A major role for prostacyclin in nitric oxide-induced ocular vasorelaxation in the piglet

Subretinal fluid accumulation in a patient with polycythemia vera after receiving a prostaglandin I2 analogue treatment

Purpose

To report a case of polycythemia vera (PV) with subretinal fluid accumulation after the administration of prostaglandin I2 (PGI2) analogue.

Observations

A 57-year-old woman diagnosed as having PV was referred to our department for the evaluation of severe metamorphopsia in the left eye, which gradually progressed after the initiation of oral administration of PGI2 mimetics. At the first visit, the patient's best-corrected visual acuities (BCVAs) were 20/20 OD and 20/30 OS. Fundus examination and optical coherence tomography revealed the presence of subretinal fluid (SRF) in the left eye and multiple serous pigment epithelial detachments (PEDs) in both eyes. Fluorescein angiography revealed central serous chorioretinopathy (CSC)-like lesions, consisting of dye pooling corresponding to the PEDs in both eyes and dye leakage in the left eye. Indocyanine green angiography and laser speckle flowgraphy revealed dilated choroidal veins and reduced choroidal blood flow, respectively. The central choroidal thickness (CCT) measured at the first visit showed a relatively thickened choroid in the left eye. Laboratory data showed mild pancytosis. The patient was diagnosed as having CSC associated with a background of PV, presumably triggered by the PGI2 analogue. One month after cessation of drug administration, the patient's BCVA improved, the CCT slightly decreased, and serous retinal detachment and PED disappeared in the left eye.

Conclusions and importance

Our case of PV presenting with CSC-like lesions after PGI2 analogue administration indicates the possible risk of SRF accumulation by PGI2 analogues in patients with PV.

Retinal Physiology and Circulation: Effect of Diabetes

In this article, we present a discussion of diabetes and its complications, including the macrovascular and microvascular effects, with the latter of consequence to the retina. We will discuss the anatomy and physiology of the retina, including aspects of metabolism and mechanisms of oxygenation, with the latter accomplished via a combination of the retinal and choroidal blood circulations. Both of these vasculatures are altered in diabetes, with the retinal circulation intimately involved in the pathology of diabetic retinopathy. The later stages of diabetic retinopathy involve poorly controlled angiogenesis that is of great concern, but in our discussion, we will focus more on several alterations in the retinal circulation occurring earlier in the progression of disease, including reductions in blood flow and a possible redistribution of perfusion that may leave some areas of the retina ischemic and hypoxic. Finally, we include in this article a more recent area of investigation regarding the diabetic retinal vasculature, that is, the alterations to the endothelial surface layer that normally plays a vital role in maintaining physiological functions. © 2020 American Physiological Society. Compr Physiol 10:933-974, 2020.

4-Aminopyridine, a Voltage-Gated K+ Channel Inhibitor, Attenuates Nitric Oxide-Mediated Vasodilation of Retinal Arterioles in Rats

Nitric oxide (NO) is an important regulator of the retinal blood flow. The present study aimed to determine the role of voltage-gated K⁺ (K_V) channels and ATP-sensitive K⁺ (K_ATP) channels in NO-mediated vasodilation of retinal arterioles in rats. In vivo, the retinal vasodilator responses were assessed by measuring changes in the diameter of retinal arterioles from ocular fundus images. Intravitreal injection of 4-aminopyridine (a K_V channel inhibitor), but not glibenclamide (a K_ATP channel blocker), significantly attenuated the retinal vasodilator response to the NO donor (±)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide (NOR3). Intravitreal injection of indomethacin (a non-selective cyclooxygenase inhibitor) also reduced the NOR3-induced retinal vasodilator response. The combination of 4-aminopyridine and indomethacin produced a greater reduction in the NOR3-induced response than either agent alone. 4-Aminopyridine had no significant effect on pinacidil (a K_ATP channel opener)-induced response. These results suggest that the vasodilatory effects of NO are mediated, at least in part, through the activation of 4-aminopyridine-sensitive K_V channels in the retinal arterioles of rats. NO exerts its dilatory effect on the retinal vasculature of rats through at least two mechanisms, activation of the K_V channels and enhancement of prostaglandin production.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Sustained Nitric Oxide-Providing Small Molecule and Precise Release Behavior Study for Glaucoma Treatment

Incidence ofglaucoma, a severe disease leading to irreversible loss of vision, is increasing with global aging populations. Lowering intraocular pressure (IOP) is the only proven treatment method for glaucoma. Nitric oxide (NO) is an emerging material targeting the conventional outflow pathway by relaxing the trabecular meshwork (TM). However, there is little understanding on the NO level effective in IOP lowering without toxicity. Here, we report a novel long-term NO-releasing polydiazeniumdiolate (NOP) that enables lowering IOP via the conventional outflow pathway. NOP is composed of carbon-bound polydiazeniumdiolate, a stable NO donor moiety. NO release was monitored with accurate parameters by real-time detection of gas and analysis of the accumulated release profile. Based on the NO release information, the selected safe level of NOP exhibited effective TM relaxation and a potential IOP lowering effect in vivo without side effects. This work provides new insights into nitric oxide release behavior that should be considered for glaucoma treatment.

Retinal biomarkers provide "insight" into cortical pharmacology and disease

The retina is an easily accessible out-pouching of the central nervous system (CNS) and thus lends itself to being a biomarker of the brain. More specifically, the presence of neuronal, vascular and blood-neural barrier parallels in the eye and brain coupled with fast and inexpensive methods to quantify retinal changes make ocular biomarkers an attractive option. This includes its utility as a biomarker for a number of cerebrovascular diseases as well as a drug pharmacology and safety biomarker for the CNS. It is a rapidly emerging field, with some areas well established, such as stroke risk and multiple sclerosis, whereas others are still in development (Alzheimer's, Parkinson's, psychological disease and cortical diabetic dysfunction). The current applications and future potential of retinal biomarkers, including potential ways to improve their sensitivity and specificity are discussed. This review summarises the existing literature and provides a perspective on the strength of current retinal biomarkers and their future potential.

The Beta-1-Receptor Blocker Nebivolol Elicits Dilation of Cerebral Arteries by Reducing Smooth Muscle [Ca2+]i

Rationale

Nebivolol is known to have beta-1 blocker activity, but it was also suggested that it elicits relaxation of the peripheral arteries in part via release of nitric oxide (NO). However, the effect of nebivolol on the vasomotor tone of cerebral arteries is still unclear.

Objective

To assess the effects of nebivolol on the diameter of isolated rat basilar arteries (BA) in control, in the presence of inhibitors of vasomotor signaling pathways of know action and hemolysed blood.

Methods and Results

Vasomotor responses were measured by videomicroscopy and the intracellular Ca²⁺ by the Fura-2 AM ratiometric method. Under control conditions, nebivolol elicited a substantial dilation of the BA (from 216±22 to 394±20 μm; p<0.05) in a concentration-dependent manner (10⁻⁷ to 10⁻⁴ M). The dilatation was significantly reduced by endothelium denudation or by L-NAME (inhibitor of NO synthase) or by SQ22536 (adenylyl cyclase blocker). Dilatation of BA was also affected by beta-2 receptor blockade with butoxamine, but not by the guanylate cyclase blocker ODQ. Interestingly, beta-1 blockade by atenolol inhibited nebivolol-induced dilation. Also, the BK_Ca channel blocker iberiotoxin and K_Ca channel inhibitor TEA significantly reduced nebivolol-induced dilation. Nebivolol significantly reduced smooth muscle Ca²⁺ level, which correlated with the increases in diameters and moreover it reversed the hemolysed blood-induced constriction of BA.

Conclusions

Nebivolol seems to have an important dilator effect in cerebral arteries, which is mediated via several vasomotor mechanisms, converging on the reduction of smooth muscle Ca²⁺ levels. As such, nebivolol may be effective to improve cerebral circulation in various diseased conditions, such as hemorrhage.

Symmetry recovery of cell-free layer after bifurcations of small arterioles in reduced flow conditions: effect of RBC aggregation

Heterogeneous distribution of red blood cells (RBCs) in downstream vessels of arteriolar bifurcations can be promoted by an asymmetric formation of cell-free layer (CFL) in upstream vessels. Consequently, the CFL widths in subsequent downstream vessels become an important determinant for tissue oxygenation (O2) and vascular tone change by varying nitric oxide (NO) availability. To extend our previous understanding on the formation of CFL in arteriolar bifurcations, this study investigated the formation of CFL widths from 2 to 6 vessel-diameter (2D-6D) downstream of arteriolar bifurcations in the rat cremaster muscle (D = 51.5 ± 1.3 μm). As the CFL widths are highly influenced by RBC aggregation, the degree of aggregation was adjusted to simulate levels seen during physiological and pathological states. Our in vivo experimental results showed that the asymmetry of CFL widths persists along downstream vessels up to 6D from the bifurcating point. Moreover, elevated levels of RBC aggregation appeared to retard the recovery of CFL width symmetry. The required length of complete symmetry recovery was estimated to be greater than 11D under reduced flow conditions, which is relatively longer than interbifurcation distances of arterioles for vessel diameter of ∼50 μm. In addition, our numerical prediction showed that the persistent asymmetry of CFL widths could potentially result in a heterogeneous vasoactivity over the entire arteriolar network in such abnormal flow conditions.

[Elucidation of dysfunctional mechanisms of retinal circulation in the rat models of glaucoma and exploration of novel therapeutic drugs]

In recent times, glaucoma has become the leading cause of acquired blindness among the Japanese. As visual disorders markedly decrease the quality of life (QOL), it is important to develop new strategies for preventing the onset of and delaying the progression of glaucoma. Glaucoma has long since been recognized as a serious disease caused by increased intraocular pressure and subsequent injury and death of the neuronal retinal cells. Therefore, numerous studies have focused on the mechanisms that damage neuronal cells and on the drugs that possess protective effects in reversing this damage. However, injury to the retinal vasculature has been recently shown in animal models of glaucoma. Hence, thus far, only few papers have been published on retinal circulation in glaucoma. These study results have indicated that retinal circulation is altered in glaucoma and that this vascular abnormality may be the cause of and/or may accelerate retinal degeneration. In this report, we have attempted to elucidate the mechanisms of retinal circulation and explore novel drugs for the treatment of retinal circulation disorders. We have also introduced here our previous research results on retinal circulation. We reported that the drugs that improved retinal circulation, by intravitreal injection, in the rat model of glaucoma also inhibited retinal nerve injury, thereby representing possibilities that they might be novel candidate drugs for glaucoma prevention and treatment.

Eicosapentaenoic acid (EPA) reduces cardiovascular events: relationship with the EPA/arachidonic acid ratio

The clinical efficacy of fish oil and high-purity eicosapentaenoic acid ethyl ester (hp-EPA-E) for treating cardiovascular disease (CVD) has been reported. Fish oil contains saturated and monounsaturated fatty acids that have pharmacological effects opposite to those of ω3 fatty acids (ω3). Moreover, ω3, such as EPA and docosahexaenoic acid (DHA), do not necessarily have the same metabolic and biological actions. This has obscured the clinical efficacy of ω3. Recently, the Japan EPA Lipid Intervention Study (JELIS) of hp-EPA-E established the clinical efficacy of EPA for CVD, and higher levels of blood EPA, not DHA, were found to be associated with a lower incidence of major coronary events. A significant reduction in the risk of coronary events was observed when the ratio of EPA to arachidonic acid (AA) (EPA/AA) was > 0.75. Furthermore, the ratio of prostaglandin (PG) I3 and PGI2 to thromboxane A2 (TXA2) ([PGI2 + PGI3]/TXA2) was determined to have a linear relationship with the EPA/AA ratio as follows: (PGI2 + PGI3)/TXA2 =λ + π＊ (EPA/AA). Like PGI2, PGI3 not only inhibits platelet aggregation and vasoconstriction, but also is assumed to reduce cardiac ischemic injury and arteriosclerosis and promote angiogenesis. Thus, the effects of EPA in reducing the risk of CVD could be mediated by biological action of PGI3 in addition to hypotriglyceridemic action of EPA. Compared with DHA, EPA administration increases the EPA/AA ratio and the (PGI2 + PGI3)/TXA2 balance to a state that inhibits the onset and/or progression of CVD.

Plasma kallikrein-kinin system and diabetic retinopathy

Diabetic retinopathy (DR) occurs, to some extent, in most people with at least 20 years' duration of diabetes mellitus. The progression of DR to its sight-threatening stages is usually associated with the worsening of underlying retinal vascular dysfunction and disease. The plasma kallikrein-kinin system (KKS) is activated during vascular injury, where it mediates important functions in innate inflammation, blood flow, and coagulation. Recent findings from human vitreous proteomics and experimental studies on diabetic animal models have implicated the KKS in contributing to DR. Vitreous fluid from people with advanced stages of DR contains increased levels of plasma KKS components, including plasma kallikrein (PK), coagulation factor XII, and high-molecular-weight kininogen. Both bradykinin B1 and B2 receptor isoforms (B1R and B2R, respectively) are expressed in human retina, and retinal B1R levels are increased in diabetic rodents. The activation of the intraocular KKS induces retinal vascular permeability, vasodilation, and retinal thickening, and these responses are exacerbated in diabetic rats. Preclinical studies have shown that the administration of PK inhibitors and B1R antagonists to diabetic rats ameliorates retinal vascular hyperpermeability and inflammation. These findings suggest that components of plasma KKS are potential therapeutic targets for diabetic macular edema.

Acute decrease of cardio-ankle vascular index with the administration of beraprost sodium

AIM

A novel arterial stiffness index, the cardio-ankle vascular index (CAVI), has been proposed. To clarify the properties of CAVI, the effects of beraprost sodium (BPS), a prostaglandin (PG) I2 analogue, which has a potent vasodilating effect, on CAVI were studied and comparing with brachialankle pulse wave velocity (baPWV) in healthy volunteers.

METHODS

Male volunteers (n=18, 46.3±4.2 yr) were enrolled in this study and administered BPS (40 µg). CAVI and baPWV were measured every hour for 4 hours.

RESULTS

When BPS was administered to 18 healthy volunteers, systolic blood pressure and diastolic blood pressure fluctuated slightly, but the means did not change. CAVI significantly decreased in the 1st hour from 8.3±0.34 (mean±SE) to 7.9±0.34 (p<0.05) and this decrease persisted for 3 hours, whereas baPWV did not significantly change. ΔbaPWV each time was significantly correlated with both Δsystolic blood pressure and Δdiastolic blood pressure, but ΔCAVI did not correlate with either Δsystolic blood pressure (r=-0.12, p=0.38) or Δdiastolc blood pressure (r=-0.22, p=0.10).

CONCLUSIONS

Beraprost sodium did not decrease blood pressure, but decreased CAVI, whereas baPWV did not change. These results indicate that CAVI partly reflected the contraction of arterial smooth muscle cells.

Noradrenaline contracts rat retinal arterioles via stimulation of α(1A)- and α(1D)-adrenoceptors

The aim of this study was to characterize the α₁-adrenoceptor subtype(s) involved in the noradrenaline-induced contraction of retinal arterioles in rats. In vivo ocular fundus images were captured with a digital camera equipped with a special objective lens. By measuring changes in diameter of retinal arterioles in the fundus images, retinal vascular response was assessed. The systemic blood pressure and heart rate in the animals were also continuously recorded. Following blockade of β₁/β₂-adrenoceptors with propranolol, noradrenaline (0.03-3 μg/kg/min, i.v.) decreased the diameter of retinal arterioles and increased the mean blood pressure in a dose-dependent manner. The highest dose (3 μg/kg/min, i.v.) of noradrenaline caused a small increase in heart rate. The α(1A)-adrenoceptor antagonist RS100329 (0.1 mg/kg, i.v.) and the α(1D)-adrenoceptor antagonist BMY 7378 (1 mg/kg, i.v.) significantly prevented noradrenaline-induced contraction of retinal arterioles and pressor responses whereas the α(1B)-adrenoceptor antagonist L-765314 (1 mg/kg, i.v.) did not. The α(1A)-adrenoceptor agonist, A 61603 (0.03-0.3 μg/kg/min, i.v.), also caused contractile responses of retinal arterioles and pressor responses. These responses were almost completely prevented by RS100329 (0.1 mg/kg, i.v.), but not by BMY 7378 (1 mg/kg, i.v.). These results suggest that the contractile effects of noradrenaline on retinal arterioles and peripheral resistance vessels are, at least in part, mediated by stimulation of α(1A)- and α(1D)-adrenoceptors. Furthermore, it is likely that the α₁-adrenoceptor subtype(s) involved in rat vascular responses are similar in both retinal and peripheral circulation.

The role of pulsatile flow in controlling microvascular retinal endothelial and pericyte cell apoptosis and proliferation

AIMS

Aberrant retinal blood flow is a hallmark of various retinopathies and may be a causative factor in the pathology associated with these conditions. We examined the effects of pulsatile flow on bovine retinal endothelial cell (BREC) and bovine retinal pericyte (BRP) apoptosis and proliferation.

METHODS AND RESULTS

Co-cultured BRECs and BRPs were exposed to low (0.3 mL/min) or high (25 mL/min) pulsatile flow for 72 h using a perfused transcapillary culture system. Pulsatile flow increased BREC nitric oxide synthase (eNOS) and cyclooxygenase-2 (COX-2) expression and activity concomitant with a significant decrease in pre-pro-endothelin-1 (ET-1) mRNA and peptide. BREC apoptosis was significantly attenuated following exposure to high flow. The inhibition of NOS, COX, and ET receptors significantly reduced the pro-survival effects of flow on BREC. In contrast, BRP apoptosis was significantly enhanced following exposure to high flow. The inhibition of COX and ET receptors significantly attenuated the high flow-induced increase in BRP apoptosis when compared with untreated controls. Treatment of static BREC with NO donor (S-nitroso-N-acetylpenicillamine, SNAP), ET-1, or iloprost inhibited serum deprivation-induced apoptosis, whereas treatment of BRP with ET-1 and iloprost, but not SNAP, was ineffective. High pulsatile flow decreased BRP proliferation, in the absence of any changes in BREC proliferation.

CONCLUSION

Increased pulsatile flow promotes BREC survival and enhances BRP apoptosis through the activation of endothelial-derived vasoactive substances. Altered pulsatile flow does not alter BREC proliferation in co-culture with BRP, whereas BRP proliferation was significantly decreased at high flow rates. These interactions have important implications for vessel growth and regression during retinal vascular pathogenesis.

Nitric oxide dilates rat retinal blood vessels by cyclooxygenase-dependent mechanisms

It has been suggested that nitric oxide (NO) stimulates the cyclooxygenase (COX)-dependent mechanisms in the ocular vasculature; however, the importance of the pathway in regulating retinal circulation in vivo remains to be elucidated. Therefore, we investigated the role of COX-dependent mechanisms in NO-induced vasodilation of retinal blood vessels in thiobutabarbital-anesthetized rats with and without neuronal blockade (tetrodotoxin treatment). Fundus images were captured with a digital camera that was equipped with a special objective lens. The retinal vascular response was assessed by measuring changes in diameter of the retinal blood vessel. The localization of COX and soluble guanylyl cyclase in rat retina was examined using immunohistochemistry. The NO donors (sodium nitroprusside and NOR3) increased the diameter of the retinal blood vessels but decreased systemic blood pressure in a dose-dependent manner. Treatment of rats with indomethacin, a nonselective COX inhibitor, or SC-560, a selective COX-1 inhibitor, markedly attenuated the vasodilation of retinal arterioles, but not the depressor response, to the NO donors. However, both the vascular responses to NO donors were unaffected by the selective COX-2 inhibitors NS-398 and nimesulide. Indomethacin did not change the retinal vascular and depressor responses to hydralazine, 8-(4-chlorophenylthio)-guanosine-3′, 5′-cyclic monophosphate (a membrane-permeable cGMP analog) and 8-(4-chlorophenylthio)-adenosine-3′, 5′-cyclic monophosphate (a membrane-permeable cAMP analog). Treatment with SQ 22536, an adenylyl cyclase inhibitor, but not ODQ, a soluble guanylyl cyclase inhibitor, significantly attenuated the NOR3-induced vasodilation of retinal arterioles. The COX-1 immunoreactivity was found in retinal blood vessels. The retinal blood vessel was faintly stained for soluble guanylyl cyclase, although the apparent immunoreactivities on mesenteric and choroidal blood vessels were observed. These results suggest that NO exerts a substantial part of its dilatory effect via a mechanism that involves COX-1-dependent pathway in rat retinal vasculature.

C-reactive protein impairs coronary arteriolar dilation to prostacyclin synthase activation: role of peroxynitrite

Endothelium-derived vasodilators, i.e., nitric oxide (NO), prostacyclin (PGI(2)) and prostaglandin E(2) (PGE(2)), play important roles in maintaining cardiovascular homeostasis. C-reactive protein (CRP), a biomarker of inflammation and cardiovascular disease, has been shown to inhibit NO-mediated vasodilation. The goal of this study was to determine whether CRP also affects endothelial arachidonic acid (AA)-prostanoid pathways for vasomotor regulation. Porcine coronary arterioles were isolated and pressurized for vasomotor study, as well as for molecular and biochemical analysis. AA elicited endothelium-dependent vasodilation and PGI(2) release. PGI(2) synthase (PGI(2)-S) inhibitor trans-2-phenyl cyclopropylamine blocked vasodilation to AA but not to serotonin (endothelium-dependent NO-mediated vasodilator). Intraluminal administration of a pathophysiological level of CRP (7 microg/mL, 60 min) attenuated vasodilations to serotonin and AA but not to nitroprusside, exogenous PGI(2), or hydrogen peroxide (endothelium-dependent PGE(2) activator). CRP also reduced basal NO production, caused tyrosine nitration of endothelial PGI(2)-S, and inhibited AA-stimulated PGI(2) release from arterioles. Peroxynitrite scavenger urate failed to restore serotonin dilation, but preserved AA-stimulated PGI(2) release/dilation and prevented PGI(2)-S nitration. NO synthase inhibitor L-NAME and superoxide scavenger TEMPOL also protected AA-induced vasodilation. Collectively, our results suggest that CRP stimulates superoxide production and the subsequent formation of peroxynitrite from basal released NO compromises PGI(2) synthesis, and thus endothelium-dependent PGI(2)-mediated dilation, by inhibiting PGI(2)-S activity through tyrosine nitration. By impairing PGI(2)-S function, and thus PGI(2) release, CRP could promote endothelial dysfunction and participate in the development of coronary artery disease.

Regulation of the fetal mouse ductus arteriosus is dependent on interaction of nitric oxide and COX enzymes in the ductal wall

Nitric oxide (NO) and cyclooxygenase (COX)-derived prostaglandins are critical regulators of the fetal ductus arteriosus. To examine the interaction of these pathways within the ductus wall, the ductus arteriosus of term and preterm fetal mice was evaluated by pressurized myography. The isolated preterm ductus was more sensitive to NOS inhibition than at term. Sequential NOS and COX inhibition caused 36% constriction of the preterm ductus regardless of drug order. In contrast, constriction of the term ductus was dependent on the sequence of inhibition; NOS inhibition prior to COX inhibition produced greater constriction than when inhibitors were given in reverse order (36+/-6% versus 23+/-5%). Selective COX-1 or COX-2 inhibition prior to N(G)-nitro-l-arginine methyl ester (l-NAME) induced the expected degree of constriction. However, NOS inhibition followed by selective COX-2 inhibition caused unexpected ductal dilation. These findings are consistent with NO-induced activation of COX in the ductus arteriosus wall and the production of a COX-2-derived constrictor prostanoid that contributes to the balance of vasoactive forces that maintain fetal ductus arteriosus tone.

Intravenously administered phosphodiesterase 4 inhibitors dilate retinal blood vessels in rats

In the present study, we examined effects of intravenously administered inhibitors of phosphodiesterase 4 (rolipram and 4-(3-butoxy-4-methoxybenzyl)-2-imidazolidinone (Ro-20-1724)) and non-selective inhibitor of phosphodiesterases (theophylline) on diameter of retinal blood vessel and fundus (retinal/choroidal) blood flow in rats. Male Wistar rats (8- to 10-week-old) were treated with tetrodotoxin (50 microg/kg, i.v.) to eliminate any nerve activity and prevent the eye movement under artificial ventilation. Methoxamine was used to maintain adequate systemic circulation. Ocular fundus images were captured with an original high-resolution digital fundus camera for small animals. Diameters of retinal blood vessels contained in the digital images were measured using image-processing softwares on a personal computer. Fundus blood flow was measured using a laser Doppler flow meter. Both rolipram (0.01-10 microg/kg/min, i.v.) and Ro-20-1724 (0.01-10 microg/kg/min, i.v.) increased diameters of retinal blood vessels in a dose-dependent manner without significant effect on systemic blood pressure, heart rate and fundus blood flow. The effects of phosphodiesterase 4 inhibitors on retinal arterioles were greater than those on retinal venules. Similarly, theophylline (0.1-10 mg/kg/min, i.v.) dilated retinal blood vessels, whereas it decreased blood pressure and increased heart rate markedly. These results suggest that phosphodiesterase 4 contributes to maintenance of retinal vascular tone. Inhibitors of phosphodiesterase 4 could be considered as a candidate for therapeutic drugs to treat diseases associated with disorders of retinal circulation without severe cardiovascular side-effects.

trans-Arachidonic acids induce a heme oxygenase-dependent vasorelaxation of cerebral microvasculature

Nitrative stress is an important regulator of vascular tone. We have recently described that trans-arachidonic acids (TAA) are major products of NO(2)(.)-mediated isomerization of arachidonic acid in cell membranes and that nitrative stress increases TAA levels leading to neural microvascular degeneration. In the present study, we explored whether TAA exert acute effects on neuromicrovascular tone and investigated potential mechanisms thereof. TAA induced an endothelium-dependent vasorelaxation of rat brain pial microvasculature. This vasorelaxation was independent of nitric oxide, prostanoids, lipoxygenase products, and CYP(450) metabolite trans-hydroxyeicosatetraenoic acids. However, inhibition of heme oxygenase (using zinc protoporphyrin IX) and of dependent soluble guanylate cyclase (sGC; using ODQ) significantly diminished (by approximately 70%) the TAA-induced vasorelaxation. Consistent with these findings, TAA stimulated heme oxygenase (HO)-2-dependent bilirubin (using siRNA HO-2) and cGMP formation, and the HO product carbon monoxide (using CO-releasing CORM-2) reproduced the sGC-dependent cGMP formation and vasorelaxation. Further exploration revealed that TAA-induced vasorelaxation and bilirubin formation (HO activation) were nearly abrogated by large-conductance calcium-dependent potassium channels (BK(Ca)) (using TEA and iberiotoxin). Opening of BK(Ca) with the selective activator NS1619 induced a concentration-dependent vasorelaxation, which was inhibited by HO and sGC inhibitors. Coimmunoprecipitation suggested a molecular complex interaction between BK(Ca) and HO-2 (but not HO-1). Collectively, these findings identify new properties of TAA, specifically cerebral vasorelaxation through interactive activation of BK(Ca) with HO-2 and, in turn, sGC. Our findings provide new insights into the characterization of nitrative stress-derived TAA products, by showing they can act as acute mediators of nitrative stress on neurovascular tone.

Vasodilator effect of nicorandil on retinal blood vessels in rats

We examined the effect of nicorandil on retinal blood vessels in rats in vivo. Male Wistar rats (8 to 10 weeks old) were anaesthetised with thiobutabarbital (120 mg/kg, intraperitoneal). Fundus images were captured with a digital camera that was equipped with a special objective lens. Diameters of retinal blood vessels were measured with a personal computer. Nicorandil (1-300 microg kg(-1) min(-1), intravenous [i.v.]) increased diameters of retinal blood vessels and decreased systemic blood pressure in a dose-dependent manner. Both responses to nicorandil were attenuated by glibenclamide (20 mg/kg, i.v.), an adenosine triphosphate (ATP)-dependent K(+) (K(ATP)) channel blocker. On the other hand, indomethacin (5 mg/kg, i.v.), a cyclooxygenase inhibitor, attenuated the vasodilation of retinal blood vessels, but not depressor response, to nicorandil and sodium nitroprusside. Pinacidil (1-300 microg kg(-1) min(-1), i.v.), a K(ATP) channel opener, also dilated retinal blood vessels and decreased systemic blood pressure. The responses to pinacidil were prevented by glibenclamide, but not by indomethacin. The vasodilation of retinal arteriole, but not depressor response, to sodium nitroprusside (1-30 microg kg(-1) min(-1), i.v.), a nitric oxide donor, was attenuated by indomethacin. These results suggest that nicorandil dilates retinal blood vessels through opening of K(ATP) channels and production of prostaglandins that are probably generated by nitric oxide.

Structure-based design and synthesis of N(omega)-nitro-L-arginine-containing peptidomimetics as selective inhibitors of neuronal nitric oxide synthase. Displacement of the heme structural water

The neuronal isoform of nitric oxide synthase (nNOS), the enzyme responsible for the production of nitric oxide in the central nervous system, represents an attractive target for the treatment of various neurodegenerative disorders. X-ray crystal structures of complexes of nNOS with two nNOS-selective inhibitors, (4S)-N-{4-amino-5-[(2-aminoethylamino]pentyl}-N'-nitroguanidine (1) and 4-N-(Nomega-nitro-l-argininyl)-trans-4-amino-l-proline amide (2), led to the discovery of a conserved structural water molecule that was hydrogen bonded between the two heme propionates and the inhibitors (Figure 2). On the basis of this observation, we hypothesized that by attaching a hydrogen bond donor group to the amide nitrogen of 2 or to the secondary amine nitrogen of 1, the inhibitor molecules could displace the structural water molecule and obtain a direct interaction with the heme cofactor. To test this hypothesis, peptidomimetic analogues 3-5, which have either an N-hydroxyl (3 and 5) or N-amino (4) donor group, were designed and synthesized. X-ray crystal structures of nNOS with inhibitors 3 and 5 bound verified that the N-hydroxyl group had, indeed, displaced the structural water molecule and provided a direct interaction with the heme propionate moiety (Figures 5 and 6). Surprisingly, in vitro activity assay results indicated that the addition of a hydroxyl group (3) only increased the potency slightly against the neuronal isoform over the parent compound (1). Rationalizations for the small increase in potency are consistent with other changes in the crystal structures.

Interaction of Endothelial Nitric Oxide and Angiotensin in the Circulation

Discovery of the unexpected intercellular messenger and transmitter nitric oxide (NO) was the highlight of highly competitive investigations to identify the nature of endothelium-derived relaxing factor. This labile, gaseous molecule plays obligatory roles as one of the most promising physiological regulators in cardiovascular function. Its biological effects include vasodilatation, increased regional blood perfusion, lowering of systemic blood pressure, and antithrombosis and anti-atherosclerosis effects, which counteract the vascular actions of endogenous angiotensin (ANG) II. Interactions of these vasodilator and vasoconstrictor substances in the circulation have been a topic that has drawn the special interest of both cardiovascular researchers and clinicians. Therapeutic agents that inhibit the synthesis and action of ANG II are widely accepted to be essential in treating circulatory and metabolic dysfunctions, including hypertension and diabetes mellitus, and increased availability of NO is one of the most important pharmacological mechanisms underlying their beneficial actions. ANG II provokes vascular actions through various receptor subtypes (AT1, AT2, and AT4), which are differently involved in NO synthesis and actions. ANG II and its derivatives, ANG III, ANG IV, and ANG-(1-7), alter vascular contractility with different mechanisms of action in relation to NO. This review article summarizes information concerning advances in research on interactions between NO and ANG in reference to ANG receptor subtypes, radical oxygen species, particularly superoxide anions, ANG-converting enzyme inhibitors, and ANG receptor blockers in patients with cardiovascular disease, healthy individuals, and experimental animals. Interactions of ANG and endothelium-derived relaxing factor other than NO, such as prostaglandin I2 and endothelium-derived hyperpolarizing factor, are also described.

Intravenously administered vasodilatory prostaglandins increase retinal and choroidal blood flow in rats

We established an experimental system for measuring blood flow in the rat fundus and examined whether intravenously administered vasodilatory prostaglandins (PGE(1), PGE(2), and PGI(2)), 8-(4-chlorophenylthio)-cAMP (a cAMP analogue), and nicardipine (a Ca(2+)-channel blocker) increase fundus blood flow (FBF). Under artificial ventilation, rats were injected with tetrodotoxin (50 microg/kg, i.v.) to eliminate any nerve activity and prevent movement of the eye. After tetrodotoxin, the rats were infused with norepinephrine (0.3 - 0.5 microg . kg(-1) . min(-1)) and epinephrine (2.7 - 4.5 microg . kg(-1) . min(-1)) simultaneously to maintain adequate systemic circulation. We found that intravenous infusion of PGE(1) (2 - 10 microg . kg(-1) . min(-1)), PGE(2) (3 - 30 microg . kg(-1) . min(-1)), and PGI(2) (1 - 10 microg . kg(-1) . min(-1)) increased the FBF in a dose-dependent manner. The vasodilatory PGs decreased arterial pressure, whereas they did not affect heart rate. Like vasodilatory PGs, 8-(4-chlorophenylthio)-cAMP (30 micromol/kg, i.v.) increased FBF and decreased arterial pressure. While infusion of nicardipine (0.3 - 3 microg . kg(-1) . min(-1)) produced comparable depressor responses with those to vasodilatory PGs and the cAMP analogue, it did not increase FBF. These results suggest that vasodilatory PGs and cAMP act more selectively than Ca(2+)-channel blockers on retinal/choroidal blood vessels. Therefore, the vasodilatory PGs might be considered to be possible candidates for the therapeutics to treat disorders of retinal/choroidal circulation.

Regulation of iodothyronine deiodinase and roles of thyroid hormones in human coronary artery smooth muscle cells

Thyroid hormones have been reported to have significant effects on the peripheral vascular system, including relaxation of vascular smooth muscle cells and prevention of atherosclerosis. To exert its biological activity, thyroxine (T4) needs to be converted to 3,5,3'-triiodothyronine (T3) by type 1 and type 2 iodothyronine deiodinases. We have previously identified type 2 iodothyronine deiodinase (D2) expression in cultured human coronary artery smooth muscle cells (hCASMCs). In the present study, we have characterized the regulation of D2 expression in hCASMCs by stable prostacyclin analogue beraprost sodium (BPS) and platelet derived growth factor (PDGF), and the roles of thyroid hormones in the functions of hCASMCs. BPS increased D2 expression, whereas PDGF suppressed BPS stimulated D2 expression without affecting cAMP production in hCASMCs. PDGF increased DNA synthesis, while BPS, T3 or T4 suppressed PDGF stimulated DNA synthesis in hCASMCs. Inhibition of D2 activity by 3,3',5'-triiodothyronine (rT3) partially restored T4 suppression of PDGF stimulated DNA synthesis in hCASMCs. PDGF increased migration activity, whereas BPS, T3 or T4 suppressed PDGF stimulated migration activity of hCASMCs. These results suggest that D2 expression is increased by BPS and suppressed by PDGF in hCASMCs, and that intracellular thyroid hormone activation may be involved in the suppression of DNA synthesis and migration activity of hCASMCs.

New insights into the retinal circulation: inflammatory lipid mediators in ischemic retinopathy

Ischemic proliferative retinopathy develops in various retinal disorders, including retinal vein occlusion, diabetic retinopathy and retinopathy of prematurity. Ischemic retinopathy remains a common cause of visual impairment and blindness in the industrialized world due to relatively ineffective treatment. Oxygen-induced retinopathy (OIR) is an established model of retinopathy of prematurity associated with vascular cell injury culminating in microvascular degeneration, which precedes an abnormal neovascularization. The retina is a tissue particularly rich in polyunsaturated fatty acids and the ischemic retina becomes highly sensitive to lipid peroxidation initiated by oxygenated free radicals. Consequently, the retina constitutes an excellent model for testing the functional consequences of membrane lipid peroxidation. Retinal tissue responds to physiological and pathophysiological stimuli by the activation of phospholipases and the consequent release from membrane phospholipids of biologically active metabolites. Activation of phospholipase A(2) is the first step in the synthesis of two important classes of lipid second messengers, the eicosanoids and a membrane-derived phospholipid mediator platelet-activating factor (PAF). These lipid mediators accumulate in the retina in response to injury and a physiologic role of these metabolites in retinal vasculature remains for the most part to be determined; albeit proposed roles have been suggested for some. The eicosanoids, in particular the prostanoids, thromboxane (TXA2) and PAF are abundantly generated following an oxidant stress and contribute to neurovascular injury. TXA2 and PAF play an important role in the retinal microvacular degeneration of OIR by directly inducing endothelial cell death and potentially could contribute to the pathogenesis of ischemic retinopathies. Despite these advances there are still a number of important questions that remain to be answered before we can confidently target pathological signals. This review focuses on mechanisms that precede the development of neovascularization, most notably regarding the role of lipid mediators that partake in microvascular degeneration.

Disturbance of vision by COX-2 inhibitors

In this review, 35 cases of acute, reversible, sometimes severe, disturbances of vision closely associated with the use of celecoxib or rofecoxib are described. These were identified from three different databases using strict selection criteria. The events included temporary blindness, visual field defect, scotoma, teichopsia, blurred vision, decreased vision and abnormal vision. The reactions had a mean onset time of 9.5 days and recovery occurred within 3 days following withdrawal of the drug. The reactions do not appear to be related to age, gender, dose, or indication for use. The incidence of reported cases is estimated to be not less than 5 per 10,000 patients. Possible mechanisms for this type of reaction are described. The most likely appears to be the result of interference with the retinal blood supply through reduced production of prostanoids. Genetic polymorphisms that affect drug metabolism or uptake could be risk factors and are discussed along with suggestions for research.

Role of PGI2 in the formation and maintenance of hyperdynamic circulatory state of portal hypertensive rats

AIM: To investigate the role of prostacyclin (PGI₂) and nitric oxide (NO) in the development and maintenance of hyperdynamic circulatory state of chronic portal hypertensive rats.

METHODS: Ninety male Sprague-Dawley rats were divided into three groups: intrahepatic portal hypertension (IHPH) group by injection of CCl₄, prehepatic portal hypertension (PHPH) group by partial stenosis of the portal vein and sham-operation control (SO) group. One week after the models were made, animals in each group were subdivided into 4 groups: saline controlled group (n = 23), Nω-nitro-L-arginine (L-NNA)group (n = 21) group, indomethacin (INDO) group (n = 22) and high-dose heparin group (n = 24). The rats were administrated 1mL of saline, L-NNA (3.3 mg/kg·d) and INDO (5 mg/kg·d) respectively through gastric tubes for one week,then heparin (200 IU/Kg/min) was given to rats by intravenous injection for an hour. Splanchnic and systemic hemodynamics were measured using radioactive microsphere techniques. The serum nitrate/nitrite(NO₂^-/NO₃^-) levels as a marker of production of NO were assessed by a colorimetric method, and concentration of 6-keto-PGF1α, a stable hydrolytic product of PGI₂, was determined by radioimmunoassay.

RESULTS: The concentrations of plasma 6-keto-PGF1α (pg/mL) and serum NO₂^-/NO₃^- (μmol/L) in IHPH rats (1123.85±153.64, 73.34±4.31) and PHPH rats (891.88±83.11, 75.21±6.89) were significantly higher than those in SO rats(725.53±105.54, 58.79±8.47) (P<0.05). Compared with SO rats, total peripheral vascular resistance (TPR) and splanchnic vascular resistance (SVR) decreased but cardiac index (CI) and portal venous inflow (PVI) increased obviously in IHPH and PHPH rats (P<0.05). L-NNA and indomethacin could decrease the concentrations of plasma 6-keto-PGF1α and serum NO₂^-/NO₃^-in IHPH and PHPH rats (P<0.05) .Meanwhile, CI, FPP and PVI lowered but MAP,TPR and SVR increased(P<0.05). After deduction of the action of NO, there was no significant correlation between plasma PGI₂ level and hemodynamic parameters such as CI, TPR, PVI and SVR. However, after deduction of the action of PGI₂, NO still correlated highly with the hemodynamic parameters, indicating that there was a close correlation between NO and the hemodynamic parameters. After administration of high-dose heparin, plasma 6-keto-PGF₁α concentrations in IHPH, PHPH and SO rats were significantly higher than those in rats administrated vehicle (P<0.05). On the contrary, levels of serum NO₂^-/NO₃^- in IHPH, PHPH and SO rats were significantly lower than those in rats administrated Vehicle (P<0.05). Compared with those rats administrated vehicle, the hemodynamic parameters of portal hypertensive rats, such as CI and PVI, declined significantly after administration of high-dose heparin (P<0.05), while TPR and SVR increased significantly (P<0.05).

CONCLUSION: It is NO rather than PGI₂ that is a mediator in the formation and maintenance of hyperdynamic circulatory state of chronic portal hypertensive rats.

Effect of estrogen on cerebrovascular prostaglandins is amplified in mice with dysfunctional NOS

Chronic estrogen treatment increases endothelial vasodilator function in cerebral arteries. Endothelial nitric oxide (NO) synthase (eNOS) is a primary target of the hormone, but other endothelial factors may be modulated as well. In light of possible interactions between NO and prostaglandins, we tested the hypothesis that estrogen treatment increases prostanoid-mediated dilation using NOS-deficient female mouse models, i.e., mice treated with a NOS inhibitor [N(G)-nitro-l-arginine methyl ester (l-NAME)] for 21 days or transgenic mice with the eNOS gene disrupted (eNOS(-/-)). All mice were ovariectomized; some in each group were treated chronically with estrogen. Cerebral blood vessels then were isolated for biochemical and functional analyses. In vessels from control mice, estrogen increased protein levels of eNOS but had no significant effect on cyclooxygenase (COX)-1 protein, prostacyclin production, or constriction of pressurized, middle cerebral arteries to indomethacin, a COX inhibitor. In l-NAME-treated mice, however, cerebrovascular COX-1 levels, prostacyclin production, and constriction to indomethacin, as well as eNOS protein, were all greater in estrogen-treated animals. In vessels from eNOS(-/-) mice, estrogen treatment also increased levels of COX-1 protein and constriction to indomethacin, but no effect on prostacyclin production was detected. Thus cerebral blood vessels of control mice did not exhibit effects of estrogen on the prostacyclin pathway. However, when NO production was dysfunctional, the impact of estrogen on a COX-sensitive vasodilator was revealed. Estrogen has multiple endothelial targets; estrogen effects may be modified by interactions among these factors.

Adenylyl and guanylyl cyclase activity in the choroid

The choroid, a low-resistance vascular structure provides nourishment to and removal of potential toxic waste products from the adjacent non-vascularized outer layers of the retina, macula, and optic disc region and may be involved in the pathology of normal tension glaucoma. This study is aimed at delineating the biochemical pharmacology of vascular tone control in this tissue. By using a procedure to harvest fresh choroidal tissue, we studied some basic characteristics of the adenylyl (AC) and guanylyl cyclase (GC) enzyme systems in this tissue using the 3H-cAMP/32P-ATP tracer method. Compared to respective baseline measurements (100+/-SEM%), AC activity was stimulated (p < 0.05) by forskolin (FSK, 477+/-59%), fluoroaluminate (AlF(4), 360+/-10.3%), isoproterenol (ISO, 129+/-5.5%), vasoactive intestinal peptide (VIP, 132+/-6.1%), calmodulin (CAM)+Mn2+ (196+/-69%), and dose-dependently by prostaglandin (PG) E2 (up to 162+/-3.6%). The antagonist drug calmidazolium inhibited the CAM-dependent increase but also blocked basal activity (47+/-2.0%) without affecting the FSK response. Other CAM blockers (TFP, W5) produced similar results but were not completely selective for CAM-activated cyclase. GDPbetaS, a blocker of G-protein coupling to AC did not affect AC responses to FSK, ISO, and AlF4, but decreased the response to PGE2. N-ethyl-carboxamido adenosine (NECA), which activates adenosine A2 receptors, did not synergize with FSK or add to ISO responses and did not activate AC by itself. In the GC system activity was stimulated (p< 0.05) by CAM+Mn2+ (239+/-27%), by atrial natriuretic peptide (ANP, up to 143+/-1.4%) and sodium nitroprusside (SNP, up to 179+/-1.6%). These results show that choroidal tissue has significant activities of the adenylyl and guanylyl cyclase second messenger systems potentially responsive to hormones/neurotransmitters that may control the degree of relaxation in this vascular tissue.

Early upregulation of kinin B1 receptors in retinal microvessels of the streptozotocin-diabetic rat

(1) Retinal microvessel responses to kinin B1 and B2 receptor agonists and antagonists were investigated in streptozotocin (STZ)-diabetic rats and age-matched controls. In addition, quantitative in vitro autoradiography was performed on retinas from control and STZ-diabetic rats with radioligands specific for B2 ([125I]HPP-Hoe 140), and B1 receptors ([125I]HPP-[des-Arg10]-Hoe 140). (2) In control rats, the B2 receptor agonist bradykinin (BK, 0.1-50 nm) vasodilated retinal vessels in a concentration and time-dependent manner. This effect was completely blocked by the B2 receptor antagonist Hoe140 (1 microm). In contrast, the B1 receptor agonist des-Arg9-BK (0.1-50 nm) was without effect. (3) Des-Arg9-BK was able to produce a concentration-dependent vasodilatation as early as 4 days after STZ injection, and the effect of 1 nm des-Arg9-BK was inhibited by the B1 receptor antagonist des-Arg10-Hoe140 (1 microm). Low-level B1 receptor binding sites were detected in control rats, but densities were 256% higher in retinas from 4- to 21-day STZ-diabetic rats. (4) In control rats, the vasodilatation in response to 1 nm BK involved neither calcium influx nor nitric oxide (NO) as GdCl3 and l-NAME were without effect. However, the vasodilatation did involve intracellular calcium mobilization as well as products of the cyclooxygenase-2 (COX-2) pathway as 2,5-di-t-butylhydroquinone (BHQ), cADP ribose and l-745 337 inhibited this response. The vasodilatation response was blocked by trans-2-phenyl cyclopropylamine (TPC) demonstrating that prostacyclins mediate this response. (5) In STZ-diabetic rats, the vasodilatation in response to des-Arg9-BK involved both calcium influx and intracellular calcium mobilization from stores both IP3 sensitive and non-IP3 sensitive. Indeed, the effect was blocked by GdCl3, BHQ and cADP ribose. Furthermore, NO production and products of the COX-2 pathway including prostacyclin are involved as the response was inhibited by l-NAME, l-745 377 and TPC. (6) Vasodilatation in response to either 1 nm BK or 1 nm des-Arg9-BK were blocked by NF023 demonstrating that a Go/Gi G-protein transduces both these effects. (7) This is the first report on the retinal circulation which provides evidence for vasodilator B2 receptors and the upregulation of B1 receptors very early following induction of diabetes with STZ rats. These results suggest that kinin receptors may be potential targets for therapeutics to treat retinopathies.

Interaction between nitric oxide and prostanoids in arterioles of rat cremaster muscle in vivo

We studied in vivo interactions of nitric oxide (NO), oxidative stress, and prostanoids derived from the cyclooxygenase pathway in the arterioles studied by intravital microscopy in peripheral muscle. Topical administration of NO synthase (NOS) inhibitor Nomega-nitro-l-arginine (l-NNA) or cyclooxygenase inhibitor mefenamic acid (MA) alone leads to vasoconstriction. We found that l-NNA after MA induced an additional constriction, whereas MA after l-NNA induced a relative dilation. Therefore, an additional constriction was found when MA was administered after l-NNA in the presence of the thromboxane A2 synthase-PGH2 (TP) receptor antagonist SQ-29548. We also found a relative dilation when the TP receptor antagonist was administered after NOS inhibition by l-NNA. In the presence of superoxide dismutase and catalase, l-NNA-induced vasoconstriction is reduced, and the dilation observed after addition of MA in presence of the reactive oxygen species is no longer present. Taken together, these results showed that NO inhibition induced a shift in the synthesis or in the effects of cyclooxygenase products, in favor of constrictor prostanoids. This effect of NO inhibition disappears when reactive oxygen species are scavenged by superoxide dismutase and catalase.

Role of cyclooxygenase and haemoxygenase products in nitric oxide-independent vasodilatation in the porcine ciliary artery

PURPOSE

Vascular endothelial cell dysfunction has been noted in patients with normal pressure glaucoma. Although nitric oxide (NO) accounts for a large proportion of vasorelaxation in the posterior ciliary artery, considerable relaxation remains unexplained. We investigated the roles of haemoxygenase (HO) and cyclooxygenase (COX), which produce the vasodilators carbon monoxide (CO) and prostacyclin, respectively, in NO-independent endothelium-dependent vasodilatation in porcine posterior ciliary arteries.

METHODS

Isolated vascular rings were mounted in a Mulvaney-Halpern small vessel myograph for the measurement of isometric tension development. Vasodilator responses to bradykinin (BK) were elicited in each ring on three separate occasions following preconstriction with prostaglandin F(2alpha): first in the absence of inhibitors, second in the presence of the NO synthase inhibitor N-nitro-L-arginine methyl ester (L-NAME, 10(-3) M), and third in the presence of L-NAME and either a COX (indomethacin, 10(-6) M) or an HO inhibitor (tin protoporphyrin-IX 10(-5) M). Results were expressed as a percentage of the maximal relaxation in the presence of L-NAME alone.

RESULTS

Incubation with indomethacin (n=6), in the presence of L-NAME, significantly reduced (P<0.01) maximum BK-induced relaxation (-103.5+/-8.8%) compared to paired rings in the presence of L-NAME alone (-130.8+/-8.8%). HO inhibition did not reduce NO-independent, BK-induced relaxation when compared to paired control vessels.

CONCLUSIONS

These data suggest that in the presence of L-NAME, a COX product accounts for a significant proportion of NO-independent vasodilatation. In contrast, endogenous CO production does not have a functionally significant role in the porcine ciliary artery. Eye (2003) 17, 628-636. doi:10.1038/sj.eye.6700437

Prostaglandins are involved in acetylcholine- and 5-hydroxytryptamine-induced, nitric oxide-mediated vasodilatation in human forearm

Both acetylcholine (ACh) and 5-hydroxytryptamine (5HT) are used to examine nitric oxide (NO)-mediated vasodilatation in humans. Animal data suggest that both substances can also induce the release of prostacyclin (PGI ). This study was designed to investigate the role of the prostaglandin pathway in Ach- and 5HT-induced vasodilation in humans. The experiments were done in three groups of healthy male volunteers. In group 1 (n = 6), ACh (100-1,000 ng/kg/min) and sodium-nitroprusside (10-100 ng/kg/min) were infused into the brachial artery alone, together with a continuous infusion of indomethacin (1.3 micro g/kg/min) and during a combined infusion of indomethacin and the competitive NO synthase inhibitor N -monomethyl-l-arginine (l-NMMA; 30 micro g/kg/min). In group 2 (n = 5), 5HT (0.3-1.0 ng/kg/min) was infused alone and together with a continuous infusion of indomethacin and l-NMMA. In group 3 (n = 6), the synthetic prostaglandin analog iloprost (0.5-4.5 ng/kg/min) was infused together with a continuous infusion of saline, l-NMMA, and l-NMMA with indomethacin, respectively. The infusions of indomethacin and l-NMMA started 10 min before the infusion of ACh, 5HT, iloprost, and sodium nitroprusside. Forearm blood flow was measured using computerized venous occlusion plethysmography. Both the Ach- and 5HT-induced vasodilator responses were significantly attenuated by indomethacin (p < 0.05 for both), but not further influenced by a concomitant infusion of l-NMMA. The vasodilatation induced by iloprost was significantly inhibited by l-NMMA (p < 0.05) and not affected by indomethacin. The sodium nitroprusside-induced vasodilation was influenced by neither l-NMMA nor indomethacin. It is concluded that in the human forearm, the prostaglandin pathway is involved in both the Ach- and 5HT-induced NO-mediated vasodilatation.

NO and prostaglandin interactions during hemodynamic stress in the fetal ovine pulmonary circulation

Nitric oxide (NO) and prostacyclin (PGI(2)) are potent fetal pulmonary vasodilators, but their relative roles and interactions in the regulation of the perinatal pulmonary circulation are poorly understood. We compared the separate and combined effects of nitric oxide synthase (NOS) and cyclooxygenase (COX) inhibition during acute hemodynamic stress caused by brief mechanical compression of the ductus arteriosus (DA) in chronically prepared fetal lambs. Nitro-L-arginine (L-NNA; NOS antagonist), meclofenamate (Mec; COX inhibitor), combined drugs (L-NNA-Mec), or saline (control) was infused into the left pulmonary artery (LPA) before DA compression. In controls, DA compression decreased pulmonary vascular resistance (PVR) by 43% (P < 0.01). L-NNA, but not Mec, treatment completely blocked vasodilation and caused a paradoxical increase in PVR (+31%; P < 0.05). The effects of L-NNA-Mec and L-NNA on PVR were similar. To determine if the vasodilator effect of PGI(2) is partly mediated by NO release, we studied PGI(2)-induced vasodilation before and after NOS inhibition. L-NNA treatment blocked the PGI(2)-induced rise in LPA blood flow by 73% (P < 0.001). We conclude that NO has a greater role than PGs in fetal pulmonary vasoregulation during acute hemodynamic stress and that PGI(2)-induced pulmonary vasodilation is largely mediated by NO release in the fetal lung.

2,3-Dinor-5,6-dihydro-15-F(2t)-isoprostane: a bioactive prostanoid metabolite

15-F(2t)-isoprostane (15-F(2t)-IsoP), also termed 8-isoprostaglandin F(2alpha), is one of a series of prostanoids formed by free radical-mediated peroxidation of arachidonic acid and exerts potent biological actions such as vasoconstriction. We recently demonstrated that 15-F(2t)-IsoP is metabolized in humans to a major metabolite, 2,3-dinor-5,6-dihydro-15-F(2t)-IsoP (15-F(2t)-IsoP-M). 15-F(2t)-IsoP-M can also potentially be formed as a product of free radical-induced oxidation of the low abundance fatty acid gamma-linolenic acid. We confirmed that 15-F(2t)-IsoP-M is generated during oxidation of gamma-linolenic acid and explored whether it may exhibit biological activity. 15-F(2t)-IsoP-M caused marked constriction of porcine surface retinal and intraparenchymal brain microvessels, comparable to that observed with 15-F(2t)-IsoP. These effects were associated with increased thromboxane A(2) (TXA(2)) formation and were virtually abolished by TXA(2)-synthase and -receptor inhibitors (CGS-12970 and L-670596). Vasoconstriction induced by either 15-F(2t)-IsoP or 15-F(2t)-IsoP-M on perfused ocular choroid was also abrogated by TXA(2)-synthase inhibition as well as by removal of endothelium. Similar to 15-F(2t)-IsoP, 15-F(2t)-IsoP-M evoked vasoconstriction and TXA(2) generation by activating Ca(2+) influx from nonvoltage-gated channels (SK&F96365 sensitive) in the retina and from both nonvoltage- and N-type voltage-gated Ca(2+) channels (omega-conotoxin MVIIA sensitive), respectively, in brain endothelial and astroglial cells; smooth muscle cells were unresponsive to both agents. Cross-desensitization experiments further suggest that 15-F(2t)-IsoP and 15-F(2t)-IsoP-M act on the same receptor mechanism. Findings reveal a novel concept by which a beta-oxidation metabolite of 15-F(2t)-IsoP that can also be formed by nonenzymatic oxidation of gamma-linolenic acid is equivalently bioactive to 15-F(2t)-IsoP and may prolong the vascular actions of F(2)-IsoPs.

Studies on the effect of diadenlyated nucleotides on calcium mobilization and prostacyclin synthesis in bovine aortic endothelial cells

Extracellular adenine dinucleotides are modulators of blood vessel tone. We have previously demonstrated that Ap(2)A and Ap(4)A induce the synthesis of nitric oxide (NO) from bovine aortic endothelial cells (BAEC) while Ap(3)A and Ap(5)A do not [FEBS Lett. 427 (1998) 320; Arch. Biochem. Biophys. 364 (1999) 280.]. In this communication we determine the effect of Ap(x)As (x=2-5) on prostacyclin (PGI(2)) synthesis and Ca(2+) mobilization in BAEC. Ap(2)A and Ap(4)A significantly enhanced the synthesis of PGI(2) while Ap(3)A and Ap(5)A do not. These data support the notion that Ap(2)A and Ap(4)A are vasodilators. All four dinucleotides significantly enhanced Ca(2+) mobilization over basal levels. Ap(5)A and Ap(3)A enhanced 2.0 and 1.6 times more Ca(2+) release than Ap(4)A, respectively. Since neither Ap(5)A nor Ap(3)A enhanced the synthesis of either PGI(2) or NO but did mobilize Ca(2+), these data support the hypothesis that in BAEC Ca(2+) release is localized or compartmentalized.

Synthesis and biological evaluation of novel pyrazoles and indazoles as activators of the nitric oxide receptor, soluble guanylate cyclase

Database searching and compound screening identified 1-benzyl-3-(3-dimethylaminopropyloxy)indazole (benzydamine, 3) as a potent activator of the nitric oxide receptor, soluble guanylate cyclase. A comprehensive structure-activity relationship study surrounding 3 clearly showed that the indazole C-3 dimethylaminopropyloxy substituent was critical for enzyme activity. However replacement of the indazole ring of 3 by appropriately substituted pyrazoles maintained enzyme activity. Compounds were evaluated for inhibition of platelet aggregation and showed a general lipophilicity requirement. Aryl-substituted pyrazoles 32, 34, and 43 demonstrated potent activation of soluble guanylate cyclase and potent inhibition of platelet aggregation. Pharmacokinetic studies in rats showed that compound 32 exhibits modest oral bioavailability (12%). Furthermore 32 has an excellent selectivity profile notably showing no significant inhibition of phosphodiesterases or nitric oxide synthases.

Retinal expression, regulation, and functional bioactivity of prostacyclin-stimulating factor

Prostacyclin-stimulating factor (PSF) acts on vascular endothelial cells to stimulate the synthesis of the vasodilatory molecule prostacyclin (PGI2). We have examined the expression, regulation, and hemodynamic bioactivity of PSF both in whole retina and in cultured cells derived from this tissue. PSF was expressed in all retinal cell types examined in vitro, but immunohistochemical analysis revealed PSF mainly associated with retinal vessels. PSF expression was constitutive in retinal pericytes (RPCs) but could be modulated in bovine retinal capillary endothelial cells (RECs) by cell confluency, hypoxia, serum starvation, high glucose concentrations, or inversely by soluble factors present in early vs. late retinopathy, such as TGF-beta, VEGF, or bFGF. In addition, RPC-conditioned media dramatically increased REC PGI2 production, a response inhibited by blocking PSF with a specific antisense oligodeoxynucleotide (ODN). In vivo, PGI2 increased retinal blood flow (RBF) in control and diabetic animals. Furthermore, the early drop in RBF during the initial weeks after inducing diabetes in rats, as well as the later increase in RBF, both correlated with levels of retinal PSF. RBF also responded to treatment with RPC-conditioned media, and this effect could be partially blocked using the antisense PSF ODN. We conclude that PSF expressed by ocular cells can induce PGI2, retinal vascular dilation, and increased retinal blood flow, and that alterations in retinal PSF expression may explain the biphasic changes in RBF observed in diabetes.