1. Nitrergic neurons regulate gastrointestinal (GI) activity and their dysfunction has been associated with various GI diseases. Nitric oxide (NO) typically relaxes GI smooth muscle, but nitrergic contractions also occur. Although guanylate cyclase is well established as mediating nitrergic GI relaxation, its role in contraction remains uncertain.
2. We used electrical field stimulation (EFS; 0.3 msec pulses, three trains of 1.2 s width, 2 Hz, at 30 s intervals) to evoke biphasic contraction–relaxation responses in rat ileum strips (longitudinal muscle–myenteric plexus preparations), mediated by the endogenous nitrergic transmitter, under non‐adrenergic, non‐cholinergic (NANC) conditions (1 μmol/L atropine and 4 μmol/L guanethidine).
3. All EFS responses were abolished by tetrodotoxin (1 μmol/L). Inhibition of NO synthase with Nω‐nitro‐l‐arginine‐methyl‐ester (l‐NAME; 100 and 300 μmol/L) prevented both EFS‐evoked contractions and relaxations. l‐Arginine (3 mmol/L) reversed l‐NAME inhibition, primarily restoring contractions and suggesting that these require lower nitrergic transmitter levels than relaxations.
4. Pretreatment of preparations with subrelaxant concentrations of sodium nitroprusside (1 μmol/L) selectively desensitized EFS‐evoked contractions without affecting relaxations, suggesting different downstream mechanisms. Nevertheless, the selective guanylate cyclase inhibitor 1H‐[1,2,4]oxadiazolo[4,3‐a]quinoxalin‐1‐one (3 and 10 μmol/L) inhibited both nitrergic contractions and relaxations, indicating that guanylate cyclase activation is required for both responses.
5. The results of the present study support the hypothesis that the endogenous nitrergic transmitter differentially regulates guanylate cyclase, leading to either contractions or relaxations depending on its concentrations, thus providing additional insight into the regulation of ileum contractility by nitrergic activity.