Effects of combined cholinergic–purinergic block upon cat carotid body chemoreceptors in vitro

Since acetylcholine (ACh) and ATP have been proposed as excitatory co-transmitters at synapses between glomus cells and sensory nerve endings of the carotid body (CB), we tested such hypothesis by studying the effects of combined cholinergic–purinergic block on the chemosensory activity recorded from cat's carotid bodies perfused and/or superfused in vitro. The preparations were bathed with Tyrode's solution, either normoxic (PO2 = 98.5 ± 13.5 Torr) or hypoxic (PO2 = 31.8 ± 5.2 Torr), and the frequency of chemosensory impulses (fχ) was recorded from the carotid (sinus) nerve. Dose–response curves for fχ increases evoked by intra-stream boluses of acetylcholine, nicotine and ATP were studied. A combination of mecamylamine 2 μM and suramin 50 μM, applied through the perfusate or superfusate, suppressed nicotine- and ATP-induced increases in fχ, but the basal chemosensory activity in normoxia and the chemosensory excitation elicited by hypoxic superfusion were preserved, although variably reduced in most preparations. Thus, in spite of the excitatory effects provoked by applying ACh and ATP to the perfused/superfused CB in vitro, a co-release of these substances cannot account entirely for the chemosensory excitation induced by hypoxic stimulation of the CB.