REGULATION OF VIP RELEASE FROM RAT ENTERIC NERVE-TERMINALS - EVIDENCEFOR A STIMULATORY EFFECT OF NO

The basal release of vasoactive intestinal polypeptide (VIP) from fres hly prepared enriched synaptosomes was 159.1 +/- 17.3 fmol/mg protein (100%), which constituted 2.5% of the total VIP content. Basal VIP rel ease was reduced by 65% by removal of external Ca2+. Release of VIP wa s stimulated by depolarization with KCl (65 mM, 143%) and in the prese nce of veratridine (10(-6) M, 184%), monensin (10(-5) M, 131%), and th e Ca2+ ionophore A-23187 (10(-6) M, 160%). Stimulation of adenosine 3' ,5'-cyclic monophosphate (cAMP)-dependent mechanisms using isoproteren ol (10(-6)-10(-4) M) and forskolin (10(-6) and 10(-5) M) had no stimul atory influence on VIP release. In contrast, sodium nitroprusside (10( -4) M, 198%), the nitric oxide (NO) donor 3-(morpholino)sydnonimine (1 0(-4) M, 155%), and the guanosine 3',5'-cyclic monophosphate (cGMP) an alogue 8-bromo-cGMP (10(-4) M, 196%) caused a significant release of V IP. L-Arginine (10(-3) M, 246%) also caused a significant increase of VIP release that was antagonized by the NO synthase inhibitor N-omega- nitro-L-arginine methyl ester (5 x 10(-4) M, 131%), which had no effec t when given alone. The results demonstrate that VIP can be released f rom enriched synaptosomes by Ca2+-dependent mechanisms by NO agonists or NO-dependent mechanisms. It is speculated that this VIP release is induced by a presynaptic stimulatory mechanism of NO and this effect c ould enhance or contribute to the action of NO.