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 microseconds, 20 Hz, 3 pulses, 10-50 V) produced inhibitory junction potentials (i.j.ps), in the presence of atropine (1 microM) and guanethidine (1 microM). NO-Cys and SNP produced hyperpolarization of the membrane in the rat gastric fundus. L-NG-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 (1 mM) but not by D-arginine. 3. Oxyhaemoglobin (Oxy-Hb; 5 microM) inhibited these hyperpolarizations, although a higher concentration of Oxy-Hb was required to inhibit the SNP-induced hyperpolarization. Hydroquinone (50 microM) inhibited only the hyperpolarization induced by NO-Cys. 4. Apamin (1 microM) 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 microM) 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 microM), TEA (1 mM) and glibenclamide (5 microM) all failed to inhibit this hyperpolarization. 6. These results indicate that NO-Cys and EFS hyperpolarize the membrane by activating apaminsensitive 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.