Inhibition of prolactin in the last trimester of gestation decreases mammary gland development in gilts

Prolactin is required for mammary development in various species but its possible role for mammogenesis in pigs is not known. The goal of the present study was therefore to determine the effect of prolactin inhibition by bromocriptine during the last third of gestation on mammary gland development in gilts. Twenty-eight primigravid gilts were assigned as controls (n = 15) or received 10 mg of bromocriptine orally thrice daily (n = 13) from d 70 to 110 of gestation. Jugular blood samples were collected on d 70 of gestation and every 8 d thereafter and were assayed for prolactin, IGF-I, estradiol, and progesterone. Gilts were slaughtered on d 110 of gestation and fetuses were counted and weighed. One row of mammary glands was used for dissection of parenchymal and extraparenchymal tissues and for determination of DNA, RNA, dry matter, protein, and fat contents. Tissue from the other row was used for measures of prolactin receptor number and affinity. Concentrations of prolactin were drastically reduced throughout the bromocriptine treatment period (P [is less than] .001), whereas there was no overall treatment effect on progesterone and IGF-I levels (P [is greater than] .10). Total weight and extraparenchymal tissue weight of the mammary glands were unaffected by treatment (P [is greater than] .1), but weight of parenchymal tissue, total DNA, and total RNA decreased (P [is less than] .01) with bromocriptine treatment. Percentages of fat and dry matter in patenchymal tissue increased with bromocriptine treatment (P [is less than] .01) and the percentage of protein decreased (P [is less than] .01). Number of prolactin receptors in parenchymal tissue decreased with bromocriptine treatment (P [is less than] .001) and receptor affinity increased (P [is less than] .001). Average fetal weight was lower in gilts receiving bromocriptine than in control gilts (P = .05), but fetal number did not differ (P [is greater than] .1). These results clearly demonstrate that prolactin is essential for normal mammary gland development and can affect fetal growth during the last third of gestation in gilts. Key Words: Bromocriptine, Mammary Development, Mammary Glands, Pregnancy, Prolactin, Sows