The Developmental Pattern of Thyrotropin-Releasing Hormone-Degrading Activity in the Plasma of Rats*

Abstract

The developmental pattern of TRH[thryotropin releasing hormone]-degrading activity in rat plasma was determined by measuring the ability of plasma from rats of various ages to degrade TRH into degradation products. From a rate of 0.519 pmol TRH degraded/.mu.l-1 .cntdot. h-1 on day 8, plasma TRH-degrading activity increased to 18.1 pmol TRH degraded/.mu.l-1 .cntdot. h-1 by day 90 in female rats. The increase in rate of formation of proline, a major plasma degradation product, was in very good agreement with the increase in rate of TRH degradation. The major increase in TRH-degrading activity occurred between the 3rd and 7th wk of life. A similar pattern of development was observed in male rats; the only significant difference was that plasma from day 90 male rats was approximately 30% more active than that from female rats (P < 0.002). Daily T3 [triiodothyronine] administration to rats from days 8-26 resulted in a 2-fold increase in plasma TRH-degrading activity (P < 0.05). The contribution of plasma degradation to the physiological control of TRH activity is not clear, but the magnitude of the increase in plasma TRH-degrading activity (approximately 30-fold) during the maturation of the rat suggests a significant biological mechanism.