Excitatory purinergic neurotransmission in smooth muscle of guinea-pig taenia caeci

Abstract

Non-adrenergic, non-cholinergic (NANC) inhibitory neurotransmission has been an area of intense interest in gut motor physiology, whereas excitatory NANC neurotransmission has received less attention. In order to further explore excitatory NANC neurotransmission, we performed conventional intracellular recordings from guinea-pig taenia caeci smooth muscle. Tissue was perfused with oxygenated Krebs solution at 35°C and nerve responses evoked by either oral or aboral nerve stimulation (NS) (4 square wave pulses, 0.3 ms duration, 20 Hz). Electrical activity was characterized by slow waves upon which one to three action potentials were superimposed. Oral NS evoked an inhibitory junction potential (IJP) at either the valley or peak of the slow wave. Application of nifedipine (1 μM) abolished slow waves and action potentials, but membrane potential flunctuations (1-3 mV) and IJPs remained unaffected. Concomitant application of apamin (300 nM), a small-conductance Ca2+-activated K+ channel blocker, converted the IJP to an EJP that was followed by slow IJP. Further administration of NG-nitro-L-arginine methyl ester (L-NAME, 200 μM), a nitric oxide synthase inhibitor, abolished the slow IJP without affecting the EJP, implying that the slow IJP is due to nitrergic innervation. The EJP was abolished by tetrodotoxin (1 μM), but was not significantly affected by atropine (3 μM) and guanethidine (3 μM) or hexamethonium (500 μM). Substance P (SP, 1 μM) desensitization caused slight attenuation of the EJP, but the EJP was abolished by desensitization with α,β-methylene ATP (50 μM), a P2 purinoceptor agonist that is more potent than ATP at the P2X receptor subtype, suramin (100 μM), a non-selective P2 purinoceptor antagonist, and pyridoxal-phosphate-6-azophenyl-2′,4′-disulphonic acid (PPADS, 100 μM), a selective P2X purinoceptor antagonist. In contrast, the EJP was unaffected by MRS-2179 (2 μM), a selective P2Y1 receptor antagonist. Aboral NS evoked an apamin- and L-NAME-sensitive IJP, but virtually no NANC EJP. These data suggest the presence of polarized excitatory purinergic neurotransmission in guinea-pig taenia caeci, which appears to be mediated by P2X purinoceptors, most likely the P2X1 subtype. © The Physiological Society 2005.