An investigation of the role of insulin in bovine milk protein gene expression in mammary explant culture

Experiments have shown that administration of insulin to dairy cattle during lactation may increase the concentration of milk protein. In the experiments reported here the requirement for insulin to stimulate expression of milk protein genes, in the presence of hydrocortisone and prolactin, was examined in a bovine mammary explant culture model, using tissue from late pregnant multiparous Holstein-Fresian cows. Stimulation of κ-, β-casein and β-lactoglobulin gene expression, in response to the addition of prolactin, was observed in media which included hydrocortisone in either the presence or absence of insulin. The magnitude and persistence of the response observed following stimulation with prolactin, was enhanced in explants cultured in the absence of insulin when preceded by a 4 day incubation in media with insulin and hydrocortisone. This response was consistent between all three milk protein genes examined. We suggest that either insulin is not required for expression of milk protein genes or that the requirement for insulin may be substituted by endogenous insulin-like growth factors/hormones produced by the bovine mammary explants.