Whole animal studies using tracer kinetics

Initiation of human lactation: secretory differentiation and secretory activation

Theories for the origin of milk have been recorded since the time of Ancient Greeks. In those times it was believed that milk was derived from special vessels that connected the uterus to the breasts. The "chyle theory" on the origin of milk was another prominent theory which persisted well into the nineteenth century before the realisation that milk components were derived from blood and some milk constituents were actually synthesized within the breasts. The demonstration that milk ejection was the expulsion of milk that had already been secreted and that milk secretion was a separate continuous process, set the background for the development for the current understanding of milk synthesis and secretion. Today we know that there are two stages in the initiation of lactation- secretory differentiation and secretory activation. Secretory differentiation represents the stage of pregnancy when the mammary epithelial cells differentiate into lactocytes with the capacity to synthesize unique milk constituents such as lactose. This process requires the presence of a 'lactogenic hormone complex' of the reproductive hormones, estrogen, progesterone, prolactin and some metabolic hormones. Secretory activation on the other hand, is the initiation of copious milk secretion and is associated with major changes in the concentrations of many milk constituents. The withdrawal of progesterone triggers the onset of secretory activation but prolactin, insulin and cortisol must also be present. This review describes the works of pioneers that have led to our current understanding of the biochemical and endocrinological processes involved in the initiation of human lactation.

Fat and carbohydrate use in posthatch chicks

Metabolism of carbohydrate and fat was examined in the hatching chick by introduction of 14C labeled oleic acid, triolein, or glucose to the gastrointestinal tract or to the yolk sac. Label in plasma and exhaled air samples were determined over a 12-h period. The activity-time curves could be fitted to double exponential equations, and rate constants for appearance and disappearance of label were calculated. Following injection to the yolk, oleic acid and triolein label showed rapid appearance in the circulation and rapid utilization. The rate of appearance of oleic acid and triolein in the circulation from the gastrointestinal tract was high at hatch and did not change with age. However, the rate constant for disappearance from the plasma increased with age and after 6 d posthatch disappearance curves fluctuated with little decline over 12 h. Following the introduction of labeled lipids more label was found in acylglycerides with time after injection but age had no effect. Lipids introduced via the yolk sac were present in higher proportions in fractions corresponding to low-density lipoprotein (LDL) than lipids introduced via the gastrointestinal tract. Glucose appearance in the circulation from the gastrointestinal tract was low at hatch and increased more than two-fold by 3 d posthatch. The rate constant for disappearance of glucose from the circulation did not change with age. It appears that lipoproteins synthesized maternally or in yolk transport lipids at hatch, but their concentration decreases posthatch, and this could reduce utilization of circulating lipids. Uptake of glucose increases after hatch, thus, allowing glucose to become a major source of energy as the yolk lipids are depleted.

Glucose kinetics, plasma metabolites, and endocrine responses during experimental ketosis in steers

Phlorizin and 1,3-butanediol were used to determine effects of glucosuria and ketonemia on concentrations of metabolites in blood plasma and on kinetics of glucose metabolism. Four steers received four treatments (control; control plus dietary 1,3-butanediol; control plus phlorizin injections; and control plus phlorizin and 1,3-butanediol) in a Latin square design. Treatments lasted 14 days. All steers received a 30% grain, 70% forage ration in equal meals every 2 h. Metabolite concentrations in blood plasma and urine and glucose kinetics were measured on each of the last 3 days of each treatment period. Phlorizin caused glucosuria; decreased plasma glucose, glucose total entry rate, and glucose recycling; and increased plasma free fatty acids and glucose irreversible loss. Glucose pool size was increased by 1,3-butanediol. Phlorizin plus 1,3-butanediol caused glucosuria and ketonuria; decreased plasma glucose; and increased blood ketone bodies, plasma free fatty acids, glucose irreversible loss, and glucose pool size. Growth hormone, insulin, and glucagon were not affected by treatment. Physiological perturbations in these steers were characteristic of some of those in ketotic cows.

Retinol homeostasis in lambs given low and high intakes of vitamin A

Four groups of lambs were fed on a low-carotene basal diet. One group received no supplemental vitamin A (mildly deficient). Remaining groups were supplemented daily with vitamin A acetate equivalent to 100 (control) 9000 (mildly intoxicated) and 18000 (severely intoxicated) microgram retinol/kg body-weight. After 16 weeks lambs received a bolus of [15-3H]retinol intravenously; blood, urine and faeces were sampled for 48 h. Plasma retinol was complexed to a protein of 20000 molecular weight (MW), which in turn was complexed to a protein of 65000 MW; these proteins correspond respectively to retinol-binding protein and prealbumin. Plasma retinol concentration reached plateau values in intoxicated lambs, but plasma retinyl ester concentrations increased rapidly when liver contents of both retinol and retinyl esters exceeded approximately 10 and 100 mg respectively and kidney contents of both retinol and retinyl esters exceeded 30 micrograms. Labelled compounds, more polar than retinol, were found in plasma; their concentration increased tenfold in intoxicated lambs within 48 h. Plasma retinol transport rates were 0.1, 10.5 and 11.8 times control values, and clearance rates were 0.3, 14.1 and 14.3 times control values in mildly-deficient, and mildly- or severely-intoxicated lambs respectively. Turnover of retinol increased rapidly when liver contents of retinol and retinyl esters exceeded approximately 10 and 100 mg respectively and kidney contents of both retinol and retinyl esters exceeded approximately 30 micrograms. Plasma clearance of retinyl esters was unchanged with intake. Faecal excretion of tracer increased linearly with plasma retinol clearance. Our findings identify several variables that appear to be involved in retinol homeostasis, including plasma retinol clearance and excretion.