The present study used the physicochemical approach to characterize the changes in plasma electrolyte and acid–base states that occur in horses in response to feeding. Jugular venous blood was sampled every 0.5–2 h over a 24-h period from two groups (n = 4 and n = 5) of Standardbreds fed a mixed hay and grain ration at 8 am and 7 pm. One group of horses was studied in October, and one in December. The time course and magnitude of feeding responses differed between groups, and between the morning and evening meals. Feeding-induced changes in plasma electrolyte and acid–base variables occurred rapidly, within the first 1–3 h of meal consumption. The plasma acidosis associated with eating the meal was marked by increased plasma [H+] and decreased TCO2. The primary contributors to the increases in plasma [H+] were the decrease in the plasma concentration of strong ions ([SID]) and the pCO2. The increase in plasma total weak acid (protein) concentration ([Atot]) post-feeding had only a minor effect on the acid–base state. The feeding-induced acidosis abated 3–6 h after the meal, showing cyclical recovery of physicochemical variables that contributed to the acid–base disturbance. It is concluded that several key plasma electrolyte and acid–base parameters undergo significant, cyclical fluctuations in response to feeding in horses.

The present study used the physicochemical approach to characterize the changes in acid–base status that occur in Standardbred racehorses during recovery from high-intensity exercise. Jugular venous blood was sampled from nine Standardbreds in racing condition, at rest and for 2 h following a high-intensity training workout. Plasma [H+] increased from 39.1±1.0 neq l−1 at rest to 44.8±2.7 neq l−1 at 1 min of recovery. A decreased strong ion difference ([SID]) was the primary contributor to the increased [H+] immediately at the end of exercise, while increased plasma weak ion concentration ([Atot]) was a minor contributor to the acidosis. A decreased partial pressure of carbon dioxide (PCO2) at 1 min of recovery had a slight alkalinizing effect. The decreased [SID] at 1 min of recovery was a result of a 15.1±3.1 meq l−1 increase in [lactate−], as [Na+] and [K+] were also increased by 6.5±0.7 and 1.14±0.06 meq l−1, respectively, at 1 min of recovery. It is concluded that high-intensity exercise and recovery is associated with significant changes in acid–base balance, and that full recovery of many parameters that determine acid–base status requires 60–120 min.