The central respiratory effects of acetylcholine vary with CSF pH.

Hydrogen ion concentration [H+] centrally is a major determinant of ventilation. Its action involves central cholinergic mechanisms. The point(s) where increased [H+] induces its changes in the cholinergic system is unclear. If H+ acts presynaptically by increasing endogenous ACh synthesis and release, its effect should be absent when ACh is supplied exogenously. If H+ acts postsynaptically by changing ACh degradation or ACh receptor sensitivity, its effect should persist in the presence of exogenous ACh. We perfused the brain ventricular system in spontaneously breathing anesthetized dogs with progressively higher concentrations of ACh (0-52.8 mM) in cerebrospinal fluid (CSF) at pH 7.4 and CSF pH 7.1. Increasing concentrations of ACh increased ventilation > 4-fold in a linear manner in the presence of non-acidic and acidic CSF. With acidic CSF the ACh ventilatory response line was shifted to a higher y-intercept, resulting in a higher ventilation at any [ACh]. These findings are consistent with the hypothesis that central acidosis augments ventilation by postsynaptic cholinergic events.