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Effect of sustained adrenergic receptors stimulation and blockade on lactate threshold in rats.

BACKGROUND
This study examined the hypothesis that prolonged (12 hrs) stimulation of adrenergic receptors by adrenaline and their blockade by regitine (alpha-blockade) or propranolol (beta-blockade) affect lactate thresh-old (T(LA)) and exercise tolerance in rats.


METHODS
Twenty-four untrained male Wistar rats performed a multistage running test on a treadmill with an intensity increased from 13 m/min to the maximum speed. After a control exercise test the animals were devided into three groups implanted s.c. with: 1) adrenaline; 2) propranolol and 3) regitine tablets. Twelve hours after the tablet implantation the exercise was repeated. Both before the exercise test and at the end of each 2-min loads blood lactate [LA] was measured to evaluate T(LA).


RESULTS
Prolonged adrenaline excess resulted in reduced maximal running speed and shifted T(LA) towards lower exercise intensities than in the control test. Both alpha- and beta-adrenergic blockade reduced the work load at which T(LA) occurred, without changing the maximum running speed and maximal blood LA concentration. Blood [LA] at the lactate threshold ([LA] at T(LA)) was significantly higher in hyperadrenalinemic than in control rats, whilst under beta-blockade it was lowered in comparison with control values. Alpha-adrenergic blockade had no effect on [LA] at T(LA), but at the higher exercise intensities blood [LA] exceeded the respective control values.


CONCLUSIONS
The present study demonstrated that prolonged excess of adrenaline reduces the maximum power output and T(LA). The former effect seems to be a result of simultaneous stimulation of both types of adrenergic receptors.