Retention of intravenously infused [13C]bicarbonate is transiently increased during recovery from hard exercise

The effects of exercise on energy substrate metabolism persist into the postexercise recovery period. We sought to derive bicarbonate retention factors ( k) to correct for carbon tracer oxidized, but retained from pulmonary excretion before, during, and after exercise. Ten men and nine women received a primed-continuous infusion of [13C]bicarbonate (sodium salt) under three different conditions: 1) before, during, and 3 h after 90 min of exercise at 45% peak oxygen consumption (V̇o2peak); 2) before, during, and 3 h after 60 min of exercise at 65% V̇o2peak; and 3) during a time-matched resting control trial, with breath samples collected for determination of 13CO2 excretion rates. Throughout the resting control trial, k was stable and averaged 0.83 in men and women. During exercise, average k in men was 0.93 at 45% V̇o2peak and 0.94 at 65% V̇o2peak, and in women k was 0.91 at 45% V̇o2peak and 0.92 at 65% V̇o2peak, with no significant differences between intensities or sexes. After exercise at 45% V̇o2peak, k returned rapidly to control values in men and women, but following exercise at 65% V̇o2peak, k was significantly less than control at 30 and 60 min postexercise in men (0.74 and 0.72, respectively, P < 0.05) and women (0.75 and 0.76, respectively, P < 0.05) with no significant postexercise differences between men and women. We conclude that bicarbonate/CO2 retention is transiently increased in men and women for the first hour of postexercise recovery following endurance exercise bouts of hard but not moderate intensity.

Acid–base and electrolyte responses to low pH maintained by phosphate–citrate buffers in the bathing medium of the larval salamander Ambystoma tigrinum

Larval Ambystoma tigrinum were exposed to an external pH of 3.5 to 7.5 using phosphate – citrate buffers in the bathing medium. Blood analysis of cannulated Ambystoma tigrinum at pH 5.5, 4.5, and 3.5 indicated their ability to maintain relatively stable arterial pH at the two higher values; however, at pH 3.5, the blood pH diminished over the 12-h period before death. The greater stability of arterial pH at higher external pH is partially due to a reversal of an initial increase in arterial [Formula: see text]. This may not be due entirely to pulmonary excretion of CO2 as it also occurred in animals that were forced to exchange gases solely across the skin – gill unit by being deprived of access to an air space. This result suggests increased skin – gill perfusion and (or) ventilation as a mechanism for lowering arterial [Formula: see text]. Sodium transport across the skin of A. tigrinum was measured over a buffered pH range of 3.5 to 7.5. Na+ influx decreased from 1.0 ± 0.1 μequiv. 10 g−1 h−1 (mean ± SEM) at pH 7.0 to 0.1 ± 0.1 μequiv. 10 g−1 h−1 at pH 3.5. Na+ efflux increased to 38.1 ± 8.7 μequiv. 10 g−1 h−1 from 4.1 ± 0.9 μequiv. 10 g−1 h−1 as pH declined from 7.0 to 3.5. Calcium added to the buffer at pH 4.5 decreased Na+ efflux at that pH. Na+ fluxes measured in nonbuffered, low-pH solutions revealed qualitatively similar patterns with lower efflux rates and lower critical pH values.