The interstitial pH of the working gastrocnemius muscle of the dog

Summary

In isolated gastrocnemius muscles from 19 dogs the interstitial H⁺ activity ([H⁺]_int) was measured with bulb-type buffer-filled glass minielectrodes. The muscles were working isotonically and perfused with blood. In addition arterial and venous pH, venous O₂ saturation, muscle temperature, and blood flow were measured continuously at rest, during 12 min of sustained exercise, and in the recovery period. Lactate (LA⁻) release and O₂ consumption were calculated by the Fick principle. The experiments were performed under normal acid-base conditions and during artificially induced metabolic acidosis and alkalosis.

• 1. In normal acid-base balance [H⁺]_int at rest was 54±3.3 neq/l (=pH 7.27), while venous H⁺ ([H⁺]_ven) was 45±4.7 neq/l (=pH 7.34) A[H⁺] gradient was always observed between interstitial fluid and venous blood.

• 2. Immediately after onset of exercise [H⁺]_int decreased transiently. After about 15 s [H⁺]_int increased rapidly up to values of 105±7 neq/l (=pH 6.98). In the recovery period [H⁺]_int diminished and reached control values after about 20–30 min. [H⁺]_ven increased up to 74.4±8.1 neq/l (=pH 7.13). Maximal gradients between [H⁺]_int and [H⁺]_ven were 36 neq/l (=pH 0.2).

• 3. During repeated exercise the decrease in [H⁺]_int at the onset of exercise was more extensive, while the subsequent increase was lowered. These changes correspond to a smaller LA⁻ release.

• 4. During metabolic alkalosis at the onset of exercise [H⁺]_int decreased less, during metabolic acidosis more than under normal acid-base conditions. Thereafter during metabolic alkalosis maximal values of 95.4±12 neq/l (=pH 7.03), during metabolic acidosis of 180±8.6 neq/l (=pH 6.74) were reached. This led to [H⁺] gradients between interstitial fluid and venous blood which were much higher in metabolic acidosis than in normal acid-base balance or in metabolic alkalosis. In metabolic acidosis [H⁺]_int decreased very slowly during recovery.

• 5. During metabolic acidosis the muscle fatigues more rapidly than during metabolic alkalosis or during normal acid-base conditions.

It is concluded that the H⁺ activity measured is that within the interstitial space. Exercise hyperemia is not caused by changes of [H⁺]_int. Mechanisms are discussed which may explain H⁺ gradients between interstitial fluid and venous blood and rapid changes of [H⁺]_int at the onset of exercise.