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The effect of goat milk fractions on synthesis of milk constituents by rabbit mammary explants and on milk yield in vivo. Evidence for autocrine control of milk secretion.

ABSTRACT: Lactose and casein synthesis by rabbit mammary explants in organ culture was inhibited when fractions of goat milk were included in the culture medium. Inhibition was dose-dependent, and readily reversed when milk fractions were removed. The pattern of effects obtained with various fractions of milk indicated that inhibition was caused by a protein of 10,000-30,000 Da, which was present in the milk serum or whey fraction. The inhibitor fraction decreased milk accumulation when injected into lactating rabbit mammary glands via the teat ducts, whereas other milk proteins had no effect. Results are discussed in terms of autocrine regulation of milk synthesis through negative feedback by milk constituents.

SUBMITTER: Wilde CJ

PROVIDER: S-EPMC1147695 | biostudies-other | 1987 Feb

REPOSITORIES: biostudies-other

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Project description:1. Explants of mammary gland from mid-pregnant rabbits were cultured in medium 199 containing insulin, prolactin and cortisol, and specific anti-casein immunoglobulin G was used to measure the amount, rate of synthesis and rate of degradation of casein in the explants in the presence of hormones and after removal of hormones from previously stimulated tissue. 2. The amount of casein in particle-free supernatants prepared from mammary explants was measured by ;rocket' immunoelectrophoresis. 3. The rate of incorporation of l-[4,5-(3)H]leucine into casein was measured after isolation of the casein by immunoadsorbent chromatography and polyacrylamide-gel electrophoresis in the presence of urea and sodium dodecyl sulphate. 4. Casein accumulates in mammary explants in the presence of insulin, prolactin and cortisol, but not in the absence of hormones. Removal of hormones after 24h in culture results in a decrease in the rate of accumulation of casein in the explants. 5. Casein-synthetic rate increases in mammary explants in the presence of insulin, prolactin and cortisol, but not in the absence of hormones. Removal of hormones after 24h in culture results in continued casein synthesis at approx. 30% of the rate in the presence of hormones. The synthetic rate does not decrease to values observed in explants cultured throughout in the absence of hormones. 6. Casein is not degraded in mammary explants during a phase of rapid casein accumulation (36-72h) in the presence of hormones. Furthermore casein is not degraded when hormones are removed from the tissue after between 36 and 72h in culture. 7. Casein is glycosylated in mammary explants; the extent of glycosylation parallels the rate of synthesis. The glycosylated protein is rapidly secreted from the tissue. 8. The results are consistent with the notion that after hormonal stimulation mammary explants from mid-pregnant rabbits synthesize, glycosylate and rapidly secrete casein. Removal of hormones decreases the synthetic rate of casein, but does not cause the accumulation of a pool of degradable casein in the lobuloalveolar cells.

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The effect of goat milk fractions on synthesis of milk constituents by rabbit mammary explants and on milk yield in vivo. Evidence for autocrine control of milk secretion.

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