Prolactin Receptor Content of Rabbit Milk*

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Abstract
Rabbit milk fat globule membranes have been shown to contain PRL receptors by a variety of criteria, including hormonal specificity and inhibition by specific anti-PRL receptor antisera. Collection of milk samples throughout the period of lactation facilitated a temporal study of the receptor content of the membrane fraction of these milk samples by Scatchard analysis after treatment with 5 M MgCb to dissociate endogenously bound PRL. Total receptor content was low after parturition (3.3 ± 1.3 fmol/mg membrane protein) but increased subsequently, reaching maximal levels (43.7 ± 3.4 fmol/mg) by day 21 of lactation. No significant difference in the Ka (4.9 × 109 ± 0.35 M-1) of the milk receptor was detected over a 4-week suckling period. No apparent relation seemed to exist between rabbit serum PRL values measured by RIA in serum samples taken just before milking and milk PRL receptor content. Milk receptor content, however, was significantly (P < 0.02) correlated with the PRL receptor content of the gland when animals were sacrificed immediately after milking.