NO releases bombesin-like immunoreactivity from enteric synaptosomes by cross-activation of protein kinase A

The effect of nitric oxide (NO) on the release of bombesin-like immunoreact ivity (BLI) was examined in synaptosomes of rat small intestine. The NO don or S-nitroso-N-acetylpenicillamine (SNAP; 10(-7) to 10(-4) M) significantly stimulated BLI release. In the presence of the NO scavenger oxyhemoglobin (10(-3) M) or the guanylate cyclase inhibitor ODQ (10(-5) M), SNAP-induced BLI release was antagonized. In addition, SNAP increased the synaptosomal c GMP content and elevation of cGMP levels by zaprinast (3 x 10(-5) M), an in hibitor of the cGMP-specific phosphodiesterase (PDE) type 5, and increased basal and SNAP-induced BLI release. NO-induced BLI release was blocked by R p-adenosine 3',5'-cyclic monophosphorothioate (3 x 10(-5) M and 10(-4) M), an inhibitor of the cAMP-dependent protein kinase A, whereas KT-5823 (3 x 1 0(-6) M) and Rp-8-(4-chlorophenylthio)-cGMP (5 x 10(-5) M), inhibitors of t he cGMP-dependent protein kinase G, had no effect. Because cGMP inhibits th e cAMP-specific PDE3, thereby increasing cAMP levels, the role of PDES was investigated. Trequinsin (10(-8) M), a specific blocker of PDES, stimulated basal BLI release but had no additive effect on NO-induced release, sugges ting a similar mechanism of action. These data demonstrate that because of a cross-activation of cAMP-dependent protein kinase A by endogenous cGMP BL I can be released by NO from enteric synaptosomes.