Postnatal developmental pattern of thyrotrophin releasing hormone-degrading activity in rat plasma, hypothalamus and liver: role of tri-iodothyronine.
Thyrotrophin releasing hormone-degrading activity (TRH-DA) is present in plasma, hypothalamus, pituitary gland, liver and kidney of adults of several species. Each of these tissues contains more than one TRH-degrading enzyme but only one, pyroglutamate aminopeptidase, isolated from the blood, is a TRH-specific enzyme. The aim of this study was to describe the developmental pattern of TRH-DA in the plasma, hypothalamus and liver and the role of tri-iodothyronine (T3) in the development of TRH-DA in the rat. Based on the hypothesis that thyroid hormones stimulate plasma TRH-DA in adult rats, degradation of TRH was studied in hypo- or hyperthyroid rats induced by 6-n-propyl-2-thiouracil or T3 respectively. Tritiated TRH was incubated with plasma and homogenates of hypothalamus or liver. After separation of degradation products by thin-layer chromatography, the amount of degraded [3H]TRH (pmol/50 microliters plasma or homogenate) was taken as a comparative index of TRH-DA. Plasma TRH-DA was not detectable before day 9 while hypothalamic and hepatic TRH-DA was already active at birth. Furthermore, the maturation pattern of total TRH-DA was different in plasma compared with other tissues and T3 appeared to play a significant role in its development. The absence of plasma TRH-DA in the neonatal period, its special thyroid-dependent developmental pattern and the presence of a specific TRH-degrading enzyme in adult blood suggest a physiological regulatory role for this activity.
D000818:Animals; D000831:Animals, Newborn / Q000378:metabolism*; D005260:Female; D007031:Hypothalamus / Q000378:metabolism*; D008099:Liver / Q000378:metabolism*; D008297:Male; D011247:Pregnancy; D011441:Propylthiouracil / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D013973:Thyrotropin-Releasing Hormone / Q000097:blood / Q000378:metabolism*; D013974:Thyroxine / Q000097:blood; D014284:Triiodothyronine / Q000097:blood / Q000378:metabolism* / Q000494:pharmacology
D014284:Triiodothyronine; D013973:Thyrotropin-Releasing Hormone; D011441:Propylthiouracil; D013974:Thyroxine