Effects of thyroid status and fasting on hepatic metabolism of apolipoprotein A-I.
Metabolism of apolipoprotein (apo)A-I was studied in normal and chow-fed hyperthyroid rats, in 24-h fasted untreated male rats, and in rats after thyroparathyroidectomy (TXPTX). Rats were made hyperthyroid by administration of T3 (9.6 micrograms/day) or T4 (30 micrograms/day) with an Alzet osmotic minipump. Hyperthyroidism produced a similar two- to threefold elevation in plasma levels of apoA-I in male or female animals. During treatment with T3, plasma levels of T3 ranged from 200 to 400 ng/dl and did not correlate with plasma apoA-I levels. The net mass secretion and synthesis ([3H]leucine incorporation) of apoA-I by perfused livers from male hyperthyroid rats was elevated, while secretion of albumin was not different than that of euthyroid rats. Furthermore, the incorporation of [3H]leucine into total perfusate and hepatic protein was not altered by hyperthyroidism. The effect of thyroid hormone on apoA-I synthesis, therefore, does not appear to be a general effect on protein synthesis. After longer periods of treatment (28 days) with T3 (9.6 micrograms/day), hepatic apoA-I production decreased from that observed after 7 or 14 days of treatment, yet plasma apoA-I concentrations remained elevated. Plasma T3 decreased from 100 ng/dl to 40 ng/dl, in the hypothyroid rat resulting from TXPTX, but the plasma concentration of apoA-I did not change during the 2-week experimental period. The net secretion of apoA-I by livers from hypothyroid animals was depressed and albumin was uneffected compared to the euthyroid. Overnight fasting of euthyroid rats did not alter hepatic apoA-I secretion or plasma apoA-I levels, although under fasting conditions we had reported that hepatic output of apoB and E of VLDL is depressed. The addition of oleic acid to the perfusion medium, sufficient to stimulate VLDL production, did not affect net hepatic secretion of apoA-I by livers from euthyroid, hyperthyroid, or hypothyroid rats. In summary, hepatic synthesis of apoA-I appears to be controlled independently of other apo-lipoproteins and secretory proteins (albumin). Hepatic apoA-I synthesis is sensitive to thyroid status, increased in the hyperthyroid and decreased in the hypothyroid state. The specific stimulation of hepatic synthesis and secretion of apoA-I in the hyperthyroid state, however, tends to normalize over an extended period, perhaps from compensatory effects of a hormonal nature.
Wilcox
H G
HG
Department of Pharmacology, University of Tennessee-Memphis, Memphis 38163.
Frank
R A
RA
Heimberg
M
M
D000818:Animals; D016632:Apolipoprotein A-I; D001054:Apolipoproteins A / Q000097:blood / Q000378:metabolism*; D005215:Fasting; D005260:Female; D006980:Hyperthyroidism / Q000378:metabolism; D007700:Kinetics; D008075:Lipoproteins, HDL / Q000378:metabolism; D008099:Liver / Q000378:metabolism*; D008297:Male; D051381:Rats; D013961:Thyroid Gland / Q000502:physiology* / Q000601:surgery; D013963:Thyroid Hormones / Q000502:physiology; D013965:Thyroidectomy; D014284:Triiodothyronine
D016632:Apolipoprotein A-I; D001054:Apolipoproteins A; D008075:Lipoproteins, HDL; D013963:Thyroid Hormones; D014284:Triiodothyronine