Reflex cardiovascular and ventilatory responses to increasing H⁺ activity in cat hindlimb muscle

Static exercise increases arterial pressure, heart rate, and ventilation, effects which are believed in part to arise reflexly from a metabolic stimulus in the working muscle. In anesthetized cats, we tested the hypothesis that intra-arterial injections of lactic and hydrochloric acid, which created levels of these substances in muscle similar to those seen during contraction, reflexly increased cardiovascular and ventilatory function. Hydrochloric acid (32 and 57 mM; 1 ml) injected into the arterial supply of the triceps surae decreased intramuscular pH from 7.26 ± 0.05 to 7.17 ± 0.05 (P < 0.01) and reflexly increased arterial pressure (23 ± 7 mmHg; P < 0.01), heart rate (11 ± 2 beats/min; P < 0.001), and ventilation (187 ± 72 ml/min; P < 0.05). Static contraction of the triceps surae decreased intramuscular pH from 7.28 ± 0.06 to 7.13 ± 0.06 (P < 0.01). Lactic acid was more potent in causing reflexes than was equimolar HCl. For example, lactic acid containing 4 mM lactate and 0.87 mM H⁺ reflexly increased arterial pressure, heart rate, and ventilation, whereas 0.87 mM HCl did not. Intra-arterial sodium lactate (13 and 33 mM) at a neutral pH had no effect on these variables. We conclude that contraction-induced accumulation of H⁺, especially that arising from lactic acid, might provide a metabolic stimulus to evoke reflex autonomic effects.