Constriction of pial arterioles produced by prostaglandin F2alpha

Prostaglandins in migraine: update

PURPOSE OF REVIEW

This review presents recent findings on the role of prostaglandins in migraine pathophysiology.

RECENT FINDINGS

Experimental studies have shown that prostaglandins are distributed in the trigeminal-vascular system and its receptors are localized in the trigeminal ganglion and the trigeminal nucleus caudalis. Prostaglandins were found in smooth muscles of cranial arteries, and functional studies in vivo showed that prostaglandins induced dilatation of cranial vessels. Human studies showed that intravenous infusion of vasodilating prostaglandins such as prostaglandin E₂ (PGE₂), prostaglandin I₂ (PGI₂) and prostaglandin D₂ (PGD₂) induced headache and dilatation of intra-cranial and extra-cranial arteries in healthy volunteers. In contrast, infusion of non-dilating prostaglandin F₂α (PGF₂α) caused no headache or any vascular responses in cranial arteries. PGE₂ and PGI₂ triggered migraine-like attacks in migraine patients without aura, accompanied by dilatation of the intra-cerebral and extra-cerebral arteries. A novel EP4 receptor antagonist could not prevent PGE₂-induced headache in healthy volunteers.

SUMMARY

Recent in-vitro/in-vivo data demonstrated presence and action of prostaglandins within the trigeminal pain pathways. Migraine induction after intravenous administration of PGE₂ and PGI₂ suggests a specific blockade of their receptors, EP and IP respectively, as a new potential drug target for the acute treatment of migraine.

Pro-inflammatory and vasoconstricting prostanoid PGF2α causes no headache in man

Background: During two decades of migraine provocation studies with naturally occurring signalling molecules, vasodilators such as prostaglandin E2, prostaglandin I2 (prostacyclin) and prostaglandin D2 were shown to be able to induce headache in man. To elucidate the role of inflammation and vasodilatation in the generation of headache, we investigated whether the pro-inflammatory and vasoconstricting prostanoid prostaglandin F2α (PGF2α) would cause headache in a human model of headache.

Methods: Twelve healthy volunteers were randomly allocated to receive 3.5 µg/kg/min PGF2α or placebo over 20 min in a two-way crossover study. We recorded headache intensity on a verbal rating scale, middle cerebral artery blood flow velocity (VMCA) and the diameters of the superficial temporal artery (STA) and radial artery (RA).

Results: We found no difference in the area under the curve (AUC) for immediate headache (0–90 min) between PGF2α and placebo ( p = 0.144). The McNemar's test showed no difference in the incidence of immediate and delayed headache between verum and placebo ( p = 0.500 and p = 1.000, respectively). There was no difference in VMCA ( p = 0.776) and in the diameter of the STA ( p = 0.460) or RA ( p = 0.780) between PGF2α and placebo.

Conclusion: The present study shows that PGF2α, unlike vasodilating prostaglandins, does not provoke headache. We suggest that the vasodilating abilities of prostaglandins are important for the induction of experimental headache in healthy volunteers.

Parathyroid hormone-related protein induction in focal stroke: a neuroprotective vascular peptide

Parathyroid hormone-related protein (PTHrP) is a multifunctional peptide that enhances blood flow in non-central nervous system (CNS) vascular beds by causing vasodilation. PTHrP expression is induced in non-CNS organs in response to ischemia. Experiments were therefore undertaken to determine whether PTHrP can be induced in brain in response to ischemic injury and whether PTHrP can act locally as a vasodilator in the cerebral vasculature, an effect that could be neuroprotective in the setting of stroke. PTHrP expression was examined by Northern analysis and immunohistochemical staining in male Sprague-Dawley rats subjected to permanent middle cerebral artery occlusion (MCAO). Vasodilatory effects of superfused PTHrP(1-34) on pial arterioles were determined by intravital fluorescence microscopy. Effects of PTHrP(1-34) peptide administration on MCAO infarction size reduction were assessed. PTHrP expression was induced in the ischemic hemisphere as early as 4 h after MCAO and remained elevated for up to 24 h. Increased immunoreactive PTHrP at sites of ischemic tissue injury was located in the cerebral microvessels. Superfusion with PTHrP(1-34) peptide for up to 25 min increased pial arteriolar diameter by 30% in normal animals. In animals with permanent MCAO, PTHrP(1-34) peptide treatment significantly decreased cortical infarct size (-47%). In summary, PTHrP expression increases at sites of ischemic brain injury in the cerebrovasculature. This local increase in PTHrP could be an adaptive response that enhances blood flow to the ischemic brain, thus limiting cell injury.

Endothelium-dependent effects of substance P and calcitonin gene-related peptide on mouse pial arterioles

BACKGROUND AND PURPOSE

The effects of substance P (SP) and calcitonin gene-related peptide (CGRP) were tested on pial arterioles of mice. This was done because (1) perivascular peptidergic nerves may play an important role in modulation of cerebrovascular responses; (2) there are conflicting data concerning the mechanism of action of CGRP; (3) there are few or no studies directly testing the endothelium dependence of dilation by these peptides in the cerebral circulation; and (4) we wished to extend previous observations of mice by comparing peptidergic responses in the mouse with those published for other species.

METHODS

The pial arterioles were monitored in vivo using video microscopy and image-shearing techniques for measuring diameter. Focal endothelial injury was produced with a laser-Evans blue technique. Responses to SP and CGRP were tested before and after endothelial injury. They were also tested before and during treatment with agents that interfere with responses mediated by endothelium-derived relaxing factor (EDRFACh). They were also tested before and during treatment with indomethacin.

RESULTS

Both CGRP and SP produced dilation that was blocked by endothelial injury and by agents interfering with responses mediated by EDRFACh. Indomethacin had no effect.

CONCLUSIONS

SP and CGRP produce endothelium-dependent dilations. These dilations are probably mediated by EDRFACh. With respect to SP, these results are similar to those reported for other vessels and species. With respect to CGRP, the finding of endothelium dependence has not been previously reported for cerebral vessels. However, very few species have been tested. Reports of other vascular beds in other species sometimes parallel and sometimes contradict our findings with CGRP.

Interaction of endothelium with dilation produced by inhibitors of cyclic nucleotide diesterases in mouse brain arterioles in vivo

BACKGROUND AND PURPOSE

In vitro evidence gathered from extracerebral conductance vessels suggests interaction between the endothelium-derived relaxing factor for acetylcholine (EDRFACh) and cyclic nucleotide action in vascular smooth muscle. The purpose of this study was to examine this interaction in vivo in pial arterioles. As had been done in vitro, we used phosphodiesterase inhibitors that elevate cyclic nucleotide levels in vascular smooth muscle.

METHODS

Pial vessels of mice were observed with television microscopy. Diameter of the arterioles was monitored with an image-splitting technique. The responses to topically applied phosphodiesterase inhibitors were tested before and after focal endothelial injury or before and during application of N-guanidino-L-monomethyl arginine (L-NMMA). Both treatments are known to eliminate the endothelium-dependent response to acetylcholine in this preparation.

RESULTS

Phosphodiesterase inhibitors dilated pial arterioles. This was true for phosphodiesterase inhibitors elevating levels of both adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) as well as for inhibitors thought to selectively affect either nucleotide. The dilation was inhibited by endothelial injury and by L-NMMA.

CONCLUSIONS

The data suggest that brain arterioles are dilated by both cAMP and cGMP but that this action is impaired if EDRFACh levels are reduced. Since EDRFACh elevates cGMP levels, these data support the hypothesis that the relaxing actions of cGMP and cAMP depend upon adequate basal levels of cGMP in vascular smooth muscle. This hypothesis, originally introduced in studies of extracerebral conductance arteries in vitro, can now be applied to brain resistance vessels in vivo.

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.

Endothelium dependence of dilation of pial arterioles in mouse brain by calcium ionophore

Previous studies have shown that local selective in situ injury of pial arteriolar endothelium eliminates the dilations produced by acetylcholine or bradykinin. One means of producing such injury employs a helium-neon laser in the presence of intravascular Evans blue. Since the endothelium-dependent dilations produced by acetylcholine or bradykinin may be initiated by interaction with endothelial surface receptors, it is possible that the light simply inactivates or destroys these receptors. We used calcium ionophore A-23187, another dilating agent known from in vitro studies of large arteries to be endothelium-dependent, which moves calcium into endothelial cells rather than interacting with surface receptors. Our data in 10 mice show that before injury, 10(-5)M A-23187 dilated arterioles to 109 +/- 2% of control diameter. After selective endothelial injury by helium-neon laser, dilation was essentially abolished (101 +/- 1% of baseline diameter; p less than 0.01, Wilcoxon test). Undamaged sites along the arteriole still dilated to A-23187. Our data indicate that the laser must do more than inactivate surface receptors and are the first in vivo microvascular (vessels of less than 100 micron diameter) data showing endothelium dependence of the response to A-23187.

Endothelium-dependent constriction demonstrated in vivo in mouse cerebral arterioles

Endothelium-dependent constriction in mouse pial arterioles was demonstrated by testing contractile responses before and after endothelial injury. All responses were monitored at the same site as the injury. Injury was produced in vivo by exposing an arteriole on the brain surface to the beam of a 6-mW helium-neon laser after first sensitizing the microvascular bed to the laser energy by injecting Evans blue intravenously. The contractile response to serotonin creatinine sulfate (20 micrograms/ml) and to sodium arachidonate (30 micrograms/ml) was monitored in vivo with an image splitter and TV microscope. The responses before laser injury were always constriction; the responses after laser injury were always relaxation. After laser injury, acetylcholine chloride (80 micrograms/ml) constricted every vessel tested at the injured site. Thus, the injured segment had not lost the capacity to constrict even though neither serotonin nor arachidonate remained able to induce a constriction. The endothelium-dependent constrictions to serotonin or arachidonate were also blocked by pretreatment of the mouse with cyclooxygenase inhibitors, acetylsalicylic acid (100 mg/kg i.p.) or indomethacin (5 mg/kg). The data suggests that endothelium-dependent constriction to serotonin is mediated by release of arachidonate from the endothelial cell and conversion of that arachidonate by cyclooxygenase to some constricting prostanoid.

In vivo evidence that an adenylate cyclase-cAMP system dilates cerebral arterioles in mice

Pial arterioles of living mice anesthetized with urethane were monitored by television microscopy. I tested the existence of an adenylate cyclase-cyclic adenosine monophosphate (cAMP) system for dilating the arterioles by topically applying the following drugs: cAMP (10(-3) M), its more potent analogue dibutyryl cAMP (10(-3) and 10(-4) M), and forskolin (10(-6) M). Forskolin activates endogenous adenylate cyclase, which leads to increases in endogenous cAMP. Each drug was applied for 30 seconds; all three produced dilation. I then applied either cAMP or forskolin in the presence or absence of 10(-4) M isobutylmethylxanthine (IMX), an inhibitor of endogenous phosphodiesterase, which destroys cAMP. The presence of IMX significantly potentiated the dilation produced by exogenous cAMP and forskolin. These data indicate that cerebral surface arterioles of mice respond to cAMP with dilation and contain the enzymes for producing and inactivating this dilator. The existence of an adenylate cyclase-cAMP dilating mechanism in pial arterioles does not rule out the simultaneous existence of other dilating mechanisms.

In vivo effect of methylene blue on endothelium-dependent and endothelium-independent dilations of brain microvessels in mice

Arterioles on the surface of the mouse brain were observed by in vivo TV microscopy. Four dilators were topically applied to relax the vessels in vivo. Two of the dilators were acetylcholine and bradykinin, whose action in this vascular bed is dependent upon production of endothelium-dependent relaxing factors. The other two dilators were sodium nitroprusside and 8-bromo-cGMP, whose action is not endothelium dependent. The dilations by acetylcholine, bradykinin, and nitroprusside were significantly depressed by 10(-4) M methylene blue applied topically for 7 minutes prior to application of the dilators. The inhibitory effect was reversible, was greatest against acetylcholine, and was least against nitroprusside. These data parallel reports of methylene blue's action against these dilators when applied to large blood vessels in vitro. Our data appear to be the first microvascular data and the first in vivo data showing this effect. The data thus suggest that the mechanisms underlying dilation of cerebral arterioles and large extracerebral vessels are similar. The literature accounts for the effect of methylene blue on the basis of its action as an inhibitor of guanylate cyclase. Our data, including the failure of methylene blue to alter dilation by 8-bromo-cGMP, are in keeping with this hypothesis and with current beliefs that guanylate cyclase and cGMP have a central role in vasodilation. The data do not rule out the possibility that methylene blue has an additional action in the case of acetylcholine and inactivates the endothelium-dependent relaxing factor for that dilator.

Loss of endothelium-dependent relaxation in mouse cerebral microvessels may be rapidly reversible

This study demonstrates that a loss of endothelium-dependent relaxation in brain microvessels can be rapidly reversible. This loss was produced by injuring the endothelium in situ with a HeNe laser in the presence of intravascular Evans blue. The noxious stimulus has previously been shown not to damage vascular smooth muscle. The loss of endothelium-dependent relaxation was manifest by loss of dilating responses to both acetylcholine (ACh) and bradykinin (BK). Both agents have been shown by others to produce an endothelium-dependent relaxation in large vessels, caused by endothelial release of one or more endothelium-dependent relaxing factor(s) or EDRF(s). Previous studies have not tested for rapid return of endothelium-dependent relaxation after initial loss. In large vessels this might not be expected if reports concerning the necessary removal of large amounts of endothelium are correct. In cerebral microvessels, however, we have already shown apparent loss of EDRF following only minimal ultrastructural alteration of intact endothelial cells. The question remained whether these morphologic alterations reflected the onset of irreversible changes. The present study suggests that these cells were not irreversibly damaged, at least with respect to apparent modulation of endothelium-dependent responses. Recovery of responses to both ACh and BK occurred within 1 hr of initial injury and initial loss of endothelium-dependent relaxation. Recovery within 1 hr did not occur at the center of laser injury, but at a site of impaired response 55 micron away. Previous studies had suggested less dramatic injury of endothelium here compared with endothelium at the center of laser impact.(ABSTRACT TRUNCATED AT 250 WORDS)

Hydroxyl radical mediates the endothelium-dependent relaxation produced by bradykinin in mouse cerebral arterioles

Bradykinin relaxes arterioles on the brain's surface. This response is endothelium-dependent. The data presented here confirm the hypothesis that hydroxyl free radical mediates this response and may be the endothelium-dependent relaxing factor for bradykinin in this microvascular bed. The response to a locally applied bolus of bradykinin (80 micrograms/ml) was monitored by intravital TV microscopy. The response was significantly inhibited or totally blocked by the presence of superoxide dismutase 60 U/ml, catalase 46 U/ml, or deferoxamine 0.1 or 0.2 mM. The superoxide dismutase scavenges superoxide radical, which is known to enter the subarachnoid space as a consequence of cyclooxygenase activation. Cyclooxygenase is activated by bradykinin. The superoxide can form H2O2, scavenged by catalase, and the two together generate hydroxyl. The formation of hydroxyl radical is catalyzed by iron. Deferoxamine 0.1 mM scavenges the iron, blocking the generation of hydroxyl. Deferoxamine 0.2 mM also directly scavenges the hydroxyl. None of the pharmacologic probes had an effect on arteriolar diameter when locally applied without bradykinin. Since the dilation produced by bradykinin was inhibited or totally blocked by probes that prevented hydroxyl formation or directly scavenged hydroxyl radical, that radical is either an essential mediator of the arteriolar relaxation, or is the endothelium-dependent relaxing factor for bradykinin in pial arterioles.

Laser-induced endothelial damage inhibits endothelium-dependent relaxation in the cerebral microcirculation of the mouse

This study demonstrates endothelium-dependent relaxation in the surface arterioles of the brain. A helium-neon laser was used to injure endothelium in situ following i.v. injection of Evans blue dye, which sensitizes the bed to the laser. Areas 18 or 36 micron in diameter were injured and no longer relaxed to either 1 ml of acetylcholine chloride or bradykinin triacetate, 80 micrograms/ml delivered for 60 seconds. Dilations to sodium nitroprusside (30 micrograms/ml) were unaffected. Normal responses to nitroprusside, plus electron microscopy, established that vascular smooth muscle was uninjured. Endothelium-dependent relaxation was impaired when only minor ultrastructural damage was present. Dilation was inhibited downstream and upstream as far as 80 micron from the center of the laser beam. This suggests a spread of endothelium injury around the site of laser impact. However, inhibition was somewhat more marked downstream than upstream, implying that a portion of the downstream response was dependent on a substance released from an upstream site. To date, very few studies have reported endothelium-dependent relaxation in vivo, especially in the microcirculation. The present study accomplishes this. Moreover, in contrast to in vitro observations of endothelium-dependent relaxation in large vessels, the in vivo elimination of endothelium-dependent relaxation in the microcirculation required neither removal of endothelium nor injury to large numbers of endothelium cells. Since endothelium-dependent relaxation in the microcirculation has now been demonstrated using three different techniques to injure endothelium, it is reasonable to conclude that the phenomenon is real.

Evidence that in vivo constriction of cerebral arterioles by local application of tert-butyl hydroperoxide is mediated by release of endogenous thromboxane

Pial arterioles of mice were suffused in situ with tert-butyl hydroperoxide (TBHP). This agent has been reported to stimulate synthesis or release of the constrictor, thromboxane. In the present study we observed pial arterioles by in vivo microscopy. Locally applied TBHP produced dose-dependent constriction, significantly inhibited by each of three drugs: acetylsalicylic acid, OKY-046, and SQ-29548. These drugs are respectively a cyclooxygenase inhibitor, a thromboxane synthetase inhibitor, and a thromboxane receptor blocker. Since each acts by a different mechanism to interfere with thromboxane-mediated responses and each inhibited the contractile response to TBHP, thromboxane appears to be a mediator of this response. Platelet aggregation was not seen after local application of TBHP, and the dwell time of platelets at the site of TBHP application is less than 1 second. Thus, platelets are an unlikely source of the thromboxane mediating the local constriction. It is much more likely that the source of thromboxane is either the wall of the pial vessels or the underlying brain and/or its vessels. These data do not distinguish between the latter two possibilities, but if this suggestion is correct, then the data show for the first time that thromboxane can be released from normal brain and/or brain vessels in amounts sufficient to cause arteriolar constriction.

Endothelial dependent relaxation demonstrated in vivo in cerebral arterioles

The endothelium of mouse pial arterioles was injured in situ with a light/dye technique. The response of the arterioles to acetylcholine or to bradykinin was compared before and after the injury. All vessels failed to dilate after injury. In fact the predominant response now became constriction. The injured vessels were still capable of dilating to papaverine. Uninjured vessels continued to dilate to acetylcholine or bradykinin. The data show that relaxation of pial arterioles to acetylcholine or bradykinin is dependent on a normal endothelium. This is in keeping with demonstrations by others that an endothelial dependent relaxing factor or factors is(are) the mediator of the dilation to either acetylcholine or bradykinin. The present demonstration of such endothelial dependence is important because in contrast with the bulk of the literature it deals with in vivo, rather than in vitro data, and with microcirculation rather than large vessels. It is also important because it concerns brain circulation. The data suggests that endothelial injury, known to occur in a wide variety of pathologic states, could enhance vasospastic potential by eliminating dilating influences and/or converting them to constricting forces.

Constricting effect of leukotrienes on cerebral arterioles of mice

Three leukotrienes (LTB4, C4, D4) were locally applied to cerebral arterioles in doses varying from 3 X 10(-8) to 4 X 10(-7) M and each produced a dose related constriction. LTC was tested in the presence and absence of FPL-55712, a relatively specific inhibitor of leukotriene receptors. A low dose (10(-8) M) of FPL-55712 did markedly reduce the response to the LTC and did not significantly alter an equally pronounced constriction produced by norepinephrine. The constricting properties of the leukotrienes shown by our data support the suggestion that these products of lipoxygenase's action on arachidonic acid have the potential to modify cerebrovascular tone. This potential may be important in view of recent evidence showing that the brain can make significant amounts of leukotrienes from arachidonic acid.

Accumulation of cyclooxygenase products of arachidonic acid metabolism in gerbil brain during reperfusion after bilateral common carotid artery occlusion

Several of the cyclooxygenase products of arachidonic acid were measured in the cerebral hemispheres of gerbils subjected to transient interruption of the cerebral circulation. The levels of PGD2, PGF2 alpha, PGE2, TXB2, 13, 14-H2-15-keto-PGE2, and the stable nonenzymic product of prostacyclin, 6-keto-PGF1 alpha, were not altered at the end of a 5-min period of ischemia. However, the onset of reperfusion was accompanied by a rapid accumulation of these products. Levels were highest during the initial period of reperfusion, then decreased to approach control levels after 120 min. PGD2, PGF2 alpha, and PGE2 were the predominant metabolites detected. This postischemic accumulation of arachidonic acid metabolites could be blocked by prior administration of inhibitors of cyclooxygenase activity.

Interaction of a potential antimigraine drug (Org GC 94) with the vasomotor action of serotonin

The interaction of a potential anti-migraine drug (Org GC 94) with the vasomotor action of 5-HT in vitro in feline, canine and human intra- and extracranial arteries, as well as in vivo in the canine nasal vascular bed as been investigated. In the two in vitro preparations, i.e. using superfusion or bath techniques, the intracranial vessels were more sensitive to 5-HT vasoconstriction than the extracranial ones. As both the maximum contraction and the slope of the dose-response curves were reduced by increasing concentrations of Org GC 94, the antagonism of 5-HT did not involve competitive blockade of 5-HT receptors. The dilator response was tested in arteries brought to a higher tone with prostaglandin F2 alpha. 5-HT produced a dose-dependent dilatation which, like the vasoconstriction, was antagonized in a non-competitive manner by Org GC 94. Intra-arterial injections of 5-HT provoked nasal vasoconstriction and this response was clearly potentiated by Org GC 94 in low doses while higher doses inhibited the vascular response to 5-HT. The specific effect of Org GC 94 in vivo may be the potentiation of 5-HT-induced vasoconstriction. The hypothesis is discussed that the so-called anti-5-HT agents act in migraine patients as partial agonists of 5-HT, mimicking rather than antagonizing 5-HT.

Serotonin neurons project to small blood vessels in the brain

Electrolytic lesions of the nucleus raphe dorsalis and medianus reduce the concentration of serotonin (5-hydroxytryptamine) within rat brain intraparenchymal blood vessels. The concentration of serotonin within these vessels increases or decreases after the administration of drugs that modify the biosynthesis and degradation of serotonin or destroy nerve terminals by an uptake-dependent mechanism. These studies provide evidence for the existence of a serotonin-containing pathway seemingly analogous to the neuronal projection that terminates on small parenchymal blood vessels from noradrenergic neurons of the locus coeruleus.

Responses of isolated dog cerebral and peripheral arteries to prostaglandins after application of aspirin and polyphloretin phosphate

In helically cut strips of dog cerebral, coronary, mesenteric and femoral arteries, the contractile response to prostaglandin (PG) F2alpha, and E2, relative to contractions induced by 30 mM K+, did not appreciably differ, whereas relaxations induced by PGE1 relative to those induced by 10(-4) M papaverine were significantly different; the least in cerebral arteries and the greatest in mesenteric arteries. The relaxation of human cerebral arteries in response to PGE1 was similar to that of dog cerebral arteries. Treatment for 60 min with polyphloretin phosphate (3 X 10(-5) and 10(-4) g/ml) suppressed the contractile response to PGF2alpha and E2 but did not alter the response to 25 mM K+. The relaxing effect of PGE1 was not influenced. Aspirin (5 X 10(-5) and 2 X 10(-4) M) significantly potentiated the contractile response to PGF2alpha and E2 but did not alter the relaxation induced by PGE1. In contrast, contractions induced by serotonin were attenuated. It is concluded that dog cerebral, coronary, mesenteric and femoral arteries relaxed differently in response to PGE1. It appears that arterial responses to vasoconstricting PGs, but not to the vasodilating PG, are significantly attenuated by polyphloretin phosphate and potentiated by aspirin.

Vasospasmogenic substance produced following subarachnoid haemorrhage, and its fate

Fresh blood and supernatants of blood-CSF mixtures incubated for 1 to 15 days were applied to the basilar artery of adult cats, and the degree of constriction was measured with a surgical microscope. The constriction due to fresh blood was weak and transient. It seems possible to assume that serotonin isolated from platelets participates greatly in the transient vasoconstriction induced by fresh blood. Supernatants of blood-CSF mixtures incubated for three days had weak activity in comparison with the powerful and long-lasting activity of those incubated for seven days. Furthermore, mixtures incubated for 15 days had little or no activity. This change in the vasoconstrictive activity was similar to, and coincides chronologically with clinical late spasm following subarachnoid haemorrhage 34. We investigated the vasospasmogenic substance in the seventh day mixture. Heat coagulation, ultrafiltration, sephadex G-100 gel-chromatography, disc-electrophoresis, and Spectrophotography show that extracellular oxyHb has a strong spasmogenic activity. In the 15th day mixture, oxyHb is spontaneously converted to metHb. Experimentally, oxyHb has a strong vasoconstrictive activity, and metHb has no vasoconstrictive activity. We have had success in oxidizing oxyHb into metHb with sodium nitrite, thus preventing experimental vasospasm.

Pial arteriolar responses in the mouse brain revisited

Arteriolar responses were measured on the cerebral surface of the mouse using an image splitter and TV monitor. The response to locally applied norepinephrine (NOR) was significantly more frequent for vessels greater than 30 mu I.D. than for smaller vessels. However, even the smaller vessels were frequently constricted by NOR in doses of 5 mug per milliliter. Reserpine (5 mg per kilogram) failed to alter the response to NOR at either 24 or 72 hours after reserpinization. At 48 hours the threshold dose of NOR was reduced, but the effect was slight (two-tailed, P = 0.08). Both propranolol (10(-6) M3 and phentolamine (10(-5M) blocked responses to 5 mug per milliliter of NOR, but neither agent altered resting arteriolar diameter. Isoproterenol, tyramine, and histamine had no effect. Serotonin (5HT) constricted the arterioles but did not potentiate the response to NOR. Additive or potentiated effects were not observed with NOR 5HT or histamine in any combination. These data indicate the presence of alpha-adrenergic receptors in murine cerebral surface arterioles, but do not establish a significant tonic effect of norepinephrine. The existence or role of a beta-receptor in these murine cerebral surface arterioles remains an unsettled question.