STIMULATION OF PULMONARY C-FIBERS BY LACTIC-ACID IN RATS - CONTRIBUTIONS OF H+ AND LACTATE IONS

1. The contributions of H+ and lactate ions to the stimulation of sing le pulmonary C fibres by lactic acid were examined in anaesthetized an d artificially ventilated rats. 2. Lactic acid injected into the right atrium caused a transient decrease in arterial blood pH (pH(a)) and a short but intense burst of afferent activities in pulmonary C fibres, whereas sodium lactate had no effect. The fibre activity usually reac hed a peak within 1-1.5 s, with an onset latency of < 1 s, and returne d to the baseline in 5 s. 3. The injection of hydrochloric acid at the same pH as that of lactic acid did not significantly decrease pH(a), nor did it stimulate any C fibres studied.4. Formic acid has a pK(a) v alue (the negative logarithm of the dissociation constant) almost iden tical to that of lactic acid; thus, its injection decreased pH(a) to t he same degree as did the injection of lactic acid. However, the respo nse of C fibres to lactic acid was 134% stronger than that to formic a cid.5. We conclude that H+ is primarily responsible for the activation of pulmonary C fibres by lactic acid, probably through a direct effec t of H+ on these afferent endings. The lactate ion, by itself, does no t activate C fibres, but it seems to potentiate the stimulatory effect of H+ on these afferents.