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1 Institute of Sports Medicine, Copenhagen, Bispebjerg Hospital, Copenhagen, Denmark
2 Department of Human Biology and Nutritional Sciences, University of Guelph, Guelph, Ontario, Canada
3 Department of Integrative Biology, University of California-Berkeley, Berkeley, California, USA
4 Department of Medicine, McMaster University, Hamilton, Ontario, Canada
* To whom correspondence should be addressed. E-mail: b.miller{at}auckland.ac.nz.
An emerging technique used for the study of metabolic regulation is the elevation of lactate concentration with a sodium-lactate infusion, the lactate clamp (LC). However, hematological and acid-base properties affected by the infusion of hypertonic solutions containing the osmotically-active, strong ions sodium (Na+) and lactate (Lac-) are a concern for clinical and research applications of LC. In the present study we characterized the hematological and plasma acid-base changes during rest and prolonged, light to moderate intensity (55% VO2peak) exercise with and without LC. During the control (CON) trial, subjects were administered an isotonic, isovolumetric saline infusion. During LC, plasma lactate concentration ([Lac-]) was elevated to 4 meq l-1 during rest and to 4-7 meq l-1 during exercise. During LC at rest, there were rapid and transient changes in plasma, erythrocyte and blood volumes. LC resulted in decreased plasma [H+] (from 39.6 to 29.6 
eq l-1) at the end of exercise while plasma [HCO3-] increased from 26 to 32.9 meq l-1. Increased plasma strong ion difference [SID], due to increased [Na+], was the primary contributor to decreased [H+] and increased [HCO3-]. A decrease in plasma total weak acid concentration also contributed to these changes, while PCO2 contributed little. The infusion of hypertonic LC caused only minor volume, acid-base, and CO2 storage responses. We conclude that a LC infusion is appropriate for studies of metabolic regulation.
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