Transmission of relaxin from lactating bitches to their offspring via suckling

The 6-kDa polypeptide hormone relaxin (Rlx) has been identified in human and bovine milk, and we recently reported its presence in canine milk We postulated that Rlx might be transferred via suckling to the newborn pups, where, by virtue of its known effects to increase the distensibility of the pelvic connective tissues, it could play a role in causing the excessive laxity of the capsule and Ligaments of the coxofemoral joint that precedes the development of hip dysplasia in genetically predisposed animals. Rlx was found in the serum of dysplastic (HD+) bitches for up to 6 wk of lactation, whereas it was detected in the serum of nondysplastic (HD-) bitches for only 1-2 wk of lactation. Rlx concentrations in milk were up to 60-fold greater than in serum. Milk Rlx levels varied markedly, but were highest during the first week of lactation and decreased thereafter. There were no significant differences in milk Rlx concentrations between HD+ and HD- bitches. Although the source of Rlx in milk is unknown, it cannot be the ovary or uterus, since hystero-ovariectomy performed at the time of cesarean section did not eliminate Rlx from milk during subsequent lactation. In serum samples taken from newborn pups before suckling, there were significant quantities of Rlx, demonstrating that the hormone enters the fetus in utero. However, Rlx rapidly disappears from serum of pups prevented from suckling for five hours. Daily ingestion of milk sustained a low but significant level of relaxin in a litter of pups for up to 30 days of lactation, and orally administered porcine relaxin was rapidly absorbed into the circulation of newborn pups. Relaxin extracted from canine placentas and tested in the mouse pubic symphysis direct measurement assay has been demonstrated to alter pelvic connective tissues. Partially purified canine placental Rlx exerted typical Rlx-like activity in the mice. In summary, Rlx is transmitted from bitches to their suckling young via their milk and is absorbed into the circulation of the pups. Canine Rlx induces a typical pelvic "relaxation" in mice. We therefore suggest that Rlx may play a role in causing the laxity of the coxofemoral joint that leads to hip dysplasia in genetically predisposed pups.