The effect of nitric oxide (NO) on the release of bombesin-like immunoreactivity (BLI) was examined in synaptosomes of rat small intestine. The NO donor 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 cGMP content and elevation of cGMP levels by zaprinast (3 × 10−5 M), an inhibitor of the cGMP-specific phosphodiesterase (PDE) type 5, and increased basal and SNAP-induced BLI release. NO-induced BLI release was blocked by Rp-adenosine 3′,5′-cyclic monophosphorothioate (3 × 10−5 M and 10−4 M), an inhibitor of the cAMP-dependent protein kinase A, whereas KT-5823 (3 × 10−6 M) and Rp-8-(4-chlorophenylthio)-cGMP (5 × 10−5M), inhibitors of the cGMP-dependent protein kinase G, had no effect. Because cGMP inhibits the cAMP-specific PDE3, thereby increasing cAMP levels, the role of PDE3 was investigated. Trequinsin (10−8 M), a specific blocker of PDE3, stimulated basal BLI release but had no additive effect on NO-induced release, suggesting a similar mechanism of action. These data demonstrate that because of a cross-activation of cAMP-dependent protein kinase A by endogenous cGMP BLI can be released by NO from enteric synaptosomes.
Role of cAMP-dependent protein kinase A in activation of a voltage-sensitive release mechanism for cardiac contraction in guinea-pig myocytes