NEURONAL MEDIATORS OF INHIBITORY JUNCTION POTENTIALS AND RELAXATION IN THE GUINEA-PIG INTERNAL ANAL-SPHINCTER

1. Inhibitory junction potentials (IJPs) and relaxations evoked in res ponse to field stimulation (supramaximal voltage, 0 . 1 ms, single sti mulus and 5 stimuli at 5-40 Hz) of non-adrenergic non-cholinergic (NAN C) nerves with atropine and phentolamine (each 1 mu M) Were measured i n the guinea-pig internal anal sphincter (gpIAS). The mean resting mem brane potential was -44 . 2 +/- 0 . 2 mV (n = 1119 cells from 260 prep arations). 2. NANC nerve stimulation evoked frequency-dependent IJPs ( 19 . 7 +/- 1 . 1 mV, n = 165, 33 tissues to a single stimulus) and rel axations. IJPs consisted of two tetrodotoxin (1 mu M)-sensitive compon ents: one was abolished by apamin (0 . 3 mu M) and the P2-purinoceptor antagonist suramin (100 mu M); the other, smaller in amplitude, was s ensitive to inhibitors of nitric oxide synthase (NOS, e.g. L-NAME, 100 mu M) and the nitric oxide (NO) scavenger oxyhaemoglobin (HbO, 10 mu M). 3. ATP (1 mM), vasoactive intestinal polypeptide (VIP, 0 . 01-0 . 25 mu M) and pituitary adenylate cyclase-activating peptide (PACAP(1-2 7), 0 . 84 mu M) each hyperpolarized and relaxed the gpIAS; only ATP r esponses resembled the evoked IJPs in time course. 4. The guanylyl cyc lase inhibitor LY83583 (10 mu M) abolished apamin-insensitive IJPs and relaxations. The cGMP phosphodiesterase inhibitor M&B 22948 (30 mu M) and 8-Br-cGMP (100 mu M) each hyperpolarized the gpIAS. 5. Two compon ents comprise the IJP and relaxation evoked in response to NANC nerve stimulation in the gpIAS. One, sensitive to apamin, resembles the resp onse to ATP and is modulated by purinoceptor antagonists; the other, a pamin and suramin insensitive, is inhibited by NO antagonists.