Induction of colostrogenesis in non-pregnant cows was used to evaluate the relationship between prolactin (PRL) and mammary immunoglobulin G1 (IgG1) receptor expression. Six of eleven non-pregnant, non-lactating Holstein cattle responded to a standard lactation induction protocol by development of elevated IgG1 concentrations in mammary secretions. In order to increase the diversity in PRL concentrations, two of the six cattle were treated with bromocriptine, and two others were treated with recombinant bovine PRL. Serum alpha-lactalbumin, serum PRL and mammary secretion IgG1 concentrations were measured throughout the experiment. Biopsies of mammary tissue were collected after induction of lactation, and after treatments to alter serum PRL. Immunohistochemistry was used to evaluate IgG1 receptor expression. Administration of recombinant bovine (rbPRL) was associated with increased lactogenic activity, decreased secretion IgG1 concentrations, and decreased IgG1 receptor expression. Decreased serum PRL, due to bromocriptine, was associated with decreased lactogenic activity and maintenance of IgG1 receptor expression. Results of this experiment are consistent with an effect of PRL in decreasing the expression of the bovine mammary IgG1 receptor at the onset of lactogenesis.

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