The effect of graded epinephrine infusion on the blood lactate response to moderate intensity exercise

The recent literature supports a hypothesis that the sudden increase in blood lactate during graded exercise is associated with the simultaneous increase in plasma epinephrine. In an attempt to determine whether this relationship between lactate and epinephrine was a causal one, eight healthy, college-aged males (22.4 ± 0.4 yrs) were recruited to participate in the research project. After completing the informed consent form, each subject performed a graded exercise test, in which the work rate was increased 30 W every three minutes, to physical exhaustion. At the end of each three-minute stage, blood samples were taken from an upper arm vein for the determination of the lactate (LT), epinephrine (EpiT), and norepinephrine (NepiT) thresholds. Within the following two weeks, each subject performed, in random order, two 30-minute submaximal exercise tests at a work rate approximately 20% below that eliciting the LT. The two 30-minute exercise tests consisted of (1) a graded infusion of epinephrine (EX) at a rate of 0.02 μg • kg^-1 • -min^-1 to 0.12 μg • kg^-1 • min^-1, and (2) a control (C) test. Throughout the submaximal exercise tests, blood was sampled at five-minute intervals for the determination of blood lactate, plasma epinephrine and plasma norepinephrine. The infusion of epinephrine resulted in plasma epinephrine concentrations similar to those observed during the graded exercise test. An increase in blood lactate during the El test was significantly greater than the C levels and exhibited a plateau throughout the final 10 minutes. Importantly, the El blood lactate values did not approach the levels exhibited during the graded exercise test. An elevation in RER during the El test suggests an increase in glycogenolysis to support the increase in lactate production. Nevertheless, the inability to observe a similar increase in blood lactate during the El when compared to the LT test suggests that other factors, in addition to plasma epinephrine, are responsible for the sudden increase in blood lactate during graded exercise.