Interplay between PACAP and NO in mouse ileum

We investigated the possibility that pituitary adenylate cyclase activating peptide (PACAP) has a role in the control of contractility in the mouse ileum. PACAP-(1-27) produced tetrodotoxin (TTX)-insensitive, concentration-dependent reduction of the amplitude of the spontaneous contractions of longitudinal muscle up to their complete disappearance. This effect was inhibited by PACAP-(6-38), PACAP receptor antagonist, and by apamin, blocker of small-conductance Ca2+-activated K+-channels. Nomega-nitro-L-arginine methyl ester (L-NAME), nitric oxide (NO) synthase inhibitor, reduced the PACAP-inhibitory response, and the joint application of apamin plus L-NAME produced additive effects. 1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), inhibitor of NO-stimulated soluble guanylate cyclase, significantly reduced the effect of PACAP. Exogenous NO, given as sodium nitroprusside (SNP), induced a concentration-dependent suppression of the phasic contractions, which was unaffected by apamin but reduced by either PACAP-(6-38) or TTX. Neurally evoked muscular relaxation was deeply antagonised by L-NAME. PACAP-(6-38) induced a reduction of the response to EFS only in the absence L-NAME. In conclusion, our results suggest that PACAP controls smooth muscle contractility, acting directly on the muscle cells through PACAP-27 preferring receptors coupled to apamin-sensitive Ca2+-dependent K+-channels and indirectly through the stimulation of NO production. In turn, NO would stimulate the release of PACAP from inhibitory neurones.