Co‐release of ATP and ACh mediates hypoxic signalling at rat carotid body chemoreceptors

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Abstract
1 Using functional co-cultures of rat carotid body (CB) O2 chemoreceptors and ‘juxtaposed’ petrosal neurones (JPNs), we tested whether ATP and ACh acted as co-transmitters. 2 Perforated-patch recordings from JPNs often revealed spontaneous and hypoxia-evoked (PO2≈5 mmHg) excitatory postsynaptic responses. The P2X purinoceptor blocker, suramin (50 μM) or a nicotinic ACh receptor (nAChR) blocker (hexamethonium, 100 μM; mecamylamine, 1 μM) only partially inhibited these responses, but together, blocked almost all activity. 3 Under voltage clamp (-60 mV), fast perfusion of 100 μM ATP over hypoxia-responsive JPNs induced suramin-sensitive (IC50= 73 μM), slowly-desensitizing, inward currents (IATP) with time constant of activation τon= 30.6 ± 4.8 ms (n= 7). IATP reversed at 0.33 ± 3.7 mV (n= 4), and the dose-response curve was fitted by the Hill equation (EC50= 2.7 μM; Hill coefficient ≈0.9). These purinoceptors contained immunoreactive P2X2 subunits, but their activation by α,β-methylene ATP (α,β-meATP; EC50= 2.1 μM) suggests they are P2X2/P2X3 heteromultimers. 4 Suramin and nAChR blockers inhibited the extracellular chemosensory discharge in the intact rat carotid body-sinus nerve preparation in vitro. Further, P2X2 immunoreactivity was widespread in rat petrosal ganglia in situ, and co-localized in neurones expressing the CB chemo-afferent marker, tyrosine hydroxylase (TH). P2X2 labelling in the CB co-localized with nerve-terminal markers, and was intimately associated with TH-positive type 1 cells. 5 Thus ATP and ACh are co-transmitters during chemotransduction in the rat carotid body.