Effects of infused epinephrine on slow phase of O₂ uptake kinetics during heavy exercise in humans

We tested the hypothesis that infused epinephrine (Epi) would augment the slow phase of oxygen uptake (V̇O₂) during heavy exercise. Six normal healthy males initially performed a ramp test on a cycle ergometer to estimate the lactate threshold (LT) and determine peak V̇O₂. Each subject then performed two 20-min constant-load tests at a power output calculated to elicit a V̇O₂ equal to estimated LT + 0.2(peak V̇O₂ - estimated LT) under control conditions throughout and with an intravenous infusion of Epi from minutes 10 to 20 at a rate of 100 ng · kg^-1 · min^-1. Pulmonary gas exchange variables were determined breath by breath. Arterialized venous blood was repeatedly sampled from the dorsum of the heated hand. Epi infusion elevated (P < 0.05) plasma Epi concentration (i.e., from 420 ± 130 pg/ml at minute 10 to 2,190 ± 410 pg/ml at minute 20) but had no effect on plasma norepinephrine or K⁺ concentrations. Concentrations of blood lactate and pyruvate were increased, pH was decreased, and base excess became more negative by infusion of Epi (P < 0.05). Epi infusion increased (P < 0.05) CO₂ production and the respiratory exchange ratio but had no effect on ventilation or V̇O₂. V̇O₂ increased (P < 0.05) to the same extent in both control (3.14 ± 0.12 l/min at minute 10, 3.28 ± 0.12 l/min at minute 20) and Epi infusion (3.10 ± 0.11 l/min at minute 10, 3.25 ± 0.11 l/min at minute 20) trials. We therefore conclude that neither Epi nor its associated humoral consequences contribute significantly to the slow phase of V̇O₂ kinetics during heavy exercise.