APAMIN-SENSITIVE AND INSENSITIVE COMPONENTS OF INHIBITORY JUNCTION POTENTIALS IN RAT CECUM - ROLE OF NITRIC-OXIDE

1 The non-adrenergic non-cholinergic (NANC) inhibitory response to ele ctrical field stimulation (EFS) in circular muscle from rat caecum was investigated using the single sucrose-gap technique. EFS with single pulses evoked hyperpolarization oral inhibitory function potential (IJ P) of the membrane associated with muscular relaxation or with transie nt inhibition of spontaneous contractile activity. 2 The amplitude and the duration of the IJPs were enhanced by using train stimulation at increasing frequency. 3 Apamin (10(-7) M) reduced the amplitude of IJP s at all frequencies tested. 4 N-omega-nitro-L-arginine methyl ester ( L-NAME) (10(-4) M, 5 x 10(-4) M), but not D-NAME, caused a concentrati on dependent decrease in the amplitude of IJPs at all frequencies test ed. L-Arginine (10(-3) M) prevented these effects. 5 L-NAME (5 x 10(-4 ) M) caused the disappearance of the apamin-resistant IJP-component, e voked by single pulse or by low frequency trains. 6 Sodium nitroprussi de (SNP) (10(-4) M), a nitric oxide (NO) donor, induced hyperpolarizat ion of membrane potential and muscular relaxation. SNP-induced effects were not affected by pretreatment of the muscle strips with effective concentrations of tetrodotoxin, apamin, and L-NAME. 7 P-2-purinergic antagonists, reactive blue 2 (up to 5 x 10(-4) M) and suramin (up to 3 x 10(-4) M), failed to affect the evoked IJPs. 8 These results show t hat, in rat caecum, the NANC response to electrical stimulation is com posed of two distinguishable components: an apamin-resistant and an ap amin-sensitive component. NO or a related compound is mainly involved in the mediation of the apamin-resistant component, while ATP is not t he mediator responsible for the apamin-sensitive component.