Exercise in the heat: effects of saline or bicarbonate infusion

Adult male rats (n = 17/group, 300-320 g, physically untrained) were exercised (9.14 m/min) in the heat (35 degrees C) to hyperthermic exhaustion (rectal temperature 43 degrees C) after infusion of 2 ml of 7.5% sodium bicarbonate (BIC) or 2 ml of 0.9% sodium chloride (SAL). Rats receiving BIC or SAL had no effects on endurance when compared with rats receiving no exogenous fluid [control (CON]), whereas the rate of heat gain, was significantly increased in the BIC-treated group. After exercise, the BIC group manifested significantly decreased hematocrit and plasma protein levels but exaggerated increments in plasma osmolality. Lactate levels were significantly increased in all three groups with no notable intergroup differences. Although venous blood pH and bicarbonate levels were decreased after exercise in the SAL and CON groups, they were unchanged in the BIC group. Although venous PCO2 was unaffected by fluid administration in all three groups, venous PO2 was significantly increased after exercise in the heat in all groups. We concluded from these experiments that even though BIC infusion prevented the acidosis and hypobicarbonatemia induced by exercise in the heat to hyperthermic exhaustion, the hypertonicity and Na+ content of the bicarbonate solution may have increased the rate of heat gain and precluded beneficial effects on physical performance.