Adrenal and sympathetic catecholamines in exercising rats

The effects of adrenodemedullation and/or adrenoceptor agonists and antagonists on plasma epinephrine (E) and norepinephrine (NE) concentrations during exercise were investigated in rats. Exercise consisted of strenuous swimming against a countercurrent for 15 min in a pool with water of 33°C. Before, during, and after swimming, blood samples were taken through a permanent heart catheter. E was not detectable in plasma of adrenodemedullated (Adm) rats. A marked reduction in the normal exercise-induced increase in plasma NE concentrations occurred in both Adm rats as well as in intact rats injected with the β2-selective adrenoceptor antagonist ICI 118551. Intravenous infusion of either E or the β2-selective agonist fenoterol restored the increase in plasma NE in Adm rats. Injection of the α2-selectivaen tagonist yohimbine in combination with infusion of the β2-selective agonist fenoterol into Adm rats caused an enormous increase in plasma NE. It is concluded that all NE in plasma as released during exercise originates from the peripheral nerve endings of the sympathetic nervous system. Adrenal E influences the release of NE via activation of presynaptic β2-adrenoceptors.