Role of Ca(2+)-dependent K+ channels in cerebral vasodilatation induced by increases in cyclic GMP and cyclic AMP in the rat

Ca(2+)-activated potassium (K(Ca)) channel inhibition decreases neuronal activity-blood flow coupling

A number of possible mediators have been proposed to couple neuronal activity with local cerebral metabolic activity and blood flow, but the mechanisms by which these mediators act is still unclear. In order to explore these coupling mechanisms, we used the rodent whisker-barrel cortex (WBC) model to test the hypothesis that modulation of K(Ca) channels is an important step in this coupling process. Anesthetized rats were prepared for laser-Doppler flowmetry (LDF) or evoked potential recordings utilizing a thinned cranial window over WBC. Superfusion of the K(Ca) channel blockers tetraethylammonium (TEA) or iberiotoxin directly onto WBC attenuated the magnitude of the whisker evoked LDF changes. Similar effects were seen after intravenous administration of TEA. Although attenuated, neither the temporal profile of the elicited blood flow responses nor the evoked electrical activity in WBC were affected by K(Ca) blockade. These data suggest that the process of cerebral metabolism/blood flow coupling in the rodent WBC involves K(Ca) channels.

Cerebral hemodynamic patterns with technetium Tc 99m exametazime single photon emission computed tomography and transcranial Doppler sonography: a validation study using visual stimulation

OBJECTIVE

To assess whether a correlation between perfusion changes to visual stimulus on the bilateral occipital areas and blood flow velocity changes to visual stimulus in both posterior cerebral arteries is present.

METHODS

Nine right-handed healthy subjects (4 women and 5 men; mean +/- SD age, 58.0 +/- 5.6 years) were included in the study. Visual stimulation was performed in room light with the subject's eyes open and looking around versus eyes closed as the stimulus-off condition. The blood flow velocities were recorded using transcranial Doppler sonography, and the regional cerebral blood flow measurements were recorded with the use of technetium Tc 99m exametazime and a single photon emission computed tomographic gamma camera system. Individual reactivity was defined as a relative increase of blood flow velocity and perfusion, which were calculated as percentage changes of baseline values.

RESULTS

Visual stimuli produced a marked increase of blood flow velocity in both posterior cerebral arteries (35.2 +/- 2.3 cm/s; P < .001) without a significant side-to-side difference in all subjects as well as a marked increase of perfusion on both occipital areas (24.9 +/- 4.8 cm/s; P < .01). Moreover, there was a positive correlation between blood flow velocity changes and perfusion changes on both sides (r = 0.833; P < .01).

CONCLUSIONS

The use of bilateral simultaneous Doppler recordings by means of a flow velocity averaging algorithm to a specific stimulus allows quantitative assessment of blood flow responses, and simple visual stimuli can be applied for different disorders to assess the vasomotor regulation that may result in measurable abnormal cerebral flow regulation even when clinically stabilized.

cGMP, but not cAMP, in rat hippocampus is involved in early stages of object memory consolidation

The present study investigates the role of cGMP and cAMP on the memory performance in the object recognition task in rats. The analogue 8-Br-GMP or 8-Br-cAMP was administered bilaterally into the hippocampus (0, 1, 3 and 10 microg in 0.5 microl saline/site) immediately after the exposure to two identical objects. After 24 h, saline-treated animals spent equal times exploring a new and the familiar object demonstrating that they did not recognize the familiar one. However, a dose-dependent improvement in object recognition was found after injection of 8-Br-cGMP. In contrast, 8-Br-cAMP did not improve the memory performance at the doses tested. These results indicate that hippocampal cGMP but not cAMP is involved in early stages of consolidation of object memory.

Role of potassium channels in the relaxation induced by the nitric oxide (NO) donor DEA/NO in the isolated rat basilar artery

This study investigates whether potassium ion (K+) channels are involved in the nitric oxide (NO)-induced relaxation in segments of the isolated rat basilar artery, mounted onto a wire myograph. A high extracellular K+ concentration partly inhibited the relaxant effects of the NO donors DEA/NO and SIN-1 (3-morpholino-sydnonimine). Whereas single applications of the K+ channel inhibitors tetraethyl-ammonium (10(-3) M), glibenclamide (10(-6) M), 4-aminopyridine (10(-3) M), or BaCl(2) (5 x 10(-5) M) did not affect the responses to DEA/NO, a combination of these inhibitors reduced the effects of DEA/NO. These data suggest, that the relaxant effects of NO donors are partly mediated via activation of K+channels. Different K+ channel types seem to be involved that function in a redundant manner and compensate for each other.

Chronic estrogen depletion alters adenosine diphosphate-induced pial arteriolar dilation in female rats

We examined pial arteriolar reactivity to a partially endothelial nitric oxide synthase (eNOS)-dependent vasodilator ADP as a function of chronic estrogen status. The eNOS-dependent portion of the ADP response was ascertained by comparing ADP-induced pial arteriolar dilations before and after suffusion of a NOS inhibitor, N(omega)-nitro-L-arginine (L-NNA; 1 mM) in intact, ovariectomized (Ovx), and 17beta-estradiol (E2)-treated Ovx females. We also examined whether ovariectomy altered the participation of other factors in the ADP response. Those factors were the following: 1) the prostanoid indomethacin (Indo); 2) the Ca2+-dependent K+ (K(Ca)) channel, iberiotoxin (IbTX); 3) the ATP-regulated K+ (K(ATP)) channel glibenclamide (Glib); 4) the K(Ca)-regulating epoxygenase pathway miconazole (Mic); and 5) the adenosine receptor 8-sulfophenyltheophylline (8-SPT). In intact females, the eNOS-dependent (L-NNA sensitive) portion of the ADP response represented approximately 50% of the total. The ADP response was retained in the Ovx rats but L-NNA sensitivity disappeared. On E2 replacement, the initial pattern was restored. ADP reactivity was unaffected by Indo, Glib, Mic, and 8-SPT. IbTX was associated with 50-80% reductions in the response to ADP in the intact group that was nonadditive with L-NNA, and 60-100% reductions in the Ovx group. The present findings suggest that estrogen influences the mechanisms responsible for ADP-induced vasodilation. The continued sensitivity to IbTX in Ovx rats, despite the loss of a NO contribution, is suggestive of a conversion to a hyperpolarizing factor dependency in the absence of E2.

Role of phospholamban in the modulation of arterial Ca(2+) sparks and Ca(2+)-activated K(+) channels by cAMP

Phospholamban (PLB) inhibits the sarcoplasmic reticulum (SR) Ca(2+)-ATPase, and this inhibition is relieved by cAMP-dependent protein kinase (PKA)-mediated phosphorylation. The role of PLB in regulating Ca(2+) release through ryanodine-sensitive Ca(2+) release channels, measured as Ca(2+) sparks, was examined using smooth muscle cells of cerebral arteries from PLB-deficient ("knockout") mice (PLB-KO). Ca(2+) sparks were monitored optically using the fluorescent Ca(2+) indicator fluo 3 or electrically by measuring transient large-conductance Ca(2+)-activated K(+) (BK) channel currents activated by Ca(2+) sparks. Basal Ca(2+) spark and transient BK current frequency were elevated in cerebral artery myocytes of PLB-KO mice. Forskolin, an activator of adenylyl cyclase, increased the frequency of Ca(2+) sparks and transient BK currents in cerebral arteries from control mice. However, forskolin had little effect on the frequency of Ca(2+) sparks and transient BK currents from PLB-KO cerebral arteries. Forskolin or PLB-KO increased SR Ca(2+) load, as measured by caffeine-induced Ca(2+) transients. This study provides the first evidence that PLB is critical for frequency modulation of Ca(2+) sparks and associated BK currents by PKA in smooth muscle.

Effects of potassium channel inhibitors on the relaxation induced by the nitric oxide donor diethylamine nitric oxide in isolated human cerebral arteries

OBJECT

The goal of this study was to investigate whether K+ channels are involved in nitric oxide (NO)-induced relaxation of isolated human cerebral arteries.

METHODS

Successive concentration-response curves relating to the use of the NO donor diethylamine NO (DEA/NO) were established in the absence and presence of different K+ channel inhibitors after mounting human cerebral arteries onto a wire myograph. The arteries were obtained from macroscopically intact tissue that had been removed during brain tumor operations. A high K+ concentration partially inhibited the relaxant effects of DEA/NO. Different K+ channel inhibitors (tetraethylammonium [TEA], 10(-3) M; charybdotoxin, 10(-7) M; glibenclamide, 10(-6) M; 4-aminopyridine [4-AP], 10(-3) M; BaCl2, 5 x 10(-5) M; and apamin, 10(-6) M) alone failed to affect the responses to DEA/NO. However, a combination of TEA, glibenclamide, 4-AP, and BaCl2 partially blocked the relaxant effects of DEA/NO. In addition, the effects of DEA/NO were inhibited by the thromboxane A2 analog U46619 (3 x 10(-7) M).

CONCLUSIONS

Inhibitors of the large-conductance or small-conductance Ca++-activated K+ channels, the adenosine triphosphate-sensitive K+ channels, and the delayed-rectifier or inward-rectifier K+ channels failed to alter the effects of DEA/NO when only one K+ channel blocker was used. However, a regimen of a combination of K+ channel blockers that possess selectivity for different channels demonstrated that different K+ channel types are involved; these channels may function in a redundant manner and compensate for each other. Selective thromboxane A2 agonists are capable of inhibiting the relaxant response to the NO donor.

Role of cGMP versus 20-HETE in the vasodilator response to nitric oxide in rat cerebral arteries

This study examined the response to nitric oxide (NO) in rat middle cerebral arteries (MCA). NO donors increased the activity of a 205-pS K(+) channel recorded from vascular smooth muscle (VSM) cells isolated from MCA 10-fold. Blockade of guanylyl cyclase activity with 1H-[1,2,4]oxadiazole[4,3-a]quinoxalin-1-one (ODQ, 10(-5) M) did not alter the effect of NO on this channel. In contrast, adding 20-hydroxyeicosatetraenoic acid (20-HETE) to the bath (10(-7) M) abolished the response to NO. NO donors also increased the diameter of serotonin-preconstricted MCA to 85% of control. Blockade of K(+) channels with iberiotoxin or a high-K(+) medium reduced this response by 50%. ODQ (10(-5) M) reduced this response by 47 +/- 3%, whereas preventing the fall of 20-HETE levels reduced the response by 59 +/- 2% (n = 5). Blockade of both pathways eliminated the response to NO donors. These results indicate that activation of K(+) channels contributes 50% to vasodilator response to NO in rat MCA. This is mediated by a fall in 20-HETE levels rather than a rise in cGMP levels or a direct effect of NO.

Potassium channels modulate cerebral autoregulation during acute hypertension

We tested the hypothesis that constriction of cerebral arterioles during acute increases in blood pressure is attenuated by activation of potassium (K(+)) channels. We tested the effects of inhibitors of calcium-dependent K(+) channels [iberiotoxin (50 nM) and tetraethylammonium (TEA, 1 mM)] on changes in arteriolar diameter during acute hypertension. Diameter of cerebral arterioles (baseline diameter = 46 +/- 2 microm, mean +/- SE) was measured using a cranial window in anesthetized rats. Arterial pressure was increased from a control value of 96 +/- 1 mmHg to 130, 150, 170, and 200 mmHg by intravenous infusion of phenylephrine. Increases in arterial pressure from baseline to 130 and 150 mmHg decreased the diameter of cerebral arterioles by 5-10%. Greater increases in arterial pressure produced large increases in arteriolar diameter (i.e., "breakthrough of autoregulation"). Iberiotoxin or TEA inhibited increases in arteriolar diameter when arterial pressure was increased to 170 and 200 mmHg. The change in arteriolar diameter at 200 mmHg was 20 +/- 3% and -1 +/- 4% in the absence and presence of iberiotoxin, respectively. These findings suggest that calcium-dependent K(+) channels attenuate cerebral microvascular constriction during acute increases in arterial pressure, and that increases in arteriolar diameter at high levels of arterial pressure are not simply a passive phenomenon.

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.

Calcium sparks in smooth muscle

Local intracellular Ca(2+) transients, termed Ca(2+) sparks, are caused by the coordinated opening of a cluster of ryanodine-sensitive Ca(2+) release channels in the sarcoplasmic reticulum of smooth muscle cells. Ca(2+) sparks are activated by Ca(2+) entry through dihydropyridine-sensitive voltage-dependent Ca(2+) channels, although the precise mechanisms of communication of Ca(2+) entry to Ca(2+) spark activation are not clear in smooth muscle. Ca(2+) sparks act as a positive-feedback element to increase smooth muscle contractility, directly by contributing to the global cytoplasmic Ca(2+) concentration ([Ca(2+)]) and indirectly by increasing Ca(2+) entry through membrane potential depolarization, caused by activation of Ca(2+) spark-activated Cl(-) channels. Ca(2+) sparks also have a profound negative-feedback effect on contractility by decreasing Ca(2+) entry through membrane potential hyperpolarization, caused by activation of large-conductance, Ca(2+)-sensitive K(+) channels. In this review, the roles of Ca(2+) sparks in positive- and negative-feedback regulation of smooth muscle function are explored. We also propose that frequency and amplitude modulation of Ca(2+) sparks by contractile and relaxant agents is an important mechanism to regulate smooth muscle function.

Involvement of K(+) channels in the relaxant effects of YC-1 in vascular smooth muscle

This study addresses the question whether K(+) channels are involved in the vasorelaxant effects of 3-(5'-hydroxymethyl-2'-furyl)-1-benzyl-indazole (YC-1 ). In rat aorta, guinea pig aorta, and guinea pig a. carotis, YC-1 inhibited contractions induced by phenylephrine (3 microM) more potently than those induced by K(+)(48 mM). In rat aorta, tetraethylammonium (10 mM), charybdotoxin (0.2 microM), and iberiotoxin (0.1 microM), but not glibenclamide (10 microM), attenuated the relaxant effects of YC-1. In guinea pig a. carotis, YC-1 (30 microM) induced a hyperpolarisation which was antagonised by 1H-[1,2,4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ; 50 microM). In rat aorta, YC-1 (30 microM) increased the rate constant of 86Rb-efflux. The effect of YC-1 was potentiated by zaprinast (10 microM), but inhibited by ODQ (50 microM) or charybdotoxin (0.2 microM). In smooth muscle cells from rat aorta, YC-1 (10 microM) increased BK(Ca) channel activity. It is suggested that YC-1-induced vasorelaxation is partially mediated by the activation of K(+) channels.

Miconazole represses CO(2)-induced pial arteriolar dilation only under selected circumstances

Previous experimental findings have led to the suggestion that guanosine 3',5'-cyclic monophosphate (cGMP) plays a permissive role in hypercapnic cerebral vasodilation. However, we recently reported that the technique used to reveal a permissive role for cGMP [cGMP repletion in the presence of nitric oxide synthase (NOS) inhibition] created a situation where CO(2) reactivity was normalized but where different mechanisms (i.e., K(+) channels) participated in the response. In the present study, we examined whether that nascent K(+)-channel dependence is related in any way to an increase in the influence of the miconazole-inhibitable cytochrome P-450 epoxygenase pathway. Using intravital microscopy and a closed cranial window system in adult rats, we measured pial arteriolar diameters during normo- and hypercapnia, first in the absence and then in the presence of a neuronal NOS (nNOS) inhibitor [7-nitroindazole (7-NI)]. This was followed by suffusion of a cGMP analog and then cGMP plus miconazole. Separate groups of rats were used to evaluate whether miconazole either alone or in the presence of 8-bromoguanosine 3', 5'-cyclic monophosphate (8-BrcGMP) or its vehicle (0.1% ethanol) had any effect on CO(2) reactivity and whether miconazole affected K(+)-channel opener-induced dilations. Hypercapnic (arterial PCO(2), congruent with65 mmHg) pial arteriolar dilations, as expected, were reduced by 70-80% with 7-NI and restored with cGMP repletion. CO(2) reactivity was again attenuated after miconazole introduction. Miconazole, with and without 8-BrcGMP, and its vehicle had no influence on pial arteriolar CO(2) reactivity in the absence of nNOS inhibition combined with cGMP repletion. Miconazole alone also did not affect vasodilatory responses to K(+)-channel openers. Thus present results suggest that the nascent K(+)-channel dependence of the hypercapnic response found in our earlier study may be related to increased epoxygenase activity. The specific reasons why the pial arteriolar CO(2) reactivity gains a K(+)-channel and epoxygenase dependence only under conditions of nNOS inhibition and cGMP restoration remain to be identified. These findings again call into question the interpretations applied to data collected in studies evaluating potential permissive actions of cGMP or NO.

Introduction of the closed cranial window technique in gerbils and verification by observation of the effects of specific drugs

The exact mechanisms of cerebral arterial hypoxia are not perfectly defined. Our purpose is to adapt and validate, with drugs well known in rats and rabbits, a closed cranial window technique in gerbils. The method was used with seventeen gerbils to measure diameter changes of the pial arterioles under normoxia (after the topical application of agonists and antagonists of ATP-sensitive and Ca2+-dependent potassium channels), as well as under hypoxia. In normoxia, aprikalim (10(-6) M), a direct activator of ATP-sensitive potassium channels, increases the diameter of pial arterioles by 10+/-2% (N = 17). This effect is inhibited by glibenclamide (10(-6) M), but not affected by iberiotoxin (10(-6) M), a specific inhibitor of Ca2+-dependent potassium channels. The adenosine-induced dilation by 19+/-5% (N = 17) is reduced by 59+/-16% with iberiotoxin, by 33+/-23% with glibenclamide and inhibited by theophylline (10(-5) M). In hypoxia (15% O2), pial arteriole diameters are increased by 24+/-5% (N = 17) and partially decreased by the application of glibenclamide and iberiotoxin to 59+/-11% and 54+/-5%, respectively. These data are similar to those obtained in other species and validate the closed cranial window technique on gerbils. They indicate that, as for rats and rabbits, both ATP-sensitive and Ca2+-dependent potassium channels are present in gerbil pial vessels and play a role in hypoxia.

Superoxide anion and K+ channels mediate electrical stimulation-induced relaxation in the rat basilar artery

Electrical field stimulation (a single pulse, 0.2 ms) caused a rapid relaxation of rat basilar artery segments precontracted with different agents, but not with 30 mM KCl. This relaxation was not modified by endothelium removal, 10 microM tetrodotoxin, 1 microM propranolol, 1 microM atropine, 30 microM indomethacin, 10 microM methylene blue, 100 microM N(G)-nitro-L-arginine methyl ester or 1 microM cimetidine but it was significantly reduced by 50 and 100 U/ml superoxide dismutase. Charybdotoxin (0.1 and 0.2 microM), a blocker of large-conductance Ca2+-activated K+ channels (BK(Ca)), decreased the relaxation elicited by electrical stimulation, whereas it was unaltered by 10 microM glibenclamide or 1 microM apamin, blockers of ATP-sensitive (K(ATP)) or small-conductance K(Ca) channels, respectively. Thapsigargin (0.01 and 0.1 microM), an inhibitor of sarcoplasmic reticulum Ca2+-ATPase, increased the electrical stimulation-induced relaxation, which was nearly abolished by charybdotoxin. These results show that electrical stimulation induces endothelium-independent and non-neurogenic relaxations in the rat basilar artery. This response appears to involve generation of superoxide anion, increase of cytosolic free Ca2+ concentration and subsequent activation of BK(Ca) channels.

Signaling mechanisms underlying the vascular myogenic response

The vascular myogenic response refers to the acute reaction of a blood vessel to a change in transmural pressure. This response is critically important for the development of resting vascular tone, upon which other control mechanisms exert vasodilator and vasoconstrictor influences. The purpose of this review is to summarize and synthesize information regarding the cellular mechanism(s) underlying the myogenic response in blood vessels, with particular emphasis on arterioles. When necessary, experiments performed on larger blood vessels, visceral smooth muscle, and even striated muscle are cited. Mechanical aspects of myogenic behavior are discussed first, followed by electromechanical coupling mechanisms. Next, mechanotransduction by membrane-bound enzymes and involvement of second messengers, including calcium, are discussed. After this, the roles of the extracellular matrix, integrins, and the smooth muscle cytoskeleton are reviewed, with emphasis on short-term signaling mechanisms. Finally, suggestions are offered for possible future studies.

Inhibitory effect of 4-aminopyridine on responses of the basilar artery to nitric oxide

1. Voltage-dependent K+ channels are present in cerebral arteries and may modulate vascular tone. We used 200 microM 4-aminopyridine (4-AP), thought to be a relatively selective inhibitor of voltage-dependent K+ channels at this concentration, to test whether activation of these channels may influence baseline diameter of the basilar artery and dilator responses to nitric oxide (NO) and cyclic GMP in vivo. 2. Using a cranial window in anaesthetized rats, topical application of 4-AP to the basilar artery (baseline diameter = 240+/-5 microm, mean +/- s.e.mean) produced 10+/-1% constriction. Sodium nitroprusside (a NO donor), acetylcholine (which stimulates endothelial release of NO), 8-bromo cyclic GMP (a cyclic GMP analogue), cromakalim (an activator of ATP-sensitive K+ channels) and papaverine (a non-NO, non-K+ channel-related vasodilator) produced concentration-dependent vasodilator responses that were reproducible. 3. Responses to 10 and 100 nM nitroprusside were inhibited by 4-AP (20+/-4 vs 8+/-2% and 51+/-5 vs 33+/-5%, respectively, n=10; P<0.05). Responses to acetylcholine and 8-bromo cyclic GMP were also partially inhibited by 4-AP. In contrast, 4-AP had no effect on vasodilator responses to cromakalim or papaverine. These findings suggest that NO/cyclic GMP-induced dilator responses of the basilar artery are selectively inhibited by 4-aminopyridine. 4. Responses to nitroprusside were also markedly inhibited by 10 microM 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (an inhibitor of soluble guanylate cyclase; 16+/-4 vs 1+/-1% and 44+/-7 vs 7+/-1%; n=10; P<0.05). 5. Thus, dilator responses of the rat basilar artery to NO appear to be mediated by activation of soluble guanylate cyclase and partially by activation of a 4-aminopyridine-sensitive mechanism. The most likely mechanism would appear to be activation of voltage-dependent K+ channels by NO/cyclic GMP.

Effect of age on the vasorelaxation elicited by cromakalim. Role of K+ channels and cyclic GMP

The objective of this study was to investigate whether age induces changes on vasodilator response induced by cromakalim, an ATP-sensitive K+ (K(ATP)) channel opener, as well as the underlying mechanism involved in this possible alteration. For this purpose, aortic segments from young (3-5 months) and old (3 years) rabbits were used, which were precontracted with noradrenaline (NA, 1 microM). The vasodilator response induced by cromakalim (0.01-100 microM) was reduced in intact segments from old rabbits, and endothelium removal reduced and did not modify this effect in young and old animals, respectively. In both groups of animals, glibenclamide (10 microM), a blocker of K(ATP) channels, significantly reduced the response elicited by cromakalim, which was not modified by the large conductance Ca2+-activated K+ channel blocker charybdotoxin (ChTX, 0.4 microM). Acetylcholine (ACh, 10 microM) and sodium nitroprusside (SNP, 100 microM) induced a lesser vasodilator effect in aortic segments from old compared with young rabbits. In segments precontracted with NA, 10 microM ACh or 100 microM SNP similarly increased cGMP levels in both groups of animals. However, basal cGMP level was reduced in segments from old rabbits. Incubation with 8-bromo-cGMP (100 microM) increased the response induced by cromakalim in both groups of animals, reaching similar maximum values in young and old rabbits. The response induced by cromakalim plus 8-bromo-cGMP was markedly decreased by glibenclamide and unmodified by ChTx in both types of animals. These results suggest that aging decreases the vasodilator response to cromakalim, mechanism in which appears to be involved the maintained low cGMP levels observed in old rabbits, and that this messenger modulates the degree of K(ATP) channel activation.

Characterization of the relaxant action of urocortin, a new peptide related to corticotropin-releasing factor in the rat isolated basilar artery

In addition to its well established neuroendocrine and neurotransmitter effects, corticotropin releasing factor (CRF) exerts a potent vasorelaxant action. Recently, a CRF-related peptide, urocortin, has been identified in the mammalian brain. In the present study, the cerebral vasomotor action of this peptide and the mechanism underlying its relaxant effect are characterized. Ring segments obtained from the rat basilar artery were used for measurement of isometric force. The relaxant action of urocortin, CRF and sauvagine was studied in segments with a functionally intact endothelium. In segments precontracted with prostaglandin F2alpha, urocortin, CRF and sauvagine induced concentration-related relaxation. The order of potency was as follows (pD2+/-s.e.m. given in brackets): urocortin (9.32+/-0.07) > sauvagine (9.08+/-0.08) > CRF (7.50+/-0.07). Complete relaxation was achieved with each agonist. Relaxation was not affected by removal of the endothelium but was markedly attenuated in segments precontracted with 50 mM K+ Krebs solution. The relaxant effect of urocortin was inhibited by astressin in an apparently competitive manner. A pA2 value of 7.52 was estimated for astressin. Inhibition of urocortin-induced relaxation was also observed in the presence of the adenylate cyclase inhibitor SQ22536 (pD2 in the presence of 300 microM SQ22536, 9.36+/-0.05) and the K+ channel blockers tetraethylammonium (10 mM; pD2, 8.65+/-0.07), iberiotoxin (100 nM; pD2, 8.88+/-0.08) and apamin (10 nM; pD2, 8.94+/-0.07). However, the inhibitory actions of SQ22536 and apamin or iberiotoxin were not additive. The results suggest that urocortin induces relaxation of cerebral arteries by activating CRF-R2 receptors present in the vascular wall. Relaxation appears to be mediated by adenylate cyclase stimulation and activation of Ca2+-dependent K+ channels.

Estrogen increases cGMP in selected brain regions and in cerebral microvessels

We have previously reported that exogenous and endogenous estrogen can amplify residual cerebral blood flow during experimental cerebral ischemia. Because estrogen has been linked to nitric oxide and cyclic guanosine monophosphate (cGMP) signaling in noncerebral tissue, we tested the hypothesis that long-term 17beta-estradiol treatment increases basal cGMP in brain homogenates and cerebral microvessels in female rabbits. We also determined whether there are important baseline gender-specific differences in regional cGMP. Adult female rabbits were implanted with 17beta-estradiol pellets, 10 mg (F10, n = 10) or 50 mg (F50, n = 13), and compared with untreated females (F, n = 19) and males with negligible estrogen (M, n = 19) (plasma 17beta-estradiol levels of 4+/-4 pg/mL in M, 7+/-5 pg/mL in F, 141+/-74 pg/mL in F10, and 289 +/-10 pg/mL in F50). Cyclic GMP was determined by radioimmunoassay in cerebellum, hypothalamus, caudate nucleus, hippocampus, and cortex. Cerebral microvessels were harvested from additional cohorts of untreated males and females or estradiol-implanted females (n = 6 per group). Basal cGMP was higher in F versus M only in cerebellum. Estrogen-induced increases in regional cGMP were prominent in hippocampus at all doses (M = 43+/-26, F = 43+/-21, F10 = 84+/-24, F50 = 117+/-55 fmol/mg protein) and in cortex at the high dose (M = 78+/-55, F = 88+/-51, F10 = 69+/-34, F50 = 143+/-52 fmol/mg protein). Similarly, microvascular cGMP increased only in females treated with the 50 mg dose (M = 77+/-13, F = 86+/-25, F10O = 106+/-35, F50 = 192+/-88 fmol/mg protein). Therefore, 17beta-estradiol increases cGMP content in parenchymal regions that are known physiologic targets for reproductive steroids but are also areas of selective vulnerability to ischemic insult. Further, high doses of estrogenic steroids could amplify cGMP signaling within the cerebral microvasculature.

Subarachnoid haemorrhage: what happens to the cerebral arteries?

1. Subarachnoid haemorrhage (SAH) is a unique disorder and a major clinical problem that most commonly occurs when an aneurysm in a cerebral artery ruptures, leading to bleeding and clot formation. Subarachnoid haemorrhage results in death or severe disability of 50-70% of victims and is the cause of up to 10% of all strokes. Delayed cerebral vasospasm, which is the most critical clinical complication that occurs after SAH, seems to be associated with both impaired dilator and increased constrictor mechanisms in cerebral arteries. Mechanisms contributing to development of vasospasm and abnormal reactivity of cerebral arteries after SAH have been intensively investigated in recent years. In the present review we focus on recent advances in our knowledge of the roles of nitric oxide (NO) and cGMP, endothelin (ET), protein kinase C (PKC) and potassium channels as they relate to SAH. 2. Nitric oxide is produced by the endothelium and is an important regulator of cerebral vascular tone by tonically maintaining the vasculature in a dilated state. Endothelial injury after SAH may interfere with NO production and lead to vasoconstriction and impaired responses to endothelium-dependent vasodilators. Inactivation of NO by oxyhaemoglobin or superoxide from erythrocytes may also occur in the subarachnoid space after SAH. 3. Nitric oxide stimulates activity of soluble guanylate cyclase in vascular muscle, leading to intracellular generation of cGMP and relaxation. Subarachnoid haemorrhage appears to cause impaired activity of soluble guanylate cyclase, resulting in reduced basal levels of cGMP in cerebral vessels and often decreased responsiveness of cerebral arteries to NO. 4. Endothelin is a potent, long-lasting vasoconstrictor that may contribute to the spasm of cerebral arteries after SAH. Endothelin is present in increased levels in the cerebrospinal fluid of SAH patients. Pharmacological inhibition of ET synthesis or of ET receptors has been reported to attenuate cerebral vasospasm. Production of and vasoconstriction by ET may be due, in part, to the decreased activity of NO and formation of cGMP. 5. Protein kinase C is an important enzyme involved in the contraction of vascular muscle in response to several agonists, including ET. Activity of PKC appears to be increased in cerebral arteries after SAH, indicating that PKC may be critical in the development of cerebral vasospasm. Recent evidence suggests that PKC activation may occur in cerebral arteries after SAH as a result of decreased negative feedback influence of NO/cGMP. 6. Cerebral arteries are depolarized after SAH, possibly due to decreased activity of potassium channels in vascular muscle. Decreased basal activation of potassium channels may be due to several mechanisms, including impaired activity of NO (and/or cGMP) or increased activity of PKC. Vasodilator drugs that produce hyperpolarization, such as potassium channel openers, appear to be unusually effective in cerebral arteries after SAH. 7. Thus, endothelial damage and reduced activity of NO may contribute to cerebral vascular dysfunction after SAH. Potassium channels may represent an important therapeutic target for the treatment of cerebral vasospasm after SAH.

Calcium-dependent and ATP-sensitive potassium channels and the 'permissive' function of cyclic GMP in hypercapnia-induced pial arteriolar relaxation

The conclusion that cyclic 3'-5 guanosine monophosphate (cGMP) functions in a 'permissive' manner in promoting cerebrovasodilation during hypercapnia was based on findings showing that the nitric oxide synthase (NOS) inhibitor-induced repression of the CO2 response could be reversed upon addition of exogenous cGMP. We hypothesized that the action of cGMP revealed in those studies does not define its normal role in hypercapnic cerebral vasodilation, but rather is a unique function of the artificial situation of NOS inhibition coupled with cGMP repletion. Thus, although CO2 reactivity may be the same in normal versus cGMP-repleted animals, the factors contributing to that response may differ. To test that possibility, the effects of calcium-dependent (KCa) or ATP-sensitive (KATP) potassium channel blockers on pial arteriolar CO2 reactivity, in vivo, were evaluated in the presence and absence of NOS inhibition plus administration of a cGMP analogue. Pial arteriolar diameter changes in hypercapnia were measured in three principal groups of anesthetized rats: (I) KCa channel-inhibited (via iberiotoxin); (II) KATP channel-inhibited (via glibenclamide); and (III) controls. Group I and II rats were further divided into: (a) those treated with the neuronal NOS (nNOS) inhibitor, 7-nitroindazole (7-NI), followed by successive suffusions of the cGMP analogue, 8-bromo-cGMP (8Br-cGMP) and 8Br-cGMP+K-channel blocker; and (b) rats where 7-NI and 8Br-cGMP applications were omitted. Group III rats were divided into time and 8Br-cGMP controls. Hypercapnia (PCO2 congruent with60 mmHg, 3 min)-induced dilations were reduced by 70-80% following 7-NI and restored by 8Br-cGMP. That restoration was reversed by both K-channel blockers. In the absence of 7-NI and exogenous cGMP, CO2 reactivity was unaffected by K-channel inhibition. These findings confirmed that nNOS-derived NO is critically important to the hypercapnic reactivity of cerebral arterioles, and that cGMP repletion, following NOS inhibition, could restore CO2 reactivity. The observation that KCa and KATP channel blockade did not alter CO2 reactivity under baseline conditions, but attenuated CO2 reactivity only in the presence nNOS inhibition (and cGMP repletion), suggests that multiple, redundant, and interactive mechanisms participate in CO2-induced vasodilation. These results also imply that current strategies for revealing permissive actions of cGMP (or NO) may need to be re-evaluated.

Frequency modulation of Ca2+ sparks is involved in regulation of arterial diameter by cyclic nucleotides

Forskolin, which elevates cAMP levels, and sodium nitroprusside (SNP) and nicorandil, which elevate cGMP levels, increased, by two- to threefold, the frequency of subcellular Ca2+ release ("Ca2+ sparks") through ryanodine-sensitive Ca2+ release (RyR) channels in the sarcoplasmic reticulum (SR) of myocytes isolated from cerebral and coronary arteries of rats. Forskolin, SNP, nicorandil, dibutyryl-cAMP, and adenosine increased the frequency of Ca(2+)-sensitive K+ (KCa) currents ["spontaneous transient outward currents" (STOCs)] by two- to threefold, consistent with Ca2+ sparks activating STOCs. These agents also increased the mean amplitude of STOCs by 1.3-fold, an effect that could be explained by activation of KCa channels, independent of effects on Ca2+ sparks. To test the hypothesis that cAMP could act to dilate arteries through activation of the Ca2+ spark-->KCa channel pathway, the effects of blockers of KCa channels (iberiotoxin) and of Ca2+ sparks (ryanodine) on forskolin-induced dilations of pressurized cerebral arteries were examined. Forskolin-induced dilations were partially inhibited by iberiotoxin and ryanodine (with no additive effects) and were entirely prevented by elevating external K+. Forskolin lowered average Ca2+ in pressurized arteries while increasing ryanodine-sensitive, caffeine-induced Ca2+ transients. These experiments suggest a new mechanism for cyclic nucleotide-mediated dilations through an increase in Ca2+ spark frequency, caused by effects on SR Ca2+ load and possibly on the RyR channel, which leads to increased STOC frequency, membrane potential hyperpolarization, closure of voltage-dependent Ca2+ channels, decrease in arterial wall Ca2+, and, ultimately, vasodilation.

Subarachnoid hemorrhage and the role of potassium channels in relaxations of canine basilar artery to nitrovasodilators

This study was designed to determine the effect of subarachnoid hemorrhage (SAH) on potassium (K+) channels involved in relaxations of cerebral arteries to nitrovasodilators. The effects of K+ channel inhibitors on relaxations to 3-morpholinosydnonimine (SIN-1) and sodium nitroprusside (SNP) were studied in rings of basilar arteries obtained from untreated dogs and dogs exposed to SAH. The levels of cyclic GMP were measured by radioimmunoassay. In rings without endothelium, concentration-dependent relaxations to SIN-1 (10(-9)-10(-4) mol/L) and SNP (10(-9)-10(-4) mol/L) were not affected by SAH, whereas increase in cyclic GMP production stimulated by SIN-1 (10(-6) mol/L) was significantly suppressed after SAH. The relaxations to SIN-1 and SNP were reduced by charybdotoxin (CTX: 10(-7) mol/L), a selective Ca(2+)-activated K+ channel inhibitor, in both normal and SAH arteries; however, the reduction of relaxations by CTX was significantly greater in SAH arteries. By contrast, the relaxations to these nitrovasodilators were not affected by glyburide (10(-5) mol/L), an ATP-sensitive K+ channel inhibitor, in both normal and SAH arteries. These findings suggest that in cerebral arteries exposed to SAH, CA(2+)-activated K+ channels may play a compensatory role in mediation of relaxations to nitric oxide. This may help to explain mechanisms of relaxations to nitrovasodilators in arteries with impaired production of cyclic GMP.

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.

Characterization and function of Ca(2+)-activated K+ channels in arteriolar muscle cells

We examined the functional role of large-conductance Ca(2+)-activated K+ (KCa) channels in the hamster cremasteric microcirculation by intravital videomicroscopy and characterized the single-channel properties of these channels in inside-out patches of membrane from enzymatically isolated cremasteric arteriolar muscle cells. In second-order (39 +/- 1 microns, n = 8) and third-order (19 +/- 2 microns, n = 8) cremasteric arterioles with substantial resting tone, superfusion with the KCa channel antagonists tetraethylammonium (TEA, 1 mM) or iberiotoxin (IBTX, 100 nM) had no significant effect on resting diameters (P > 0.05). However, TEA potentiated O2-induced arteriolar constriction in vivo, and IBTX enhanced norepinephrine-induced contraction of cremasteric arteriolar muscle cells in vitro. Patch-clamp studies revealed unitary K(+)-selective and IBTX-sensitive currents with a single-channel conductance of 240 +/- 2 pS between -60 and 60 mV (n = 7 patches) in a symmetrical 140 mM K+ gradient. The free Ca2+ concentration ([Ca2+]) for half-maximal channel activation was 44 +/- 3, 20 +/- 1, 6 +/- 0.4, and 3 +/- 0.5 microM at membrane potentials of -60, -30, +30, and +60 mV, respectively (n = 5), with a Hill coefficient of 1.9 +/- 0.2. Channel activity increased e-fold for a 16 +/- 1 mV (n = 6) depolarization. The plot of log[Ca2+] vs. voltage for half-maximal activation (V1/2) was linear (r2 = 0.9843, n = 6); the change in V1/2 for a 10-fold change in [Ca2+] was 84 +/- 5 mV, and the [Ca2+] for half-maximal activation at 0 mV (Ca0; the Ca2+ set point) was 9 microM. Thus, in vivo, KCa channels are silent in cremasteric arterioles at rest but can be recruited during vasoconstriction. We propose that the high Ca0 is responsible for the apparent lack of activity of these channels in resting cremasteric arterioles, and we suggest that this may result from expression of unique KCa channels in the microcirculation.

Calcium-activated K+ channels in cerebral arterioles in piglets are resistant to ischemia

Our previous studies indicate that function of ATP-dependent K+ channels (K(ATP)) in cerebral arterioles is suppressed after ischemia. In the current study, we examined pial arteriolar responses to forskolin, dibutyryl-cAMP, NS-1619, and methionine (met)-enkephalin, activators of calcium-dependent K+ channels (K(Ca)) before and 1 hour after 10 minutes of total, global ischemia in anesthetized piglets. Arteriolar diameters were measured using a closed cranial window and intravital microscopy. All pharmacologic agents were given topically. Baseline diameters were approximately 100 microm, and diameters had returned to normal by 1 hour after ischemia. Forskolin dilated arterioles by 9 +/- 3%, 18 +/- 4%, and 31 +/- 12% at 5 x 10(-8), 5 x 10(-7), and 10(-6) mol/L, respectively (P < 0.05, n = 10). In addition, dibutyryl-cAMP dilated arterioles by 8 +/- 2% at 10(-4) mol/L and 14 +/- 2% at 3 x 10(-4) mol/L (P < 0.05, n = 6). Also, NS-1619 increased diameter of arterioles by 9 +/- 2% at 10(-7) mol/L and 17 +/- 9% at 10(-5) mol/L (P < 0.05, n = 5). Finally, met-enkephalin dilated arterioles by 9 +/- 2% at 10(-8) mol/L and 16 +/- 3% at 10(-6) mol/L (P < 0.05, n = 5). At 1 hour after ischemia, arteriolar dilator effects to forskolin, dibutyryl-cAMP and NS-1619, and met-enkephalin were intact. Thus, in contrast to K(ATP), K(Ca) in cerebral arterioles are resistant to ischemic stress.

Inhibition of cGMP mediated relaxation in small rat coronary arteries by block of CA++ activated K+ channels

The functional importance of Ca++ activated K+ (K(Ca)) channels in cGMP mediated relaxation of pressurized septal arteries (internal basal diameter 213 +/- 4 microm) was investigated. Vascular tone was increased by the thromboxane A2 analogue, U-46619 and internal pressure was maintained at 60 mmHg. Vessels were tested with an endothelium independent agonist (nitroprusside) and endothelium dependent agonist (acetylcholine) of nitric oxide which activates soluble guanylate cyclase. Receptor activation of particulate guanylate cyclase was tested by atrial natriuretic peptide. Direct changes in intracellular cGMP concentration were done with the cell permeable analog, 8-Bromo-cGMP. Tetraethylammonium ion (TEA+), 1 mM, significantly inhibited relaxation to nitroprusside from 10(-7) to 10(-3) M with a maximal inhibition of 53 +/- 8% at 10(-3) M. Relaxation to acetylcholine from 10(-9) M to 10(-5) M was significantly inhibited by TEA+ with a maximal inhibition of 52 +/- 13% at 10(-7) M. TEA+ significantly inhibited relaxation to 8-Bromo-cGMP from 10(-6) M to 10(-3) M with a maximal inhibition of 59 +/- 14% at 10(-4) M. The relaxation response to atrial natriuretic peptide from 10(-12) M to 10(-7) M was significantly inhibited by TEA+ with a maximal inhibition of 84 +/- 5% at 10(-11) M. The large conductance K(Ca) channel blocker, iberiotoxin, eliminated the relaxation response to 8-Bromo-cGMP (10(-3) M). The results suggest that a large portion of the dilator action of cGMP is mediated by effects on K+ membrane channels.

Potassium channels and the cerebral circulation

1. Hyperpolarization of vascular muscle in response to activation of potassium channels is a major mechanism of vasodilatation. 2. In cerebral blood vessels, two potassium channels have received considerable study recently: ATP-sensitive and calcium-dependent potassium channels. Activation of these potassium channels appears to play a major role in the relaxation of cerebral arteries and arterioles in response to diverse stimuli, including receptor-mediated agonists, intracellular second messengers, reactive oxygen species and hypoxia. 3. The functional influence of ATP-sensitive and calcium-dependent potassium channels may be altered in disease states, including hypertension, diabetes and subarachnoid haemorrhage.