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

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Abstract

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 V̇O2max. 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.

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