PACAP and nitric oxide inhibit contractions in the proximal intestine of the atlantic cod, Gadus morhua.

Abstract

The possible inhibitory roles of pituitary adenylate cyclase-activating polypeptide (PACAP), vasoactive intestinal polypeptide (VIP) and nitric oxide in the control of intestinal motility were investigated in the Atlantic cod, Gadus morhua. Circular and longitudinal smooth muscle preparations developed spontaneous contractions that were inhibited by atropine (10(-)(5 )mol l(-)(1)). PACAP 27 and PACAP 38 (10(-)(7 )mol l(-)(1)) reduced the amplitude of the contractions but did not usually affect the resting tension. In the circular preparations, the mean active force developed (above resting level; +/- s.e.m.) was reduced from 0. 62+/-0.18 mN to 0.03+/-0.03 mN (N=10) by PACAP 27 and from 0.53+/-0. 20 mN to 0.31+/-0.13 mN (N=7) by PACAP 38, while neither cod nor mammalian VIP (10(-)(10)-10(-)(6 )mol l(-)(1)) had any effect. In the longitudinal preparations, PACAP 27 reduced the force developed from 1.58+/-0.22 mN to 0.44+/-0.25 mN (N=8) and PACAP 38 reduced it from 1.61+/-0.47 mN to 0.75+/-0.28 mN (N=5). The nitric oxide donor sodium nitroprusside (NaNP) almost abolished the contractions in the circular preparations, reducing the mean force developed from 0. 47+/-0.05 mN to 0.02+/-0.06 mN (10(-)(6 )mol l(-)(1); N=9) and 0+/-0. 07 mN (10(-)(5 )mol l(-)(1); N=8). In the longitudinal preparations, NaNP reduced the force developed from 2.03+/-0.36 mN to 0.33+/-0.22 mN (10(-)(6 )mol l(-)(1); N=8) and 0.19+/-0.30 mN (10(-)(5 )mol l(-)(1); N=8). The L-arginine analogue N(G)-nitro-L-arginine methyl ester (L-NAME; 3x10(-)(4 )mol l(-)(1)) enhanced the contractions in both circular and longitudinal preparations, increasing the mean force developed from 0.51+/-0.12 mN to 0.94+/-0.21 mN (N=8) and from 1.49+/-0.36 mN to 3.34+/-0.67 mN (N=7), respectively. However, preincubation with L-NAME before a second addition of PACAP 27 (10(-)(7 )mol l(-)(1)) did not affect the response to PACAP, neither did preincubation with the guanylate cyclase inhibitor 6-anilinoquinoline-5,8-quinone (LY83583; 10(-)(5 )mol l(-)(1)), while the inhibitory response to NaNP (3x10(-)(7 )mol l(-)(1)) was abolished by LY83583. The PACAP analogue PACAP 6-27 (3x10(-)(7 )mol l(-)(1)) had no effect on the response to either NaNP (3x10(-)(7 )mol l(-)(1)) or PACAP 27 (10(-)(8 )mol l(-)(1)) in the circular preparations. These findings indicate the presence of both a cholinergic and a nitrergic tonus in the smooth muscle preparations of the cod. Although PACAP and NaNP both inhibit contractions, there is no evidence of any interactions between the two substances. In addition, NaNP, but not PACAP, probably acts via stimulating the production of cyclic GMP. In conclusion, both PACAP and nitric oxide may act as inhibitory transmitters, using distinct signalling pathways, in the control of intestinal motility in the Atlantic cod.