Protein turnover in gravid and nongravid horns of uterus in pregnant rats.
Changes in uterine protein, RNA, and DNA contents in gravid and nongravid horns from the same animal were studied after 14 and 16 days of pregnancy. At 16 days, all of these parameters of growth had increased between three- and sevenfold in the gravid horn while remaining unchanged in the nongravid horn compared with the uterus of nonpregnant control animals. In the nongravid horn, the hormonal changes associated with pregnancy induced antagonistic decreases in both the average rates of protein synthesis and protein breakdown, with the net result being no change in its growth. In the gravid horn, however, the hormonal influences are coupled with the stretching effects produced by the enlarging fetuses. Here the net effect was a marked suppression of protein breakdown, resulting in a fourfold increase in its rate of growth. These data, together with those derived from analysis of both fetal and nonfetal sites from the gravid horn, led to the following conclusions. Changes in circulating hormones during pregnancy do not alone appear to influence the overall size of the uterus. In contrast, mechanical distension of the uterus promotes rapid and extensive growth.