Proceedings of The Physiological Society
Kings College London (2005) J Physiol 565P, PC169
Effect of combined cholinergic-purinergic block on carotid body chemosensory activity in situ
Reyes, E P; Fernandez, R ; Larrain, C ; Zapata, P ;
1. Lab. Neurobiology, P. Catholic University of Chile, Santiago, Chile.
It has been reported that the combined administration of cholinergic and purinergic antagonists totally suppresses the response to hypoxia observed in co-cultures of glomus cells from carotid bodies (CBs) with sensory ganglion neurons from nodose-petrosal complex of rats (Zhang et al., 2000). This suggests that ACh and ATP are the excitatory co-transmitters involved in CB chemoexcitation by hypoxia, and would explain the failure of previous attempts to block CB chemoreceptor activity by separate applications of cholinergic or purinergic antagonists. We studied the effect of combined cholinergic-purinergic block on CB preparations in situ, in adult cats anaesthetized with sodium pentobarbitone 40 mg/kg I.P., and supplemented I.V. as required. Cats breathed spontaneously through a cannula inserted per os, connected to a pneumotachograph to measure airflows, which were integrated to obtain ventilatory volumes. The thyroid artery was retrogradely catheterized to perform intracarotid (I.C.) injections of ACh, nicotine and epibatidine, and ATP, β,γ-methylene-ATP and γ-S-ATP as cholinergic and purinergic agonists, respectively, and mecamylamine and suramine, as their respective antagonists, all given in 0.1 ml boluses. Changes in ventilatory tidal volume (VT) were studied in preparations where the ipsilateral carotid (sinus) nerve was intact (n=7); changes in the frequency of chemosensory nerve discharges (fχ) were recorded in preparations where this nerve had been cut (n=8). Hypoxic stimulation was performed by : (a) breathing 100% N2 for 5, 10 and 15 s, resulting in increases of VT to 138%, 195% and 232%, respectively; (b) breathing 17.5, 15.0, 12.5, 10.0, 7.5 and 5.0% O2 for 1 min periods, resulting in increases of fχ from basal levels of 55 ± 8 Hz (mean ± SEM) to maxima of 210 Hz. Cats were killed by an overdose of pentobarbitone given I.V. at the end of experiments. Nicotine (0.1-200 μg) evoked dose-dependent increases in VT (up to 280%) as in fχ (up to 580 Hz) were abolished by 7.5 mg I.C. mecamylamine, while ATP (1-1000 μg) induced dose-dependent increases in fχ (up to 96 Hz) and, at higher doses, abrupt rises in VT (up to 185%), were blocked by 1 mg I.C. suramine. The combined administration of both antagonists blocked entirely the chemosensory and reflex actions of their respective analogues, but failed to suppress the increases in chemosensory nerve discharges or the ventilatory reflex responses elicited by the hypoxic stimuli. Thus, present results indicate that the chemosensory response to hypoxia observed in CBs in situ is not exclusively mediated by co-activation of cholinergic and purinergic mechanisms.
Where applicable, experiments conform with Society ethical requirements