Lactogenic hormone receptors in mammary membrane preparations from prepartum and 60 and 180 day post-partum Holstein cattle.

Mammary tissue from nine Holstein cows was collected within 1 week of parturition and at 60 and at 180 days post partum. Blood samples were collected by puncture of the coccygeal vein or artery at 6-h intervals from 2 days before to 2 days after surgery. A membrane-enriched fraction of tissue homogenates was prepared by differential centrifugation. A microcomputer program (LIGAND) provided estimates of the dissociation constant (Kd) and receptor concentrations using Scatchard analysis of competition of radioiodinated human GH by bovine prolactin (NIH-bPRL-6), ovine prolactin (NIH-oPRL-15) and unlabelled human GH (NIH-hGH-I-1). Split-plot analysis of variance of hormone data indicated that mean prolactin concentrations during the periparturient period were greater than those at 60 or 180 days post partum. However, no differences were evident in prolactin content of blood samples collected immediately before biopsy. Analysis of variance of Scatchard data indicated that the Kd of the lactogenic hormone receptor did not differ at different stages of lactation, and averaged 89.7 nmol/l. Receptor concentrations were lower during the prepartum period than at 60 and 180 days post partum (0.65 vs 1.2 and 1.5 fmol/mg membrane protein respectively). The Kd of the lactogenic hormone receptor was similar when estimated with NIH-bPRL-6 or NIH-oPRL-15 competition, but 100-fold greater when estimated with NIH-hGH-I-1. It is concluded that lactogenic hormone receptor concentrations in bovine mammary tissue increase with the onset of lactation, with a pattern similar to that observed in non-ruminants.