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Division of Neuroscience and Biomedical Systems, Glasgow University, UK.

1. Inhibitory junction potentials (IJPs) and relaxations evoked in response to field stimulation (supramaximal voltage, 0.1 ms, single stimulus and 5 stimuli at 5-40 Hz) of non-adrenergic non-cholinergic (NANC) nerves with atropine and phentolamine (each 1 microM) were measured in the guinea-pig internal anal sphincter (gpIAS). The mean resting membrane potential was -44.2 +/- 0.2 mV (n = 1119 cells from 260 preparations). 2. NANC nerve stimulation evoked frequency-dependent IJPs (19.7 +/- 1.1 mV, n = 165, 33 tissues to a single stimulus) and relaxations. IJPs consisted of two tetrodotoxin (1 microM)-sensitive components: one was abolished by apamin (0.3 microM) and the P2-purinoceptor antagonist suramin (100 microM); the other, smaller in amplitude, was sensitive to inhibitors of nitric oxide synthase (NOS, e.g. L-NAME, 100 microM) and the nitric oxide (NO) scavenger oxyhaemoglobin (HbO, 10 microM). 3. ATP (1 mM), vasoactive intestinal polypeptide (VIP, 0.01-0.25 microM) and pituitary adenylate cyclase-activating peptide (PACAP(1-27), 0.84 microM) each hyperpolarized and relaxed the gpIAS; only ATP responses resembled the evoked IJPs in time course. 4. The guanylyl cyclase inhibitor LY83583 (10 microM) abolished apamin-insensitive IJPs and relaxations. The cGMP phosphodiesterase inhibitor M&B 22948 (30 microM) and 8-Br-cGMP (100 microM) each hyperpolarized the gpIAS. 5. Two components comprise the IJP and relaxation evoked in response to NANC nerve stimulation in the gpIAS. One, sensitive to apamin, resembles the response to ATP and is modulated by purinoceptor antagonists; the other, apamin and suramin insensitive, is inhibited by NO antagonists.

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