Impeding O 2 unloading in muscle delays oxygen uptake response to exercise onset in humans

• Published in: American journal of physiology. Regulatory, integrative and comparative physiology Vol. 277; no. 5; pp. R1274 - R1281
• Main Authors: Hayashi, Naoyuki, Ishihara, Mutsuhisa, Tanaka, Ayumu, Yoshida, Takayoshi
• Format: Journal Article
• Language: English
• Published: United States 01.11.1999
• Subjects: hypocapnia; oxygen dissociation curve of hemoglobin; hyperpnea
• ISSN: 0363-6119; 1522-1490
• DOI: 10.1152/ajpregu.1999.277.5.R1274

We tested whether the leftward shift of the oxygen dissociation curve of hemoglobin with hyperpnea delays the oxygen uptake (V˙o ) response to the onset of exercise. Six male subjects performed cycle ergometer exercise at a work rate corresponding to 80% of the ventilatory threshold (VT) V˙o of each individual after 3 min of 20-W cycling under eupnea [control (Con) trial]. A hyperpnea procedure (minute ventilation = 60 l/min) was undertaken for 2 min before and during 80% VT exercise in hypocapnia (Hypo) and normocapnia (Normo) trials. In the Normo trial, the inspired CO fraction was 3% to prevent hypocapnia. The subjects completed two repetitions of each trial. To determine the kinetic variables ofV˙o and heart rate (HR) at the onset of exercise, a nonlinear least-squares fitting was applied to the data averaged from two repetitions by a monoexponential model. The end-tidal CO partial pressure before the onset of exercise was significantly lower in the Hypo trial than in the Con and Normo trials (22 ± 1 vs. 38 ± 3 and 36 ± 1 mmHg, respectively, P < 0.05). The time constant ofV˙o and HR was significantly longer in the Normo trial (28 ± 7 and 39 ± 18 s, respectively) than in the Con trial (21 ± 7, 34 ± 16 s, respectively, P < 0.05). TheV˙o time constant of the Hypo trial (37 ± 12 s) was significantly longer than that of the Normo trial, although no significant difference in the HR time constant was seen (Hypo, 41 ± 28 s). These findings suggested that respiratory alkalosis delayed the kinetics of oxygen diffusion in active muscle as a result of the leftward shift of the oxygen dissociation curve of hemoglobin. This supports an important role for hemoglobin-O offloading in setting the V˙o kinetics at exercise onset.