EXPERIMENTS ON THE MECHANISM OF ACTION OF RELAXIN

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Abstract
Virgin female guinea pigs, weighing about 250 g., were spayed and a few weeks later given 2 mg. of estradiol benzoate daily for 3 weeks. An X-ray photograph of the pelvis was then taken and the animals were injd. subcut. with 1 ml. of serum from a rabbit in the last few days of pregnancy. The serum contains relaxin. Six hours after the injn.another X-ray picture was taken, and the widths of the symphyses before and after the injn. were com-pared. The relaxation of the symphysis pelvis was found to be markedly decreased when the animals were anesthetized with urethane, amytal, or Na barbitai. A similar result was obtained when the spinal cord was sectioned at the 12th thoracic vertebra. In the latter case the symphysis, which showed no relaxation, could be made to widen by stretching with a load of 50 g. Isolated symphyses incubated with relaxin could not be extended by stretching. The authors conclude that the stretching by muscle tone and/or static forces of the skeleton is an essential factor for the relaxation of the symphysis pubis. The other factor is a histolog-ical change in the connective tissue, which increases its extensibility.