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Apamin- and nitric oxide-sensitive biphasic non-adrenergic non-cholinergic inhibitory junction potentials in the rat anococcygeus muscle

S. Selemidis, J. Ziogas and T. M. Cocks

Changes in membrane potential following electrical field stimulation (EFS; 1, 2 and 5 pulses at 5 Hz, 0·5 ms duration, 60-80 V) of non-adrenergic non-cholinergic (NANC) inhibitory nerves in the rat isolated anococcygeus muscle were measured using standard intracellular recording techniques. Resting membrane potential ranged between -60 and -70 mV.

In the presence of guanethidine (30 µM), atropine (1 µM), propranolol (1 µM) and phentolamine (0·05 µM) to establish NANC conditions, the membrane potential depolarized to between -40 and -50 mV. Under these conditions, EFS caused pulse-dependent, tetrodotoxin (1 µM)-sensitive biphasic inhibitory junction potentials (IJPs) comprising a fast onset and time-to-peak phase followed by a second, slower phase that delayed repolarization. The duration of NANC IJPs ranged between 10 and 20 s.

Inhibition of small-conductance Ca²⁺-activated K⁺ channels with apamin (0·1 µM) selectively blocked the first fast phase of the NANC IJP, whereas inhibitors of large-conductance Ca²⁺-activated K⁺ channels (charybdotoxin and iberiotoxin) and ATP-sensitive K⁺ channels (glibenclamide) all had no effect on NANC IJPs.

Both the nitric oxide synthase inhibitor N ^G-nitro-L-arginine (L-NOARG; 100 µM) and the inhibitor of soluble guanylate cyclase 1-H-oxodiazol-[1,2,4]-[4,3-a] quinoxaline-1-one (ODQ; 10 µM) had no effect on the first fast phase of the NANC IJP. Each treatment, however, markedly inhibited the slow phase with the duration of the IJP reduced to between 1 and 3 s. The L-NOARG-resistant fast phase of the NANC IJP was almost abolished by the subsequent addition of apamin (0·1 µM).

In conclusion, the present study demonstrates unequivocal NANC nerve-mediated biphasic IJPs in the rat isolated anococcygeus. We propose that nitric oxide (NO), via activation of cGMP-dependent K⁺ channels, and a non-NO inhibitory factor which activates apamin-sensitive K⁺ channels contribute to NANC nerve-evoked IJPs in the rat anococcygeus.

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