Response of Four Inbred Types of Mice to Relaxin **

Relaxin, when administered to mice pretreated with estrogen, induces a transformation of the joint cartilage into ligamentous tissue. This transformation , which results in an increased distance between the pubic bones that border the joint, is similar to that occurring during normal pregnancy."'2 The increase in interpubic distance following relaxin treatment has been the chief criterion used by different investigators to measure the response of a mouse to this hormone.8 It is also the basis upon which the hormone is bio-assayed."' Although individual investigators have achieved reproducible increases in interpubic distance following treatment with estrogen and relaxin, wide variations in increases have been reported by different investigators. These differences can be accounted for by a number of factors, e.g., the amount of hormone administered at a given time, the time interval between each administration of the hormone, the type of hormone used (estrone vs. estradiol, pregnant rabbit serum relaxin vs. sow ovary relaxin, etc.), and the responsiveness of the animals due to variations in age, weight, sex, diet, season of the year, and strain. The strain differences have not been evaluated. Therefore, the purpose of this investigation was to determine whether differences exist among different types of inbred mice in the response of their pubic joint tissue to both exogenous estrogen and relaxin and to the hormones of pregnancy. A total of 200 mice of two inbred strains (brother-sister mated), Bagg-Albino and Albany, and two inbred stocks (not brother-sister mated), CHI and Brown-belt, was used. The Bagg-Albino strain was originally obtained from the R. B. Jackson Memorial Laboratory and the Albany strain from Dr. E. H. Horn of the Albany Medical College. The CHI and Brown-belt stocks were brother-sister mated strains originally obtained from Dr.