmented plasma insulin response to a glucose load. It has been suggested that the insulin resistance and resultant reduced adipose tissue lipogenesis and glucose oxidation are largely responsible for the reduction in feed intake observed in response to pST treatment.[14]

Treatment with somatotropin also causes an increase in lipolytic response to adrenergic stimulation in pigs and cattle.[10,15] In animals that are in a positive energy balance, where fat synthesis in adipose tissue is high and fat mobilization is low, effects of ST are predominantly a reduction in lipogenesis with little effect on lipolysis. On the other hand, if animals are in negative energy balance or need to draw on energy reserves, ST-treated animals have an enhanced ability to mobilize fat. Thus, the overall effect is that less nutrients are partitioned to body fat and more nutrients are available for productive functions.

Protein

Although it is well established that ST increases protein deposition and milk protein synthesis in growing and lactating animals, respectively, the precise mechanisms and extent to which effects on protein metabolism are direct or indirect via IGF-I remains an active area of investigation. Most studies suggest that in growing animals, the increase in protein deposition is due primarily

I 25

I 20

Fig. 2 Effect of bovine somatotropin (bST) on milk yield in lactating dairy cows. Commencing at week 0 (84 ±10 days postpartum) cows received a daily injection of excipient (dotted line) or bST (27 mg/day; solid line) for 26 weeks. (Adapted from Ref. 12.)

Table 1 Biological and production effects of somatotropin in farm animals during growth and lactation

Tissue Physiological process affected

Table 1 Biological and production effects of somatotropin in farm animals during growth and lactation

Tissue Physiological process affected

Skeletal muscle (growth)

" Protein gain

" Protein synthesis

" Amino acid and glucose uptake

" Partial efficiency of amino acid utilization

Bone (growth)

" Mineral gain (paralleling tissue growth)

Mammary tissue (lactation)

" Milk with normal composition

" Uptake of nutrients used for milk synthesis

" Maintenance and activity of secretory cells

" Blood flow consistent with milk yield

Adipose tissue

# Lipid gain

# Lipid synthesis

" Lipid mobilization (especially if in negative energy balance)

" Catecholamine stimulated lipolysis

# Insulin stimulated lipid synthesis and glucose metabolism

Liver

# Ability of glucose to inhibit gluconeogenesis

" Glucose output (in lactation) to meet glucose needs

# Glucose output (in growth of pigs) to meet glucose needs

Systemic effects

# Glucose clearance and oxidation

# Insulin sensitivity

# Amino acid oxidation and catabolism

" Plasma glucose, insulin, IGF I, and IGFBP 3

# Plasma urea and IGFBP 2

" increase; # decrease; IGF I

insulin like growth factor I; IGFBP insulin like growth factor binding protein.

(Adapted from Refs. 10 and 15.)

to an increase in protein synthesis, with little effect on protein degradation. The increase in protein synthesis is also associated with a reduction in amino acid oxidation, so that a greater proportion of absorbed amino acids is used for protein accretion.[6,15]

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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