Changes in arterial, mixed venous and intraerythrocytic concentrations of ions in supramaximally exercising horses

Reasons for performing study: Horses experience major perturbations in acid-base balance during supramaximal exercise. Ion movement in and out of erythrocytes (RBCs) is believed to be important in maintaining acid-base balance but it is unclear as to the extent to which this happens, nor how it affects single measurements of ion concentrations in arterial and venous blood. Objectives: To clarify the role RBCs play in mitigating perturbations in acid-base balance during high speed exercise in horses, and to describe associated differences in arterial (a) and mixed venous (v) concentrations of key ions. Methods: Six exercise-trained Thoroughbreds galloped to fatigue at speeds calculated to have an oxygen demand that was 115% of the VO_2max. Blood samples (a and v) were collected pre-exercise, during warm-up, at fatigue, and immediately post exercise. Packed cell volume (PCV), pH, PCO₂, and plasma concentrations of bicarbonate (HCO₃^-), chloride (Cl^-), sodium (Na⁺), potassium (K⁺), and lactate (Lac^-) and strong ion difference (SID) were determined, and RBC concentrations of Lac^- and electrolytes calculated for each sample. Data were analysed using a 2-way ANOVA for repeated measures testing for effects of sampling time and site (P<0.05). Results: Plasma and RBC [Cl^-] were increased with hypercapnoea and acidaemia. [HCO₃^-]v was greater than pre-exercise values at fatigue, although [HCO₃^-]a was lower. Hyperkalaemia and decreased RBC [K⁺] were evident at fatigue, as was an increased RBC [Na⁺]. Plasma [K⁺] started to decrease as soon as exercise ceased and Na⁺ began to move back onto RBCs in exchange for K⁺. Concentrations of all measures of Lac^- rose from fatigue to post exercise. The SID decreased with exercise and was higher in v at fatigue and post exercise, reflecting the decrease in pH. Conclusions: RBCs act as a repository for lactate, and therefore the increase in PCV facilitates the maintenance of the muscle to plasma Lac^- diffusion gradient during exercise. Potential relevance: This serves to keep intramuscular [Lac^-] lower than it would otherwise be and, because of the link between Lac^- accumulation, pH decrease and the onset of fatigue, may help delay the onset of fatigue.