Analysis of vasodilator responses to novel nitric oxide donors in the hindquarters vascular bed of the cat.

Controlled release of nitric oxide (NO*) may be useful in the treatment of a variety of vascular disorders. NO* donors of the diazeniumdiolate family with different rates of spontaneous NO* release have been synthesized. In the current study responses to seven diazeniumdiolate NO* donors (DEA/NO*, DETA/NO*, OXI/NO*, PIPERAZI/NO*, PROLI/NO*, SPER/NO*, and SULFI/NO*) were investigated in the hindquarters vascular bed of the cat. Intravenous injections of all NO* donors caused dose-dependent decreases in systemic arterial pressure and the rank order of potency was SNP > DEA/NO* > PIPERAZI/NO* > SPER/NO* > PROLI/NO* > OXI/NO*. Injections of all NO* donors into the hindlimb perfusion circuit caused dose-related decreases in hindquarters perfusion pressure that were similar to the order of potency in decreasing systemic arterial pressure. The rank order of the time required for the response to return to 50% of the maximal decrease in pressure (T(1/2)) and total duration of action of the NO* donors was SPER/NO* > PIPERAZI/NO* > DEA/NO* > OXI/NO* > DETA/NO* > PROLI/NO* > SULFI/NO*. After treatment with the NO* synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (100 mg/kg, i.v.), hindlimb vasodilator responses to the NO* donors were not significantly different, but vasodilator responses to acetylcholine were significantly reduced. After treatment with zaprinast (2 mg/kg, i.v.), a type V cyclic 3',5'-guanosine monophosphate-specific phosphodiesterase inhibitor, the duration of vasodilator responses to the NO* donors, as measured by T(1/2), was increased significantly, whereas the duration of the response to the beta2-adrenergic receptor agonist albuterol was unchanged. These data suggest that diazeniumdiolate NO* donors are endothelium-independent, directly stimulate soluble guanylate cyclase, and decrease vascular resistance by increasing cyclic 3',5'-guanosine monophosphate levels in the hindquarters vascular bed of the cat.     

Purinergic responses in the feline pulmonary vascular bed

Adenosine and ATP produce dose-dependent, tone-dependent response in the pulmonary vascular (PV) bed, under conditions of controlled pulmonary blood flow and constant left atrial pressure in vivo in intact-chest, spontaneously breathing cats. At low, resting PV tone, adenosine produces dose-dependent vasoconstrictor responses by acting on “A₁-like” adenosine receptors which activate a phospholipase and thromboxane release. ATP Produces dose-dependent vasoconstrictor responses, in part, following its metabolism to adenosine, by acting on “A₁-like” adenosine receptors and by acting on P_2x receptors. At elevated PV tone, data to date support that adenosine and ATP produce dose-dependent vasodilator responses by acting on A₂ and P_2Y receptors, respectively. Mechanisms to explain how a change in vascular tone alters the response of the vascular bed to a substance are not known. Moreover, the role of purinoceptors in pulmonary vascular disease is not known. Identification of selective pharmacological probes for purinoceptors in the pulmonary vascular bed is necessary to investigate their role in the development of pulmonary hypertension of acute and chronic lung disease. © 1993 Wiley-Liss, Inc.     

Comparison of responses to siguazodan, rolipram, and zaprinast in the feline pulmonary vascular bed

The present study was undertaken to investigate and compare responses to the cyclic nucleotide phosphodiesterase inhibitors siguazodan (type III, guanosine 3',5'-cyclic monophosphate (cGMP)-inhibited adenosine 3',5'-cyclic monophosphate (cAMP)), rolipram (type IV, cAMP-specific), and zaprinast (type V, cGMP-specific) in the feline pulmonary vascular bed. When tone in the pulmonary vascular bed was raised to a high steady level with a constant infusion of the thromboxane mimic U46619 (9,11-dideoxy-11, alpha9alpha-epoxymethano prostaglandin F(2alpha)), intralobar injections of the three phosphodiesterase inhibitors caused dose-related decreases in lobar arterial pressure. In terms of relative vasodilator activity, rolipram was more potent at higher doses than siguazodan, which was more potent than zaprinast. The duration of the pulmonary vasodilator response to zaprinast was shorter than for siguazodan or rolipram. Furthermore, siguazodan and rolipram, but not zaprinast, decreased systemic arterial pressure when injected into the perfused lobar artery in the range of doses studied. The present data demonstrate that the three phosphodiesterase inhibitors have potent, long-lasting vasodilator activity in the pulmonary vascular bed of the cat. These data suggest that there is rapid turnover of cAMP and cGMP in the pulmonary circulation and indicate that phosphodiesterase enzyme types III, IV, and V may play an important role in the regulation of vasomotor tone in the feline lung.     

Therapeutic potential of endothelin receptor antagonists and nitric oxide donors in pulmonary hypertension

Pulmonary hypertension can occur idiopathically as a primary disorder of the pulmonary circulation or more commonly, it can exist as a haemodynamic manifestation of a wide variety of pulmonary and cardiovascular diseases, including acute lung injury, chronic obstructive lung disease, congenital heart disease, mitral stenosis, chronic left-sided congestive heart failure and connective tissue diseases such as scleroderma. Pulmonary hypertension is associated with changes in vascular tone as well as vascular structure, with the relative contribution of each dependent upon the aetiology of the increased pulmonary vascular resistance. Most currently available treatments utilise anticoagulants as well as vasodilator drugs that only attenuate the vasoconstrictive component of the disease. The latter category includes oral calcium channel blockers, iv. and aerosolised prostacyclin analogues and inhaled nitric oxide but all three classes of vasodilators have disadvantages and limitations. Treatment with vasodilators is often ineffective in patients with longstanding pulmonary hypertension in which structural changes contribute significantly to the pulmonary hypertension, blood flow obstruction and right heart failure. In view of the immense clinical need, new therapies are being developed by pharmaceutical companies to treat pulmonary hypertension. This update will focus on the current development status of endothelin receptor antagonists and nitric oxide donors for the treatment of pulmonary hypertension.     

The effects of sufentanil in the feline pulmonary vascular bed

The purpose of this prospective vehicle controlled study was to test the hypothesis that sufentanil induces a depressor response in the pulmonary vascular bed of the cat and identify the receptors involved in the mediation or modulation of these effects. In separate experiments, the effects of diphenydramine (histamine receptor blocker), glibenclamide (ATP-sensitive K+ channel blocker), L-N5-(1-Iminoethyl) ornithine hydrochloride (L-NIO) (nitric oxide synthase inhibitor), nimesulide (selective cyclooxygenase (COX)-2 inhibitor), and naloxone (opiate receptor antagonist) were investigated on pulmonary arterial responses to sufentanil and other agonists in the feline pulmonary vascular bed. The lobar arterial perfusion pressures were continuously monitored, electronically averaged, and recorded. In the feline pulmonary vascular bed of the isolated left lower lobe, sufentanil induced a dose-dependent vasodepressor response that was not significantly altered after administration of glibenclamide, L-NIO, and nimesulide. However, the responses to sufentanil were significantly attenuated following administration of diphenhydramine and naloxone. The results of the present study suggest that sufentanil has potent vasodepressor activity in the pulmonary vascular bed of the cat and that this response may be mediated or modulated by both histaminergic and opioid receptor sensitive pathways.     

Altered vascular permeability responses to substance P in diabetic rats: Interactions with a nitric oxide synthesis inhibitor

The effect of inhibiting nitric oxide synthase (N^ω-nitro-L-arginine) on plasma extravasation induced by intravenously administered substance P, [pGlu⁵,Me-Phe⁸,Sar⁹]substance P-(5–11) or prostaglandin E₂ was examined. Control rats were more responsive than diabetic rats to both substance P and [pGlu⁵,Me-Phe⁸,Sar⁹]-substance P-(5–11). N^ω-Nitro-L-arginine blocked the actions of substance P on dorsal skin, but potentiated those of [pGlu⁵,Me-Phe⁸,Sar⁹]substance P-(5–11) in control rats. In diabetic rats N^ω-nitro-L-arginine, which did not affect the actions of the substance P analogue, exerted complex effects on substance P induced plasma extravasation giving potentiation in the tongue, inhibition in bronchioles, and no effect in other tissues. N^ω-Nitro-L-arginine inhibited prostaglandin E₂ induced extravasation in control, but not diabetic rats. The altered plasma extravasation in diabetic rats may be due to diabetes induced alterations in nitric oxide synthesis or in the responses of the endothelial cells to nitric oxide.     

Contribution of nitric oxide produced by inducible nitric oxide synthase to vascular responses of mesenteric arterioles in streptozotocin-diabetic rats

1. The functional changes in mesenteric arterioles of streptozotocin-induced diabetes were investigated by intravital microscopy. The mesentery was exteriorized from anesthetized rats, spread in a chamber, and superfused with Tyrode solution. All drugs tested were applied to the superfusing Tyrode solution. 2. Compared with age-matched controls, the diabetic rats showed enhanced vascular sensitivity to phenylephrine, an alpha(1)-adrenoceptor agonist. The preincubation of the mesentery with N(G)-nitro-l-arginine (l-NNA), a nitric oxide synthase (NOS) inhibitor, shifted the phenylephrine-concentration-response curves to the left in both the diabetic and control rats. Even in the presence of l-NNA, the sensitivity to phenylephrine was higher in the diabetic rats than in the control. 3. Acetylcholine relaxed the mesenteric arterioles in both groups, but to a significantly greater extent in the control than in the diabetic rats. However, the l-NNA-induced constriction of arterioles did not differ significantly between the groups. In contrast, the amplitude of the constrictions of mesenteric arterioles induced by S-ethylisothiourea, an inducible NOS (iNOS) inhibitor, was significantly greater in the diabetic rats than in the control. 4. Immunostaining of the mesentery with a specific antibody for iNOS revealed iNOS in the microvessels of only the diabetic rats. 5. These results suggest that constrictor responses to alpha(1)-adrenoceptor stimulation are sensitized in the mesenteric arterioles of STZ-diabetic rats, and that iNOS expressed in the arteriolar smooth muscle plays a role in suppressing the basal tone and the reactivity of the arterioles in STZ-diabetic rats.     

Comparison of α-MSH and several vasoactive substances on vascular resistance in the feline mesenteric vascular bed

The effects of α-MSH and several other vasoactive substances on the mesenteric vascular bed were studied in the anesthetized cat under conditions of controlled blood flow. Intra-arterial injections of α-MSH in doses of 10, 30, and 100 μg resulted in significant dose-related decreases in mesenteric arterial perfusion pressure but little or no effect on systemic arterial pressure. The vasodilator response to α-MSH was brief in duration and resistance to flow was decreased 10, 18, and 26 percent at 10, 30, and 100 μg. These significant changes after α-MSH were of a much smaller magnitude than were observed after prostaglandins E₁ and E₂, isoproterenol, bradykinin or glyceryl trinitrate and differed completely from the increased resistance after angiotensin II and norepinephrine.     

The influence of nitric oxide donors on the responses to nitrergic nerve stimulation in the mouse duodenum

We investigated whether exogenous nitric oxide (NO) donors have a prejunctional and/or postjunctional inhibitory effect on the nitrergic responses and whether this inhibitory effect was mediated by NO itself and in part, by cyclic GMP in mouse duodenal strips. N(omega)-nitro-L-arginine inhibited relaxations induced by electrical field stimulation of nitrergic nerves, but not those with acidified NaNO2. Furthermore, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) inhibited both types of relaxations while 2-amino-5,6-dihydro-6-methyl-4H-1,3-thiazine (AMT) and N-ethylmaleimide were ineffective. NO donors, nitroglycerin and sodium nitroprusside, inhibited relaxations induced by nitrergic nerve stimulation, but not those with acidified NaNO2. Hemoglobin, exogenous Cu(2+)/Zn(2+) superoxide dismutase, diethyldithiocarbamic acid and pyrogallol did not influence the relaxation with nitrergic nerve stimulation. However, hemoglobin, diethyldithiocarbamic acid, pyrogallol and diethyldithiocarbamic acid plus pyrogallol attenuated the inhibitory effect of NO donors on relaxation with nitrergic nerve stimulation, and exogenous superoxide dismutase potentiated this inhibitory effect. Moreover, nitrergic nerve-mediated relaxations were inhibited by 8-bromo-cyclic GMP, but not by 8-bromo-cyclic AMP. These results suggest that exogenous NO donors have a prejunctional inhibitory effect on the nerve-mediated nitrergic relaxation and that the inhibitory effects of nitroglycerin and sodium nitroprusside are NO-dependent, but not related to NO metabolites such as peroxynitrite or a nitrosothiol intermediate. However, a contribution of S-nitrosothiol formed intracellularly cannot be entirely ruled out. Also, this prejunctional inhibition is mediated, at least in part, by the cyclic GMP, but not the cyclic AMP, pathway.     

Physiological release of nitric oxide is dependent on the level of vascular tone

The presser effect of N^G-methyl-L-arginine (NMA) was tested in urethane-anesthetized rats which were untreated (control) or devoid of sympathetic tone. In contrast with controls, the NMA response was attenuated by pithing or ganglionic blockade. In pithed rats, the induction of moderate or intense vasoconstriction with constant phenylephrine infusion restored or augmented, respectively, the NMA pressor response. Our data suggest that vascular tone may physiologically regulate the release of nitric oxide in vivo.     

Tolerance to the vascular effect of a novel nitric oxide-donating vasodilator, FK409

We investigated whether tolerance develops to the vasorelaxant effects of a new vasodilator, (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (FK409), in isolated canine coronary artery strips and to its hypotensive effect in rats, and whether FK409 activates soluble guanylate cyclase isolated from vascular tissues in the absence of -cysteine. No tolerance to FK409 (0.46 nM to 0.46 μM or 1–1000 μg/kg, i.v.) or cross-tolerance between FK409 and glyceryl trinitrate was demonstrated in vitro and in vivo experiments, whereas the tolerance to glyceryl trinitrate (0.44 nM to 4.4 μM or 1–1000 μg/kg, i.v.) was marked in both conditions. In addition, FK409 (0.1–10 μM) activated soluble guanylate cyclase without -cysteine, but glyceryl trinitrate (1–100 μM) required the addition of -cysteine (5 mM) for the activation of the enzyme. The results suggest that FK409 may be advantageous compared to tolerance-producing nitrates currently in clinical use, and that this property of FK409 is probably due to its independence of a sulfhyhydryl donor.     

Nitric oxide scavenger carboxy-PTIO potentiates the inhibition of dopamine uptake by nitric oxide donors

2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO) has been increasingly used as nitric oxide (NO) scavenger. Carboxy-PTIO reacts with NO to form nitric dioxide and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl (carboxy-PTI). In rat C6 glioma cells expressing human dopamine transporter, carboxy-PTIO paradoxically potentiated the inhibition of [(3)H]dopamine uptake by two NO donors, diethylamine/NO and (Z)-1-[N-(3-ammoniopropyl)-N-(n-propyl)-amino]/NO. Further examinations revealed that carboxy-PTI concentration-dependently reduced dopamine uptake, indicating that the formation of carboxy-PTI may account for the failure of carboxy-PTIO to abolish NO elicited effects. These results suggest that caution should be taken in interpreting data obtained using carboxy-PTIO and probably other NO scavengers.     

Stimulatory effects of nitric oxide donors on gastric acid secretion in isolated mouse stomach

We previously reported on the stimulatory role of endogenous nitric oxide (NO) in gastric acid secretion. In the present study, we investigated the effects of NO donors on acid secretion in isolated mouse stomach. Nitroprusside (100 μM–1 mM) inhibited the gastric acid secretion induced by histamine (500 μM) in a concentration-dependent manner. In addition, nitroprusside abolished the acid secretion induced by bethanechol (100 μM) and by electrical stimulation (10 Hz) of the vagus nerve. On the other hand, nitroprusside, 75 μM, which did not affect the acid secretion induced by histamine, itself elicited an increase in acid secretion. The acid secretion induced by 75 μM nitroprusside was inhibited by 10 μM famotidine, a histamine H₂ receptor antagonist. These results suggest that NO donors at high doses act on gastric parietal cells, resulting in inhibition of the stimulated acid secretion, and, at lower doses, facilitate histamine release from histamine-containing cells, leading to the increased acid secretion.     

Basal tonic release of nitric oxide coupled to cGMP production regulates the vascular reactivity of the mesenteric bed.

To reveal a basal production of nitric oxide (NO) and guanosine 3',5' cyclic monophosphate (cGMP) in the rat arterial mesenteric bed, mesenteries were perfused in the absence and in the presence of selective blockers of the L-arginine cascade. Endothelium removal or inhibition of NO synthase significantly reduced the release of NO and tissue cGMP. A significant correlation between these messengers was shown. Blockade of soluble guanylyl cyclase with 0.3-10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) only reduced basal cGMP production; 1-100 nM sildenafil (Sild), an inhibitor of phosphodiesterase V, increased basal tissue cGMP without modifying the release of NO. Acetylcholine (0.01-10 microM) caused a concentration-dependent rise in NO and cGMP evoking a proportional vasodilatation, demonstrating the interdependence between these messengers and vascular reactivity. Endothelium removal or NO synthase blockade reduced the acetylcholine-induced increase of messengers and the vasodilatation. ODQ attenuated only the increase in cGMP and the vasodilatation, while sildenafil increased cGMP without significantly altering luminal NO release. The present results highlight a tonic release of NO and its involvement in endothelial-smooth muscle signaling; NO and cGMP are determinants of vascular reactivity.     

Evidence that different mechanisms underlie smooth muscle relaxation to nitric oxide and nitric oxide donors in the rabbit isolated carotid artery

1. The endothelium-dependent relaxants acetylcholine (ACh; 0.03–10 μM) and {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187 (0.03–10 μM), and nitric oxide (NO), applied either as authentic NO (0.01–10 μM) or as the NO donors 3-morpholino-sydnonimine (SIN-1; 0.1–10 μM) and S-nitroso-N-acetylpenicillamine (SNAP; 0.1–10 μM), each evoked concentration-dependent relaxation in phenylephrine stimulated (1–3 μM; mean contraction and depolarization, 45.8±5.3 mV and 31.5±3.3 mN; n=10) segments of rabbit isolated carotid artery. In each case, relaxation closely correlated with repolarization of the smooth muscle membrane potential and stimulated a maximal reversal of around 95% and 98% of the phenylephrine-induced depolarization and contraction, respectively.
2. In tissues stimulated with 30 mM KCl rather than phenylephrine, smooth muscle hyperpolarization and relaxation to ACh, {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187, authentic NO and the NO donors were dissociated. Whereas the hyperpolarization was reduced by 75–80% to around a total of 10 mV, relaxation was only inhibited by 35% (n=4–7 in each case; P<0.01). The responses which persisted to ACh and {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187 in the presence of 30 mM KCl were abolished by either the NO synthase inhibitor L-N^G-nitroarginine methyl ester (L-NAME; 100 μM) or the inhibitor of soluble guanylyl cyclase 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 μM; 10 min; n=4 in each case; P<0.01).
3. Exposure to ODQ significantly attenuated both repolarization and relaxation to ACh, {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187 and authentic NO, reducing the maximum changes in both membrane potential and tension to each relaxant to around 60% of control values (n=4 in each case; P<0.01). In contrast, ODQ almost completely inhibited repolarization and relaxation to SIN-1 and SNAP, reducing the maximum responses to around 8% in each case (n=3–5; P<0.01).
4. The potassium channel blockers glibenclamide (10 μM), iberiotoxin (100 nM) and apamin (50 nM), alone or in combination, had no significant effect on relaxation to ACh, {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187, authentic NO, or the NO donors SIN-1 and SNAP (n=4 in each case; P>0.05). Charybdotoxin (ChTX; 50 nM) almost abolished repolarization to ACh (n=4; P<0.01) and inhibited the maximum relaxation to ACh, {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187 and authentic NO each by 30% (n=4–8; P<0.01). Application of ODQ (10 μM; 10 min) abolished the ChTX-insensitive responses to ACh, {"type":"entrez-nucleotide","attrs":{"text":"A23187","term_id":"833253","term_text":"A23187"}}A23187 and authentic NO (n=4 in each case; P<0.01
5. When the concentration of phenylephrine was reduced (to 0.3–0.5 μM) to ensure the level of smooth muscle contraction was the same as in the absence of potassium channel blocker, ChTX had no effect on the subsequent relaxation to SIN-1 (n=4; P>0.05). However, in the presence of tone induced by 1–3 μM phenylephrine (51.2±3.3 mN; n=4), ChTX significantly reduced relaxation to SIN-1 by nearly 50% (maximum relaxation 53.2±6.3%, n=4; P<0.01).
6. These data indicate that NO-evoked relaxation of the rabbit isolated carotid artery can be mediated by three distinct mechanisms: (a) a cyclic GMP-dependent, voltage-independent pathway, (b) cyclic GMP-mediated smooth muscle repolarization and (c) cyclic GMP-independent, ChTX-sensitive smooth muscle repolarization. Relaxation and repolarization to both authentic and endothelium-derived NO in this large conduit artery appear to be mediated by parallel cyclic GMP-dependent and -independent pathways. In contrast, relaxation to the NO-donors SIN-1 and SNAP appears to be mediated entirely via cyclic GMP-dependent mechanisms.

     

Modulation of vasorelaxant responses to potassium channel openers by basal nitric oxide in the rat isolated superior mesenteric arterial bed.

1. We have used the isolated buffer-perfused mesenteric arterial bed of the rat to assess the modulation of vasorelaxation to potassium channel openers (KCOs) by basal nitric oxide. 2. The dose-response curves to the KCOs, levcromakalim and pinacidil, in preconstricted preparations were significantly shifted to the left in the presence of the nitric oxide synthase inhibitor (100 microM) NG-nitro-L-arginine methyl ester (levcromakalim, ED50 = 4.47 +/- 0.70 nmol vs. 1.73 +/- 0.26 nmol, P < 0.001; pinacidil, ED50 = 16.1 +/- 4.8 nmol vs. 5.43 +/- 1.10 nmol, P < 0.001). The vasorelaxant responses to papaverine, a vasodilator which acts independently of potassium channels was unaffected by NG-nitro-L-arginine methyl ester (L-NAME). 3. Removal of the endothelium, by perfusion with the detergent CHAPS (0.3%), significantly (P < 0.001) increased the potency of levcromakalim as a vasodilator (ED50 4.47 +/- 0.70 nmol vs. 2.59 +/- 0.31 nmol). The subsequent administration of L-NAME following perfusion with CHAPS did not lead to any additional enhancement of responses to levcromakalim. 4. The presence of the non-selective adenosine antagonist, 8-phenyltheophylline (8-PT, 10 microM) significantly (P < 0.001) shifted the dose-response curve to levcromakalim to the left (ED50 4.47 +/- 0.70 nmol vs. 1.11 +/- 0.32 nmol). In the presence of both L-NAME and 8-PT, the dose-response curve to levcromakalim was also significantly (P < 0.01) shifted to the left compared with control (ED50 in the presence of both L-NAME and 8-PT was 0.42 +/- 0.08 nmol). 5. The presence of 8-bromo cyclic GMP (10 microM) reversed the increase potency of levcromakalim, observed following inhibition of nitric oxide synthase (ED50 in the presence of L-NAME was 0.59 +/- 0.01 nmol and in the presence of 8-bromo cyclic GMP plus L-NAME the ED50 was 3.17 +/- 0.80 nmol). However in the absence of L-NAME, the cell permeable analogue of cyclic GMP, 8-bromo cyclic GMP, did not affect the dose-response curve to levcromakalim compared with control (control ED50 value was 4.16 +/- 0.52 nmol vs. 3.85 +/- 1.13 nmol in the presence of 8-bromo cyclic GMP). 6. The present investigation demonstrates that both basal nitric oxide and adenosine modulate vasorelaxation to the KCOs levcromakalim and pinacidil. The modulatory effect of nitric oxide may be mediated via cyclic GMP.     

Involvement of neuronal processes and nitric oxide in the inhibition by endotoxin of pentagastrin-stimulated sastric acid secretion

Administration of E. coli endotoxin (1 mg kg^–1, i.v.) abolished the acid response induced by the i.v. infusion of pentagastrin (8 g kg^–1, h^-1) in the continuously perfused stomach of the anaesthetized rat. Local serosal application of tetrodotoxin (36 ng per rat) completely restored acid responses to pentagastrin in endotoxin-treated rats. However, pretreatment with atropine (0.5 mg kg^–1, s.c.), capsaicin (20, 30, and 50 mg kg^–1, s.c. 2 weeks before the study) or guanethidine (16 mg kg^–1, s.c. 3 and 16h before) did not influence the inhibitory effects of endotoxin. Continuous i.v. infusion with N^G-nitro arginine methyl ester (L-NAME, 10 mg kg^–1, h^–1) restored the secretory responses to pentagastrin in endotoxin treated rats. The effects of L-NAME were reversed by L-arginine (100 mg kg^–1, h^–1, i.v.), but not by its enantiomer D-arginine (100 mg kg^–1, h^–1, i.v.). The secretory responses elicited by pentagastrin (10^–10–10^–6 M) in the isolated lumen perfused stomach of the rat were not influenced by incubation (100 min) with endotoxin (10 g ml ^–1). These observations with tetrodotoxin indicate that inhibition of acid secretion by endotoxin in vivo involves neuronal activity, while inhibition of NO synthesis had a comparable inhibitory action. Activation of a systemic non-adrenergic non-cholinergic neuronal pathway involving NO could thus mediate the acute acid inhibitory effects of endotoxin. Correspondence to: J. V. Esplugues at the above address     

Analysis of vasodilator responses to peroxynitrite in the hindlimb vascular bed of the cat

The free radical peroxynitrite (ONOO-) is formed in biological systems from the reaction of nitric oxide (NO) with superoxide (O2-) and can react with protein and nonprotein thiol groups to produce tissue injury. However, these pathologic actions of (ONOO-) may have been overemphasized, in that (ONOO-) has vasorelaxant properties through activation of soluble guanylate cyclase; inhibits leukocyte-endothelial cell interactions; and reduces ischemia-reperfusion injury in the heart, lung, and liver. It has been reported that tolerance develops to the vasodilator actions of (ONOO-) and that (ONOO-) impairs vascular function. However, little, if anything, is known about responses to (ONOO-) in the hindlimb circulation of the cat. To better understand the effects of (ONOO-) on responses to vasoactive agonists and the mechanism by which (ONOO-) induces vasodilation, the effects of short-term exposure to (ONOO-) were investigated under constant-flow conditions in the hindlimb vascular bed of the cat.In these studies, direct intraarterial injections of (ONOO-) produced dose-dependent decreases in hindquarters perfusion pressure. The vasodilator responses to (ONOO-) were rapid in onset, were short in duration, and could be repeated without exhibiting tachyphylaxis. Vasodilator responses to (ONOO-) were not changed in the presence of inhibitors of nitric-oxide synthase, cyclooxygenase, or K+-ATP (adenosine triphosphate-sensitive potassium) channels. Furthermore, responses to (ONOO-) were enhanced in duration by the type 5-cGMP (cyclic guanosine monophosphate) phosphodiesterase inhibitor zaprinast, whereas rolipram, a type 4-cGMP phosphodiesterase inhibitor, was without effect. Repeated administration of (ONOO-) had no significant effect on responses to vasoconstrictor or to vasodilator agents including acetylcholine. These results show that (ONOO-) has significant vasodilator activity in the hindlimb vascular bed of the cat and suggest that the response is mediated by a cGMP- dependent mechanism.The results of experiments with repeated injections of (ONOO-) indicate that (ONOO-) does not impair vasoconstrictor and endothelium-dependent or endothelium-independent vasodilator responses. Furthermore, tolerance did not develop with repeated short-term exposure to (ONOO-). Moreover, the results of experiments with inhibitors suggest that responses to (ONOO-) are not dependent on K-ATP (adenosine triphosphate-sensitive potassium) channel activation, increased NOS activity, or the formation of products in the cyclooxygenase pathway. The results of these studies are consistent with the hypothesis that (ONOO-) is rapidly converted in the hindlimb circulation to a substance that has the properties of an NO donor. These studies suggest that under physiologic conditions, the cytotoxic effects of (ONOO-) on a short-term basis may have been overemphasized.     

Effects of a nitric oxide donor on behavior and interaction with nitrous oxide in the mouse light/dark exploration test

There is controversy whether endogenous nitric oxide (NO) is involved in anxiogenesis or anxiolysis. This study was conducted to determine the influence of the NO donor, 3-morpholinosyndnonimine (SIN-1), on resting and nitrous oxide (N(2)O)-induced behaviors in the mouse light/dark exploration test. I.c.v. doses of 0.3 and 1.0 microg SIN-1 both increased the time spent in the light compartment. When pretreated with 0.1 microg SIN-1, mice responded to N(2)O with an apparent additive increase in the time spent in the light compartment. These findings further support a functional role of NO in regulation of anxiety and mediation of N(2)O-induced behavior.