INFLUENCE OF THYROID-HORMONES ON EXERCISE TOLERANCE AND LACTATE THRESHOLD IN RATS

Effects of thyroid hormone deficit, and triiodothyronine (T-3) treatme nt on exercise performance, blood lactate (LA) concentrations and LA t hreshold (T-LA) were studied in trained and untrained rats. Fourteen r ats were thyroidectomized and then treated with propylthiouracil for 3 0 days (THY + PTU group). Fourteen sham operated rats served as contro ls. In each group there were 7 sedentary and 7 endurance-trained anima ls. Six weeks after thyroidectomy or sham operation the rats were subj ected to a multistage running test with speed increasing from 13 m/min at 10 degrees treadmill inclination till maximum. Blood samples for L A were taken from the rats' tail after each 3-min exercise stage. Duri ng 3 days following this test rats from all groups were injected (i.p. ) with 75 mu g/100 g of triiodothyronine (T-3), and 24 hrs afterwards the second exercise test was performed. In THY + PTU rats maximal runn ing speed (RSmax) and the speed at which T-LA occurred were markedly d ecreased in comparison with control group. The level of LA at the maxi mal speed (LAmax) and that corresponding to T-LA were higher in THY PTU rats than in controls. T-3 injection to control animals diminished their RSmax and T-LA, whereas in THY + PTU rats it increased RSmax an d shifted T-LA to a higher speed. Both in THY + PTU and control animal s T-3 elevated LAmax and the threshold LA concentration. Endurance tra ining in control and THY + PTU animals markedly enhanced RSmax and T-L A. This was accompanied by increases in LAmax and concentration of LA at T-LA only in control group. After T-3 injection to control trained rats RSmax and T-LA were diminished, whereas in THY + PTU trained grou p RSmax was unchanged and T-LA was elevated. Maximal blood LA increase d only in THY + PTU trained rats whilst the threshold blood LA was ele vated in both groups. It is concluded, that both the T-3 deficiency an d its excess reduce maximal exercise performance and shift T-LA to low er workloads. Enduarance training or administration of T-3 to hypothyr oid rats markedly improve their exercise performance and elevate T-LA, however, T-3 treatment markedly increases maximal and submaximal LA l evels.