Mammary function and its control at the cessation of lactation in the goat.

Abstract

The changes in mammary function following cessation of milking during declining lactation were studied in conscious goats. No significant changes in the rate of milk secretion, mammary blood flow or metabolism occurred in the first 24 h after cessation of milking. After then, secretory rate, mammary blood flow, O2 consumption, glucose uptake and acetate uptake decreased markedly over the next 3 days. Up to the time of maximum udder distension on day 3, there were no major changes in milk composition. The rate of milk secretion declined when the calculated pressure within the alveoli became positive. After 3 days, mammary volume and intramammary pressure decreased, and the composition of milk changed slowly to resemble that of extracellular fluid, i.e., [Na+], [Cl-], [HCO3-] and pH increased while [K+], [lactose] and [citrate] decreased. During this time [lactose] and [K+] were positively correlated, and [lactose] and [Na+], and [lactose] and [Cl-] negatively correlated. The changes in milk composition, the decreases in mammary volume and in intramammary pressure after day 3 are apparently due to the loss of integrity of the mammary epithelium. By about 7 wk after the cessation of milking the udder volume was less than the empty udder volume before milking was stopped, indicating a loss of mammary tissue as well as the resorption of fluid. When milking of an autotransplanted gland was stopped, while milking of the control gland in situ was continued, the rate of secretion in the transplant fell while that of the control did not change. In goats milked normally but in which a volume of isosmotic lactose equal to the volume of milk removed at that milking was injected into the lumen of 1 gland at each milking, the rate of secretion of that gland, but not that of the other, decreased.