Prolactin and the swine mammary gland.

Prolactin is a hormone that is most important for mammary development in swine. It is also essential for both the onset and the maintenance of lactation. In early studies, exogenous recombinant porcine prolactin was used to detect its biological effects on mammary tissue. In these studies, a stimulatory role of prolactin for mammary development of prepubertal gilts was demonstrated. However, when injected throughout lactation, prolactin did not increase sow milk yield likely because mammary receptors were saturated. The secretion of prolactin is largely under negative regulation via dopamine. Inhibition studies using the dopamine agonist bromocriptine showed that prolactin is required in the last third of gestation to sustain mammary development in gilts. When creating a hyperprolactinemic state during that same period, with the dopamine antagonist domperidone, differentiation of mammary epithelial cells was stimulated and milk yield in the subsequent lactation was increased. Domperidone given throughout lactation also led to greater milk yield. A study using domperidone in prepubertal gilts, however, did not reproduce the stimulatory effect of exogenous prolactin on mammary development and demonstrated that timing of the prolactin increase in relation to age of the animals is most important to elicit a response on mammary tissue. Attempts were made to use feed ingredients such as the plant extract from milk thistle, sylimarin, to stimulate prolactin secretion in sows. However, even though prolactin concentrations were increased, this augmentation was not important enough to have an impact on mammary development in late gestation or to increase milk yield. The current knowledge that hyperprolactinemia has beneficial effects for mammary development at various physiological stages and can enhance milk yield in swine leads to new avenues in elaborating strategies that could be used at the farm level to improve sow lactation performance.