John A. Bevan

Functional arterial changes in chronic cerebrovasospasm in monkeys: an in vitro assessment of the contribution to arterial narrowing.

Cerebral arteries from monkeys with chronic cerebral vasospasm arising from experimental subarachnoid hemorrhage produced 5-6 days previously were examined for changes in their functional properties in an attempt to understand the basis of the narrowing. Hemorrhage was caused by puncture of the internal carotid artery just proximal to the circle of Willis. Segments taken close to the origins of the anterior and middle cerebral arteries consistently showed decreased distensibility. In addition, they exhibited large, prolonged, spontaneous increases in muscle tone. Other alterations observed include a marked reduction in the capacity of the vessel wall to contract, reduction in constrictor and dilator nerve influences on vascular tone, and some increased sensitivity to serotonin. Small pial arteries (150-200 micron o.d.) from the side of the injury showed large spontaneous irregular increases in tone. It is proposed that 5-6 days after experimental subarachnoid hemorrhage in monkeys the change most responsible for persistent narrowing in the larger arteries is an increased rigidity of the vessel wall. This is probably caused by an inflammatory response. In the smaller arteries, abnormal spontaneous contractile activity is a major factor in narrowing. This activity is not stretch-dependent. We suggest that the initial cause of the arterial narrowing after hemorrhage is the action of vasoactive substances released in the close vicinity of the arterial wall, which lead to tissue damage, abnormal tone, and an inflammatory response with fibrosis.

Some bases of differences in vascular response to sympathetic activity.

Basic patterns of neuroeffector organization vary widely in the vasculature, in general, with vessel diameter and type, and confer distinctive properties. The smaller the vessel, the more intimate the neuroeffector relationship, the more localized the action of the released transmitter, and the more important myogenic conduction compared to transmitter diffusion for the coordination of vascular effector response. Seemingly superimposed upon these basic general patterns are other variable features, conferring upon vessels of similar size and type diversity of function. These variables include sensitivity and magnitude and possible location of alpha- and beta-receptors and their subtypes, presence and nature of intrinsic vascular tone, and the density and pattern of adrenergic innervation to mention the more important. Functional diversity in neuroeffector characteristics can, to some extent, be understood in relation to embryological development, neurotrophic influences, effector regulation of innervation, and the mural response to an increase in intravascular pressure.

Nitric Oxide Inhibits α2-Adrenoceptor–Mediated Endothelium-Dependent Vasodilation

This study was designed to investigate the interaction between the NO/L-arginine pathway and the alpha2-adrenoceptor-mediated endothelium-dependent vasorelaxation. Reactivity of isolated resistance mesenteric arterial segments from mice lacking the gene for constitutive endothelial NO synthase (eNOS- mice, n=14) and from their wild-type controls (WT mice, n=46) was studied in isometric conditions in the presence of indomethacin (blocker of cyclooxygenase). Oxymetazoline (OXY, 0.01 to 30 micromol/L; a selective alpha2-adrenoceptor agonist) induced an endothelium-dependent relaxation of eNOS- but not WT arteries preconstricted either with phenylephrine or serotonin. In the presence of Nomega-nitro-L-arginine (l-NNA, 100 micromol/L), an inhibitor of NOS, OXY induced an endothelium-dependent relaxation of WT mesenteric arteries. l-NNA had no effect on the relaxation caused by OXY in eNOS- arterial rings. Therefore, the relaxation caused by OXY was independent of NO formation. To demonstrate the inhibitory role of NO on the alpha2-adrenoceptor-mediated relaxation, subthreshold (0.1 nmol/L) to threshold (1 nmol/L) concentrations of sodium nitroprusside (donor of NO) were added to l-NNA-treated arteries before OXY challenges: in these conditions, the alpha2-adrenoceptor-mediated relaxation of eNOS- and WT arteries was inhibited. OXY-induced relaxation was restored on readdition of methylene blue (1 micromol/L, inhibitor of guanylate cyclase), suggesting that cGMP may be the mechanism of inhibition of the alpha2-adrenergic pathway in the presence of NO. Finally, OXY-mediated relaxation was blocked by tetraethylammonium (1 mmol/L) but not glibenclamide (1 micromol/L), suggesting the involvement of an endothelium-derived hyperpolarizing factor that activates Ca2+-activated K+ channels. In conclusion, alpha2-adrenoceptor activation caused relaxation of isolated murine mesenteric arteries that was functionally blocked by NO through a mechanism that may involve activation of the soluble guanylate cyclase and cGMP formation. The endothelium-dependent alpha2-adrenoceptor-mediated relaxation is likely to be due to an endothelium-derived hyperpolarizing factor, whose release and/or production is reduced by concurrent NO formation.

Selective action of diltiazem on cerebral vascular smooth muscle in the rabbit: Antagonism of extrinsic but not intrinsic maintained tone

Diltiazem selectively reduces the second equilibrium phase of contraction of several arteries and veins in the rabbit to a number of agonists, including norepinephrine, serotonin, histamine and potassium. Its effect is consistent with blockage of the entry of activator calcium from the extracellular space into the vascular smooth muscle cell. It has a preferential action on responses of the basilar artery in comparison with those of the ear and mesenteric arteries and saphenous vein. Although the spontaneously developing, maintained intrinsic tone of the basilar artery and facial vein are very resistant to diltiazem, the spontaneous rhythmic activity of the portal vein is sensitive to its antagonistic action. Findings reported here are consistent with the possibility that diltiazem preferentially blocks receptor-operated and potential-sensitive slow calcium channels of the cerebral artery compared with other systemic vessels tested but has little effect on the mechanisms responsible for maintained intrinsic tone development.

Weakness of Sympathetic Neural Control of Human Pial Compared With Superficial Temporal Arteries Reflects Low Innervation Density and Poor Sympathetic Responsiveness

BACKGROUND AND PURPOSE The primary goal of these studies was to understand and investigate the capacity of perivascular nerves to influence the tone of human pial arteries and to compare them with other human cephalic arteries, the superficial temporal and middle meningeal. METHODS Responses to electrical activation of intramural nerves and related features of fresh segments of human cephalic arteries-the pial (PA; 478+/-34 microm ID), middle meningeal (MMA; 540+/-41 microm ID), and superficial temporal (STA; 639+/-49 microm ID)-obtained from patients aged 15 to 82 years during surgical procedures were studied on a resistance artery myograph. RESULTS The PA segment responses to electrical nerve activation and to norepinephrine (NE; 10[-5] mol/L) were 1% and 21% of tissue maximum, respectively, compared with 6% and 34% for the MMA and 14% and 90% for the STA. Tissue maximum was defined as the force increase to 127 mmol/L KCl plus arginine vasopressin (1 microm). All arteries dilated well to acetylcholine. Possible explanations for the PA marginal neurogenic responses were assessed. NE ED50 was 5.4+/-2.2 X 10(-7) mol/L and did not vary with age or diameter. NE responsiveness did not increase in vessels with spontaneous or raised potassium-induced tone. Relaxation to isoproterenol was variable and propranolol did not increase the neurogenic response. Neither N(G)-monomethyl-L-arginine, N(G)-nitro-L-arginine methyl ester, endothelium removal, nor indomethacin consistently influenced the contractions to NE or neurogenic reactivity. The weak PA neurogenic response is in keeping with its poor innervation. As determined by catecholamine histofluorescence, innervation in the PA is sparse, with density increasing in the order PA, MMA, and STA. The incidence of nerve structures in the PA adventitio-medial junction was only 3% of those in the STA, and these were situated more than 3 microm from the closest smooth muscle cell. CONCLUSIONS We conclude that the weak neurogenic response of adult human pial artery reflects its poor innervation and responsiveness to NE, implying that these features are not important in the regulation of its diameter.

Role of Ca2+-Activated K+ Channels in the Regulation of Membrane Potential and Tone of Smooth Muscle in Human Pial Arteries

Smooth muscle cells (SMCs) in 58% of human pial arteries obtained during surgery showed no spontaneous contractions and displayed a stable resting membrane potential (MP) of -54.7 +/- 1.5 mV. Those that exhibited periodic spontaneous contractions associated with periodic depolarization and generation of spontaneous action potentials (APs) had a less negative MP of -43.1 +/- 0.5 mV (42%). Inhibition of calcium-activated potassium (KCa) channels in the silent arteries by charybdotoxin (CTX) and tetraethylammonium ions (TEA) induced dose-dependent depolarization, AP generation, and contraction. TEA and CTX enhanced the spontaneous depolarization and force in arteries that exhibited spontaneous activity. They also prolonged the spontaneous APs up to several times and increased their upstroke amplitude. Both TEA and CTX failed to produce significant depolarization in arteries treated with nifedipine. It is concluded that KCa channels are important regulators of human pial artery SMC resting MP and tone. They are also involved in the control of AP amplitude and duration and the associated contractions. These data suggest that alterations in the activity of SMC KCa channels could be responsible for the appearance of spontaneous activity in human pial arteries in vitro and that impaired function of these channels might be related to vasospastic phenomena in human cerebral circulation.

Innervation Pattern and Neurogenic Response of Rabbit Veins

The adrenergic neuroeffector mechanism has been assessed in 14 different veins of the rabbit chosen to represent vessels of different, function and regional location. The pattern, distribution and den

Responsiveness of human infant cerebral arteries to sympathetic nerve stimulation and vasoactive agents.

Responses of segments of basilar and middle cerebral arteries of eight human infants to activation of perivascular nerves and to vasoactive drugs were studied using a resistance artery myograph. The infants ages ranged from 23 wk of gestation to 34 postnatal days. Neurogenic vasoconstriction occurred in all segments and at 8 Hz was 12.7 ± 3.5% (11%) of tissue maximum and was blocked by phentolamine (10-6 M). There was no evidence of a neurogenic dilator response. Catecholamine histofluorescence was seen in nerves in the adventitia at all ages studied. Norepinephrine ED50 was 7.6 ± 1.8 × 10-7 M, and its maximum effect was 43.1 ± 5.7% of tissue maximum. Both neural and norepinephrine responses were greater than those of the proximal parts of adult human middle cerebral arteries obtained postmortem and surgically removed adult human pial arteries. Electron microscopy demonstrated that neural density at the adventitiomedial junction in the infant vessels was greater than in the pial arteries. Constrictor responses to serotonin and prostaglandin F2α were minimal in the two infants of 23 and 24 wk of gestation but were clearly present in the older infants. Histamine and acetylcholine were potent vasodilators. Indomethacin potentiated agonist-induced contraction. In a limited number of trials angiotensin II, neuropeptide Y, caused contraction and bradykinin, relaxation. It is concluded that there is a quantitative similarity between the studied responses of infant cerebral artery segments and human pial arteries of similar diameter. However, sympathetic nerves may potentially play a more important role in the regulation of cerebrovascular tone in the infant compared with the adult, and during the gestational period examined these vessels possess an indomethacin-sensitive system that buffers agonist tone.

Localized Neurogenic Vasoconstriction of the Basilar Artery

Ring segments from the caudal end of the basilar artery of the rabbit contract to electrical stimulation of their intramural nerve supply more than those from adjacent parts of the vertebral and the rostral three-fourths of the basilar arteries. The magnitude of developed neurogenic tension is approximately one-fourth that of the ear artery, a highly reactive muscular artery. The position of this reactive arterial segment suggests its role in the regulation of cerebral blood flow.

An in vitro study of prolonged vasospasm of a rabbit cerebral artery.

Longitudinal stretch of the rabbit basilar artery produces local injury followed by prolonged circular constriction. After stretching and rapid release in vitro localized constrictions promptly occurred. This could be prevented by prior treatment with cyanide or calcium-free solution. Once produced, constrictions persisted for more than 72 hours. Previously induced constriction was not reversed by treatment for two hours with cyanide or by removing calcium. Histological observation indicated that constricted areas were associated with a discrete circumferential rupture of the internal elastic lamina and disruption and thinning of the underlying media. Specific catecholamine fluorescence at the adventitio-medial junction was unchanged in constricted areas. The relationship between smooth muscle cell length and resting tension of artery segments with and without constrictions was compared. Segments with constrictions had a shorter muscle length for any given resting tension, which confirms that constriction was not due to passive collapse of the vessel wall. These findings suggest that injury of cerebrovascular smooth muscle may result in essentially irreversible vasoconstriction. Such a mechanism could contribute to the pathogenesis of prolonged cerebral vasospasm after SAH or traumatic injury to the cerebrum.