Influence of Breed and Hormones on Production of Milk Proteins by Mammary Explants from Prepubertal Heifers

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Abstract
Effects of hormone treatment and breed on lactogenic responses were studied in organ culture of mammary tissue obtained from prepubertal Angus or Holstein heifers. Donor animals were treated with injections of estradiol and progesterone for 7 d and were killed on d 15 after initial injection. Explants were cultured for up to 96 h in a basal medium containing insulin, hydrocortisone, and triiodothyronine or a stimulatory medium, further supplemented with prolactin. Concentrations of .alpha.-lactalbumin and .alpha.s1-casein were measured in media and explant homogenates by radioimmunoassay, as an index of lactogenesis. In selected cultures, 3H-labeled amino acids were added to assess de novo protein synthesis. Addition of prolactin to medium elicited marked increases in accumulation of .alpha.s1-casein but had less effect on content of .alpha.-lactalbumin or [3H]protein in media or explant homogenates. Explants from Holstein heifers consistently produced more .alpha.-lactalbumin, casein, and total protein than those from Angus heifers, reflecting their inherent superiority in ability to produce milk. Breed differences were more readily detected among cultures exposed to prolactin. Demonstration of breed differences in biosynthetic capacity of mammary tissue suggests a possible means for early selection of dairy heifers.