The aim of the present study was to characterize the action of NO, a membrane permeable neurotransmitter, on rat myenteric neurons. An NO donor such as sodium nitro prusside (SNP; 10^-4 mol/l) induced a hyperpolarization of the membrane from –30.6 4.3 to –42.4 4.2 mV (n=7), caused by the activation of Ca²⁺-dependent K⁺ channels, which were inhibited by intracellular Cs⁺. This activation is caused by a stimulation of voltage-dependent Ca²⁺ channels (Sitmo et al. 2007). In order to investigate the mechanism of action of NO, inhibitors of different signalling pathways were used.

YC-1 (3-(5-Hydroxymethyl-2-furyl)-1-benzylindazole; 1 mM), an NO-independent stimulator of soluble guanylyl cyclase, mimicked the effect of SNP on membrane potential. The membrane hyperpolarized upon administration of YC-1 from –31.4 7.0 mV to –42.1 7.3 mV (n = 8). In general, protein kinases are involved in the sGC-pathway. Therefore, staurosporine (1 mM) was used as an unspecific inhibitor of several protein kinases. The NO-induced hyperpolarization was abolished by this drug. H-89 (N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfon-amide; 1 mM) a more specific protein kinase A inhibitor, prevented the NO effect, too, suggesting an involvement of the cAMP pathway.

A point, were cGMP- and cAMP-signalling converge, are cGMP-sensitive phosphodiesterases (PDE3A, PDE3B), which are responsible for the degradation of cAMP, so that an increase in the cGMP concentration is followed by a secondary rise of the cytosolic cAMP concentration. Indeed, two inhibitors of PDE3, trequinsin as well as amrinone, prevented the NO-induced hyperpolarization. Both isoforms of PDE3 were detected immunhistochemically in rat myenteric ganglia.

Taken together, the results revealed that NO serves as a messenger via the cGMP-sensitive phosphodiesterases and subsequent activation of protein kinase A.

Supported by Deutsche Forschungsgemeinschaft (Di 388/10–1).