Nitric Oxide

Although it is unlikely to be directly involved in pressure autoregulation itself, NO is the subject of intense scrutiny as a mediator of vascular tone and as a neurotransmitter. ^, The interest in NO results from the identification of the multiple biologic roles it plays as a messenger molecule. Although until recently no evidence had shown it to have any biologic function at all in vertebrates, NO now appears to have at least the following major roles: (1) bactericidal and tumoricidal effects in white blood cells, (2) a neurotransmitter, and (3) a moderator/mediator of vascular tone, functioning as an “endothelium-derived relaxing factor.”

NO is synthesized from l -arginine by nitric oxide synthase (NOS). There are at least three isoforms of NOS: endothelial (eNOS), neuronal (nNOS), and inducible (iNOS). Of these, eNOS and nNOS exist in the normal brain, whereas iNOS synthesis can be induced by endotoxins and cytokines. Endogenous inhibitors of NOS, such as asymmetric dimethyl- l -arginine (ADMA), are produced during protein catabolism and may reach concentrations sufficient to inhibit NOS activity in the brain. NO action has been studied through the use of arginine analogues such as NG-nitro- l -arginine methyl ester ( l -NAME), 7-nitroindazole, and aminoguanidine, which can nonselectively or selectively block NO synthesis. NO appears to influence basal tone, as well as the endothelium-dependent response to acetylcholine in cerebral arteries and vasogenic dilation from stimulation of nonadrenergic, noncholinergic nerves. In general, topical, systemic, and intra-arterial application of NO donors increases CBF in several animal species. ^, Intra-arterial injection of the NO donor nitroprusside into angiographically normal territories in patients with cerebral arteriovenous malformations failed to augment CBF. A similar failure of intra-arterial nitroprusside was seen in healthy primates. ^, In contrast, a study in human volunteers found that systemic and intra-arterial administration of NG-monomethyl- l -arginine ( l -NMMA), a nonspecific inhibitor of eNOS, decreases CBF. ^, The latter findings suggest that NO may be involved in regulation of basal cerebrovascular tone. After synthesis, NO diffuses into the vascular myocyte and activates guanylate cyclase, forming cyclic guanosine monophosphate (cGMP). A protein kinase is stimulated by cGMP, resulting in phosphorylation of the light chain of myosin and thus vascular relaxation. NO may also act partly through calcitonin gene-related peptide (CGRP) and ATP-sensitive potassium (KATP) channels. NO partly acts also by suppressing endothelial generation of vasoconstrictors such as thromboxane A ₂ . In pathologic settings, such as vasospasm and hypoxia, Rho kinase, a serine threonine kinase, is emerging as a potent mediator of sustained vasoconstriction that in part acts through the NO pathway. Inhibition of Rho kinase increases cerebral blood flow. In middle cerebral artery occlusion models, Rho kinase inhibition improves neurologic outcome. ^, Rho kinase inhibition increases eNOS synthesis, and Rho kinase seems to negatively regulate eNOS activity. Calcium is intimately involved in vascular relaxation by NO. NO appears to be formed on demand and is not stored in vesicles—the traditional fate of neurotransmitters.

The role of NO in the vasodilitatory response due to changes in perfusion pressure or carbon dioxide (CO ₂ ) remains to be coherently defined. For example, nonspecific inhibition of NOS in primates does not affect pressure autoregulation but impairs response to CO ₂ . However, in humans, nonspecific inhibition of NOS results in a decrease in CBF but does not affect response to hypercapnia. In rodents, nonspecific inhibition of NOS impairs autoregulatory response to hypotension in basilar artery irrigation. While selective nNOS inhibition by 7-nitroindazole has no effect on baseline blood flow, 7-nitroindazole can prevent the increase in blood flow due to neural activation. In canines, 7-nitroindazole decreases collateral blood flow during middle cerebral artery occlusion.

Some investigators have reported that NO appears to play a role in dilation in response to CO ₂ . In other experiments, however, its participation in hypocapnia-induced vasoconstriction could not be demonstrated. Iadecola and Zhang proposed that NO plays either an “obligatory” or a “permissive” role in CO ₂ -induced vasodilation. Obligatory implies that NO directly mediates vasodilation through that mechanism. For example, topical application of glutamate agonists results in vasodilation that can be markedly attenuated by inhibition of NOS. Therefore, NO seems to play an obligatory role in glutamate-mediated vasodilation. Permissive implies that NO facilitates relaxation but near-complete inhibition of NOS only partly attenuates the vasodilator response. Because hypercapnic response is only partly attenuated by NOS inhibition, NO’s role is described as permissive, with other mechanisms also contributing to hypercapnic dilation. NO appears to play a much greater role in hypercapnic vasodilation in adults than in neonates. The site of action for CO ₂ -induced NO production may not be in the endothelium but, rather, in the perivascular structures, such as astrocytes.

The participation of NO in hypoxia-induced vasodilation does not appear to be physiologically important. ^{, ,} In regard to anesthetic effects on CBF, NO appears to interact with the cerebral vasodilatory effects of both halothane and isoflurane. The role of NO as a neurotransmitter undoubtedly will prove to be significant for care of the patient with neurologic disease through its interactions with anesthetic depth and cerebral ischemic states, ^, in particular the pathogenesis of vasospasm after subarachnoid hemorrhage (SAH). Inhibition of NO synthesis leads to vasoconstriction due to unopposed effects of endothelial prostanoids, such as thromboxane A ₂ and prostaglandin F _2α . Vascular abnormalities in disease states that significantly predispose the brain to damage, such as diabetes mellitus, may also be related to an NO-mediated mechanism.

Vasoactive Peptides

In the cerebral circulation, perivascular nerves contain several vasodilator peptides, including CGRP, substance P, and neurokinin A. Vasodilation with CGRP, unlike with substance P and neurokinin A, is independent of endothelin. CGRP acts by increasing intracellular cyclic adenosine monophosphate (cAMP) concentrations and partly mediates cerebral vasodilation in response to hypotension, cortical spreading depression, and cerebral ischemia. Vasodilation by NO is in part mediated by CGRP. ^, CGRP probably does not play a role in vasodilator response to hypoxia or hypercapnia. The physiologic roles of substance P and neurokinin A are not yet understood. Substance P may mediate vasodilation during pathologic derangements such as cerebral and meningeal inflammation and edema.

Potassium Channels

Of the several potassium channels in the cerebral vessels, two are of particular importance in the regulation of vascular tone: KATP channel and calcium-activated potassium (KCa) channel. A third potassium channel, pH-sensitive delayed rectifier potassium channel, may play a role in hypercapnia. Opening of potassium channels triggers potassium efflux from the vascular smooth muscle cell, hyperpolarizes the cell membrane, closes the voltage-dependent calcium channels, decreases calcium entry into the cells, and ultimately relaxes the muscles. KATP channels are opened by a decrease in intracellular pH and are inhibited by an increase in intracellular ATP concentrations and by sulfonylureas. Activation of KATP channels may partly mediate vasodilation by acetylcholine, CGRP, or norepinephrine (noradrenaline). KATP channels may play some role in vasodilation during hypotension, hypercapnia, acidosis, and hypoxia. KCa channel-mediated vasodilation is due partly to astrocyte-derived carbon monoxide, which diffuses into the smooth muscle cells. Large-conductance KCa (BKCa) channels are the most important of the several KCa channels found in the cerebral circulation. These channels can be selectively blocked by tetraethylammonium, charybdotoxin, and iberiotoxin. Inhibition of BKCa channels results in cerebral vasoconstriction in the large arteries, suggesting that BKCa channels may be involved in the regulation of basal cerebrovascular tone in these vessels. BKCa channels are activated by cGMP, cyclic adenosine monophosphate, and NO and are partly responsible for hypoxia-induced vasodilation of cerebral arteries. ^{, , ,}

Prostaglandins

Prostaglandins such as PGE ₂ and PGI ₂ are vasodilators but thromboxane A ₂ and PGF _2α are vasoconstrictors in the cerebral circulation. Synthesis of prostaglandin H ₂ from membrane phospholipids involves two critical enzymes, phospholipase and cyclooxygenase. Prostaglandin H ₂ is converted into other prostaglandins by subsequent enzymatic steps. Although cyclooxygenase can be inhibited by aspirin, naproxen, and indomethacin, only indomethacin impairs hypercapnic vasodilation in humans. ^,

Prostaglandins probably play a more significant role in the regulation of CBF in neonates than in adults. Inhibition of phospholipase by quinacrine hydrochloride abolishes the cerebrovascular response to hypercapnia, and hypoxia in newborn animals. Endothelial damage and indomethacin also abolish hypercapnia-induced vasodilation and the increase in cerebrospinal fluid (CSF) PGI ₂ concentrations. However, indomethacin-impaired CO ₂ reactivity can be restored by very low concentrations of PGE ₂ . This suggests that prostaglandins may not be direct mediators of hypercapnic vasodilation but that small amounts of prostaglandins are necessary for the CO ₂ response to hypercapnia to occur and that prostaglandins thus play a so-called permissive role.

Endothelin

Endothelin is a vasoactive peptide that is synthesized by the brain and the vascular endothelium. There are three isoforms of endothelin. The brain synthesizes endothelin-1 (ET-1) and endothelin-3 (ET-3) but not endothelin-2 (ET-2). The vascular endothelium synthesizes ET-1. The two receptors for endothelin are endothelin A (ETA) and endothelin B (ETB). Activation of ETA receptors causes vasoconstriction, and activation of ETB receptors may cause either vascular relaxation or constriction. Vascular relaxation is thought to be mediated by endothelin receptors on the endothelium, whereas constriction is probably mediated by endothelin receptors located on the smooth muscle cells. ETA receptors are probably more sensitive to ET-1 and ET-2 than to ET-3. The ETB receptor is equally sensitive to all isoforms of endothelin. ^, Endothelin most likely acts through influx of extracellular calcium, which is probably mediated by protein kinases. The vascular smooth muscle contraction caused by endothelin is sustained, suggesting that endothelin is not involved in rapid adjustment of cerebrovascular resistance (CVR). Topical applications of endothelin receptor antagonists do not alter resting CVR. Endothelin has been implicated in vascular spasm after SAH. ^, In experimental models of SAH, ETA and ETB receptor antagonists prevent evolution vasospam. Endothelin-induced vasospasm can also be reversed nonspecifically by calcium channel blockade and seems to be more responsive to intra-arterial nicardipine than to verapamil. ^, Early results of clinical trials showed that intravenous infusion of an ETA receptor antagonist, clazosentan, resulted in a decrease in the incidence of vasospasm after SAH. Intravenous clazosentan infusion reduces the severity of the established cerebral vasospasm. However, despite improvements in angiographic vasospasm with ETA receptor antagonists, more recent clinical analyses have shown no improvement in vasospasm-related cerebral infarction, new cerebral infarction, or case-fatality. Furthermore, treatment with ETA receptor antagonists was associated with higher incidence of pulmonary complications, hypotension, and anemia. Thus, enthusiasm for using these agents has waned.