Functional coupling between nitric oxide synthesis and VIP release within enteric nerve terminals of the rat: involvement of protein kinase G and phosphodiesterase 5

1. The subcellular mechanisms involved in the effect of nitric oxide (NO) o n the release of vasoactive intestinal poly peptide (VIP) were examined in synaptosomes isolated from rat small intestine. 2. VIP release was stimulated by the NO donor SNAP (10(7)-10(-4) M) in an o xyhaemoglohin-sensitive manner. The presence of the guanylate cyclase inhib itor ODQ (10(-5) M), or inhibition of protein kinase G (PKG) by KT 5828 (3 x 10(-6) M) or Rp-8Br-PET-cGMPS (5 x 10(-7) M), antagonized the SNAP-induce d VIP release, suggesting a regulatory role of PKG, confirming previously p ublished data from enteric ganglia. This finding was further supported by t he fact that. direct PKG activation by the stable cGMP analogue 8-pCPT-cGMP stimulated VIP secretion to the same extent as SNAP. 3. Basal VIP secretion was enhanced in the presence of zaprinast, an inhibi tor of cGMP-dependent phosphodiesterase 5 (PDE 5), suggesting a functional role of PDE 5 in NO-cGMP signalling. Supportive evidence for this finding w as obtained by demonstration of the presence of PDE 5 using RT-PCR. 4. Stimulation of endogenous NO production by L-arginine was also effective in releasing VIP. The effect was abolished in the presence of KT 5823, but was insensitive to oxyhaemoglobin (10(-3) M), suggesting that, an interact ion between NO and VIP is likely to occur within the same nerve terminal ra ther than between terminals. 5. NO synthesis was not, affected by VIP (10(-8)-10(-5) M), suggesting that there is no feedback regulation between the NO and the VIP pathways. 6. These findings support, the notion that an anatomical and functional int errelationship exists between NO and VIP in enteric nerve terminals and tha t complex signalling mechanisms involving PKG and PDE 5 contribute to NO-in duced VIP release.