Studies on the inhibitory effect of apomorphine and bromocryptine on basal and TRH induced level of TSH and PRL in hypothyroid rats under pentobarbiturate anesthesia.

Groups of male rats were inserted with polyethylene tubings into femoral artery and vein under pentobarbiturate anesthesia and small blood samples were frequently taken for the estimation of TSH and PRL under maintaining isovolemia. After a single injection of apomorphine (12 mg kg-1) or bromocryptine (20 mg kg-1) much more expressed effect of these drugs on a decrease of PRL level in plasma was found than that on a decrease of TSH level and similar observation was made with the use of continuous i.v. infusion of apomorphine (50 micrograms in 20 microliter per min for 180 min). Finally, under the above dose of infused apomorphine, the effect of TRH on the increase of TSH level was depressed at the 30th min as compared to that 0 and 120th min of infusion. In addition, at 120 min of infusion the effect of TRH was significantly higher than that at 0 min. These results suggest that the effect of apomorphine may take place at the pituitary level.

In vivo study of iodothyronine deiodination in rat liver: effect of salicylate on biliary excretion of several iodothyronines.

Biliary excretion of total (i.e. conjugated plus unconjugated) thyroxine (T4), 3,5,3'-triiodothyronine (T3), 3,3',5'-triiodothyronine (rT3), 3,5-diiodothyronine (3,5-T2), 3,3'-diiodothyronine (3,3'-T2) and 3',5'-diiodothyronine (3',5'-T2) was measured with the aid of specific radioimmunoassay in 17 control rats and in 31 rats injected sodium salicylate i.v. (200 mg kg-1). The animals were anesthetized with pentobarbiturate and the samples of bile were taken in subsequent 2 h periods from a drained bile duct. In controls a gradual decrease of excretion of all compounds measured was found during 10 h observation period. In contrast, after salicylate injection a transient increase of total bile volume and of T4, T3 and 3,5-T2 excretion was observed followed by a remarkable decrease, while a multifold and prolonged increase of the excretion of rT3, 3,3'-T2 and 3',5'-T2 was found. These data suggest an acute and remarkable effect of salicylate on T4 deiodinating pathway in the liver.

Preliminary observations on the absorption of biliary iodothyronines from the intestine in vivo in rats.

From a drained bile duct of the rats injected i. v. with various doses of L-thyroxine (50 to 200 microgram per animal) bile was obtained which was then injected to various parts of intestine in recipient rats. The recipient animals were anesthetized with pentobarbiturate and their bile duct as well as femoral artery and vein were cannulated with polyethylene tubings. The level of thyroxine (T4), 3,5,3'-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) in plasma and biliary excretion of these compounds was estimated by specific radioimmunoassay, the samples of plasma and bile being taken up to 4 h after the administration of donor bile. In one experiment also concentration of three different diiodothyronines (e.e. 3,5-T2, 3,3'T2 and 3',5'T2) in recipient bile was measured. A dose dependent increase of T4 excretion was fond in recipient bile. In some experiments also an increase of 3,3'-T2 and 3',5'-T2 excretion was observed and a higher ratio between the excreted and administered amount of rT3 was found than that of T3. An increase of T4 level in plasma was also directly related to the administered dose of this compound, while the increase of T3 and rT3 under the conditions used was observed only in some experiments where relatively higher doses of these substances were administered. It was also observed that the absorption of T4 from the administered bile was higher in fed than in fasted animals.

Direct quantitative estimation of several iodothyronines in rat bile by radioimmunoassay and basal data on their biliary excretion.

A method was developed for the hydrolysis of conjugated iodothyronines in bile with the aid of beta-glucuronidase/arylsulfatase and for subsequent direct estimation of total and free iodothyronines with the aid of specific radioimmunoassay. The amount of conjugated fraction could then be calculated from the difference. Thus, basal biliary excretion of several iodothyronines was measured in 31 normal, fed rats in which the bile duct was drained with polyethylene tubing under pentobarbiturate anesthesia and the bile was collected for 2 h. The free fraction of thyroxine, 3,5,3'-triiodothyronine and 3,3'-diiodothyronine was approx. 30% of total content, while that of 3,3',5'-triiodothyronine and 3,5-diiodothyronine was approx. 20% and that of 3',5'-diiodothyronine was less than 10%. This suggests some considerable differences in the conjugation of individual iodothyronines in the liver. The concentration of T4 in bile was about the same as in plasma, while that of other iodothyronines was about 3-8 times higher than in plasma. This shows close interrelations between the iodothyronine deiodinating pathway in liver cells in vivo and the spectrum of iodothyronine in bile. The average ratio of T3/rT3 as found in bile was about 4.

Immediate opposite effect of salicylate on thyroxine and 3,5,3'-triiodothyronine versus 3,3',5'-triiodothyronine level in plasma in rats.

In a total of 46 male rats polyethylene tubings were introduced into femoral artery and vein under pentobarbiturate anesthesia. Then heparin (300 U kg-1) was injected at 60-90 min after pentobarbiturate and two control blood samples were subsequently taken. After that sodium salicylate (200 mg kg-1) was injected i.v. and blood samples were taken at 30-420 min later. An immediate decrease of the thyroxine (T4) level in plasma to about 20% of original level and that of 3,5,3'-triiodothyronine (T3) to about 60% of that was found, while the level of 3,3',5'-triiodothyronine (rT3) was increased 20%. It was concluded that the administration of salicylate results in an immediate displacement of T4 and T3 from plasma protein binding and possibly inhibits the conversion of T4 to T3 and of rT3 to 3,3'-diiodothyronine which results in an increase of rT3 level in plasma. This might by partially prevented by an inhibiting effect of salicylate on the binding of rT3 to plasma proteins.