Production of VLDL and vitellogenin(s) in oestrogen-challenged hypothyroid chickens

Compared with normal control birds of the same breed, obese strain (OS) White Leghorn chickens, exhibiting classical symptoms of hypothyroidism, were generally half as responsive to an oestrogen challenge in terms of synthesis of VLDL and vitellogenins, despite having livers which were, on the average, twice as large. Even the few OS birds which were laying eggs and therefore enjoying a degree of thyroid function, failed to exhibit normal responses to hyperdynamic doses of oestrogens. However, simultaneous injections of triiodothyronine (T3), in the physiological range, and hyperdynamic doses of oestrogenic substances (Ayerst Co., mainly oestrone) into OS chickens resulted in serum levels of VLDL and vitellogenins which were more than double those achieved without thyroid hormone treatment. Untreated OS birds were found typically to exhibit a unique hyperlipaemia (i.e. increased synthesis of serum triglyceride and phospholipid without corresponding increases in synthesis of apoprotein B (normally associated with VLDL and LDL) or apoprotein A-1 (normally associated with HDL). This type of hyperlipaemia resulted in formation of large lipid micelles which imparted turbidity to the serum. Injection of oestrogenic substances into two OS chickens especially hyperlipaemic in this unique fashion resulted in rapid liver production of apoprotein B. By 2 days post injection the levels of apoprotein B in the sera were increased fifteen times over original levels. The initially turbid sera were completely cleared by 36 h post oestrogen treatment. Since the VLDL present during the first 2 days floated at faster rates than those present at 6 days following injection of oestrogen, it appears that newly synthesized apoprotein B (very likely as lipoprotein B) can combine with existing lipid micelles in the plasma, thus ultimately dispersing the larger lipid micelles into lipoproteins of a size consistent with the loss of turbidity.