The effect of infusions of glucose, acetate and amino acids on hourly milk yield in fed, fasted and insulin-treated goats.

1. Experiments have been carried out in lactating goats milked hourly to assess the value of this technique in studies of milk secretion.2. On refeeding 24 hr-fasted goats there was an increase in arterial concentration and mammary uptake of volatile fatty acids within an hour, but little increase in hourly milk and lactose yield until the mammary uptake of glucose had also increased (after 2-3 hr).3. Intravascular infusions of acetate had no effect on milk secretion in 24 hr-fasted goats but glucose infusions increased milk yield by 62 +/- 5% and lactose yield by 87 +/- 12% within 3 hr, with no effect on fat secretion. The addition of acetate or acetate plus amino acids had no more effect than glucose alone.4. The yield of milk and lactose could be reduced within an hour by insulin (2 u./kg I.V.) and this was prevented or reversed by injecting glucose. In one goat, where in spite of a fall in blood sugar, mammary arteriovenous difference and blood flow did not fall, there was little or no fall in milk yield.5. In fasted or insulin treated goats an increase in milk and lactose secretion could be obtained within an hour by infusing glucose into the artery of one gland autotransplanted to the neck, which responded before the control gland in situ, thus showing that the effect of glucose is directly on the mammary tissue.6. In two normally fed goats with a low blood sugar, glucose infusions increased the milk or lactose yield by 30% within 3 hr.7. It is concluded that frequent milking, using minimal doses of oxytocin, is a valid method of studying factors controlling milk secretion and that, in the lactating goat, the availability of glucose to the mammary gland can be a limiting factor for maximum milk secretion.