TRANSMISSION OF RELAXIN FROM LACTATING BITCHES TO THEIR OFFSPRING VIASUCKLING

The 6-kDa polypeptide hormone relaxin (Rlx) has been identified in hum an and bovine milk, and we recently reported its presence in canine mi lk We postulated that Rlx might be transferred via suckling to the new born pups, where, by virtue of its known effects to increase the diste nsibility of the pelvic connective tissues, it could play a role in ca using the excessive laxity of the capsule and Ligaments of the coxofem oral joint that precedes the development of hip dysplasia in genetical ly 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 s erum 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 differen ces in milk Rlx concentrations between HD+ and HD- bitches. Although t he source of Rlx in milk is unknown, it cannot be the ovary or uterus, since hystero-ovariectomy performed at the time of cesarean section d id not eliminate Rlx from milk during subsequent lactation. In serum s amples 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 f rom suckling for five hours. Daily ingestion of milk sustained a low b ut significant level of relaxin in a litter of pups for up to 30 days of lactation, and orally administered porcine relaxin was rapidly abso rbed into the circulation of newborn pups. Relaxin extracted from cani ne placentas and tested in the mouse pubic symphysis direct measuremen t assay has been demonstrated to alter pelvic connective tissues. Part ially purified canine placental Rlx exerted typical Rlx-like activity in the mice. In summary, Rlx is transmitted from bitches to their suck ling young via their milk and is absorbed into the circulation of the pups. Canine Rlx induces a typical pelvic ''relaxation'' in mice. We t herefore suggest that Rlx may play a role in causing the laxity of the coxofemoral joint that leads to hip dysplasia in genetically predispo sed pups.