[Effects of low pH and nicotine on carotid body chemoreceptor activity in the rabbit]

The unit discharges of carotid chemosensory afferent fibers were recorded in the in vitro carotid body (CB)--sinus nerve preparations (n = 55) from 29 rabbits. The results were as follows: (1) The discharge of all the units increased from 0.86 +/- 0.21 to 1.75 +/- 0.40 imp/s (P < 0.001) upon lowering the pH of modified Locke solution. (2) Addition of nicotine at doses of 1, 3, 6, 10 and 15 micrograms/ml to the solution led to a dose-dependent increase in the discharge from 0.60 +/- 0.21 to 0.96 +/- 0.21, 1.19 +/- 0.30. 1.24 +/- 0.29, 1.48 +/- 0.41 and 1.82 +/- 0.39 imp/s, respectively (r = 0.94, P < 0.001, n = 10). Nicotinic stimulation was antagonized by D-tubocurarine but the basal discharge was not significantly affected. (3) Superfusion of the CB with acid solution increased the discharge from 0.95 +/- 0.34 to 1.84 +/- 0.55 imp/s (P < 0.01, n = 19); After injections of nicotine 1-5 micrograms/ml to acid perfusate the unit discharge showed an increase from 0.96 +/- 0.25 to 1.53 +/- 0.24 imp/s. The degree of increase was less as compared with that due to mere acidification (P < 0.05). (4) D-tubocurarine did not alter the action of acid solution on chemosensory discharge (P > 0.05, n = 17). The present results suggest that ACh might act only as a modulator on carotid chemosensory activity without involving N-cholinergic receptor directly as did by low pH perfusion of CB.