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


OBJECTIVE
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 VO2max. 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 (HCOP3-), 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 [HCO3l]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.


CONCLUSIONS
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.
Bayly
W M
WM
Department of Veterinary Clinical Sciences, College of Veterinary Medicine. Washington State University, Pullman, Washington 99164-6610, USA.
Kingston
J K
JK
Brown
J A
JA
Keegan
R D
RD
Greene
S A
SA
Sides
R H
RH
D000136:Acid-Base Equilibrium / Q000502:physiology*; D000818:Animals; D001639:Bicarbonates / Q000097:blood; D001774:Blood Chemical Analysis / Q000662:veterinary; D001784:Blood Gas Analysis / Q000662:veterinary; D002245:Carbon Dioxide / Q000097:blood; D002712:Chlorides / Q000097:blood; D004573:Electrolytes / Q000097:blood*; D004912:Erythrocytes / Q000378:metabolism*; D006400:Hematocrit / Q000662:veterinary*; D006736:Horses / Q000097:blood*; D006863:Hydrogen-Ion Concentration; D019344:Lactic Acid / Q000097:blood*; D010101:Oxygen Consumption; D010805:Physical Conditioning, Animal / Q000502:physiology*; D011188:Potassium / Q000097:blood; D012964:Sodium / Q000097:blood
D001639:Bicarbonates; D002712:Chlorides; D004573:Electrolytes; D002245:Carbon Dioxide; D019344:Lactic Acid; D012964:Sodium; D011188:Potassium