Casein turnover in rabbit mammary explants in organ culture

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Abstract
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. The amount of casein in particle-free supernatants prepared from mammary explants was measured by rocket immunoelectrophoresis. The rate of incorporation of L-[4,5-3H]L into casein was measured after isolation of the casein by immunoadsorbent chromatography and polyacrylamide-gel electrophoresis in the presence of urea and sodium dodecyl sulfate. Casein accumulated in mammary explants in the presence of the hormones, but not in their absence. Removal of hormones after 24 h in culture resulted in a decrease in the rate of accumulation of casein in the explants. Casein-synthetic rate increased in mammary explants in the presence of the hormones but not in their absence. Removal of hormones after 24 h in culture resulted in continued casein synthesis at approximately 30% of the rate in the presence of hormones. The synthetic rate did not decrease to values observed in explants cultured throughout in the absence of hormones. Casein was not degraded in mammary explants during a phase of rapid casein accumulation (36-72 H) in the presence of hormones. Furthermore casein was not degraded when hormones were removed from the tissue after between 36 and 72 h in culture. Casein was glycosylated in mammary explants; the extent of glycosylation paralleled the rate of synthesis. The glycosylated protein was rapidly secreted from the tissue. After hormonal stimulation mammary explants from mid-pregnant rabbits apparently 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.