Blockade of nitrergic neuroeffector transmission in guinea-pig colon by a selective inhibitor of soluble guanylyl cyclase

Inhibitors of phosphodiesterase 5 (PDE 5) inhibit the nerve-induced release of nitric oxide from the rabbit corpus cavernosum

BACKGROUND AND PURPOSE

Nitrergic neurons are important for erectile responses in the corpus cavernosum and impaired signalling results in erectile dysfunction, today treated successfully by oral administration of the selective phosphodiesterase 5 (PDE 5) inhibitors sildenafil, tadalafil and vardenafil. Although the importance of nitrergic neurons in urogenital function has become evident, it has not been investigated if the PDE 5 inhibitors affect the nerve-induced release of nitric oxide (NO). In a previous study we found that the soluble guanylate cyclase (sGC)/cyclic guanosine 3',5'-monophosphate (cGMP) pathway might modulate nerve-induced release of NO in isolated cavernous tissue.

EXPERIMENTAL APPROACH

Electrical field stimulation (EFS 5 Hz, 40 V, 0.3 ms pulse duration, 25 pulses at intervals of 2 min) of rabbit isolated cavernous tissue elicited reproducible, nerve-mediated relaxations in the presence of scopolamine (10(-5) M), guanethidine (10(-5) M) and phenylephrine (3 x 10(-6) M). In superfusion experiments, nerve stimulation (20 Hz, 40 V, 1 ms) of the cavernous tissue evoked release of NO/NO2-, measured by chemiluminescence.

KEY RESULTS

Sildenafil, tadalafil and vardenafil decreased the muscular tone and prolonged the relaxations to nerve stimulation. The evoked release of NO decreased to 72+/-11%, 55+/-16% and 61+/-14% of control, respectively after addition of sildenafil, tadalafil or vardenafil (all 10(-4) M, n=6-8, p<0.05).

CONCLUSIONS AND IMPLICATIONS

Selective PDE 5 inhibitors influence the nerve-induced release of NO, probably via cGMP-mediated negative feedback. This negative feedback might explain why priapism is not seen during monotherapy with the PDE inhibitors.

Nitric oxide induces endometrial secretion at implantation time

BACKGROUND

Uterine cervical secretory cells receive a sympathetic cholinergic secretomotor innervation. Glandular nitric oxide (NO) production has been proposed to be a prerequisite for muscarine-induced carbohydrate secretion from endometrial glands and cervical glands at ovulation time and from the seminal vesicle glands. Nitric oxide has also been suggested to have a significant role in the process of implantation and early pregnancy in the mouse, a process, which has also been compared with an inflammatory response.

METHODS

The carbohydrate secretion from everted guinea pig uterine horns placed in organ baths was estimated. Polymerase chain reaction was performed in order to identify the isoforms of nitric oxide synthase (NOS). results. Carbamylcholine chloride (Carbachol) induced carbohydrate secretion of the endometrium, whereas L-NNA and L-NAME inhibited the Carbachol-induced secretion. The isomer D-NAME had no effect on Carbachol-induced secretion. The NO donor GTN induced carbohydrate secretion of the endometrium. The addition of the nitrergic inhibitor of soluble guanylyl cyclase (sGC) ODQ to Carbachol and to the NO donor GTN gave a reduced response. No synergism was seen when the sGC stimulator YC-1 was applied together with Carbachol. Three isoforms of NOS - endothelial NOS (eNOS), cytokine-inducible NOS (iNOS), and neuronal (nNOS) - were identified at implantation time and may take place in the endometrial cell.

CONCLUSIONS

The results of this study suggest that glandular NO production is a prerequisite for the autonomic nervous modulation of endometrial secretion in the guinea pig and that NO may play a role in the implantation time.

Mechanism of a nitric oxide donor NOR 1-induced relaxation in longitudinal muscle of rat proximal colon

We previously suggested that nitric oxide (NO)-mediated relaxation of the rat proximal colon is not associated with change in cyclic GMP content. We further studied the intracellular mechanism of NO-induced relaxation by measuring changes in tension and intracellular Ca2+ concentration ([Ca2+]i), simultaneously. NOR 1, NO donor, relaxed the longitudinal muscle of the rat proximal colon, which was precontracted by carbachol, with a concomitant decrease in [Ca2+]. ODQ, an inhibitor of soluble guanylate cyclase, partially inhibited the relaxant effect of only higher concentrations of NOR 1, but Rp-8-Br-cGMPS, an inhibitor of cyclic GMP-dependent protein kinase (PKG), did not have any effects on the relaxant effect of NOR 1. When the preparations were transferred to normal solution after the treatment with thapsigargin, an inhibitor of sarcoplasmic reticulum (SR) Ca2+-ATPase, in the absence of Ca2+, contraction with a concomitant increase in [Ca2+]i occurred. NOR 1 did not show significant effects on the tension and [Ca2+]i in thapsigargin-treated preparations. In high K+-precontracted preparations, NOR 1 relaxed the preparations with a slight change in [Ca2+]i. The relaxant effect was significantly inhibited by ODQ and Rp-8-Br-cGMPS. These results suggest that NO induces the relaxation preferentially by acting thapsigargin-sensitive function of SR and in turn decreasing [Ca2+]i, although a cyclic GMP-PKG pathway is suggested under the experimental conditions of a high K+ concentration.

Modulation of neuronal nitric oxide release by soluble guanylyl cyclase in guinea pig colon

Peripheral autonomic neurones release nitric oxide (NO) upon nerve activation. However, the regulation of neuronal NO formation is poorly understood. We used the cyclic guanosine 3',5'-monophosphate (cGMP) analogue 8-Br-cGMP, the soluble guanylyl cyclase (sGC) stimulator YC-1, the phosphodiesterase inhibitor zaprinast and the sGC inhibitor ODQ to study whether the sGC/cGMP pathway is involved in regulation of neuronal NO release in nerve plexus-containing smooth muscle preparations from guinea pig colon. Electrical stimulation of the preparation evoked release of NO/NO(-)(2). In the presence of 8-Br-cGMP, YC-1 and zaprinast (all at 10(-4) M) the NO/NO(-)(2)-release increased to 152 +/- 16% (P < 0.05), 164 +/- 37% (P < 0.05) and 290 +/- 67% (P < 0.05) of controls, respectively. Conversely, ODQ (10(-5) M) decreased the evoked release of NO/NO(-)(2) to 49 +/- 7% (P < 0.05) of controls. Our data suggest that the sGC/cGMP pathway modulates NO release. Thus it is likely that NO exerts a positive feedback on its own release from peripheral autonomic neurones.

Inhibition of the NANC relaxation of the guinea-pig proximal colon longitudinal muscle by the purinoceptor antagonist PPADS, inhibition of nitric oxide synthase, but not by a PACAP/VIP antagonist

The effects of the P(2)-purinoceptor antagonist pyridoxal-phosphate-6-azophenyl- 2('),4(')- disulphonic acid (PPADS), the nitric oxide (NO) synthase inhibitor N(G)-nitro- l -arginine (l -NOARG), the K(+)-channel blocker apamin, the pituitary adenylate cyclase activating peptide (PACAP) antagonist PACAP(6-38) and the sensory neuron-blocking drug capsaicin were examined on the non-adrenergic, non-cholinergic (NANC) relaxation evoked by electrical field stimulation in the longitudinal muscle of the guinea-pig proximal colon. Both PPADS (50 microm) and l -NOARG (100 microm) significantly inhibited the NANC relaxation. In the presence of l -NOARG, PPADS inhibited and apamin (100 nm) practically abolished the response. Capsaicin slightly but significantly enhanced the NANC relaxation at 10, but not at 1 Hz stimulation frequency. PACAP(6-38) (3 microm) had no effect on the NANC relaxation, although it abolished the relaxant effect of exogenous PACAP(1-27) (10 nm) and reduced that of exogenous vasoactive intestinal polypeptide (VIP, 30-100 nm) by about 60 %. PPADS (50 microm) inhibited the relaxant action of exogenous adenosine 5(')-triphosphate (ATP; 1 and 10 microm), the inhibition being stronger at 1 microm ATP. These data indicate that an exogenous P(2)-purinoceptor stimulant (possibly ATP) and NO are involved in the NANC relaxation of the guinea-pig colon. The 'non-nitrergic' apamin-sensitive component of the response might also include an unidentified transmitter. No evidence has been found for a mediating role of PACAP/VIP-like peptides.

Role of cGMP as a mediator of nerve-induced motor functions of the opossum esophagus

Stimulation of esophageal nerves produces biphasic relaxation of the lower esophageal sphincter (LES) and an off response of circular esophageal muscle. Previously, we proposed that cGMP mediates nerve-induced hyperpolarization of circular LES muscle but not LES relaxation. These experiments explore whether cGMP mediates LES relaxation or the off response. Strips of muscle from the opossum esophagus and LES were connected to force-displacement transducers, placed in tissue baths containing oxygenated Krebs solution at 37 degrees C, and stimulated by an electrical field. 1H-[1,2, 4]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), a selective inhibitor of guanylyl cyclase, antagonized the off response, shortened its latency, and blocked the first phase of LES relaxation. ODQ also antagonized LES relaxation by exogenous nitric oxide (NO) but not relaxations by vasoactive intestinal polypeptide (VIP). Part of the nerve-induced LES relaxation and the off response appear to be mediated by the second messenger cGMP. These studies indicate that VIP-induced LES relaxation is not mediated by cGMP and therefore do not support the hypothesis that VIP produces LES relaxation by causing the generation of NO.

Evidence for nonvesicular nitric oxide release evoked by nerve activation

The gaseous nature of nitric oxide (NO) has led to the general assumption that its release from neurons during nerve stimulation is independent of vesicular storage. However, recent findings have shown that NO can exist intracellularly as part of more stable bioactive molecules, suggesting that the role of vesicular exocytosis for NO release cannot be excluded simply based on the chemical nature of NO itself. We have used botulinum toxin B (BTX B) to directly address the role of vesicular exocytosis for NO release. BTX B cleaves the synaptic vesicle protein synaptobrevin/VAMP, and by this inhibits Ca++-mediated exocytic release of neurotransmitters. As a target organ we used the guinea-pig enteric nervous system, which innervates the gastrointestinal tract, and in which both classical neurotransmitters as well as NO are released and influence smooth muscle activity. As expected, BTX B (0.1 microM) blocked the nerve stimulation-induced cholinergic and tachykininergic smooth muscle contractions, and markedly inhibited the nerve stimulation-evoked release of [3H]-choline. In contrast, BTX B (0.1 microM) had no effect on nerve stimulation-evoked relaxations, which were equally inhibited by an NO-synthase inhibitor as well as by a selective inhibitor of soluble guanylyl cyclase. In addition, nerve stimulation-evoked NO synthase-dependent outflow of NO/NO2- was unaffected by BTX B (0.1 microM). These findings suggest that the neuronal release of endogenous NO is independent of intact synaptobrevin/VAMP, and therefore provide further evidence that nerve-mediated release of further NO is nonvesicular.