Capsaicin-sensitive nerves and endurance exercise in the rat.

The purpose of our study was to clarify the role of capsaicin-sensitive nerves in the control of plasma catecholamine and glucose concentrations during exercise. In vehicle-treated rats, plasma epinephrine (E) and norepinephrine (NE) levels were significantly higher in animals exercised to exhaustion than in the group sacrificed at rest. However, it was not the case for the neonatally capsaicin-treated animals. The epinephrine and norepinephrine levels were not significantly higher in the capsaicinized animals exercised to exhaustion than in those studied at rest. As a result, plasma epinephrine and norepinephrine levels were higher in control than in capsaicinized exhausted animals. Impairment of capsaicin-sensitive nerves by the neonatal capsaicin treatment prevented the exercise-induced increase of catecholamine output despite a significant decrease in plasma glucose levels and a lower liver glycogen content at rest. We suggest that this impairment of catecholamine output during exercise was caused by depletion of substance P in C-fibers directed to the adrenal medulla. This is supported by the observation of a lower plasma epinephrine level in capsaicin-treated rats. We conclude that C-fibers are therefore involved in the control of catecholamine secretion by the adrenal medulla during exercise to exhaustion. However, such an impairment of catecholamine output was not associated with a further decrease in plasma glucose levels or a shorter time-to-exhaustion. This also suggests that a partial dysfunction of the adrenal medulla is not sufficient to alter exercise endurance and plasma glucose levels.