CHARACTERIZATION OF THE MEMBRANE CONDUCTANCE CHANGES UNDERLYING THE APAMIN-RESISTANT NANC INHIBITORY JUNCTION POTENTIAL IN THE GUINEA-PIG PROXIMAL AND DISTAL COLON

The nature of the electrically- or stretch-evoked nonadrenergic, nonch olinergic (NANC) inhibitory junction potentials (IJPs) in circular smo oth muscle cells of the guinea-pig proximal and distal colon were inve stigated using standard intracellular microelectrode recording techniq ues. We have confirmed that the NANC LIP, recorded in the presence of hyoscine (1 mu M) and nifedipine (1 mu M), can be divided into two com ponents with apamin (250 nM), a blocker of the small conductance Ca2+- activated K+ channels, Both the apamin-sensitive and the apamin-resist ant components of the IJP were blocked by tetrodotoxin (1.6 mu M) or b y lowering the external Ca2+ concentration (to 0.25 mM). The apamin-se nsitive IJP was also blocked by omega-conotoxin GVIA (100 nM), a block er of 'N-type' Ca2+ channels. The apamin-resistant IJP and rebound pos t-stimulus depolarization (PSD) were reduced upon exposure to either N -G-L-arginine (NOLA), an inhibitor of nitric oxide synthase (NOS), or the nitric oxide (NO) scavenger, haemoglobin. The effects of NOLA were partially reversed in the presence of excess L-arginine, a substrate for NOS, suggesting that NO, or a related NO-donor compound, is likely to be the apamin-resistant inhibitory transmitter, Blockade of either the apamin-sensitive or apamin-resistant IJP was associated with memb rane depolarization and a decrease in the membrane conductance in the absence of nerve stimulation. In the proximal colon, the apamin-resist ant IJP acid PSD could both be demonstrated to arise from an increase in the membrane conductance after subtraction of a non-linear backgrou nd conductance. The hyperpolarization upon repetitive NANC nerve stimu lation was mimicked by the NO donor, S-nitroso-L-cysteine (2.5-25 mu M ), which evoked a transient apamin-sensitive, but omega-conotoxin. GVI A resistant, component followed by a slower apamin-resistant component . These results suggest that neurally-released NO has a number of acti ons in the guinea-pig colon, causing apamin-resistant hyperpolarizatio n and depolarization, as well as directly opening apamin-sensitive Kchannels.