Electrophysiology of cerebral blood vessels

Chloride Ions, Vascular Function and Hypertension

Blood pressure is determined by cardiac output and systemic vascular resistance, and mediators that induce vasoconstriction will increase systemic vascular resistance and thus elevate blood pressure. While peripheral vascular resistance reflects a complex interaction of multiple factors, vascular ion channels and transporters play important roles in the regulation of vascular tone by modulating the membrane potential of vascular cells. In vascular smooth muscle cells, chloride ions (Cl⁻) are a type of anions accumulated by anion exchangers and the anion–proton cotransporter system, and efflux of Cl⁻ through Cl⁻ channels depolarizes the membrane and thereby triggers vasoconstriction. Among these Cl⁻ regulatory pathways, emerging evidence suggests that upregulation of the Ca²⁺-activated Cl⁻ channel TMEM16A in the vasculature contributes to the increased vascular contractility and elevated blood pressure in hypertension. A robust accumulation of intracellular Cl⁻ in vascular smooth muscle cells through the increased activity of Na⁺–K⁺–2Cl⁻ cotransporter 1 (NKCC1) during hypertension has also been reported. Thus, the enhanced activity of both TMEM16A and NKCC1 could act additively and sequentially to increase vascular contractility and hence blood pressure in hypertension. In this review, we discuss recent findings regarding the role of Cl⁻ in the regulation of vascular tone and arterial blood pressure and its association with hypertension, with a particular focus on TMEM16A and NKCC1.

Molecular keys to the problems of cerebral vasospasm

The mechanisms responsible for subarachnoid hemorrhage (SAH)-induced vasospasm are under intense investigation but remain incompletely understood. A consequence of SAH-induced vasospasm, cerebral infarction, produces a nonrecoverable ischemic tissue core surrounded by a potentially amenable penumbra. However, successful treatment has been inconsistent. In this review, we summarize the basic molecular biology of cerebrovascular regulation, describe recent developments in molecular biology to elucidate the mechanisms of SAH-induced vasospasm, and discuss the potential contribution of cerebral microcirculation regulation to the control of ischemia. Our understanding of the pathogenesis of SAH-induced vasospasm remains a major scientific challenge; however, molecular biological techniques are beginning to uncover the intracellular mechanisms involved in vascular regulation and its failure. Recent findings of microvascular regulatory mechanisms and their failure after SAH suggest a role in the development and size of the ischemia. Progress is being made in identifying the various components in the blood that cause SAH-induced vasospasm. Thus, our evolving understanding of the underlying molecular mechanism may provide the basis for improved treatment after SAH-induced vasospasm, especially at the level of the microcirculation.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.

Electrical activity underlying rhythmic contraction in human pial arteries

Human pial arteries obtained during surgery frequently exhibit spontaneous periodic contractions. Simultaneous measurements of membrane potential and vessel wall force were used to examine whether these contractions are associated with electrical activity of smooth muscle cells (SMCs). A total of 53 segments from 38 patients were studied, and of these, 26 showed spontaneous contractions related to periodic depolarization and generation of action potentials (APs). The resting membrane potential during the silent periods was -44.0 +/- 0.5 mV. APs without "overshoot'' were observed when spontaneous depolarization reached levels of -40 to -35 mV. Just over half of the arterial segments failed to exhibit spontaneous activity; however, APs could be generated during K+-induced depolarization. The mean SMC resting membrane potential of these vessels was -53.5 +/- 1.5 mV, and this value differed significantly from that of SMCs in spontaneously active arteries. Application of tetrodotoxin did not change the amplitude and duration of APs. Removal of Ca2+ from the bathing solution and addition of nifedipine completely inhibited the spontaneous APs and associated contractions. K+ depolarization failed to induce APs and contraction in the presence of nifedipine. We conclude that periodic spontaneous depolarization and AP generation underlie the periodic spontaneous contractions of human pial arteries. Both the APs and associated contractions are related to the activation of dihydropyridine-sensitive voltage-dependent Ca2+ channels. It is suggested that AP generation can be responsible for vasomotion of human pial arteries in vivo.

Role of potassium channels in cerebral blood vessels

BACKGROUND

Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. In cerebral blood vessels, four different potassium channels have been described: ATP-sensitive potassium channels, calcium-activated potassium channels, delayed rectifier potassium channels, and inward rectifier potassium channels.

SUMMARY OF REVIEW

Activation of ATP-sensitive and calcium activated potassium channels appears to play a major role in relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, and hypoxia. Both calcium-activated and delayed rectifier potassium channels may contribute to a negative feedback system that regulates tone in large cerebral arteries. The influence of ATP-sensitive and calcium-activated potassium channels is altered in disease states such as hypertension, diabetes, and atherosclerosis.

CONCLUSIONS

Activation of potassium channels is a major mechanism of cerebral vasodilatation. Alteration of activity of potassium channels and impairment of vasodilatation may contribute to the development or maintenance of cerebral ischemia or vasospasm.

Cerebral vascular smooth muscle potassium channels and their possible role in the management of vasospasm

One of the promising therapeutic uses of the potassium channel openers is in the management of cerebral vasospasm, a prolonged vasoconstriction of major cerebral arteries which follows aneurysmal subarachnoid haemorrhage. In this review, we first summarize the properties of potassium channels in cerebral vascular smooth muscle. Calcium-activated and ATP-dependent potassium channels are the major potassium channels identified in the cerebrovascular smooth muscle and both are believed to play a role in the regulation of cerebrovascular smooth muscle tone. The calcium-activated potassium channels can be activated by depolarization, by elevation of internal calcium and by some vasodilators. Some neuropeptides and potassium channel openers open the ATP-dependent potassium channels and produce vasodilation. We then review the effects of both synthetic and endogenous potassium channel openers in the cerebrovascular system, discuss their efficacy in the management of models of cerebrovascular spasm, and outline the clinical promise of these agents.

Contribution of both nitric oxide and a change in membrane potential to acetylcholine-induced relaxation in the rat small mesenteric artery

1. Acetylcholine stimulated repolarization and relaxation in isolated segments of rat small mesenteric artery (D100 = 325 +/- 9 microns) in which the smooth muscle cells were depolarized and contracted by submaximal concentrations of noradrenaline (0.75-2.5 microM). There was no significant difference either in the time taken to initiate relaxation or hyperpolarization, or for these parameters to reach maximum in response to acetylcholine. 2. The nitric oxide synthase inhibitor, NG-nitro L-arginine methyl ester (L-NAME, 100 microM) reduced the pD2 for acetylcholine-induced relaxation from 7.5 to 7 and depressed the maximum relaxation from 89% to 68% in tissues stimulated with noradrenaline. The pD2 for smooth muscle repolarization in these experiments was also reduced (7.4 to 6.6) but the maximum change in membrane potential in response to acetylcholine was unaltered. The increase in potential now clearly preceded relaxation by 3.7 s (to initiation) and 4.7 s (to maximum). 3. In the presence of noradrenaline and a raised potassium concentration (25 mM), the repolarization to acetylcholine was markedly attenuated. Simultaneous tension measurements also revealed a marked reduction in the maximal relaxation to acetylcholine, but the pD2 was unchanged at 7.4. 4. The residual relaxation recorded in the absence of marked repolarization (in the presence of noradrenaline and 25 mM potassium) was abolished by the addition of 100 microM L-NAME. 5. Nitric oxide gas in solution (0.2-2.2 microM; NOg) relaxed artery segments precontracted with noradrenaline. The magnitude of relaxation to NOg was not altered in the presence of noradrenaline and 25 mM potassium. 6. These data provide additional evidence that acetylcholine-evoked endothelium-dependent increases in membrane potential provide a major mechanism for smooth muscle relaxation in the mesenteric artery.They also show that voltage-dependent and independent (initiated by NO) mechanisms can both contribute to relaxation, and suggest that NO may modulate the increase in membrane potential or the release of a hyperpolarizing factor.