The developmental pattern of thyrotropin-releasing hormone-degrading activity in the plasma of rats

The developmental pattern of TRH-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/microliter-1.h-1 on day 8, plasma TRH-degrading activity increased to 18.1 pmol TRH 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 third and seventh week 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 less than 0.002). Daily T3 administration to rats from days 8--26 resulted in a 2-fold increase in plasma TRH-degrading activity (P less than 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 is suggestive of a mechanism of biological significance.