Effect of intravenous bovine growth hormone or human pancreatic growth hormone-releasing factor on milk production and plasma hormones and metabolites in sheep

Responses to daily injections of bovine growth hormone (GH, 0·15 mg kg-1 liveweight), beginning on day 10 of lactation, were measured in lactating ewes. Milk yields of GH-treated ewes increased soon after commencement of injections and continued to increase for some 25 days before reaching plateau levels. By comparison, yields of ewes injected with excipient (controls) decreased over the experiment. There was a tendency for contents of milk fat to be higher and milk protein to be lower for GH-treated than for control ewes during the first 15-20 days after injections were started.At the beginning and over the first 15-20 days of the experiment feed intakes of both groups of ewes were similar, but thereafter intakes of GH-treated ewes gradually increased to reach plateau levels some 2OG-300 g day-l higher than for control ewes by about day 35. Liveweights of both groups of ewes decreased during the first 2 weeks of treatment then increased, with GH-treated ewes losing, then gaining, more weight than control ewes. The efficiency of food utilization for milk production was higher for GH-treated than control ewes throughout the experiment but digestibility of food organic matter was not different during the eighth week of the experiment. At the end of the experiment, body composition, assessed by dilution of tritiated water, was similar for both groups of ewes. Differences in milk production were not sustained after withdrawal of GH injections.Measurements of tissue uptake of key metabolites were made on days 3 and 45 of GH treatment. On day 3, GH lowered uptake of glucose and non-esterified fatty acids by leg muscle tissue and increased mammary uptake of non-esterified fatty acids. By day 45 there were no apparent differences of tissue uptake of key metabolites.The results indicate that there is a biphasic response to exogenous GH in the lactating ruminant. It appears that initially GH affects nutrient partition thereby increasing supplies to the mammary gland of key nutrients for milk synthesis. In the longer term, GH increases feed intake, which provides sufficient nutrients to sustain increased milk production and also Iiveweight gain.Extra keywords: Milk yield and composition, feed intake and digestibility, Iiveweight and body composition, arterial metabolites and hormones, arterio-venous differences, leg muscle and mammary tissue, non-esterified fatty acid kinetics, tissue blood flow.

Section: Discussionsupporting

confidence: 92%

Responses of Lactating Ewes to Exogenous Growth Hormone: Short- and Long-term Effects on Productivity and Tissue Utilization of Key Metabolites

Sandles¹,

Sun²,

D'Cruz³

et al. 1988

“…The present results are consistent with results of a limited number of previous studies which have shown that exogenous GH is galactopoietic in the lactating ewe (Dracy and Jordan 1954;Jordan and Shaffhausen 1954;Hart et al 1985;McDowell et al 1987c). Thus, milk yield and the yields of the major milk constituents were increased and milk fat content was raised during injections of GH.…”

Section: Tissue Blood Flowssupporting

confidence: 93%

Short-term Effects of Exogenous Growth Hormone: Effects on Milk Production and Utilization of Nutrients in Muscle and Mammary Tissues of Lactating Ewes

McDowell

Leenanuruksa²,

Niumsup³

et al. 1988

Exogenous bovine growth hormone at a dose of 0·1 mg kg-1liveweight increased yields of milk and milk constituents and milk fat content when injected over 5 days into ewes in mid-lactation. These changes in milk production were associated with changes in the supply to, and utilization of, nutrients by leg muscle and mammary tissues.Arterial concentrations of glucose and non-esterified fatty acids increased significantly, concentrations of lactate and 3-hydroxybutyrate tended to increase, and concentrations of triglycerides associated with very low-density lipoproteins decreased significantly. Growth hormone increased mammary uptake of nonesterified fatty acids, decreased mammary uptake of very low-density lipoproteins and tended to reduce the release of lactate from leg muscle. Oxidation of non-esterified fatty acids in the whole body and mammary tissue was increased by growth hormone and there was a tendency for reduction of glucose oxidation in mammary tissues.During injection of growth hormone, blood flow to leg muscle and mammary tissues increased as did the calculated ratio of blood flow : milk yield. These changes in blood flow, together with changes in arterial concentrations and tissue utilizations of key metabolites, were sufficient to account for the synthesis of extra milk and milk constituents.Extra keywords: milk yield, milk components, arterio-venous differences of nutrients across leg muscle and mammary tissue, oxidation of glucose and non-esterified fatty acids, plasma triglycerides, blood flow.

“…Although it is well documented that growth hormone (GH) increases milk production in cows Bines et al 1980;Peel et al 1981Peel et al , 1982, sheep (Dracy and Jordan 1954;Jordan and Shaffhausen 1954;Hart et al 1985) and goats Mepham et al 1984), the mechanisms by which the hormone exerts galactopoietic effects remain unclear. Bines et al (1980) and also raised the possibilities that GH acts directly or indirectly on the mammary gland or, alternatively, influences the supplies of key nutrients to the mammary gland, thereby influencing the capacity of the mammary gland to synthesize milk.…”

Section: Introductionmentioning

confidence: 99%

Local Intra-arterial Infusion of Growth Hormone into the Mammary Glands of Sheep and Goats: Effects on Milk Yield and Composition, Plasma Hormones and Metabolites

McDowell¹,

Hart²,

Ac³

1987

Lactating goats and sheep were fitted with catheters in the external pudendal arteries supplying both mammary glands. Saline was infused continuously into one artery whereas the other artery received continuous infusions, over successive 4-day periods, of either saline or growth hormone (GH)-doses increasing twofold between successive periods from 100 to 400 ltg/day in goats and 400 to 3200 Itg/ day in sheep.Local infusion of GH at up to 1600 ltg/day in sheep did not affect milk yield or composition nor peripheral plasma concentrations of GH, insulin, glucose, urea and non-esterified fatty acids (NEFA). Infusion of GH at 3200 ltg/day in sheep increased peripheral plasma concentrations of GH, tended to increase milk yield and peripheral plasma NEF A but there were no changes in peripheral plasma insulin, glucose and urea. It is concluded that GH does not exert direct effects on the mammary glands of sheep and goats in situations where the hormone is administered over short periods.