INCREASED GROWTH PROMOTING SUBSTANCE IN THE PLASMA OF PREGNANT RATS1

Abstract

Normal rat plasma contains growth promoting substance(s) as judged by the increase in the tibial width, body weight and tail length of the injected hypophysectomized rats. This activity in normal plasma is equivalent to 1-2.5 [mu]g of purified bovine growth hormone per cc of plasma. Growth promoting activity is increased during pregnancy, to an equivalent of 3.5-7.5 [mu]g of purified bovine growth hormone per cc of plasma, a 3-fold increase. No evidence of TSH in plasma of normal or pregnant rats was found as judged by the histology, I131 uptake and weight of the thyroid of injected hypophysectomized rats. The role of the maternal and fetal pituitary, the placenta and thyroid in causing this increased activity is discussed.