Canine hip dysplasia is a polygenically inherited developmental abnormality that is associated with osteoarthritis in adulthood.1,2 Prevalence of hip displasia can be close to 50% in some breeds, according to statistics compiled by the Orthopedic Foundation for Animals.3 The hip joint is supported by muscles, ligaments, and a capsule, all of which must develop congruously to ensure joint stability, especially during the first 60 days after birth, prior to ossification, when the tissues are still pliable and readily deformed. Thus, it is likely that the factors that initiate the dysplastic process are expressed during this time period.
One of the early abnormalities detected in canine pups destined to develop hip dysplasia is a laxity of the hip joints that permits excessive movement of the femoral head in its acetabular socket, resulting in joint damage.4,5 Dysplasia frequently occurs in more than 1 joint, suggesting a possible involvement of blood-borne mediators,6 and a specific role of maternal hormones in abnormal hip development has long been suspected. Injection of large doses of estrogens into bitches in late stages of pregnancy or estrogens and a crude relaxin extract injected directly into newborn pups induce a dysplasia-like skeletal abnormality.7,8
Relaxin is a 6-kd polypeptide hormone of mammalian pregnancy that, in concert with estrogens, induces profound changes in the composition and tensile properties of connective tissues of the pubic symphysis and sacroiliac joints. In several species, estrogens and relaxin induce formation of interpubic ligaments, increasing the diameter and flexibility of the birth canal9; in others, estrogens and relaxin increase the extensibility of the pubic or sacroiliac joints.10 These hormonally induced changes in flexibility facilitate parturition and result from breakdown of connective tissues in association with increased activity of several proteases, including collagenase.9 The similarity of the relaxin-induced increase in flexibility of the pelvic ligaments to the abnormal laxity of predysplastic hip joints observed in HD+ pups5,6 led us to postulate that maternal estrogens and relaxin might be the prime initiators of hip disease in these animals.
Relaxin receptors are members of the leucine-rich, repeat-containing, G-protein–coupled receptor group and are designated as LGR7 and LGR8.11 A previous study12 revealed that estrogens promote the synthesis of relaxin receptors and, conversely, that relaxin activates estrogen receptors.13 In addition, estrogens prime connective tissues for relaxin's action by inducing synthesis and activation of key enzymes and mediators required for collagen and proteoglycan breakdown.14
Crelin15 discovered that genetically normal neonates are protected against untoward effects of estrogen and relaxin. Thus, the pubic symphysis of newborn mice does not respond to exogenous estrogen and relaxin and remains refractory until the time of puberty. Moreover, pubic symphyses transplanted from nurslings to adults respond to estrogen and relaxin only if excised after the 12th day of birth.15 The refractoriness of newborn mice to estrogen and relaxin suggests that the pubic symphysis may lack specific receptors for one or both of these hormones.
In other mammalian species, specific estrogen binding sites have been found in adult articular cartilage, but not in epiphyseal plate or embryonic cartilage, suggesting that mature cartilage, but not transforming or developing cartilage, responds to estrogens through a receptor mechanism.16,17 In harmony with this finding, estradiol-17β enhanced, whereas an antiestrogen inhibited, cartilage destruction in experimental osteoarthritis in adult rabbits, purportedly acting through a receptor mechanism.18 Although radiolabeled relaxin specifically bound to connective tissues of the pubic symphyses of adult mice and rats,12,19 the relationship between age and first appearance of chemically identified relaxin receptors has not yet been investigated in rodents. However, LGR7 relaxin receptors were found in the cervices of neonatal pigs, which responded to injection of porcine relaxin as evidenced by induced tissue growth.20 This was proven a relaxin-specific effect because it was neither enhanced by a concomitant estrogen injection nor inhibited by an estrogen antagonist.20
Crelin and Lavin's21 mouse experiments suggest that the connective tissues of the neonatal pelvis remain refractory to estrogen and relaxin during the lactation interval, when these hormones could easily be absorbed from the milk across the immature gut to enter the circulation. If this is true for other species as well, then a genetic defect resulting in premature appearance of estrogen, relaxin receptors, or both might enable an inappropriate response to these hormones before puberty. In support of this argument, Andren22 found instability of the pubic symphysis in newborn children in association with congenital dislocation of the hip, suggesting sensitivity to a common blood-borne factor.
Immunoreactive relaxin persisted in the circulation of dysplasia-prone Labrador Retriever bitches for 5 to 6 weeks during the lactation period, whereas its concentration decreased below detectable limits after 1 to 2 weeks in nondysplastic Labrador Retriever bitches or in dysplasia-resistant Beagles.23,24 Thus, there may be a departure from normal in the secretion or metabolism of relaxin associated with the genetic predisposition for hip dysplasia in Labrador Retrievers.
By use of a heterologous porcine relaxin RIA, immunoactive relaxin was previously found in dog milk at concentrations 5 to 20 times those in dog serum.24 More recently, by use of a homologous canine relaxin RIA,25 immunoactive relaxin concentration in milk was found to be in the microgram per milliliter range; the pups thus ingested milligram quantities during suckling. The source of relaxin in dog milk is unknown. However, relaxin and its receptors have been immunolocalized and relaxin genes expressed in mammary glands of other species.26-28 Moreover, normal concentrations of relaxin were found in milk of 2 bitches following ovariohysterectomy at the time of cesarean section, a procedure that eliminated the ovaries and uteri as sources of relaxin.24 These observations suggest that the mammary glands, per se, may be the source of relaxin found in milk.
Estrogens and estrogen precursors have also been identified in milk of several species.29,30 Pups born to parents with hip dysplastia but hand-reared from birth, and therefore deprived of their dam's colostrum and milk, had a lower prevalence and severity of hip disease than did their suckled counterparts.1 This finding supports the hypothesis that the transmission of milk-born factors is related to the development of hip dysplasia in HD+ pups.
The objectives of the study reported here were to provide new data on milk-borne factors that might induce hip joint laxity in canine HD+ pups by use of RIAs for estrogens and estrogen precursors and a homologous canine relaxin RIA; to modulate hip laxity by treatment of HD+ pups with an estrogen synthesis inhibitor; and to attempt to induce laxity in hip joints of HD− pups by injections of the hormones postulated to be the causative milk-borne factors.
Genetically predisposed to hip dysplasia
Not genetically predisposed to hip dysplasia
Depo-Estradiol, Upjohn, Kalamazoo, MI.
ICN Biomedicals Diagnostic Division, Carson, Calif.
Whatman glass fiber paper, Whatman, Inc. Florham Park, NJ.
PennHip, provided by Dr. G. K. Smith, University of Pennsylvania, Philadelphia, PA.
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