A comparison of the effect of L-triiodothyronine on the plasma lipid and cholesterol concentrations of the rat, chicken and guinea pig.

The normal concentration of FFA in plasma of the rat, guinea pig and chicken was found to be very similar (47 to 51 M-Eq/100 ml). However, after a single injection of T3, the concentration in the chicken was elevated to greater extent than in the rat or guina pig. Normal concentration of plasma phospholipid, fatty acid ester and cholesterol of the guinea pig were found to be much lower than those of the rat or chicken. These plasma components of the rat and chicken were decreased by thyroid hormone administration, although the time of onset or duration of the response was not equal. Conversely, thyroid hormone elicited a great elevation in the levels of these plasma components of the guinea pig. {Endocrinology 90: 830, 1972) I WAS REPORTED that the concentration of cholesterol in human serum was elevated in hypothyroidism and slightly below the normal value in hyperthyroidism (1). The rise of plasma cholesterol level of humans and dogs in the hypothyroid state was found to be much greater than that of rats (2). In rabbits, it was reported that removal of the thyroid gland failed to induce any rise in plasma cholesterol concentration (3), although it was later reported that a small rise might occur in this species too (4,5). On the contrary, the plasma cholesterol concentration of the guinea pig was decreased in 3 weeks after the treatment with I (6). The concentration of other lipids in plasma is also known to be influenced by the functional state of the thyroid (2). A rise in concentration of phospholipids and neutral fats in plasma of myxedema patients was shown (7,8), whereas we observed a rise in the plasma lipids of the guinea pig after the administration of thyroid hormone (6). In this paper we present further observation on plasma lipid and cholesterol levels of the guinea pig and the chicken for comparison with those of the rat. Materials and Methods Experimental animals. Male Sprague-Dawley rats, weighing between 200 to 250 g, were used. The rats were fed Purina laboratory chow ad lib, and tap water was given as their drinking water. Male guinea pigs, weighing from 240 to 300 g, which were purReceived July 30, 1971. chased from Gopher State Caviary, St. Paul, Minn., were fed guinea pig chow ad lib and tap water was given as their drinking water. Baby chicks (Gallus domesticus) were purchased from a poultry house in Missouri and fed the crushed Purina laboratory chow and tap water ad lib until use. The final body wts of the chickens used in this experiment were a little over 3 kg. T3, which was dissolved in a minimum amount of 1 N NaOH and diluted to 2 mM with 0.154 M NaCl, was injected ip in a single dose (1 mnole/200 g of body wt). The pH of the solution was approximately 9. The control animals received the same amount of 0.154 M NaCl (pH 9) ip. All the experimental animals were killed between noon and 1 PM. Blood of rats and guinea pigs was obtained by heart puncture under ether anesthesia with a heparinized syringe. Chicken blood was withdrawn from vessels localized in wings into a heparinized syringe. The plasma was separated by centrifugation at 800 X g for 10 min at 3 ± 1 C and then stored at -20 ± 1 C until use. Materials. All chemicals used were analytical reagent grade and purchased from Mallinckrodt Chemical Works, Baker Chemical Co., Fisher Scientific Co. or Sigma Chemical Company. Methods. Plasma cholesterol and lipids were determined as follows. FFA of plasma were determined with a modified Dole's method described by Trout et al. (9). A one-ml aliquot of plasma was extracted with anhydrous methanol and methanol-chloroform successively, and the combined extract was completely dried under low pressure (10 mm Hg) at room temperature after chloroform and methanol were removed by flash-rotary evaporation. The residue was then completely extracted by anhydrous pure chloroform for phospholipid, fatty acid ester