S-NITROSOCYSTEINE, BUT NOT SODIUM-NITROPRUSSIDE, PRODUCES APAMIN-SENSITIVE HYPERPOLARIZATION IN RAT GASTRIC FUNDUS

1 To investigate the pharmacological properties of the membrane hyperpolarization induced by electrical field stimulation (EFS), sodium nitroprusside (SNP) and S-nitrosocysteine (NO-Cys) in circular smooth muscle cells of the rat gastric fundus (forestomach), the effects of various potassium channel blockers on these hyperpolarizations were investigated. 2 EFS (50 mus, 20 Hz, 3 pulses, 10-50 V) produced inhibitory junction potentials (i.j.ps), in the presence of atropine (1 muM) and guanethidine (1 muM). NO-Cys and SNP produced hyperpolarization of the membrane in the rat gastric fundus. L-N(G)-nitroarginine (L-NNA) inhibited the i.j.ps, but not the hyperpolarization induced by NO-Cys and SNP. This inhibitory action of L-NNA on the i.j.ps was partly reversed by subsequent application Of L-arginine (I mm) but not by D-arginine. 3 Oxyhaemoglobin (Oxy-Hb; 5 muM) inhibited these hyperpolarizations, although a higher concentration of Oxy-Hb was required to inhibit the SNP-induced hyperpolarization. Hydroquinone (50 muM) inhibited only the hyperpolarization induced by NO-Cys. 4 Apamin (1 muM) partly inhibited i.j.ps and NO-Cys-induced hyperpolarization, but not the SNP-induced hyperpolarization. Tetraethylammonium (TEA; 1 mM), 4-aminopyridine (4-AP; 1 mM) or glibenclamide (1 muM) did not affect hyperpolarization induced by NO-Cys and SNP. 5 8-Bromo cyclic guanosine 3':5'-monophosphate (1 mM) also produced hyperpolarization. Apamin (1 muM), TEA (1 mM) and glibenclamide (5 muM) all failed to inhibit this hyperpolarization. 6 These results indicate that NO-Cys and EFS hyperpolarize the membrane by activating apamin-sensitive and TEA-resistant K+ channels and favour the hypothesis that a NO-liberating substance may act as a neurotransmitter in non-adrenergic, non-cholinergic (NANC) neurones in the rat forestomach. Our results also suggest that increase in cyclic GMP may cause apamin-resistant hyperpolarization but the apamin-sensitive hyperpolarization is mediated by another mechanism.