Endothelium-derived relaxing factor in brain blood vessels is not nitric oxide.

BACKGROUND: The endothelium-derived relaxing factor that mediates the actions of acetylcholine is now most frequently identified as nitric oxide. Nitric oxide is believed to have numerous important regulating actions in neurons, blood vessels, and several other biological systems. SUMMARY OF REVIEW: The literature concerning tissue other than cerebral blood vessels supports the conclusion that the endothelium-derived relaxing factor for acetylcholine is either nitric oxide or a compound formed from and containing nitric oxide (for example, a nitrosothiol). However, papers can be found indicating that this endothelium-derived mediator is not nitric oxide. In brain blood vessels the evidence is strongly against the conclusion that nitric oxide is the endothelium-derived mediator for acetylcholine. If this mediator is formed from nitric oxide, either in brain vessels or in other vessels, no data are available delineating how this synthesis is regulated or whether and where nitric oxide leaves the nitroso compound to initiate dilation. Indeed, cerebrovascular data now cast doubt on the commonly held belief that nitrosovasodilators regulate vascular tone by giving off nitric oxide to vascular smooth muscle. CONCLUSIONS: In brain blood vessels the chemical identity of the endothelium-derived relaxing factor mediating the action of acetylcholine is unknown, but this relaxing factor does not appear to be nitric oxide. If the mediator contains nitric oxide, as is probably the case, the means by which it activates vascular guanylate cyclase and/or produces dilation is unknown. Since this relaxing factor inhibits platelet adhesion/aggregation in cerebral vessels as well as relaxing these vessels, the chemical identification of this relaxing factor and the elucidation of its mode of action are extremely important to our understanding and control of cerebrovascular phenomena in health and disease.     

Endothelium-derived relaxing factor modulates noradrenergic constriction of cerebral arterioles in rabbits.

BACKGROUND AND PURPOSE: Cerebral arterioles are relatively unresponsive to norepinephrine. We tested the hypothesis that release of endothelium-derived relaxing factor is stimulated by norepinephrine and attenuates adrenergic constriction of pial arterioles. METHODS: In seven anesthetized New Zealand White rabbits, diameter of pial arterioles was measured through a cranial window. Responses to topical application of norepinephrine and arginine vasopressin were examined before and during application of NG-nitro-L-arginine, which inhibits synthesis of endothelium-derived relaxing factor. RESULTS: Norepinephrine (10(-6) M) had no effect (0 +/- 3%, mean +/- SE) on arteriolar diameter under basal conditions. Norepinephrine decreased arteriolar diameter by 15 +/- 4% during application of nitro-L-arginine (10(-4) M) (p less than 0.05 versus basal response). L-arginine inhibited the effect of nitro-L-arginine on responses of pial arterioles to norepinephrine. In contrast to norepinephrine, constrictor responses of pial arterioles to vasopressin were not potentiated by nitro-L-arginine. CONCLUSIONS: Norepinephrine, but not arginine vasopressin, releases endothelium-derived relaxing factor, which inhibits constrictor responses of cerebral arterioles in rabbits. This mechanism contributes to the finding that cerebral vessels in rabbits are relatively unresponsive to noradrenergic stimuli.     

Transient cerebral vasodilatory effect of neuropeptide y mediated by nitric oxide

The effects of intracarotidly injected neuropeptide Y (NPY; 0.1 μg/kg) on the local cerebral blood volume (CBV) and blood flow (CBF) in the parieto-temporal cortex were examined by the photoelectric method in 17 anesthetized cats. CBV reflects the cumulative crosssectional area of the cerebral microvascular beds. NPY immediately caused transient but significant increases in CBV and CBF, which lasted for less than 5 min. Thereafter, CBV returned to and remained at the control level, although CBF was decreased by 30–40% for 60 min during the monitoring period. The CBV increases after NPY were prevented by a 15-min preinjection of 0.35 mg/kg/min of N^g-monomethyl-L-arginine (L-NMMA), which is a competitive blocker of nitric oxide synthesis. The CBV increases after NPY reappeared following a 15-min administration of 0.25 mg/kg/min of L-arginine, which is a precursor of nitric oxide. We conclude that NPY administered in vivo exerts a previously unreported effect of transient vasodilatation on the cerebral microvessels. This action appears to be mediated by nitric oxide, which is a major candidate as an endothelium-derived relaxing factor (EDRF).     

Endothelium-derived relaxing factor inhibits constrictor responses of large cerebral arteries to serotonin.

Endothelium-derived relaxing factor [EDRF, nitric oxide (NO) or a NO-containing compound] influences basal tone of cerebral blood vessels and mediates vasodilation in response to several stimuli. It is not known whether EDRF also modulates responses to cerebral vasoconstrictor stimuli in vivo. Our goal was to determine whether formation of EDRF inhibits constrictor responses of large cerebral arteries to serotonin. We measured cerebral blood flow (microspheres) and pial microvascular pressure (servo null) in anesthetized rabbits and calculated resistance of large cerebral arteries. Responses to an inhibitor of NO formation, NG-nitro-L-arginine (L-NNA, 3 mg/kg i.v.), were examined. L-NNA produced an increase in resistance of large arteries and total cerebral vascular resistance of approximately 15% (p less than 0.05 for both variables) and a small decrease in cerebral blood flow (35 +/- 9 vs. 32 +/- 7 ml min-1 100 g-1, mean +/- SD, p less than 0.05). Under control conditions, infusion of serotonin (10 micrograms kg-1 min-1, into the left atrium) produced an increase in resistance of large arteries. Following treatment with L-NNA, the change in resistance of large arteries in response to serotonin was increased more than twofold (0.20 +/- 0.17 vs. 0.43 +/- 0.21 mm Hg ml-1 min 100 g, p less than 0.05). In contrast, L-NNA did not alter the increase in resistance of large arteries during hypocapnia. L-arginine inhibited the effects of L-NNA on baseline cerebral vascular resistance and on responses of large arteries to serotonin.(ABSTRACT TRUNCATED AT 250 WORDS)     

Role of soluble guanylate cyclase in dilator responses of the cerebral microcirculation

Responses of cerebral blood vessels to nitric oxide (NO) are mediated by soluble guanylate cyclase (sGC)-dependent and potentially by sGC-independent mechanisms. One sGC-independent mechanism by which NO may produce vasodilatation is inhibition of formation of a vasoconstrictor metabolite produced through the cytochrome P450 pathway. In these experiments, we examined the hypothesis that dilatation of cerebral microvessels in response to NO is dependent on activation of sGC. Diameters of cerebral arterioles (baseline diameter=94+/-5 micrometers, mean+/-S.E.) were measured using a closed cranial window in anesthetized rabbits. Under control conditions, YC-1 [3-(5'-hydroxymethyl-2'-furyl)-1-benzyl indazole], an NO-independent activator of sGC, produced vasodilation that was blocked by ODQ (1H-[1,2,4]oxadiazolo[4,3,-a]quinoxalin-1-one)(10 microM), an inhibitor of sGC. These findings indicate that sGC is functionally important in cerebral arterioles. In addition, acetylcholine (which stimulates endogenous production of NO by endothelium) produced dilatation of cerebral arterioles that was inhibited by ODQ. For example, 1 microM acetylcholine dilated cerebral arterioles by 34+/-7 and 5+/-1% in the absence and presence of ODQ (10 microM), respectively. Increases in arteriolar diameter in response to sodium nitroprusside (1 microM, an NO donor) were inhibited by approximately 80% by ODQ, but were not affected by 17-ODYA (10 microM) or clotrimazole (10 microM), inhibitors of the cytochrome P450 pathway. Thus, dilatation of the cerebral microcirculation in response to exogenously applied and endogenously produced NO is dependent, in large part, on activation of sGC.     

Preganglionic and postganglionic neurons responsible for cerebral vasodilation mediated by nitric oxide in anesthetized dogs.

The authors performed investigations to functionally determine the route of efferent innervation in vivo responsible for cerebral vasodilation mediated by nitric oxide (NO). In anesthetized beagles, electrical stimulation of the pterygopalatine ganglion vasodilated ipsilateral cerebral arteries such as the middle cerebral and posterior communicating arteries. Intravenous injections of NG-nitro-L-arginine (L-NA) markedly inhibited the response to nerve stimulation, and the effect was reversed by L-arginine. Stimulation of the proximal portion of the greater superficial petrosal nerve, upstream of the pterygopalatine ganglion, also produced cerebral vasodilation, which was abolished by L-NA and restored by L-arginine. Treatment with hexamethonium abolished the response to stimulation of the petrosal nerve but did not affect the response to pterygopalatine ganglion stimulation. Destruction of the pterygopalatine ganglion by cauterization constricted the cerebral arteries. Postganglionic denervation abolished the vasodilation, lacrimation, and nasal secretion induced on the ipsilateral side by stimulation of the pterygopalatine ganglion and petrosal nerve. The vasodilator response was suppressed by L-NA but unaffected by atropine, whereas lacrimation and nasal secretion were abolished solely by atropine. It is concluded that postganglionic neurons from the pterygopalatine ganglion play crucial roles in cerebral vasodilation mediated by NO from the nerve, and preganglionic neurons, possibly from the superior salivatory nucleus through the greater superficial petrosal nerve, innervate the pterygopalatine ganglion. Tonic discharges from the vasomotor center participate significantly in the maintenance of cerebral vasodilation.     

Analysis of acetylcholine-induced relaxation of rabbit isolated middle cerebral artery: effects of inhibitors of nitric oxide synthesis, Na,K-ATPase, and ATP-sensitive K channels.

The functional importance of membrane hyperpolarization through activation of ATP-sensitive K channels, or activation of the Na,K-ATPase, was investigated for acetylcholine (ACh)-induced relaxation of the rabbit isolated middle cerebral artery (MCA) precontracted with uridine triphosphate. Incubation with glibenclamide (1 microM), a known blocker of ATP-sensitive K channels, or precontraction with a high concentration of KCl (50 mM) had no effect on ACh-induced relaxation. Similarly, inhibition of the Na,K-ATPase with ouabain (10 microM) or incubation with a potassium-free solution had either no or only a small effect on ACh-induced relaxation. In contrast, NG-nitro-L-arginine (NOLAG) (1 to 10 microM), a structural analogue of L-arginine and an inhibitor of nitric oxide synthesis, produced concentration-dependent although apparently noncompetitive inhibition of ACh-induced relaxation. This inhibition was partially reversed by application of L-arginine (100 microM), a putative precursor for nitric oxide synthesis. It is concluded that membrane hyperpolarization induced by activation of ATP-sensitive K channels or Na,K-ATPase does not play a major functional role in ACh-induced relaxation of rabbit MCA. The potent inhibitory actions of NOLAG would suggest that the major mechanism of ACh-induced relaxation is by release of nitric oxide as in other cerebral and peripheral arteries.     

Role of NO in endothelium-dependent relaxation of rabbit basilar artery in situ

The purpose of this study was to investigate the possible involvement of endothelium-derived hyperpolarizing factor in endothelium-dependent relaxation of the cerebral vasculature by testing the effectiveness of NO synthase inhibitors at inhibiting endothelium-dependent relaxation in the rabbit basilar artery. Acetylcholine (1.0 microM) and 0.1/0.2 microM sarafotoxin S6c, an endothelinB receptor agonist, relaxed serotonin constricted basilar artery in situ by 100% and 70%, respectively. NG-monomethyl-L-arginine (L-NMMA; 0.1 mM) and 0.3 mM NG-nitro-L-arginine (L-NNA), NO synthase inhibitors, decreased the 1.0 microM acetylcholine- and 0.1/0.2 microM sarafotoxin S6c-induced relaxations by 75% and 45%, respectively. Unexpectedly, the relaxations were abolished by the combination of L-NMMA plus L-NNA. Furthermore, L-arginine (1.0 mM), but not D-arginine, restored the relaxations. Sodium nitroprusside-induced relaxation was also inhibited by L-NMMA plus L-NNA, and the inhibition was reversed by L-arginine. KCl constricted vessels only minimally relaxed in response to sodium nitroprusside, acetylcholine, and sarafotoxin S6c. These results demonstrate that combined NO synthase inhibitors more effectively inhibit endothelium-dependent relaxation than a single inhibitor. The mechanism underlying the greater inhibition due to the combined NO synthase inhibitors may result from both decreased NO release and secondary effects caused by decreased NO release, such as membrane depolarization. The results further suggest that caution should be used with respect to suggestions of the involvement of endothelium-derived hyperpolarizing factor in endothelium-dependent relaxation based upon the partial inhibitory effects of NO synthase inhibitors.     

Nitric oxide mediates the neurogenic vasodilation of bovine cerebral arteries

Nitric oxide (NO) is a mediator of the vasodilation induced by a variety of physiological and pharmacological stimuli. The possible role of NO in the relaxation elicited in cerebral arteries by perivascular nerve stimulation has been investigated. Electrical field stimulation of precontracted bovine cerebral arteries induced a relaxation that was blocked by tetrodotoxin, but not by adrenergic or muscarinic receptor antagonists, suggesting the existence of noradrenergic, noncholinergic dilator nerves, as has been shown in other species. The relaxation was significantly reduced by the inhibitors of NO synthesis, NG-monomethyl-L-arginine and nitro-L-arginine methyl ester, but not by the enantiomer, NG-monomethyl-D-arginine. Such a reduction was reversed by L-arginine. In addition, transmural nerve stimulation (TNS)-induced relaxation was potentiated by superoxide dismutase. No response to TNS was observed in arteries without endothelium. These results suggested that neurogenic relaxation of bovine cerebral arteries is mediated by endothelium-derived NO.     

Endothelium-derived nitric oxide synthase inhibition. Effects on cerebral blood flow, pial artery diameter, and vascular morphology in rats.

BACKGROUND AND PURPOSE: We determined the effects of inhibiting the production of cerebral endothelium-derived nitric oxide on pial artery diameter, cortical blood flow, and vascular morphology. METHODS: An inhibitor of endothelium-derived nitric oxide synthesis, NG-nitro-L-arginine methyl ester hydrochloride (L-NAME), or an equivalent volume of 0.9% saline was infused into rats intra-arterially in a retrograde fashion via the right external carotid artery at a rate of 3 mg/kg/min to a total dose of 190 mg/kg or intravenously at 1 mg/kg/min to a total dose of 15 mg/kg. Large pial arteries were continuously visualized through an operating microscope, and cortical cerebral blood flow was monitored by laser-Doppler flowmetry. To localize areas of morphological interest, the protein tracer horseradish peroxidase was injected 15 minutes before termination of the L-NAME infusion and the rats were perfusion-fixed 15 minutes later for light and electron microscopic analysis. RESULTS: Infusion of L-NAME significantly raised arterial blood pressure at both doses (for 190 mg/kg, from 103.2 +/- 3.4 to 135 +/- 3.4 mm Hg; for 15 mg/kg, from 125 +/- 2.8 to 144.4 +/- 4.0 mm Hg). Pial arteries constricted within 10 minutes after the start of the intracarotid infusion to 40% of the preinfusion diameter, while cortical cerebral blood flow decreased to an average of 72.5% of that at baseline. Morphological abnormalities in the experimental rats included microvascular stasis and focal areas of blood-brain barrier disruption to protein. Ultrastructural examination of cortical leaky sites revealed constricted arterioles with many endothelial pinocytotic vesicles and microvilli. CONCLUSIONS: These observations suggest that inhibition of endothelium-derived nitric oxide synthesis affects the relation between cerebral arterial diameter and cerebral blood flow and can lead to subtle cerebral vascular pathological changes consistent with focal brain ischemia.     

Calcium ionophore and acetylcholine dilate arterioles on the mouse brain by different mechanisms

Pial arterioles on the surface of the mouse brain were observed in vivo under a chamber with a glass window. When placed under the window, calcium ionophore, acetylcholine, and previously acidified sodium nitrite each dilated the arterioles. If the cyclooxygenase inhibitors indomethacin or acetylsalicylic acid were first placed in the chamber, subsequent dilation of the arterioles by calcium ionophore was reduced to essentially zero. Similar blockade of cyclooxygenase failed to significantly reduce dilation by acetylcholine or sodium nitrite. We have previously shown that dilations by calcium ionophore and acetylcholine were endothelium dependent. Our present experiments show that the endothelium-dependent mechanism for dilation by calcium ionophore is cyclooxygenase dependent, while that for acetylcholine is not. This implies that, in pial arterioles, the endothelium-derived relaxing factor for acetylcholine differs from that for calcium ionophore. This agrees with data from other microvascular beds.     

Endothelium-independent contractions to NG-monomethyl-L-arginine in canine basilar artery.

BACKGROUND AND PURPOSE: NG-substituted analogues of L-arginine are potent and selective inhibitors of nitric oxide synthase(s). The present study was designed to determine the effects of these analogues on the vascular smooth muscle of isolated canine basilar arteries. METHODS: Basilar artery rings without endothelium were suspended for isometric tension recording in Krebs-Ringer bicarbonate solution bubbled with 94% O2-6% CO2 (temperature = 37 degrees C, pH = 7.4). A radioimmunoassay technique was used to determine the levels of guanosine 3',5'-cyclic monophosphate (cyclic GMP). RESULTS: NG-Monomethyl-L-arginine (L-NMMA) caused concentration-dependent contractions, whereas the D-enantiomer and NG-nitro-L-arginine did not. Contractions to L-NMMA were reduced in the presence of L-arginine but not in the presence of D-arginine. Superoxide anions generated by xanthine plus xanthine oxidase in the presence of catalase abolished contractions to L-NMMA but did not affect contractions to the prostaglandin H2/thromboxane A2 agonist U46619. Zaprinast, a selective cyclic GMP phosphodiesterase inhibitor, caused concentration-dependent relaxations. L-NMMA selectively inhibited these relaxations. The inhibitory effect of L-NMMA was reversed in the presence of L-arginine. L-NMMA selectively reduced the basal production of cyclic GMP. CONCLUSIONS: These studies suggest that in cerebral arteries, contractions of smooth muscle cells to L-NMMA are mediated by inhibition of nitric oxide synthase with a resultant decrease in the basal production of nitric oxide.     

P2u receptor-mediated release of endothelium-derived relaxing factor/nitric oxide and endothelium-derived hyperpolarizing factor from cerebrovascular endothelium in rats

BACKGROUND AND PURPOSE: Stimulation of P2u purinoceptors by UTP on endothelium dilates the rat middle cerebral artery (MCA) through the release of endothelium-derived relaxing factor/nitric oxide (EDRF/NO) and an unknown relaxing factor. The purpose of this study was to determine whether this unknown relaxing factor is endothelium-derived hyperpolarizing factor (EDHF). METHODS: Rat MCAs were isolated, cannulated, pressurized, and luminally perfused. UTP was added to the luminal perfusate to elicit dilations. RESULTS: Resting outside diameter of the MCAs in one study was 209+/-7 micrometer (n=10). The MCAs showed concentration-dependent dilations with UTP administration. Inhibition of NO synthase with NG-nitro-L-arginine methyl ester (L-NAME) (1 micromol/L to 1 mmol/L) did not diminish the maximum response to UTP but did shift the concentration-response curve to the right. Scavenging NO with hemoglobin (1 or 10 micromol/L) or inhibition of guanylate cyclase with ODQ (1 or 10 micromol/L) had effects on the UTP-mediated dilations similar to those of L-NAME. In the presence of L-NAME, dilations induced by 10 micromol/L UTP were accompanied by 13+/-2 mV (P<0.009) hyperpolarization of the vascular smooth muscle membrane potential (-28+/-2 to -41+/-1 mV). Iberiotoxin (100 nmol/L), blocker of the large-conductance calcium-activated K channels, sometimes blocked the dilation, but its effects were variable. Charybdotoxin (100 nmol/L), also a blocker of the large-conductance calcium-activated K channels, abolished the L-NAME-insensitive component of the dilation to UTP. CONCLUSIONS: Stimulation of P2u purinoceptors on the endothelium of the rat MCA released EDHF, in addition to EDRF/NO, and dilated the rat MCA by opening an atypical calcium-activated K channel.     

The endothelium-dependent effects of thimerosal on mouse pial arterioles in vivo: evidence for control of microvascular events by EDRF as well as prostaglandins.

Thimerosal causes synthesis and/or release of both endothelium-derived relaxing factor (EDRF) and prostaglandins from conductance vessels in vitro. We tested its effects and mechanism of action on mouse pial arterioles in vivo using intravital microscopic techniques. Topical thimerosal dilated pial arterioles. This effect was eliminated by endothelial injury produced by a laser/Evans blue technique. Dilation was also eliminated by topical L-NMMA, a reported inhibitor of EDRF synthesis. Topical thimerosal also reduced the incidence of platelet adhesion/aggregation ("capture") at a site of minimal endothelial damage. This effect was eliminated by L-NMMA pretreatment. The ability of thimerosal to dilate arterioles was eliminated not only by treatments thought to eliminate synthesis/release of EDRF, but also by cyclooxygenase inhibitors. However, inhibition of platelet adhesion/aggregation was not affected by cyclooxygenase inhibition. Thimerosal significantly increased production of prostaglandin E2 recovered from a closed cranial window. We conclude that the dilating effects of thimerosal on diameter require two endothelium-derived agents: EDRF and one or more prostaglandins acting in concert. However, the inhibiting effect of thimerosal on local platelet adhesion/aggregation appears to be caused only by an increase in EDRF at the injured site.     

Estimation of EDRF and nitric oxide release using [3H]GTP-labeled human platelets

We have developed a new bioassay for endothelium-derived relaxing factor (EDRF) or nitric oxide (NO) using human [³H]guanosine triphosphate (GTP)-labeled platelets. The labeled platelets were preincubated with isobutyl-methylxanthine and co-cultured with endothelial cells and the [³H]cyclic guanosine monophosphate (cGMP) formed was isolated by ion-exchange chromatography. Endothelial cells, either in monolayer cells or in suspension, increased platelet cGMP accumulation dose-dependently, a significant increase being detected with 5,000 endothelial cells or more/assay when suspended cells were used. Co-culturing with the same number of skin fibroblasts failed to elevate platelet cGMP. Preincubation of endothelial cells with bradykinin and superoxide dismutase (SOD) synergistically potentiated the increase in platelet cGMP, but was attenuated by N^ω-nitro-L-arginine, with partial restoration by L-arginine but not by D-arginine. These compounds, however, did not affect cGMP accumulation by sodium nitroprusside. Moreover, preincubation of the labeled platelets with the NO synthase inhibitor prior to EDRF assay had no effect. We conclude that [³H]GTP-labeled platelets could provide a simple, sensitive and specific bioassay for estimating EDRF or NO release.     

Dysfunction of nitric oxide synthases as a cause and therapeutic target in delayed cerebral vasospasm after SAH

Nitric oxide (NO), also known as endothelium-derived relaxing factor, is produced by endothelial nitric oxide synthase (eNOS) in the intima and by neuronal nitric oxide synthase (nNOS) in the adventitia of cerebral vessels. It dilates the arteries in response to shear stress, metabolic demands, pterygopalatine ganglion stimulation and chemoregulation. Subarachnoid hemorrhage (SAH) interrupts this regulation of cerebral blood flow. Hemoglobin, gradually released from erythrocytes in the subarachnoid space, destroys nNOS-containing neurons in the conductive arteries. This deprives the arteries of NO, leading to initiation of delayed vasospasm. But such vessel narrowing increases shear stress, which stimulates eNOS. This mechanism normally would lead to increased production of NO and dilation of arteries. However, a transient eNOS dysfunction evoked by an increase in the endogenous competitive NOS inhibitor, asymmetric dimethylarginine (ADMA), prevents this vasodilation. eNOS dysfunction has been recently shown to be evoked by increased levels of ADMA in cerebrospinal fluid (CSF) in response to the presence of bilirubin-oxidized fragments (BOXes). A direct cause of the increased ADMA CSF level is most likely decreased ADMA elimination owing to disappearance of ADMA-hydrolyzing enzyme [dimethylarginine dimethylaminohydrolase II (DDAH II)] immunoreactivity in the arteries in spasm. This eNOS dysfunction sustains vasospasm. CSF ADMA levels are closely associated with the degree and time course of vasospasm; when CSF ADMA levels decrease, vasospasm resolves. Thus, exogenous delivery of NO, inhibiting the L-arginine-methylating enzyme or stimulating DDAH II, may provide new therapeutic modalities to prevent and treat vasospasm. This paper will present results of pre-clinical studies supporting the NO-based hypothesis of delayed cerebral vasospasm development and its prevention by increased NO availability.     

Astrocyte-derived lipoxygenase product evokes endothelium-dependent relaxation of the basilar artery

The goal of this study was to examine the possible production of vasoactive factors by astrocytes. We consistently observe that rat astroglial cells in suspension produce marked relaxation when added to precontracted rings of intact (but not endothelium-denuded) rabbit basilar artery. The ultimate mediator of this relaxation was endothelium-derived nitric oxide whose synthesis is activated by an as yet unidentified factor(s) produced tonically by astrocytes. The factor is relatively stable, and is not arachidonate, or a product of cyclooxygenase or P450 metabolism. Based upon studies with selective inhibitors, the factor appears to result from 12- or 15-lipoxygenase metabolism, the products of which are known to be vasoactive. In a separate series of experiments, astrocyte-conditioned medium stimulated the production of citrulline from L-arginine by nitric oxide synthase in bovine aortic endothelial cells. The possible significance for central nervous system (CNS) pathophysiology of an astrocyte-derived vasodilator is discussed. © 1994 Wiley-Liss, Inc.     

Intracisternal versus intracarotid infusion of L-arginine in experimental cerebral vasospasm.

AIM: The effect of short term intracisternal and intracarotid L-arginine infusion on experimental cerebral acute phase vasospasm in a rabbit subarachnoid haemorrhage model is investigated, and the two groups compared. MATERIALS AND METHOD: Subarachnoid haemorrhage was produced by intracisternal injection of autologous blood in New Zealand rabbits. On the fourth day after subarachnoid haemorrhage, cerebral blood flow was monitored using transcranial Doppler ultrasonography during intracisternal and intracarotid saline and L-arginine infusions. RESULT: Cerebral blood flow measurements revealed resolution of vasospasm with short-term intracisternal and intracarotid L-arginine infusion. No significant difference was found between the effects of intracisternal and intracarotid L-arginine infusions, however intracarotid L-arginine infusion created a more potent vasodilatation towards the end of infusion. CONCLUSION: Both intracisternal and intracarotid short term L-arginine infusion significantly improve acute phase cerebral vasospasm after experimental subarachnoid haemorrhage. Intracarotid L-arginine infusion is more potent and safer as large amounts of intracisternal L-arginine may lead to overproduction of nitric oxide by inducible nitric oxide synthase with the production of free radicals.     

Superoxide anion radical does not mediate vasodilation of cerebral arterioles by vasoactive intestinal polypeptide

We examined the hypothesis that oxygen radicals may mediate the vasodilator effect of VIP on cerebral arterioles in cats equipped with cranial windows. The appearance of superoxide anion radical in cerebral extracellular space during VIP application was examined by measuring the rate of superoxide dismutase (SOD)-inhibitable reduction of nitroblue tetrazolium (NBT). Although VIP (1 and 10 micrograms/ml) caused substantial reduction of NBT, the rate of the SOD-inhibitable portion was not significantly different from zero. We also examined the effect of scavenging of superoxide and hydrogen peroxide by topical application of SOD plus catalase on the vasodilator effect of VIP (0.05-1.0 microgram/ml). The dilation in response to VIP was not significantly affected in either large or small arterioles by scavenging of superoxide and hydrogen peroxide. We conclude that VIP does not cause generation of superoxide and that superoxide or other reactive oxygen species derived from it, such as hydrogen peroxide and hydroxyl radical, are not mediators of the cerebral vasodilation caused by VIP.     

Recovery of impaired endothelium-dependent relaxation after fluid-percussion brain injury in cats

The effect of a moderate level of fluid-percussion brain injury on acetylcholine-induced cerebral arteriolar vasodilation was examined for 12 hours after trauma in anesthetized cats equipped with cranial windows. The cats were then perfused with aldehydes, and the pial arteries were prepared for electron microscopy. Immediately after brain injury, the normal vasodilator response to topical application of acetylcholine was converted to vasoconstriction. By 4 hours after trauma, the ability of small pial arterioles (diameters less than 100 microns) to dilate after acetylcholine application had returned to the pretrauma level and was observed to be normal at both 8 and 12 hours after trauma (p less than 0.05). The vasodilator response of large caliber arterioles (diameters greater than or equal to 100 microns) at 4, 8, and 12 hours after injury was reduced relative to the pretrauma response but was significantly improved relative to their response at 30 minutes after trauma (p less than 0.05). Moreover, the response of large vessels at 4, 8, and 12 hours in injured animals was equal to that observed in noninjured control animals assessed at 4, 8, and 12 hours after window implantation. At 12 hours after injury, the ultrastuctural characteristics of both large and small vessels resembled their preinjury state. These data suggest that the impairment of acetylcholine-induced endothelium-dependent relaxation observed in cats after fluid-percussion brain injury is not irreversible but returns to normal (small arterioles) or exhibits significant recovery (large arterioles) within 4 hours after injury.