Influence of acid-base status on plasma catecholamines during exercise in normal humans

The influence of acid-base status on plasma catecholamines during exercise was investigated in six healthy volunteers. Incremental exercise to 175 W was performed on a bicycle ergometer under four conditions: 1) control, 2) during forced hyperventilation (HV), 3) after pretreatment with acetazolamide (AZE), and 4) while breathing 4% CO₂. Resting plasma norepinephrine (PNE) and epinephrine (PE) were not different among the four protocols despite higher resting pH during HV and lower resting pH after AZE [control, 7.4 ± 0.02; HV, 7.48 ± 0.03 (P < 0.005); AZE, 7.36 ± 0.01 (P < 0.005) (P values indicate significant differences from the control protocol)]. Resting pH was not different from control during the 4% CO₂ study (7.4 ± 0.01). At the 175-W exercise load, there were significant differences in both pH and PNE. During the control test, pH was 7.38 ± 0.02, PNE was 951 ± 164 pg/ml, and PE was 264 ± 132 pg/ml. During HV, pH was 7.46 ± 0.5 (P < 0.001), PNE was 976 ± 67 pg/ml, and PE was 210 ± 27 pg/ml. After AZE, pH was 7.31 ± 0.2 (P < 0.001), PNE was 1,866 ± 561 pg/ml (P < 0.005), and PE was 382 ± 264 pg/ml. While subjects breathed 4% CO₂, pH was 7.29 ± 0.02 (P < 0.001), PNE was 1.842 ± 617 pg/ml (P < 0.01), and PE was 467 ± 275 pg/ml. There were no differences in heart rate, O₂ consumption, or CO₂ production during exercise among the four groups. Also, in an additional five subjects studied under control conditions, during 4% Co₂, and with AZE, no differences in exercise systolic pressure at 175 W were evident. These results demonstrate that, at a constant power level, acidosis significantly enhances the response of PNE to dynamic exercise in normal humans, whereas alkalosis has no effect. A similar but less consistent effect is present for PE. These results may have implications for mechanisms known to influence sympathetic stimulation during exercise and indicate that knowledge to acid-base status is crucial in the interpretation of PNE and PE during exercise in health and perhaps also in disease.