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

Abstract

We tested the hypothesis that infused epinephrine (Epi) would augment the slow phase of oxygen uptake (VO2) 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 VO2. Each subject then performed two 20-min constant-load tests at a power output calculated to elicit a VO2 equal to estimated LT + 0.2(peak VO2--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 VO2. VO2 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 concluded that neither Epi nor its associated humoral consequences contribute significantly to the slow phase of VO2 kinetics during heavy exercise.