Heterogeneity in the response to vasoconstrictors of isolated dog proximal and distal middle cerebral arteries

Ethanol-induced contractions in cerebral arteries: role of tyrosine and mitogen-activated protein kinases

BACKGROUND AND PURPOSE

The relationship between alcohol consumption and stroke appears complex; moderate ingestion is associated with reduced stroke risk, while heavy intake is associated with increased stroke risk. Ethanol has been shown both experimentally and epidemiologically to induce hemorrhagic and ischemic strokes, which are associated with cerebral vasoconstriction. Ethanol is known to induce contraction in isolated cerebral arteries and intact microvessels from diverse mammalian animals. The relationships between ethanol-induced contractions in cerebral arteries, intracellular free Ca(2+) ([Ca(2+)](i)), tyrosine kinases (including the src family), and mitogen-activated protein kinases (MAPK) were investigated in the present study.

METHODS

Cerebral arterial muscle tension and [Ca(2+)](i) were quantified by an isometric contraction technique and direct visualization of Ca(2+) in single cells.

RESULTS

Ethanol induces concentration-dependent contractions in intact canine basilar arteries, which are attenuated significantly by pretreatment of the arteries with low concentrations of an antagonist of protein tyrosine kinases (genistein); an src homology 2 (SH2) domain inhibitor peptide; a highly specific antagonist of p38 MAPK (SB-203580); a potent, selective antagonist of MEK1/MEK2 (U0126); and a selective antagonist of mitogen-activated protein kinase kinase (MAPKK) (PD-98059). IC(50) levels obtained for these 5 antagonists are consistent with reported K:(i) values for these tyrosine kinase, MAPK, and MAPKK antagonists. Ethanol induces transient and sustained increases in [Ca(2+)](i) in primary single smooth muscle cells from canine basilar arteries, which are markedly attenuated in the presence of genistein, an SH2 domain inhibitor peptide, SB-203580, U0126, and PD-98059. Several specific antagonists of known endogenously formed vasoconstrictors do not inhibit or attenuate either the ethanol-induced contractions or the elevation of [Ca(2+)](i).

CONCLUSIONS

The present study suggests that activation of protein tyrosine kinases (including the src family) and MAPK appear to play important roles in the ethanol-induced contractions and the elevation of [Ca(2+)](i) in smooth muscle cells from canine basilar arteries. The results could be used to suggest that selective antagonists of protein tyrosine kinases and MAPK may be useful both prophylactically and therapeutically in alcohol-induced strokes.

Effect of the H1 blocker d-chlorpheniramine on cerebral blood flow in the conscious dog during normocapnic hypoxia

The mechanism causing cerebral vasodilatation during hypoxia remains unclear. A role for histamine is suspected because H2 receptor-blocking drugs blunt the hypoxia-induced increase in cerebral blood flow (CBF). Moreover, in vitro blockade of H1 receptors by chlorpheniramine decreases the vasodilatation of cerebral arteries that is induced by histamine. The present study tested the hypothesis that an H1 receptor blocker (d-chlorpheniramine) would have a similar effect in vivo during hypoxia. Isocapnic hypoxia (inspired oxygen fraction, FIO2 = 0.10; inspired carbon dioxide fraction, FICO2 = 0.035) was induced in 16 conscious dogs randomly divided into two groups: eight dogs received saline intravenously (controls) at time 0 (normoxia) and after 2 h and 4 h hypoxia, and the other eight dogs received d-chlorpheniramine intravenously (0.5 mg kg-1) to block the H1 receptors. Regional CBF was measured by the radioactive microspheres technique 15 min after each injection of d-chlorpheniramine or saline. In the control group, CBF increased during hypoxia in all regions of the brain. In the d-chlorpheniramine group, total CBF increased similarly after 2 h of hypoxia. After 4 h of hypoxia, the increase was limited, especially in the pons, cerebral peduncles, hippocampus, hypothalamus, thalamus, and occipital lobes (six out of 12 studied regions). It is concluded that the H1 blocker d-chlorpheniramine did not strongly inhibit the increase in CBF during hypoxia. After cumulative doses, however, as in the fourth hour of hypoxia, the increase in total CBF was limited.

In vitro relaxation of dog cerebral veins in response to histamine is mediated by histamine H2 receptors

There is little information on the histamine receptor mechanisms involved in cerebral venodilation, thus the role of histamine present in human cerebrospinal fluid is difficult to assess. In isolated canine pial veins, concentration-response curves to histamine (10[-7]-10[-3] M), the histamine H1 receptor agonist, 2-pyridylethylamine (10[-6]-10[-2] M), the histamine H2 receptor agonist, dimaprit (S-(3-dimethylaminopropyl) isothiourea dihydrochloride, 10[-6]-10[-2] M), and the histamine H3 receptor agonist, imetit (S-[2-(1 midazol-4-yl)ethyl]isothiourea dihydrobromide, 10[-7]-10[-3] M) were isometrically determined. In resting veins, histamine, 2-pyridylethylamine and dimaprit had no significant effect, whereas in endothelin-1-precontracted veins, these drugs produced concentration-dependent relaxation (Emax in % of active tone and pD2 were: for histamine, 72 +/- 6 and 5.36 +/- 0.09; for 2-pyridylethylamine, 59 +/- 5 and 3.28 +/- 0.05; for dimaprit, 65 +/- 7 and 4.81 +/- 0.10, respectively). The relaxations in response to histamine and dimaprit were competitively antagonized by the histamine H2 receptor antagonist, cimetidine (3 x 10[-6]-10[-4] M) (pA2 = 6.07 +/- 0.03 for histamine, and 6.09 +/- 0.07 for dimaprit), but were not affected by the histamine H1 receptor antagonist, chlorpheniramine (10[-6] M) or the histamine H3 receptor antagonist, thioperamide (N-cyclohexyl-4-(1-H-imidazol-4-yl)-1-piperidine-carbothioamide maleate, 10[-6] M). The relaxation in response to 2-pyridylethylamine was inhibited by cimetidine (10[-5] M), but not by chlorpheniramine (10[-6] M). Imetit produced a small contraction in resting veins (14 +/- 4 mg) and precontracted veins (20 +/- 3 mg), which was not modified by thioperamide (10[-6] M). The relaxation of veins in response to histamine was not modified by endothelium removal, nor by the inhibitor of nitric oxide synthase, N(G)-nitro-L-arginine methyl ester (10[-4] M), or the cyclooxygenase inhibitor, meclofenamate (10[-5] M). Therefore, in pial veins: (1) histamine produces relaxation by activation of histamine H2 receptors, probably located in the smooth musculature, with no participation of histamine H1 and H3 receptors, and (2) endothelium, nitric oxide and prostanoids are probably not involved in the relaxation in response to histamine.

Chronic treatment of cultured cerebral vascular smooth cells with low concentration of ethanol elevates intracellular calcium and potentiates prostanoid-induced rises in [Ca2+]i: relation to etiology of alcohol-induced stroke

The influence of chronic treatment of cultured canine cerebral vascular smooth muscle cells, with low concentrations of ethanol, on the intracellular concentrations of free calcium ([Ca2+]i) was studied by use of the fluorescent indicator, fura-2, and digital imaging microscopy. The resting level of [Ca2+]i in the cerebral vascular smooth muscle cells was 89 +/- 3.2 nM. Exposure of these cells to 10 and 25 mM ethanol for 5 days resulted in significant elevation of [Ca2+]i (mean rises to 208 +/- 11.4 and 307 +/- 14.0 nM, respectively), and potentiated the transient rise in [Ca2+]i induced by 10(-7) M PGF2 alpha. However, exposure of these cerebral cells to a high-concentration ethanol (100 mM) resulted in only a slight increase of [Ca2+]i (106 +/- 6.9 nM) and lack of effects on the [Ca2+]i response to PGF2 alpha. Irrespective of the different ethanol treatments, the subcellular distribution of [Ca2+]i was heterogeneous in all the cells tested. Our data suggest that chronic exposure of cerebral vascular smooth muscle cells to ethanol, particularly at low concentrations, results in dramatic increases in [Ca2+]i and the responses of these vascular smooth muscle cells to prostanoids. These results support an hypothesis whereby ethanol induces stroke by causing spasm and rupture of cerebral blood vessels as a consequence of large rises in intracellular Ca2+.

Altered reactivity of human cerebral arteries after subarachnoid hemorrhage

To investigate the effects of subarachnoid hemorrhage (SAH) on the responsiveness of human cerebral arteries to vasoactive substances, the authors measured the isometric tension generated in helical strips of basilar and middle cerebral arteries isolated from human cadavers. Contractions caused by KCl, prostaglandin F2 alpha, noradrenaline, and serotonin were reduced in arteries obtained from cadavers with aneurysmal SAH damage and compared to those obtained from cadavers with no indication of intracranial diseases. Endothelium-dependent relaxation elicited by substance P and bradykinin, and endothelium-independent relaxation induced by prostaglandin I2 and nitroglycerin were also markedly decreased in arteries affected by SAH. However, the reduction in relaxation response to prostaglandin I2 was significantly less than that to the other vasodilator agents. These results indicate that human cerebral artery functions are severely impaired after SAH and that poor responses to vasoactive agents may result primarily from dysfunction of smooth-muscle cells.

Innervation of cerebral arteries by nerves containing 5-hydroxytryptamine and noradrenaline

Noradrenaline (NA)-containing nerves, mainly originating in the sympathetic superior cervical ganglia, supply large and small cerebral arteries. In large cerebral arteries, nerves containing serotonin (5-hydroxytryptamine, 5-HT) may represent neuronal uptake of circulating 5-HT by sympathetic nerves. 5-HT-containing nerves supplying small pial vessels probably have a central origin in the dorsal raphe nucleus. In most species, NA is a weak vasoconstrictor (alpha 1- or alpha 2-adrenoceptors), while 5-HT is a potent vasoconstrictor (5-HT2 or 5-HT1-like receptors) of large cerebral arteries. In contrast, both NA and 5-HT tend to cause vasodilatation in small pial vessels and arterioles. Adrenergic and serotonergic transmission can be modulated by pH, a range of putative neurotransmitters and neuromodulators, and by the endothelium. Sumatriptan, a 5-HT1-like receptor agonist, has been shown to be effective in the treatment of migraine. Changes in NA- or 5-HT-containing nerves and/or in the responses of cerebral vessels to NA and 5-HT have been observed in a variety of vascular disorders, including cerebral vasospasm following subarachnoid haemorrhage, hypertension, and atherosclerosis.

Ethanol-induced contraction of cerebral arteries in diverse mammals and its mechanism of action

Acute ethanol exposure (8-570 mM) induced potent contractile responses of rings in both basilar and middle cerebral arteries, from dogs, sheep, piglets and baboons, in a dose-dependent manner. The contractions were reproducible and not tachyphylactic. The middle cerebral arteries were found to be more sensitive to ethanol than the basilar arteries. No known pharmacological antagonist, tested, exerted any effects on ethanol-induced contractions. No differences in responsiveness to ethanol in canine cerebral arteries were found between male and female animals or between the presence and the absence of endothelial cells. Removal of extracellular Ca2+ ([Ca2+]o) partially attenuated ethanol-induced contractions, while withdrawal of extracellular Mg2+ ([Mg2+]o) potentiated such contractions. In the complete absence of [Ca2+]o, caffeine and ethanol induced similar, transient contractions followed by relaxation in K(+)-depolarized cerebral vascular tissue. Ethanol-induced contractions were completely abolished by pretreatment of tissues with caffeine. Our results suggest that: (a) acute ethanol intoxication can induce direct contractions (independent of amine, prostanoid or opioid mediation) of diverse mammalian cerebral vascular tissues, including those from primates; (b) these contractile responses are heterogeneous along the cerebrovascular tree and independent of endothelial cells; (c) in addition to a need for [Ca2+]o, an intracellular release of Ca2+ is needed for ethanol to induce contractions; and (d) hypomagnesemia or Mg deficiency potentiates the contractile effects of ethanol on brain vessels and may be a risk factor for ethanol-related, ischemic stroke events.

Cerebral vasoconstrictor mediators

Endogenous cerebral vasoconstrictor mediators regulate vascular resistance and blood flow in the brain as a whole and in various regions and participate in the pathogenesis of cerebral circulatory disturbances. Vasoconstrictors are effective in the treatment of diseases associated with cerebral vasodilatation. There are variations in the response of cerebral arteries from primate and subprimate mammals; therefore, information as to similarities and differences in their response is quite important in evaluating the physiological role, involvement in pathogenesis and therapeutic usefulness of the mediators in healthy men and patients. In this review we described characteristics of the action of vasoconstrictors (amines, peptides, prostanoids, and others) on isolated cerebral arteries from mammals, including humans and monkeys.

Regional difference in the response mediated by beta 1-adrenoceptor subtype in bovine cerebral arteries

Helical strips of bovine rostral cerebral arteries (anterior cerebral, middle cerebral, and internal carotid artery) responded to norepinephrine with contractions, whereas the caudal cerebral arteries (posterior communicating, posterior cerebral, and basilar artery) relaxed in response to the amine. After blockade of alpha-adrenoceptors, norepinephrine-induced rostral artery contractions were reversed to relaxations, which were smaller than those in the caudal arteries. Isoproterenol, dobutamine, and terbutaline produced greater relaxations in caudal than in rostral arteries, but forskolin relaxed these arteries to a similar magnitude. The isoproterenol-induced relaxation was not affected by removal of the endothelium. Maximal relaxations induced by terbutaline in caudal arteries were much inferior to those by isoproterenol, norepinephrine, and dobutamine. Relaxations caused by isoproterenol, norepinephrine, and terbutaline in the caudal arteries were attenuated by metoprolol, but not influenced by butoxamine. Relaxations mediated possibly by beta 1-adrenoceptor subtypes are greater in bovine caudal cerebral arteries than in the rostral arteries. The heterogeneity does not appear to be associated with the different ability of cyclic AMP to relax arterial smooth muscle but with the difference of beta-adrenoceptor populations and/or processes from the receptors to adenylate cyclase.

Does vasospasm occur in small pial arteries and arterioles of rabbits?

BACKGROUND

Vasospasm is a serious complication associated with subarachnoid hemorrhage. Successful management of vasospasm will ultimately depend on a clear understanding of the scope of this phenomenon, including whether arterial elements of different calibers are equally affected. We therefore examined the responses to subarachnoid hemorrhage in rabbit basilar arteries, small pial arteries, and arterioles.

SUMMARY OF REPORT

We compared the brain stem pial arteries of 10 perfusion-fixed male New Zealand White rabbits after experimental subarachnoid hemorrhage to those of five control rabbits using morphological analysis of cross-sections of plastic-embedded vessels. After subarachnoid hemorrhage, the internal elastic lamina was highly corrugated in all basilar arteries (mean diameter 319 +/- 51 microns). These arteries were severely constricted in comparison with the control group, in which the mean diameter was 691 +/- 17 microns, and corrugation of the internal elastic lamina was not present. In contrast, small pial arteries and arterioles very rarely demonstrated a vasoconstrictive configuration after subarachnoid hemorrhage. The contractility of the smaller vessels was confirmed by injecting 2 mg/kg BaCl2 intracisternally. Following BaCl2 injection, corrugation of the internal elastic lamina was detected in the small arteries and arterioles as well as the basilar arteries.

CONCLUSIONS

We conclude that experimental chronic vasospasm after subarachnoid hemorrhage in rabbits tends to occur in large conducting arteries rather than in smaller pial arteries and arterioles.

Toxic effects of somatostatin in the cerebellum and vestibular nuclei: multiple sites of action

This study demonstrates that somatostatin (SRIF), an endogenous peptide in vestibular nuclei and cerebellum, can produce both a dose-dependent death of Purkinje cells in distinct sagittal regions of cerebellar cortex and vascular infarcts centered selectively in the inferior vestibular nucleus. Alert, adult male rats were given a 5 microliters intracerebroventricular (i.c.v.) bolus of either SRIF alone (20 or 40 micrograms) or a combined dose of SRIF plus either arginine-vasopressin (AVP, 1 micrograms) or an AVP V1 antagonist, (1-(beta-mercapto-beta,beta-cyclopentamethylene propionic acid), 2-(O-methyl)-tyrosine)-arginine 8-vasopressin (mcAVP, 1 micrograms), through an implanted cannula. After a 4-5 day survival, the brains were stained with the cupric-silver selective degeneration method. Two types of dose-dependent lesions were observed in the cerebellar and vestibular nuclei of these animals: degeneration of Purkinje cell responses in the cerebellar cortex and vascular infarcts in vestibular nuclei. These toxic responses were unaffected by application of AVP or mcAVP; hence, they can be attributed to actions of SRIF. The distribution of Purkinje cell degeneration varied with the SRIF dose in different cerebellar regions. Purkinje cell responses in lobules I-III were equivalent at both SRIF doses, and degeneration in the copula pyramis, paraflocculus and paramedian lobule emerged at the higher SRIF dose. Purkinje cells in the medial aspect of lobules IX-X had an intermediate sensitivity to SRIF intoxication. Degenerating Purkinje cells tended to be arranged in parasagittal bands in each region, suggesting parasagittal zonal variations in susceptibility to SRIF intoxication. By contrast, infarctions in the vestibular nuclei only appeared at the higher SRIF dose. These infarcts could be unilateral or bilateral and always involved the inferior vestibular nucleus at the level of the caudal margin of the acoustic tubercle; they often extended into the medial and lateral vestibular nuclei. The infarcts had a necrotic core that was infiltrated by non-neuronal elements. Thus, they appear to reflect a direct or neurally-mediated vascular response to the peptide.

Serotonergic innervation of the cerebral vasculature: relevance to migraine and ischaemia

Multiple and complex interactions exist between the cerebral circulation and a potent vasoactive (and neurotransmitter) agent, serotonin. The nature and bases of the real and potential relationships are often hotly contested, for example, the serotonergic innervation of brain conducting and resistance vessels. In this review, an attempt is made to reconcile the available literature and to indicate future and possibly fruitful research directions. It appears that, by its very nature, the pattern of the serotonergic innervation is singular to blood vessels of the brain and could provide a neuronal link (or coupling) between functional events within the central nervous system and its perfusion which subserves changes in brain function. Finally, there are sufficient data to suggest an involvement of 5-hydroxytryptamine in different cerebrovascular pathologies.

Ischemia-reperfusion does not affect reactivity of isolated canine basilar artery

The effect of ischemia-reperfusion on endothelium-dependent relaxations and reactivity of vascular smooth-muscle cells was studied in rings of basilar arteries obtained from six dogs exposed to 12 min of complete global cerebral ischemia followed by 100 min of reperfusion. Three sham-operated control dogs served as controls. Ischemia was induced either by an increase in intracranial pressure or by aortic occlusion. The rings were suspended for isometric tension recording in physiological salt solution. Ischemia-reperfusion did not affect endothelium-dependent relaxations to vasopressin and bradykinin. In rings without endothelium relaxations to sodium nitroprusside, molsidomine (SIN-1), and papaverine as well as contractions to 5-hydroxytryptamine and KCl were preserved. These results demonstrate that in large canine cerebral arteries, ischemia-reperfusion of these durations does not affect relaxations mediated by activation of endothelium or direct relaxations and contractions of vascular smooth-muscle cells.

High sensitivity of porcine cerebral arteries to endothelin

The threshold concentration of endothelin to induce contractions in porcine cerebral arteries (anterior cerebral, Willis ring and basilar artery) was lower than those for coronary and renal arteries. The median effective concentrations (ED50) of endothelin in cerebral arteries were also significantly lower than those for coronary and renal arteries. There was no significant difference in the sensitivity to endothelin among cerebral arteries, or between coronary and renal arteries. The maximal percentage of contractions induced by endothelin, as compared to that induced by 10(-1) M potassium chloride, was not significantly different between the arteries.

Comparison of endothelium-dependent responses of monkey cerebral and temporal arteries

1. Endothelium-dependency of vasodilator responses was compared in helical strips of monkey cerebral and superficial temporal arteries contracted with prostaglandin F2 alpha. Acetylcholine produced an endothelium-dependent relaxation in the temporal arteries, but did not consistently alter the tone of cerebral arteries. 2. Adenosine 5'-triphosphate (ATP) produced a transient contraction followed by a relaxation in the temporal and cerebral arteries; removal of the endothelium partially attenuated the relaxation of the cerebral arteries and markedly suppressed the relaxation in the temporal arteries. The dependency of adenosine 5'-diphosphate (ADP)-induced relaxations on the endothelium was also greater in temporal arteries than in cerebral arteries. 3. Histamine-induced relaxations in the temporal arteries were independent of the endothelium and were reversed to contractions by cimetidine. Cerebral arterial relaxations induced by histamine were partly dependent on the endothelium. Relaxations caused by substance P were reversed to contractions by removal of the endothelium in the temporal arteries, whereas the peptide did not consistently alter the tone of cerebral arteries. 4. The Ca2+ ionophore, A23187, relaxed the temporal and cerebral arteries to a similar extent; removal of the endothelium abolished these relaxations. Glyceryl trinitrate elicited similar relaxation of cerebral and temporal arteries, and these were independent of the endothelium. 5. These findings clearly indicate heterogeneity in the endothelium-dependency of several vasodilator responses in monkey intra- and extracranial arteries, although the ability of these arteries to respond to A23187 and glyceryl trinitrate does not appear to differ. The heterogeneous responses observed so far could therefore be due to different distributions of receptors or to variation in receptor-effector coupling in endothelial cells.

Isolated bovine cerebral arteries from rostral and caudal regions: distinct responses to adrenoceptor agonists

Responsiveness to norepinephrine and related agents was compared in isolated bovine anterior cerebral (ACA). middle cerebral (MCA), intracranial internal carotid (ICA), posterior communicating (PCOM), posterior cerebral (PCA) and basilar arteries (BA). Norepinephrine contracted the strips from ACA, MCA and ICA, but relaxed those from PCOM, PCA and BA. In the presence of propranolol, the amine-induced contractions tended to be potentiated in ACA, MCA and ICA, and the relaxations in PCOM, PCA and BA were reversed to contractions. The maximum contractions induced by norepinephrine in ICA and ACA treated with propranolol were significantly greater than those in PCA and BA, but the EC50 values did not differ among arteries. In ACA and MCA, the contractions induced by phenylephrine were greater than those induced by clonidine. The contractions induced by norepinephrine and phenylephrine were attenuated by prazosin but not influenced by yohimbine in ACA and MCA treated with propranolol. These findings indicate that the responses to norepinephrine evidently differ in bovine cerebral arteries of rostral (ACA, MCA and ICA) and caudal regions (PCOM, PCA and BA), possibly due to different functioning of alpha/beta receptors. The amine-induced contraction predominantly seen in the rostral arteries appears to be associated with activation of the alpha 1 adrenoceptor subtype.

Endothelium-dependent and -independent responses to vasodilators of isolated dog cerebral arteries

We compared responses to calcium ionophore A23187, vasopressin, and substance P in helical strips of dog middle cerebral, basilar, and posterior communicating arteries to obtain a better understanding of humoral control of cerebrovascular tone in different brain regions and its potential impact on mechanisms of cerebral vasospasm. A23187 relaxed these different arterial strips partially precontracted with prostaglandin F2 alpha to a similar extent. Vasopressin produced concentration-dependent relaxation in basilar and posterior communicating arterial strips, whereas middle cerebral arterial strips either contracted or relaxed slightly. Relaxations induced by A23187 and vasopressin were either abolished or converted to contractions by removal of the endothelium. In contrast, the relaxation of cerebral arterial strips to substance P was markedly attenuated but not abolished by endothelium denudation; the remaining relaxation was suppressed by indomethacin. In some cerebral arterial strips with intact endothelium, substance P caused a transient contraction that was reversed to a relaxation by indomethacin or ONO-3708, a prostaglandin antagonist. In arterial strips denuded of endothelium from the same dogs, substance P always produced relaxations. Relaxations of cerebral arterial strips to A23187 and vasopressin appear to be mediated by endothelium-derived relaxing factor. The function of vasopressin receptors in endothelial cells differs markedly in basilar and posterior communicating arteries versus middle cerebral arteries. Substance P-induced relaxations appear to be primarily associated with endothelium-derived relaxing factor and with prostaglandin I2, whereas contractions appear to be mediated by endothelium-derived prostaglandins.

Heterogeneous vasomotor responses of anatomically distinct feline cerebral arteries

1. The vasomotor reactivity to a number of neurotransmitters and blood-borne substances was evaluated in several anatomically distinct arteries of the cat cerebral circulation. Few regional differences were observed in their vasoconstrictor responses to noradrenaline, dopamine, 5-hydroxytryptamine and prostaglandin F2 alpha. Only the anterior cerebral artery reacted strongly to all vasoconstrictor agents. 2. Adenosine, acetylcholine and histamine induced pronounced relaxation in the vast majority of the major cerebral arteries. The relaxation elicited by adenosine showed a slight degree of heterogeneity between the arteries and the overall response accounted for 81 +/- 6% of the pharmacologically-induced tone. On the other hand, the dilatation induced by acetylcholine and histamine varied as a function of the anatomical localization of the cerebral arteries. The acetylcholine-induced vasodilatation was significantly more pronounced in the middle cerebral, anterior communicating and anterior cerebellar arteries, with respective responses of 72, 66 and 83% of the induced tone as compared to 43% in the other vessels. However, all arteries were equally sensitive to acetylcholine with an overall mean pD2 value of 7.47 +/- 0.06. The most heterogeneous results were obtained with histamine and applied both to the magnitude of the maximal response and the sensitivity of the various arteries to this amine. The intensity of the relaxation varied from 20% (anterior communicating artery) to 118% (posterior cerebellar artery). 3. Among the neuropeptides studied, substance P and bradykinin were considerably less potent than vasoactive intestinal peptide on all the cerebral arteries. The least responsive vessel to bradykinin was the anterior cerebral artery with a maximal response of 22 +/- 5% of the induced-tone and a pD2 value of 7.56 +/- 0.24. All vessels responded weakly to substance P and those from the vertebrobasilar circulation were significantly less sensitive to this neuropeptide with pD2 values around 8.07 as compared to 9.82 in the more rostral arteries. Although all vessels were equally sensitive to vasoactive intestinal peptide, the dilator responses were significantly less pronounced in the middle cerebral and basilar arteries (maximal response of 86 +/- 5% and 69 +/- 6% of the induced-tone, respectively, as compared to 110 +/- 9% in the other vessels). 4. The vertebrobasilar arteries were as reactive, if not more reactive, to vasoconstrictors than the vessels originating from the carotid circulation. In contrast, the dilator responses were less marked in most caudal arteries. Such dichotomies may be important in the regulation of local cerebral blood flow. 5. The results emphasize the considerable heterogeneity in the vasomotor responses to a given substance among the various cerebral arteries. Further, they suggest the presence of multiple receptor populations which mediate opposite effects and which are distributed in different proportions among the cephalic arteries.

Mechanism of histamine actions in human coronary arteries

Helical strips of human coronary arteries contracted in response to histamine concentration dependently, they relaxed with low concentrations and contracted with high concentrations. Treatment with cimetidine potentiated contraction in the strips with intact and damaged endothelium to a similar extent and attenuated relaxation. Removal of endothelium abolished relaxation and potentiated contraction in the cimetidine-treated strips. Methylene blue increased the contractile response to histamine in the strips with endothelium but did not alter the response in the damaged-endothelium strips. Histamine-induced relaxations in the intact strips were suppressed or abolished by treatment with ETYA, AA861, a lipoxygenase inhibitor, and by chlorpheniramine but were unaffected by indomethacin. Chlorpheniramine also abolished amine-induced contraction. It may be concluded that histamine-induced contraction in human coronary arteries is mediated by H1 receptors in smooth muscle, and relaxation is mediated by H2 receptors in smooth muscle and H1 receptors in endothelium. Also, stimulation of the endothelial H1 receptor liberates vasodilator substance and possibly activates smooth muscle guanylate cyclase to accumulate cellular cyclic guanosine monophosphate.

Blood-borne factors regulating microvascular constriction in the rat hippocampal slice

Previous work shows that blood serum contains a factor that elicits constriction of large cerebral arteries and may be responsible for cerebral vasospasm in subarachnoid hemorrhage patients. However, few studies have considered the action of potential spasmogens on intraparenchymal resistance vessels. We used a common preparation for neurophysiology, the rat hippocampal slice, to study penetrating arterioles and their response to a variety of potential vasoconstrictors. Dilute serum from clotted rat blood evoked profound microvascular constriction when applied to the slice, but plasma incubated with heparin to prevent coagulation was unable to do so. A variety of potential vasoconstrictor substances were tested to see if they could duplicate the effects of serum. Norepinephrine (10 microM), serotonin (one microM) and prostaglandin E2 (one nM) were ineffective in this regard. However, when a stable eicosanoid, thromboxane B2, was applied at a concentration achieved in the cerebrospinal fluid of subarachnoid hemorrhage patients, it duplicated a portion of the vasoconstriction produced by serum. It is concluded that thromboxane B2 may account for part of the spasmogenic property of serum and that unidentified molecules in the coagulation pathway may also play a role.