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, noncholinergic (NANC) inhibitory junction potentials (IJPs) in circular smooth muscle cells of the guinea-pig proximal and distal colon were investigated using standard intracellular microelectrode recording techniques. We have confirmed that the NANC IJP, recorded in the presence of hyoscine (1 microM) and nifedipine (1 microM), can be divided into two components with apamin (250 nM), a blocker of the small conductance Ca2(+)-activated K+ channels. Both the apamin-sensitive and the apamin-resistant components of the IJP were blocked by tetrodotoxin (1.6 microM) or by lowering the external Ca2+ concentration (to 0.25 mM). The apamin-sensitive IJP was also blocked by omega-conotoxin GVIA (100 nM), a blocker of 'N-type' Ca2+ channels. The apamin-resistant IJP and rebound post-stimulus depolarization (PSD) were reduced upon exposure to either NG-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 membrane depolarization and a decrease in the membrane conductance in the absence of nerve stimulation. In the proximal colon, the apamin-resistant IJP and PSD could both be demonstrated to arise from an increase in the membrane conductance after subtraction of a non-linear background conductance. The hyperpolarization upon repetitive NANC nerve stimulation was mimicked by the NO donor, S-nitroso-L-cysteine (2.5-25 microM), which evoked a transient apamin-sensitive, but omega-conotoxin GVIA resistant, component followed by a slower apamin-resistant component. These results suggest that neurally-released NO has a number of actions in the guinea-pig colon, causing apamin-resistant hyperpolarization and depolarization, as well as directly opening apamin-sensitive K+ channels.