Responses to exogenous growth hormone were measured in lactating dairy cows surgically prepared to allow measurement of nutrient exchanges across mammary and hind-limb muscle tissues. Cows were injected daily with either saline or growth hormone, at a dose of 0·1 mg/kg liveweight, over periods of 6 days.During administration of growth hormone milk yield, milk fat content and yields of milk fat protein and lactose increased. Arterial plasma concentrations of glucose and non-esterified fatty acids were increased, uptake of glucose by leg muscle tissue decreased, lactate release from leg muscle tended to increase, mammary uptake of non-esterified fatty acids increased, blood flow to leg muscle tended to increase and blood flow to mammary tissue increased during injection of growth hormone.The results show that growth hormone affects supply to and utilization of key nutrients by tissues, resulting in the supply to the mammary gland of additional precursors for milk synthesis.

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.

After intravenous administration of alloxan (50 mg kg-1 liveweight) to lactating ewes, there were triphasic changes in plasma glucose and insulin. Almost immediately, plasma insulin decreased and hyperglycaemia occurred, then, between c. 5-12 h, insulin increased and ewes became hypoglycaemic. Thereafter, insulin decreased and glucose increased from c. 20 h after alloxan and the diabetic state was established. Changes in glucose production and utilization correlated with changes in plasma glucose.Exogenous insulin was administered from 30 h after alloxan, and it took some 2 weeks to stabilize ewes. During this period, when mild hyperglycaemia persisted, milk yields and feed intakes were decreased but milk fat content was elevated. Once ewes were stabilized, plasma glucose, milk yield, feed intake and milk fat content returned to levels prior to alloxan. These observations are consistent with insulin playing a role in the aetiology of the 'low milk fat syndrome' in the ruminant.It appears that the alloxan-treated, insulin-stabilized ewe would be a useful model for studying the role of insulin during lactation, but it is necessary to allow time for animals to overcome effects of administration of alloxan.Extra keywords: glucose production and utilization, milk yield and fat content, feed intake.

Lactating ewes received continuous intravenous infusions of adrenaline (0�05 Itg/kg liveweight) for 4 days. Prior to, during and after adrenaline infusions, milk yield and composition were monitored. Plasma concentrations of metabolites and hormones were measured each day and glucose biokinetics were measured in non-steady state at the start and end of adrenaline infusions.

No abstract

An antiserum to purified bovine C-peptide was used to develop a sensitive radioimmunoassay for C-peptide in sheep. The assay was used to measure kinetics of C-peptide and insulin in non-pregnant and non-lactating sheep. Injected, purified C-peptide was distributed in pools comprising c. 11·4% of liveweight, the half time of C-peptide was estimated as 13·7 min and its clearance rate was c. 5 ml kg-I min-I. In lactating ewes exogenous recombinant bovine growth hormone (rebGH) increased both plasma insulin and C-peptide as did glucose challenge given before and during administration of rebGH. Estimates of insulin secretion rate in lactating ewes were c. 7 X 10-3 and

Effects of insulin on exchanges of glucose across skeletal muscle and mammary tissue were measured in short-term studies in lactating ewes. Insulin secretion was suppressed by a primed/continuous infusion of somatostatin, then insulin was administered by continuous intravenous infusion of doses that were increased, in a step-wise manner, from 0 to 2 U h -I. Plasma glucose was maintained essentially constant by frequent monitoring and intravenous administration of exogenous glucose.Somatostatin suppressed but did not completely inhibit insulin secretion as shown by maintenance of plasma concentration of C-peptide. As plasma insulin was increased, while arterial glucose was maintained stable, uptake of glucose by skeletal muscle increased and glucose uptake by the mammary gland decreased. These observations confirm the role of insulin in regulating glucose uptake by skeletal muscle and raise the possibility that insulin also regulates glucose uptake by the mammary gland.

Ewes induced to lactate artificially were given continuous intravenous infusions of either saline or glucose in saline during the first 5 days of lactation. Glucose-infused ewes secreted significantly greater (P < 0·05) amounts of milk than control ewes during the first 10 days of lactation. On the tenth day of lactation glucose-infused ewes secreted approximately twice as much milk as control ewes (c. 320 v. 160 g/day). Plasma concentrations of glucose and insulin were significantly higher (P < O· 05) on the first 2 days of lactation for glucose-infused than control ewes.The results suggest that glucose supply to the mammary gland at lactogenesis may be rate-limiting for milk production during early lactation.