The influence of thyroxine on the plasma post-heparin lipolytic activity in rats.

10 min after heparin administration (25 IU per 100 g body weight) the plasma post-heparin lipolytic activity (PHLA) was increased in thyreoidectomized (TECT), hypophysectomized (HX) and hypophysectomized + thyreoidectomized (HX + TECT) rats to the same extent as in intact and sham hypophysectomized controls. However, 60 min after heparin the increase of PHLA persisted only in the TECT, HX and HX + TECT rats. After an administration of thyroxine to TECT rats (25 microgram per 100 g body weight) and also to HX + TECT rats (25 microgram or 100 microgram per 100 g body weight) the PHLA decreased similarly as in controls 60 min after heparin administration. The results suggest that the increased PHLA in hypothyroid rats is caused by the deficiency of thyroid hormones, while TSH released after TECT seems not involved in the regulation of enzymes degrading plasma lipoproteins.