Differential effects of acetylcholine, nitric oxide and levcromakalim on smooth muscle membrane potential and tone in the rabbit basilar artery

The vascular effect of glibenclamide: A systematic review

Objective:

To systematically review the vascular effects of glibenclamide.

Background:

Infusion of adenosine triphosphate (ATP)-sensitive potassium (KATP) channel opener (KCO) levcromakalim dilates cranial arteries and induces headache and migraine attacks. Recent data show that levcromakalim-induced vasodilation is associated with headache. Glibenclamide is a KATP channel blocker that may alter the vascular tone and thus has an impact on headache or migraine prevention.

Methods:

A search through PubMed was undertaken for studies investigating the vascular effects of glibenclamide in vitro as well as in vivo published until July 2019.

Results:

We identified 58 articles; 31 in vitro studies, 24 in vivo studies and 3 studies with both. The main findings were that glibenclamide inhibited levcromakalim-induced and other KCOs-induced vasodilation, while the basal vascular tone remained unchanged.

Conclusion:

Glibenclamide could inhibit vasodilation by KCOs, and further studies are needed to clarify the vascular effect of glibenclamide on human cranial arteries.

Impaired Endothelium-Dependent Hyperpolarization Underlies Endothelial Dysfunction during Early Metabolic Challenge: Increased ROS Generation and Possible Interference with NO Function

Endothelial dysfunction is a hallmark of diabetic vasculopathies. Although hyperglycemia is believed to be the culprit causing endothelial damage, the mechanism underlying early endothelial insult in prediabetes remains obscure. We used a nonobese high-calorie (HC)-fed rat model with hyperinsulinemia, hypercholesterolemia, and delayed development of hyperglycemia to unravel this mechanism. Compared with aortic rings from control rats, HC-fed rat rings displayed attenuated acetylcholine-mediated relaxation. While sensitive to nitric oxide synthase (NOS) inhibition, aortic relaxation in HC-rat tissues was not affected by blocking the inward-rectifier potassium (Kir) channels using BaCl₂ Although Kir channel expression was reduced in HC-rat aorta, Kir expression, endothelium-dependent relaxation, and the BaCl₂-sensitive component improved in HC rats treated with atorvastatin to reduce serum cholesterol. Remarkably, HC tissues demonstrated increased reactive species (ROS) in smooth muscle cells, which was reversed in rats receiving atorvastatin. In vitro ROS reduction, with superoxide dismutase, improved endothelium-dependent relaxation in HC-rat tissues. Significantly, connexin-43 expression increased in HC aortic tissues, possibly allowing ROS movement into the endothelium and reduction of eNOS activity. In this context, gap junction blockade with 18-β-glycyrrhetinic acid reduced vascular tone in HC rat tissues but not in controls. This reduction was sensitive to NOS inhibition and SOD treatment, possibly as an outcome of reduced ROS influence, and emerged in BaCl₂-treated control tissues. In conclusion, our results suggest that early metabolic challenge leads to reduced Kir-mediated endothelium-dependent hyperpolarization, increased vascular ROS potentially impairing NO synthesis and highlight these channels as a possible target for early intervention with vascular dysfunction in metabolic disease. SIGNIFICANCE STATEMENT: The present study examines early endothelial dysfunction in metabolic disease. Our results suggest that reduced inward-rectifier potassium channel function underlies a defective endothelium-mediated relaxation possibly through alteration of nitric oxide synthase activity. This study provides a possible mechanism for the augmentation of relatively small changes in one endothelium-mediated relaxation pathway to affect overall endothelial response and highlights the potential role of inward-rectifier potassium channel function as a therapeutic target to treat vascular dysfunction early in the course of metabolic disease.

Acute activation of endothelial AMPK surprisingly inhibits endothelium-dependent hyperpolarization-like relaxations in rat mesenteric arteries

BACKGROUND AND PURPOSE

Endothelium-dependent hyperpolarizations (EDHs) contribute to the regulation of peripheral resistance. They are initiated through opening of endothelial calcium-activated potassium channels (K_Ca ); the potassium ions released then diffuse to the underlying smooth muscle cells, causing hyperpolarization and thus relaxation. The present study aimed to examine whether or not AMPK modulates EDH-like relaxations in rat mesenteric arteries.

EXPERIMENTAL APPROACH

Arterial rings were isolated for isometric tension recording. AMPK activity and protein level were measured by ELISA and western blotting respectively.

KEY RESULTS

The AMPK activator, AICAR, reduced ACh-induced EDH-like relaxations and increased AMPK activity in preparations with endothelium; these responses were prevented by compound C, an AMPK inhibitor. AICAR inhibited relaxations induced by SKA-31 (opener of endothelial K_Ca ) but did not affect potassium-induced, hyperpolarization-attributable relaxations or increase AMPK activity in preparations without endothelium. A769662, another AMPK activator, not only caused a similar inhibition of relaxations to ACh and SKA-31 in preparations with endothelium but also inhibited hyperpolarization-attributable relaxations and augmented AMPK activity in rings without endothelium. Protein levels of total AMPKα, AMPKα1, or AMPKβ1/2 were comparable between preparations with and without endothelium.

CONCLUSIONS AND IMPLICATIONS

Activation of endothelial AMPK, by either AICAR or A769662, acutely inhibits EDH-like relaxations of rat mesenteric arteries. Furthermore, A769662 inhibits signalling downstream of smooth muscle hyperpolarization. In view of the major blunting effect of AMPK activation on EDH-like relaxations, caution should be applied when administering therapeutic agents that activate AMPK in patients with endothelial dysfunction characterized by reduced production and/or bioavailability of NO.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

EDH: endothelium-dependent hyperpolarization and microvascular signalling

Endothelium-dependent hyperpolarizing factor (EDHF) is a powerful vasodilator influence in small resistance arteries and thus an important modulator of blood pressure and flow. As the name suggests, EDHF was thought to describe a diffusible factor stimulating smooth muscle hyperpolarization (and thus vasodilatation). However, this idea has evolved with the recognition that a factor can operate alongside the spread of hyperpolarizing current from the endothelium to the vascular smooth muscle (VSM). As such, the pathway is now termed endothelium-dependent hyperpolarization (EDH). EDH is activated by an increase in endothelial [Ca²⁺ ]_i , which stimulates two Ca²⁺ -sensitive K channels, SK_Ca and IK_Ca . This was discovered because apamin and charybdotoxin applied in combination blocked EDHF responses, but iberiotoxin - a blocker of BK_Ca - was not able to substitute for charybdotoxin. SK_Ca and IK_Ca channels are arranged in endothelial microdomains, particularly within projections towards the adjacent smooth muscle, which are rich in IK_Ca channels and close to interendothelial gap junctions where SK_Ca channels, are prevalent. K_Ca activation hyperpolarizes endothelial cells, and K⁺ efflux through them can act as a diffusible 'EDHF' by stimulating VSM Na⁺ ,K⁺ -ATPase and inwardly rectifying K channels (K_IR ). In parallel, hyperpolarizing current spreads from the endothelium to the smooth muscle through myoendothelial gap junctions located on endothelial projections. The resulting radial EDH is complemented by the spread of 'conducted' hyperpolarization along the endothelium of arteries and arterioles to affect conducted vasodilatation (CVD). Retrograde CVD effectively integrates blood flow within the microcirculation, but how the underlying hyperpolarization is sustained is unclear.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

Potassium channels importantly contribute to the regulation of vascular smooth muscle (VSM) contraction and growth. They are the dominant ion conductance of the VSM cell membrane and importantly determine and regulate membrane potential. Membrane potential, in turn, regulates the open-state probability of voltage-gated Ca²⁺ channels (VGCC), Ca²⁺ influx through VGCC, intracellular Ca²⁺, and VSM contraction. Membrane potential also affects release of Ca²⁺ from internal stores and the Ca²⁺ sensitivity of the contractile machinery such that K⁺ channels participate in all aspects of regulation of VSM contraction. Potassium channels also regulate proliferation of VSM cells through membrane potential-dependent and membrane potential-independent mechanisms. VSM cells express multiple isoforms of at least five classes of K⁺ channels that contribute to the regulation of contraction and cell proliferation (growth). This review will examine the structure, expression, and function of large conductance, Ca²⁺-activated K⁺ (BK_Ca) channels, intermediate-conductance Ca²⁺-activated K⁺ (K_Ca3.1) channels, multiple isoforms of voltage-gated K⁺ (K_V) channels, ATP-sensitive K⁺ (K_ATP) channels, and inward-rectifier K⁺ (K_IR) channels in both contractile and proliferating VSM cells.

Activation of endothelial IKCa channels underlies NO-dependent myoendothelial feedback

Agonist-induced vasoconstriction triggers a negative feedback response whereby movement of charged ions through gap junctions and/or release of endothelium-derived (NO) limit further reductions in diameter, a mechanism termed myoendothelial feedback. Recent studies indicate that electrical myoendothelial feedback can be accounted for by flux of inositol trisphosphate (IP3) through myoendothelial gap junctions resulting in localized increases in endothelial Ca(2+) to activate intermediate conductance calcium-activated potassium (IKCa) channels, the resultant hyperpolarization then conducting back to the smooth muscle to attenuate agonist-induced depolarization and tone. In the present study we tested the hypothesis that activation of IKCa channels underlies NO-mediated myoendothelial feedback. Functional experiments showed that block of IP3 receptors, IKCa channels, gap junctions and transient receptor potential canonical type-3 (TRPC3) channels caused endothelium-dependent potentiation of agonist-induced increase in tone which was not additive with that caused by inhibition of NO synthase supporting a role for these proteins in NO-mediated myoendothelial feedback. Localized densities of IKCa and TRPC3 channels occurred at the internal elastic lamina/endothelial-smooth muscle interface in rat basilar arteries, potential communication sites between the two cell layers. Smooth muscle depolarization to contractile agonists was accompanied by IKCa channel-mediated endothelial hyperpolarization providing the first demonstration of IKCa channel-mediated hyperpolarization of the endothelium in response to contractile agonists. Inhibition of IKCa channels, gap junctions, TRPC3 channels or NO synthase potentiated smooth muscle depolarization to agonists in a non-additive manner. Together these data indicate that rather being distinct pathways for the modulation of smooth muscle tone, NO and endothelial IKCa channels are involved in an integrated mechanism for the regulation of agonist-induced vasoconstriction.

Regulation of cellular communication by signaling microdomains in the blood vessel wall

It has become increasingly clear that the accumulation of proteins in specific regions of the plasma membrane can facilitate cellular communication. These regions, termed signaling microdomains, are found throughout the blood vessel wall where cellular communication, both within and between cell types, must be tightly regulated to maintain proper vascular function. We will define a cellular signaling microdomain and apply this definition to the plethora of means by which cellular communication has been hypothesized to occur in the blood vessel wall. To that end, we make a case for three broad areas of cellular communication where signaling microdomains could play an important role: 1) paracrine release of free radicals and gaseous molecules such as nitric oxide and reactive oxygen species; 2) role of ion channels including gap junctions and potassium channels, especially those associated with the endothelium-derived hyperpolarization mediated signaling, and lastly, 3) mechanism of exocytosis that has considerable oversight by signaling microdomains, especially those associated with the release of von Willebrand factor. When summed, we believe that it is clear that the organization and regulation of signaling microdomains is an essential component to vessel wall function.

Hydrogen sulfide: a novel signaling molecule in the vascular system

Hydrogen sulfide (H2S) is an endogenous gasotransmitter produced in mammalian cells. It is responsible for physiological functions in many organs and systems, with attention focused mainly on the cardiovascular and nervous systems. In the vascular system, H2S produces biphasic effects in regulation of vascular tone. At lower concentrations, it induces vasoconstriction predominantly via decreasing cyclic adenosine monophosphate in smooth muscle cell and inhibiting the production and bioavailability of nitric oxide. At higher concentrations, it produces vasorelaxation mainly through opening of KATP channels and induction of intracellular acidification. Scavenging reactive oxygen species and elevation of cyclic guanosine monophosphate are also implicated in the vasorelaxant response. This review presents an overview of the current knowledge of H2S in the vascular system, with special emphasis and discussion on the involvement of various signaling pathways and ion channels based on current understanding and reported literature till date.

EDHF: spreading the influence of the endothelium

Our view of the endothelium was transformed around 30 years ago, from one of an inert barrier to that of a key endocrine organ central to cardiovascular function. This dramatic change followed the discoveries that endothelial cells (ECs) elaborate the vasodilators prostacyclin and nitric oxide. The key to these discoveries was the use of the quintessentially pharmacological technique of bioassay. Bioassay also revealed endothelium-derived hyperpolarizing factor (EDHF), particularly important in small arteries and influencing blood pressure and flow distribution. The basic idea of EDHF as a diffusible factor causing smooth muscle hyperpolarization (and thus vasodilatation) has evolved into one of a complex pathway activated by endothelial Ca(2+) opening two Ca(2+) -sensitive K(+) -channels, K(Ca)2.3 and K(Ca)3.1. Combined application of apamin and charybdotoxin blocked EDHF responses, revealing the critical role of these channels as iberiotoxin was unable to substitute for charybdotoxin. We showed these channels are arranged in endothelial microdomains, particularly within projections towards the adjacent smooth muscle, and close to interendothelial gap junctions. Activation of K(Ca) channels hyperpolarizes ECs, and K(+) efflux through them can act as a diffusible 'EDHF' stimulating Na(+) /K(+) -ATPase and inwardly rectifying K-channels. In parallel, hyperpolarizing current can spread from the endothelium to the smooth muscle through myoendothelial gap junctions upon endothelial projections. The resulting radial hyperpolarization mobilized by EDHF is complemented by spread of hyperpolarization along arteries and arterioles, effecting distant dilatation dependent on the endothelium. So the complexity of the endothelium still continues to amaze and, as knowledge evolves, provides considerable potential for novel approaches to modulate blood pressure.

Possible Role for K + in Endothelium-Derived Hyperpolarizing Factor–Linked Dilatation in Rat Middle Cerebral Artery

Background and Purpose— Endothelium-derived hyperpolarizing factor (EDHF) and K + are vasodilators in the cerebral circulation. Recently, K + has been suggested to contribute to EDHF-mediated responses in peripheral vessels. The EDHF response to the protease-activated receptor 2 ligand SLIGRL was characterized in cerebral arteries and used to assess whether K + contributes as an EDHF.

Methods— Rat middle cerebral arteries were mounted in either a wire or pressure myograph. Concentration-response curves to SLIGRL and K + were constructed in the presence and absence of a variety of blocking agents. In some experiments, changes in tension and smooth muscle cell membrane potential were recorded simultaneously.

Results— SLIGRL (0.02 to 20 μmol/L) stimulated concentration and endothelium-dependent relaxation. In the presence of N G -nitro- l -arginine methyl ester, relaxation to SLIGRL was associated with hyperpolarization and sensitivity to a specific inhibitor of IK Ca , 1-[(2-chlorophenyl)diphenylmethyl]-1 H -pyrazole (1μmol/L), reflecting activation of EDHF. Combined inhibition of K IR with Ba 2+ (30μmol/L) and Na + /K + -ATPase with ouabain (1 μmol/L) markedly attenuated the relaxation to EDHF. Raising extracellular [K + ] to 15 mmol/L also stimulated smooth muscle relaxation and hyperpolarization, which was also attenuated by combined application of Ba 2+ and ouabain.

Conclusions— SLIGRL evokes EDHF-mediated relaxation in the rat middle cerebral artery, underpinned by hyperpolarization of the smooth muscle. The profile of blockade of EDHF-mediated hyperpolarization and relaxation supports a pivotal role for IK Ca channels. Furthermore, similar inhibition of responses to EDHF and exogenous K + with Ba 2+ and ouabain suggests that K + may contribute as an EDHF in the middle cerebral artery.

The relaxant effect of vasoactive intestinal polypeptide in the isolated canine uterine artery: the role of endothelium

The purpose of this study was to examine the effect of vasoactive intestinal polypeptide (VIP) on the uterine artery obtained from non-pregnant dogs. VIP (3 x 10(-9)-3 x 10(-7) M) induced concentration-dependent relaxation in canine uterine arteries with intact endothelium, pre-contracted with 10(-5) M phenylephrine (pEC(50) = 7.52 +/- 0.02, maximal response was 82.19 +/- 2.15%, n = 36). The administration of the cyclooxygenase inhibitor indomethacin (10(-5) M) or 4-aminopyridine (4-AP), a blocker of potassium channels (10(-5) M), did not modify the relaxation induced by VIP. Contrary to this, N(G)-nitro-L-arginine (L-NOARG) (10(-5) M) inhibited relaxation is evoked by VIP. Indomethacin applied with L-NOARG did not provoke further inhibition of VIP-induced relaxation. In the presence of both L-NOARG and L-NOARG + indomethacin, 4-AP led to the further inhibition of VIP-induced relaxation of canine uterine artery. It is concluded that VIP induces endothelium-dependent relaxation of uterine arteries of non-pregnant dogs, which can be entirely explained by the production of nitric oxide (NO) from the endothelial cells. We proposed that when NO synthesis is inhibited, VIP induces further relaxation, independent of the edothelium-derived relaxing factors, probably through activation of K(+) channels.

Evaluation of the rat bladder-derived relaxant factor by coaxial bioassay system

The release of bladder-derived relaxant factor in a coaxial bioassay system and the effects of reactive oxygen species were studied. After precontraction with phenylephrine (10(-6)-3x10(-6)) or 50 mM K+, acetylcholine (10(-8)-10(-3) M) induced relaxation in rat anococcygeus muscle mounted within rat bladder in a tissue bath. This relaxation was not altered by the removal of the urothelium or incubation with tetrodotoxin (10(-6) M). However, bupivacaine (10(-4) M) and lidocaine (3 x 10(-4) M) inhibited this response after raising the pH of the nutrient solution to 7.8, and oxybuprocaine (10(-4) M) exerted inhibitory effect at both physiological pH (7.4) and at pH 7.8. Exposure to electrolysis-generated reactive oxygen species or incubation with hydrogen peroxide and pyrogallol did not alter the acetylcholine response. Present results indicate that the bladder-derived relaxant factor does not behave like endothelium-derived hyperpolarizing factor, but its release may be associated with tetrodotoxin-resistant Na+ channels, which are probably in the neurons of the bladder rather than in the urothelium or detrusor muscle. Furthermore, reactive oxygen species do not interact with this relaxing factor, the exact nature and the physiological importance of which, however, remains to be established.

Basal nitric oxide production contributes to membrane potential and vasotone regulation of guinea pig in vitro spiral modiolar artery

Nitric oxide (NO) is a potent vasodilating agent implicated in cochlear blood flow regulation. We recently demonstrated that exogenously applied NO donor DPTA-NONOate hyperpolarizes both endothelial and smooth muscle cells of in vitro spiral modiolar artery (SMA) via activation of ATP-sensitive K+ channels (K(ATP)). Also, NO was detected in the SMA cells by NO indicator dye in the in vitro basal condition. Using intracellular recording techniques, electrochemical NO-sensing measurement, and a vaso-diameter video tracking method, we investigated the basal release of NO from the in vitro SMA and its role in the vascular function. We found that (1) 300 microM L-NAME, a NO synthase inhibitor, and 3 microM glipizide caused a depolarization of approximately 4.5 and approximately 3.2 mV, respectively, in cells with a resting potential less negative than -60 mV; (2) NO sensor in the close vicinity of the SMA detected a NO concentration of approximately 50 nM that was suppressed by L-NAME and enhanced by L-arginine (1-1000 microM); (3) NO donor DPTA-NONOate (0.1-30 microM) applications produced about 8-245 nM of NO in the recording bath. These data indicate a NO concentration-hyperpolarization relation, with an EC50 of 22 nM. (4) Finally, L-NAME but not glipizide produced a 4.8% reduction in SMA diameter (approximately 50 microm) in the majority of SMAs, whereas NONOate (10 microM) always caused a dilation. Both the induced constriction and dilation were not significantly affected by 3 microM glipizide. We conclude that a significant amount of NO (> 50 nM) is tonically released from the in vitro SMA, which is above the EC50 for activation of K(ATP), and thus contributes to the membrane polarization. The basal release of NO also contributes to vasotone relaxation, but the K(ATP) activation appears to play little role in the relaxation of the in vitro SMA.

Endothelium-dependent Relaxation of Canine Uterine Artery in Response to Acetylcholine: the Possible Involvement of Alternative Pathways

The effect of acetylcholine on the isolated, pre-contracted, uterine artery of non-pregnant dog was investigated. Acetylcholine-induced concentration-dependent relaxation of isolated canine uterine artery with endothelium (pEC50 = 6.48 +/-0.01, n = 37) and was without effect on arterial segments denuded of endothelium. Indomethacin, 4-aminopyridine (10-5 m) and pre-contraction with K+-rich Krebs-Ringer bicarbonate solution had no effect on acetylcholine-induced relaxation. NG-nitro-l-arginine (l-NOARG) (10-5 m) inhibited relaxation evoked by acetylcholine. Indomethacin applied with l-NOARG led to further inhibition of acetylcholine-induced relaxation. In the presence of both l-NOARG and indomethacin, 4-aminopiridine did not provoke further inhibition of acetylcholine-induced relaxation of canine uterine artery. It is concluded that the acetylcholine-induced relaxation of canine uterine artery is probably mediated by endothelial production of nitric oxide (NO). However, if NO-synthase is inhibited, acetylcholine-induced vasorelaxation may be, in part, mediated through activation of cyclooxygenase pathway.

ATP-sensitive potassium channels in the cerebral circulation

BACKGROUND

In brain blood vessels, electrophysiological studies proving the existence of ATP-sensitive potassium channels (KATP) are scarce. However, numerous pharmacological studies establish the importance of KATP channels in these blood vessels. This review emphasizes the data supporting the importance of vascular KATP in the responses of brain blood vessels.

SUMMARY OF REVIEW

Electrophysiological data show the existence of KATP in smooth muscle and endothelium of brain vessels. A much larger number of studies in virtually all experimental species have shown that classic openers of KATP dilate brain arteries and arterioles. This response can by blocked by glibenclamide, a selective inhibitor of KATP opening. Several physiological or pathophysiological responses are also blocked by glibenclamide. KATP contains a multiplicity of potential sites of interaction with drugs of diverse, sometimes unrelated, structures. Drugs with imidazole or guanidinium groups are particularly likely to have effects on KATP. This complicates interpretation of the actions of such drugs when used as supposedly selective pharmacological probes for other putative targets. A pH-sensitive site on the internal surface of cloned channels may explain the glibenclamide-inhibitable dilation produced by intracellular acidosis and perhaps by CO2. In some situations KATP appears to be involved in either the synthesis/release or action of endothelium-derived mediators of cerebrovascular tone. The importance of KATP may be dependent on the portion of the cerebrovascular tree being studied and on diverse experimental conditions, age, species, and the presence of disease.

CONCLUSIONS

KATP have been shown to mediate a wide range of cerebrovascular response in physiologic or pathologic circumstances in a variety of experimental conditions. Their relevance to cerebrovascular responses in humans remains to be explored.

NO and the vasculature: where does it come from and what does it do?

Nitric oxide (NO) is involved in a large number of cellular processes and dysfunctions in NO production have been implicated in many different disease states. In the vasculature NO is released by endothelial cells where it modulates the underlying smooth muscle to regulate vascular tone. Due to the unique chemistry of NO, such as its reactive and free radical nature, it can interact with many different cellular constituents such as thiols and transition metal ions, which determine its cellular actions. In this review we also discuss many of the useful pharmacological tools that have been developed and used extensively to establish the involvement of NO in endothelium-derived relaxations. In addition, the recent literature identifying a potential source of NO in endothelial cells, which is not directly derived from endothelial nitric oxide synthase is examined. Finally, the photorelaxation phenomena, which mediates the release of NO from a vascular smooth muscle NO store, is discussed.

Blockade of K(ATP) channels with glibenclamide does not alter functional activation of cerebral blood flow in the unanesthetized rat

Possible involvement of ATP-sensitive K(+) (K(ATP)) channels in the cerebral blood flow (CBF) response to neuronal functional activation was investigated in unanesthetized rats. Glibenclamide (1, 2, or 10 micromol/l), a specific inhibitor of K(ATP) channels, was infused intracisternally continuously for 30 min prior to and during the 1-min period of measurement of CBF. Unilateral functional activation was maintained throughout the measurement of CBF by continuous stroking of the vibrissae on the left side of the face. Local CBF was determined bilaterally by the quantitative autoradiographic [14C]iodoantipyrine method in four structures of the whisker-to-barrel cortex pathway and in 18 structures unrelated to the pathway. Glibenclamide tended to lower baseline CBF in almost all regions examined, statistically significantly (P<0.05) in the cerebellar lobules with all doses, in the cerebellar cortex with 10 micromol/l, in the pontine nuclei with 2 and 10 micromol/l, and in the spinal trigeminal nucleus of the unstimulated side with all doses. Vibrissal stimulation increased CBF unilaterally in all the stations of the pathway, but the percent increases were not statistically significantly affected by the glibenclamide treatment, except in the spinal trigeminal nucleus where it was reduced statistically significantly (P<0.05) only by 2 micromol/l glibenclamide. These results indicate that K(ATP) channels may play a role in the tonic regulation of baseline CBF in some regions but provide no support for their role in the increases in CBF evoked by functional activation.

Nitric oxide induces hyperpolarization by opening ATP-sensitive K(+) channels in guinea pig spiral modiolar artery

Nitric oxide (NO) hyperpolarizes vascular smooth muscle cells and dilates blood vessels of various beds, but little is known on cochlear vasculatures. Using in vitro preparations of the spiral modiolar artery (SMA), intracellular electrical recording and labeling techniques, we found that the NO donor DPTA-NONOate (10 microM) caused a hyperpolarization of approximately 9 mV in all the cells that had a low resting potential (RP) level near -40 mV. The hyperpolarization amplitude was concentration-dependent, with a 50% effect concentration (EC(50)) of 1 microM. The responses occur in both smooth muscle and endothelial cells, neither of which was blocked by 18beta-glycyrrhetinic acid. The induced hyperpolarization was completely blocked by glipizide, but not by charybdotoxin, apamin, barium, 4-aminopyridine or tetraethylammonium. The hyperpolarizing responses were imitated by pinacidil (EC(50)=30 microM). The pinacidil-induced response was also blocked by glipizide but not by the other K(+) channel blockers mentioned above. Both DPTA-NONOate and pinacidil had little membrane potential effect on cells that had a high RP level near -75 mV. However, when the high RP cells were depolarized to a level beyond -45 mV by barium, both DPTA-NONOate and pinacidil hyperpolarized these cells not differently from those that initially had a low RP. It is concluded that NO hyperpolarizes the SMA primarily by activating K(ATP) channels in both muscle and endothelial cells.

Relaxation to authentic nitric oxide and SIN-1 in rat isolated mesenteric arteries: variable role for smooth muscle hyperpolarization

Authentic nitric oxide (NO; 0.1 - 10 micromoles) caused transient, dose-dependent relaxation of phenylephrine-induced tone without changing membrane potential in mesenteric arteries. Larger doses, above 10 micromoles, did not evoke more relaxation (maximal relaxation to 150 micromoles NO in denuded arteries, 69+/-7%, n=8) but stimulated muscle hyperpolarization (maximum 19+/-3 mV, n=5). The soluble guanylyl cyclase inhibitor, 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ; 10 microM), abolished relaxation to low doses of NO (n=4), but did not modify hyperpolarization with higher doses of NO (n=4). The potassium channel blocker charybdotoxin (ChTX; 50 nM) abolished hyperpolarization to high doses of NO and significantly reduced the maximal relaxation (to 43+/-6%, n=4; P<0.01). ODQ and ChTX together abolished tension and membrane potential change to all doses of NO (n=4). All relaxations to 3-morpholino-sydnonimine (SIN-1; 0.01 - 10 microM) were associated with hyperpolarization. When the endothelium was intact, ChTX inhibited hyperpolarization and relaxation to SIN-1 (n=5), while iberiotoxin (IbTX; 50 nM) or 4-aminopyridine (4-AP; 500 microM) reduced relaxation by 40% and 20%, respectively and by 80% in combination (n=6 in each case). In denuded arteries, relaxation to SIN-1 was unaffected by either ChTX or ODQ alone, but abolished by the inhibitors together (n=6). Alone, 4-AP did not alter relaxation, but in the presence of ODQ it reduced the maximal response by around 45% (n=6; P<0.01). 4-AP, ODQ and IbTX together inhibited relaxation to SIN-1 by 75% (n=6; P<0.01). Therefore, cyclic guanosine 3',5'-monophosphate (cyclic GMP)-independent smooth muscle hyperpolarization, possibly involving direct activation of calcium-activated and voltage-sensitive potassium channels, contributes to relaxation evoked by authentic NO and SIN-1. However, the importance of each pathway depends on the source of NO and with SIN-1 the relative contribution from each pathway is modified by the endothelium.

Arachidonate dilates basilar artery by lipoxygenase-dependent mechanism and activation of K(+) channels

Dilatation of cerebral arterioles in response to arachidonic acid is dependent on activity of cyclooxygenase. In this study, we examined mechanisms that mediate dilatation of the basilar artery in response to arachidonate. Diameter of the basilar artery (baseline diameter = 216 +/- 7 micrometer) (means +/- SE) was measured using a cranial window in anesthetized rats. Arachidonic acid (10 and 100 microM) produced concentration-dependent vasodilatation that was not inhibited by indomethacin (10 mg/kg iv) or N(G)-nitro-L-arginine (100 microM) but was inhibited markedly by baicalein (10 micrometerM) or nordihydroguaiaretic acid (NDGA; 10 microM), inhibitors of the lipoxygenase pathway. Dilatation of the basilar artery was also inhibited markedly by tetraethylammonium ion (TEA; 1 mM) or iberiotoxin (50 nM), inhibitors of calcium-dependent potassium channels. For example, 10 microM arachidonate dilated the basilar artery by 19 +/- 7 and 1 +/- 1% in the absence and presence of iberiotoxin, respectively. Measurements of membrane potential indicated that arachidonate produced hyperpolarization of the basilar artery that was blocked completely by TEA. Incubation with [(3)H]arachidonic acid followed by reverse-phase and chiral HPLC indicated that the basilar artery produces relatively small quantities of prostanoids but large quantities of 12(S)-hydroxyeicosatetraenoic acid (12-S-HETE), a lipoxygenase product. Moreover, the production of 12-HETE was inhibited by baicalein or NDGA. These findings suggest that dilatation of the basilar artery in response to arachidonate is mediated by a product(s) of the lipoxygenase pathway, with activation of calcium-dependent potassium channels and hyperpolarization of vascular muscle.

Endothelium-dependent relaxation in perforating branch of human internal mammary artery

The purpose of the present study was to examine the effect of acetylcholine on perforating branch of the human internal mammary artery (HIMA). Acetylcholine (10(-9)-10(-5)M) induced concentration- and endothelium-dependent relaxation (pEC(50)=7.54+/-0.03, maximal response was 98+/-1.3%) of the precontracted arterial segments. Indomethacin, 4-aminopyridine (10(-5)M) and precontraction with K(+)-rich Krebs-Ringer-bicarbonate solution had no effect on acethylcholine-induced relaxation. N(G)-monomethyl-L-arginine (L-NMMA) (10(-5)M) inhibited relaxation evoked by acetylcholine. Indomethacin applied together with L-NMMA lead to further inhibition of acethylcholine-induced relaxation. Even in the presence of both L-NMMA and indomethacin, 4-aminopyridine had no provoked further inhibition of acetylcholine-induced relaxation of perforating branch of HIMA. It was concluded that the acethylcholine-induced relaxation of isolated perforating branch of HIMA is probably mediated via endothelial production of nitric oxide. However, when NO-synthase is inhibited, acetylcholine-induced vasorelaxation may be, in part, mediated through activation of cyclooxygenase pathway and consequent production and release of prostacyclin or some other cyclooxygenase products.

Action of a NO donor on the excitation-contraction pathway activated by noradrenaline in rat superior mesenteric artery

The aim of the present study was to investigate the actions of NO donors in ratsuperior mesenteric artery stimulated with noradrenaline by studying their effects on isometric tension, membrane potential (Vm), cytosolic calcium concentration ([Ca2+]cyt) and accumulation of inositol phosphates. In unstimulated arteries, SNAP (S-nitroso-N-acetylpenicillamine, 10 microM) hyperpolarised Vm by 3.0 +/- 0.5 mV (n = 9). In KCl-stimulated arteries, SNAP relaxed contraction without changing Vm and [Ca2+]cyt. In noradrenaline-stimulated arteries, SNAP relaxed tension, repolarised Vm and decreased [Ca2+]cyt with the same potency. Responses to SNAP were unaffected by the following K+ channel blockers: glibenclamide, 4-aminopyridine, apamin and charybdotoxin, and by increasing the KCl concentration to 25 mM. In SNAP-pretreated arteries, the production of inositol phosphates and the contraction stimulated by noradrenaline were inhibited similarly. The guanylate cyclase inhibitor ODQ abolished the increase in cyclic GMP content evoked by SNAP and inhibited the effects of SNAP on contraction, Vm and accumulation of inositol phosphates in noradrenaline-stimulated artery. These results indicate that, in rat superior mesenteric arteries activated by noradrenaline, inhibition of production of inositol phosphates is responsible for the effects of the NO donor SNAP on membrane potential, [Ca2+]cyt and contraction through a cyclic GMP-dependent mechanism.

Levcromakalim causes indirect endothelial hyperpolarization via a myo-endothelial pathway

1. Effects of K+ channel opener, levcromakalim, on vascular endothelial cells were examined. Under voltage- and current-clamp conditions, application of acetylcholine to dispersed endothelial cells isolated from rabbit superior mesenteric artery (dispersed RMAECs) produced hyperpolarization and outward currents. On the other hand, dispersed RMAECs did not respond to levcromakalim. 2. When membrane potential was recorded from endothelium in a mesenteric arterial segment, exposure to levcromakalim in a concentration range of 0.1 to 3 microM caused concentration-dependent hyperpolarization. The hyperpolarization was observed in the absence of external Ca2+ and was inhibited by 10 microM glibenclamide. 3. The presence of 1 mM heptanol did not affect the levcromakalin-induced hyperpolarization, whereas treatment of the mesenteric arterial segment with 20 microM 18 beta-glycyrrhetinic acid significantly reduced the hyperpolarization. The response to acetylcholine of RMAECs in an arterial segment with 18 beta-glycyrrhetinic acid was, however, similar to that without 18 beta-glycyrrhetinic acid. 4. These suggest that although RMAECs themselves are functionally insensitive to levcromakalim, those in an arterial segment are hyperpolarized by levcromakalim via myo-endothelial electrical communication.

Effect of nitro-L-arginine on electrical and mechanical responses to acetylcholine in the superior mesenteric artery from stroke-prone hypertensive rat

1. High salt diet is known to aggravate the vascular pathology in spontaneously hypertensive stroke-prone rats (SHR-SP). The aim of the present study was to assess the involvement of endothelial dysfunction in this effect. Contractile tension and membrane potential were simultaneously recorded in superior mesenteric artery rings of untreated and NaCl-loaded (1% NaCl in the drinking water) SHR-SP and normotensive Wistar Kyoto rats (WKY). 2. In unstimulated artery, hyperpolarization evoked by acetylcholine was not different in WKY and in NaCl-loaded WKY; it was reduced in SHR-SP and further reduced in NaCl-loaded SHR-SP. Hyperpolarization was unaffected by N(omega)-nitro-L-arginine (L-NA) but was abolished in high-KCl solution. 3. In noradrenaline-stimulated artery, ACh-evoked hyperpolarization and relaxation were not different in WKY and in SHR-SP. NaCl-treatment did not affect the responses to ACh in WKY but decreased maximum relaxation in SHR-SP from 93+/-2% to 72+/-7% of the contraction. In WKY, in NaCl-loaded WKY and in SHR-SP, L-NA similarly shifted the concentration-relaxation curve to ACh to the right and depressed its maximum but L-NA did not affect the hyperpolarization to ACh. In NaCl-loaded SHR-SP, L-NA blunted the effects of ACh on membrane potential and on contraction. 4. The NO donor SNAP abolished the depolarization and the contraction evoked by noradrenaline with the same potency in WKY and in untreated SHR-SP but was more potent in NaCl-loaded SHR-SP. 5. In KCl-contracted arteries the relaxations to ACh were not different in WKY and SHR-SP but NaCl-loaded SHR-SP were more sensitive to ACh. 6. The results showed that NaCl-rich diet markedly reduced the L-NA-resistant responses to ACh and increased the sensitivity to NO in SHR-SP.

Role of K+ channels and sodium pump in the vasodilation induced by acetylcholine, nitric oxide, and cyclic GMP in the rabbit aorta

The endothelium-dependent relaxation caused by acetylcholine (ACh) in rabbit aorta segments was reduced by the nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester and by blockade of: Na+ pump with ouabain, large-conductance Ca2+-activated K+ (BK(Ca)) channels with charybdotoxin (ChTx), or voltage-dependent K+ (Kv) channels with 4-aminopyridine (4-AP). ACh relaxation was unaltered by glibenclamide, apamin, and Ba2+, blockers of ATP-sensitive K+ channels, small-conductance Ca2+-activated K+ channels, and inward rectifier K+ channels, respectively. The relaxation induced by exogenous NO and 8-bromocyclic GMP (8-BrcGMP) was similar in intact and endothelium-denuded segments, and it was reduced or unaltered by the same drugs used in the case of ACh. However, a 4-AP concentration 20-fold higher was necessary to reduce exogenous NO relaxation. These data suggest a resemblance in the mechanisms implicated in the relaxation elicited by ACh, exogenous NO, and 8-BrcGMP. Therefore, the relaxation caused by ACh is mainly mediated by endothelial NO, which in turn, enhances cGMP levels; this messenger appears to be the major one responsible for the smooth muscle cell hyperpolarization in the relaxation elicited by ACh, which is mediated by activation of the Na+ pump and ChTx- and 4-AP-sensitive K+ channels, likely BK(Ca) and Kv channels.

Levcromakalim decreases cytoplasmic Ca2+ and vascular tone in basilar artery of SAH model dogs

We investigated the effects of levcromakalim, a K+ channel opener, on [Ca2+]i and the contractile force of basilar arteries obtained from normal dogs and subarachnoid hemorrhage (SAH) dogs. The responsiveness to serotonin was increased more in the SAH group than in the control group. Levcromakalim decreased the resting [Ca2+]i and force more profoundly than did a Ca2+ channel blocker, nicardipine, and these effects were more prominent in the SAH group than in the control group. Levcromakalim diminished the increases in [Ca2+]i and contractile force induced by serotonin more profoundly than nicardipine did, and these effects were equal in both groups. The effects of levcromakalim were not inhibited by iberiotoxin but were antagonized completely by glibenclamide. These results suggest that levcromakalim-induced opening of adenosine triphosphate (ATP)-sensitive K+ (K(ATP)) channels reduces [Ca2+]i more effectively than does nicardipine and that levcromakalim exerts the vasodilator effects under the condition in which large conductance Ca2+-activated K+ (BK) channels are blocked with iberiotoxin. Consequently, K+ channel openers like levcromakalim may be useful drug candidates to treat delayed cerebral vasospasm after SAH.

P2u receptor-mediated release of endothelium-derived relaxing factor/nitric oxide and endothelium-derived hyperpolarizing factor from cerebrovascular endothelium in rats

BACKGROUND AND PURPOSE

Stimulation of P2u purinoceptors by UTP on endothelium dilates the rat middle cerebral artery (MCA) through the release of endothelium-derived relaxing factor/nitric oxide (EDRF/NO) and an unknown relaxing factor. The purpose of this study was to determine whether this unknown relaxing factor is endothelium-derived hyperpolarizing factor (EDHF).

METHODS

Rat MCAs were isolated, cannulated, pressurized, and luminally perfused. UTP was added to the luminal perfusate to elicit dilations.

RESULTS

Resting outside diameter of the MCAs in one study was 209+/-7 micrometer (n=10). The MCAs showed concentration-dependent dilations with UTP administration. Inhibition of NO synthase with NG-nitro-L-arginine methyl ester (L-NAME) (1 micromol/L to 1 mmol/L) did not diminish the maximum response to UTP but did shift the concentration-response curve to the right. Scavenging NO with hemoglobin (1 or 10 micromol/L) or inhibition of guanylate cyclase with ODQ (1 or 10 micromol/L) had effects on the UTP-mediated dilations similar to those of L-NAME. In the presence of L-NAME, dilations induced by 10 micromol/L UTP were accompanied by 13+/-2 mV (P<0.009) hyperpolarization of the vascular smooth muscle membrane potential (-28+/-2 to -41+/-1 mV). Iberiotoxin (100 nmol/L), blocker of the large-conductance calcium-activated K channels, sometimes blocked the dilation, but its effects were variable. Charybdotoxin (100 nmol/L), also a blocker of the large-conductance calcium-activated K channels, abolished the L-NAME-insensitive component of the dilation to UTP.

CONCLUSIONS

Stimulation of P2u purinoceptors on the endothelium of the rat MCA released EDHF, in addition to EDRF/NO, and dilated the rat MCA by opening an atypical calcium-activated K channel.

Calcium channels, potassium channels and membrane potential of smooth muscle cells of human allantochorial placental vessels

The membrane potential (Um), the main factor of the excitation-contraction coupling, of human allantochorial placental vascular smooth muscle cells (VSMCs) has been previously shown to depend on voltage-sensitive K+ channels. These channels were blocked by high external K+. To characterize other channels which regulated Um, various constrictor or/and vasodilators and channel blockers were used. Serotonin depolarized VSMCs, in normal medium, but induced a more marked depolarization in VSMCs predepolarized by high external K+. This depolarization was inhibited by nifedipine, a blocker of voltage-gated Ca2+ channels. Acetylcholine, sodium nitroprusside (without effect on Um in normal medium), hyperpolarized the predepolarized-high K+ medium VSMCs. This hyperpolarization was inhibited after addition of charybotoxin (a blocker of Ca2+-activated K+ channels) or/and glibenclamide (a blocker of ATP-sensitive K+ channels). A similar effect was obtained with isoproterenol. These results indicated that membrane potential of human placental allantochorial VSMCs was regulated by voltage-gated, Ca2+- and ATP-sensitive K+ channels and by voltage-dependent Ca2+ channels.

Nitric oxide-dependent and -independent mechanisms in the relaxation elicited by acetylcholine in fetal rat aorta

The aim of the present study was to analyze the mechanisms involved in the relaxation induced by 1 microM acetylcholine (ACh) in aortic segments from fetal rats at term precontracted with 3 microM prostaglandin F2alpha (PGF2alpha) and incubated with 1 microM indomethacin. The endothelium-dependent relaxation caused by ACh was reduced by the nitric oxide (NO) synthase inhibitor NG-monomethyl-L-arginine (L-NMMA, 0.1 mM), such an effect was reversed by 0.1 mM L-arginine (L-Arg). After precontraction of segments with 50 mM KCl the relaxant response to ACh was smaller than that after precontraction with PGF2alpha; this reduction was increased by L-NMMA, whereas L-NMMA plus L-Arg potentiated the relaxation. Thiopentone sodium (0. 1 mM), ouabain (10 microM), tetraethylammonium (TEA, 0.5 mM) and apamin (1 microM), inhibitors of cytochrome P450 monooxygenases, Na+ pump, Ca2+-activated (KCa) and small-conductance (SKCa) K+ channels, respectively, reduced the relaxation to ACh, which was unaffected by charybdotoxin (0.1 microM) and glibenclamide (1 microM), inhibitors of large-conductance BKCa and ATP-sensitive K+ channels. The L-NMMA/indomethacin-resistant relaxation to ACh was markedly reduced by thiopentone sodium, and similarly decreased by either ouabain or TEA. The endothelium-independent relaxation induced by exogenous NO (10 microM) in segments precontracted with PGF2alpha was unaltered by ouabain, glibenclamide, TEA and after precontraction with 50 mM KCl, and potentiated by L-NMMA. The potentiation of NO responses by L-NMMA was also observed in segments precontracted with KCl. These results suggest that ACh relaxes the fetal rat aorta by endothelial release of both NO and endothelium-derived hyperpolarizing factor (EDHF), a metabolite derived from cytochrome P450 monooxygenases, that hyperpolarizes smooth muscle cells by activation of KCa, essentially SKCa channels, and Na+ pump. It seems that when the effect of EDHF is abolished, the formation of NO could be increased.

Levcromakalim decreases vascular tone, cytoplasmic Ca2+ and Ca2+ sensitivity in canine basilar artery

The involvement of large conductance Ca(2+)-activated K+ channels (BK) and ATP-sensitive K+ (KATP) channels in the regulation of canine basilar arterial tone was estimated in the presence of the agonist and blockers of these channels, by simultaneously measuring the changes in intracellular Ca2+ concentration ([Ca2+]i) with the fura-2 microfluorimetric method. In the resting condition, levcromakalim reduced [Ca2+]i and vascular tone. Levcromakalim suppressed the serotonin-induced increases in [Ca2+]i and force of contraction, the maximum effects of which were much greater than those of nicardipine. The inhibitory effects of levcromakalim were blocked by glibenclamide but not by tetraethylammonium (TEA) or iberiotoxin (IbTX). In the presence of levcromakalim, the curve relating [Ca2+]i with force in the presence of serotonin at different extracellular Ca2+ concentration ([Ca2+]o) was shifted down- and right-ward compared with that in the absence of levcromakalim, suggesting that levcromakalim may reduce the Ca(2+)-sensitivity of the contractile proteins. Thus, levcromakalim may be a good candidate to suppress delayed cerebral vasospasm after subarachnoid hemorrhage.

The endothelium-dependent, substance P relaxation of porcine coronary arteries resistant to nitric oxide synthesis inhibition is partially mediated by 4-aminopyridine-sensitive voltage-dependent K+ channels

We examined the role of K+ channels in the endothelium-dependent relaxation which is resistant to nitric oxide (NO) synthase inhibition in porcine coronary artery. In the presence of 0.2 mM NG-nitro-L-arginine (L-NNA), a potent inhibitor of NO synthase, 10 nM substance P (SP) added to 9,11-dideoxy-11alpha,9alpha-epoxymethano-prostaglandin F2alpha (U46619) contractures elicited a relaxation. The L-NNA-resistant relaxation induced by SP was strongly inhibited by 5 mM tetrabutylammonium chloride (TBA), a non-specific inhibitor of K+ channels. Interestingly, 4-aminopyridine (4-AP, 1 mM), a relatively specific inhibitor of voltage-sensitive K+ channels, shortened the duration of SP response, but it had no effect on the peak of SP response. Although 4-AP has also been shown to inhibit Ca2+-activated K+ channels, the shortening effect of 4-AP in SP response was observed in the presence of 1 microM apamin, an inhibitor of small conductance Ca2+-activated K+ channels, or 100 nM charybdotoxin, and inhibitor of large conductance Ca2+-activated K+ channels. Moreover, although SP stimulates both L-NNA-resistant relaxation and endothelium-derived NO-dependent relaxation (EDNO) in porcine coronary arteries, a low concentration of 4-AP (1 mM) affected only the L-NNA-resistant response, but not the EDNO response. These are the first results to show that the L-NNA-resistant relaxation induced by SP, probably, endothelium-derived hyperpolarizing factor(s) (EDHF) response, is dependent on voltage-dependent K+ channels in porcine coronary artery.

Hydroxylamine-induced relaxation inhibited by K+ channel blockers in rat aortic rings

Hydroxylamine, a putative endogenous nitric oxide donor, relaxed rat aorta in a concentration-dependent manner (0.01-30 microM). Removal of endothelium or pretreatment of aortic tissue with N(G)-nitro-L-arginine (L-NOARG, 100 microM) did not affect the relaxant effect of hydroxylamine but L-NOARG at 100 microM abolished the acetylcholine-induced relaxation. Methylene blue (10 microM) significantly reduced the relaxant effect of hydroxylamine in endothelium-denuded arteries. Tetrapentylammonium ions (0.3-3 microM), tetraethylammonium ions (1-3 mM) and charybdotoxin (100 nM) reduced the relaxant effect of hydroxylamine in the endothelium-denuded arteries while glibenclamide (3 microM) had no effect. Neither tetrapentylammonium nor tetraethylammonium ions affected relaxations induced by forskolin and verapamil. The effects of tetrapentylammonium ions (3 microM) and charybdotoxin (100 nM) were additive. Tetrapentylammonium ions (3 microM), tetraethylammonium ions (3 mM) and charybdotoxin (100 nM) decreased the relaxation induced by sodium nitroprusside in the endothelium-denuded arteries while glibenclamide (3 microM) had no effect. The concentration-relaxation curve for the relaxant effect of hydroxylamine was shifted to the right in the presence of high extracellular K+ (15-60 mM). Neither tetrapentylammonium ions (3 microM) nor charybdotoxin (100 nM) affected hydroxylamine-induced relaxation of the endothelium-denuded aorta precontracted with 60 mM K+. These results indicate that hydroxylamine relaxes the rat aorta partially through activation of tetrapentylammonium-, tetraethylammonium- and charybdotoxin-sensitive K+ channels and its action is comparable with that of sodium nitroprusside, an exogenous nitric oxide donor. The endothelium is not involved in the aortic response to hydroxylamine.

Cerebral vasodilators

The vascular tone, vascular resistance and blood flow in the brain are regulated by neural and humoral factors in quite a different way from those of peripheral organs and tissues. In contrast to the dominant vasoconstrictor control in the periphery, the intracranial vascular tone is predominantly influenced by vasodilator mediators over vasoconstrictor ones. Recent studies have revealed that nitroxidergic vasodilator nerve and endothelium-derived hyperpolarizing factor (EDHF) or K+ channel opening substance appear to play important roles in the regulation of cerebral arterial and arteriolar tone in primate and subprimate mammals, in addition to the accepted information concerning the crucial contribution of endothelium-derived relaxing factor (EDRF) or nitric oxide (NO), polypeptides, prostanoids, etc. This article summarizes characteristic properties of vasodilator factors in controlling the cerebral arterial and arteriolar tone that undoubtedly contribute to circulatory homeostasis. The content includes vasodilator nerve, endogenous vasodilator substances, and vasodilator interventions such as hypoxia, hypercapnia and hyperosmolarity.

Effect of acidosis on tension and [Ca2+]i in rat cerebral arteries: is there a role for membrane potential?

The cellular mechanism responsible for the reduction of tension in cerebral small arteries to acidosis is not known. In this study the role of smooth muscle intracellular Ca2+ concentration ([Ca2+]i) and membrane potential for the relaxation to acidosis was investigated in isolated rat cerebral small arteries. Isometric force was measured simultaneously with [Ca2+]i (fura 2) or with membrane potential (intracellular microelectrodes), and acidosis was induced by increasing PCO2 or reducing HCO3- of the bathing solution. Both hypercapnic and normocapnic acidosis were associated with a reduction of intracellular pH [measured with 2',7'-bis-(carboxyethyl)-5 (and -6)-carboxyfluorescein], caused relaxation, and reduced [Ca2+]i. However, whereas hypercapnic acidosis caused hyperpolarization, normocapnic acidosis was associated with depolarization. It is concluded that a reduction of [Ca2+]i is in part responsible for the direct effect of the acidosis on the vascular smooth muscle both during normo- and hypercapnia. The mechanism responsible for the reduction of [Ca2+]i differs between the hypercapnic and normocapnic acidosis, being partly explained by hyperpolarization during hypercapnic acidosis, whereas it is seen despite depolarization during normocapnic acidosis.

The effects of levcromakalim and pinacidil on the human internal mammary artery

The present study was undertaken to examine the effects of pinacidil and levcromakalim, two potassium, channel openers, on human internal mammary artery (HIMA) obtained from patients undergoing coronary artery bypass surgery, and to clarify the contribution of different K+ channel subtypes in pinacidil and levcromakalim action in this blood vessel. Pinacidil and levcromakalim induced a concentration-dependent relaxation of the precontracted arterial segments (pEC50 = 5.77 +/- 0.05 and 6.89 +/- 0.03, respectively), 4-Aminopyridine (3 mM), a non-selective blocker of K+ channels, induced significant shifts to the right of the concentration-response curves for pinacidil and levcromakalim. Tetraethylammonium (6 mM), charybdotoxin (0.4 microM) and apamin (0.1 microM), blockers of Ca(2+)-sensitive K+ channels, had no effect on the pinacidil- and levcromakalim-evoked relaxation. Glibenclamide (0.1-10 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels, competitively antagonized the response to levcromakalim (pKB = 7.92 +/- 0.07). In contrast, glibenclamide, in significantly higher concentrations (3-30 microM), non-competitively antagonized the response to pinacidil. High concentrations of pinacidil (> 10 microM) relaxed arterial rings bathed by a medium containing 100 mM K+ with maximum response 83 +/- 6%. Under the same conditions, the maximum levcromakalim-induced relaxation on HIMA was almost abolished (15 +/- 2%). It is concluded that pinacidil and levcromakalim do not relax the HIMA through the same subtype of K+ channel. ATP-sensitive K+ channels are probably involved in levcromakalim- but not in a pinacidil-induced relaxation in the HIMA. In addition, in pinacidil-induced relaxation of the HIMA, K+ channel-independent mechanisms seem to be involved.

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.

Role of endothelium in regulation of smooth muscle membrane potential and tone in the rabbit middle cerebral artery

1. The characteristic features of the endothelium-mediated regulation of the electrical and mechanical activity of the smooth muscle cells of cerebral arteries were studied by measuring membrane potential and isometric force in endothelium-intact and -denuded strips taken from the rabbit middle cerebral artery (MCA). 2. In endothelium-intact strips, histamine (His, 3-10 microM) and high K+ (20-80 mM) concentration-dependently produced a transient contraction followed by a sustained contraction. Noradrenaline (10 microM), 5-hydroxytryptamine (10 microM) and 9,11-epithio-11, 12-methano-thromboxane A2 (10 nM) each produced only a small contraction (less than 5% of the maximum K+-induced contraction). 3. N(G)-nitro-L-arginine (L-NOARG, 100 microM), but not indomethacin (10 microM), greatly enhanced the phasic and the tonic contractions induced by His (1-10 microM) in endothelium-intact, but not in endothelium-denuded strips, suggesting that spontaneous or basal release of nitric oxide (NO) from endothelial cells potently attenuates the His-induced contractions. Acetylcholine (ACh, 0.3-3 microM) caused concentration-dependent relaxation (maximum relaxation by 89.7 +/- 7.5%, n=4, P<0.05) when applied to endothelium-intact strips precontracted with His. L-NOARG had little effect on this ACh-induced relaxation (n=4; P<0.05). Apamin (0.1 microM), but not glibenclamide (3 microM), abolished the relaxation induced by ACh (0.3-3 microM) in L-NOARG-treated strips (n=4, P<0.05). 4. In endothelium-intact tissues, His (3 microM) depolarized the smooth muscle membrane potential (by 4.4 +/- 1.8 mV, n = 12, P < 0.05) whereas ACh (3 microM) caused membrane hyperpolarization (-20.9 +/- 3.0 mV, n = 25, P< 0.05). The ACh-induced membrane hypepolarization persisted after application of L-NOARG (-23.5 +/- 5.9 mV, n=8, P<0.05) or glibenclamide (-20.6 +/- 5.4 mV, n=5, P<0.05) but was greatly diminished by apamin (reduced to - 5.8 +/- 3.2 mV, n = 3, P< 0.05). 5. Sodium nitroprusside (0.1-10 microM) did not hyperpolarize the smooth muscle cell membrane potential (0.2 +/- 0.3 mV, n=4, P>0.05) but it greatly attenuated the His-induced contraction in endothelium-denuded strips (n-4, P<0.05). 6. These results suggest that, under the present experimental conditions: (i) spontaneous or basal release of NO from endothelial cells exerts a significant negative effect on agonist-induced contractions in rabbit MCA, and (ii) ACh primarily activates the release of endothelium-derived hyperpolarizing factor (EDHF) in rabbit MCA.

Characterization of endothelium-derived relaxing factors released by bradykinin in human resistance arteries

1. Relaxing factors released by the endothelium and their relative contribution to the endothelium-dependent relaxation produced by bradykinin (BK) in comparison with different vasodilator agents were investigated in human omental resistance arteries. 2. BK produced an endothelium-dependent relaxation of arteries pre-contracted with the thromboxane A2 agonist, U46619. The B2 receptor antagonist, Hoe 140 (0.1, 1 and 10 microM), produced a parallel shift to the right of the concentration-response curve to BK with a pA2 of 7.75. 3. Neither the cyclo-oxygenase inhibitor, indomethacin (10 microM) alone, the nitric oxide synthase inhibitor, N omega-nitro-L-arginine methyl ester (L-NAME, 300 microM) alone, the nitric oxide scavenger, oxyhaemoglobin (Hb, 10 microM) alone, nor the combination of L-NAME plus Hb affected the concentration-response curve to BK. Conversely, the combination of indomethacin with either L-NAME or Hb attenuated but did not abolish the BK-induced relaxation. By contrast, the relaxations produced by the Ca2+ ionophore, calcimycin (A23187), and by the inhibitor of sarcoplasmic reticulum Ca(2+)-ATPase, thapsigargin (THAPS), were abolished in the presence of indomethacin plus L-NAME. Also, the presence of indomethacin plus L-NAME produced contraction of arteries with functional endothelium. 4. The indomethacin plus L-NAME resistant component of BK relaxation was abolished in physiological solution (PSS) containing 40 mM KCl and vice versa. However, in the presence of KCl 40 mM, indomethacin plus L-NAME did not affect the nitric oxide donor, S-N-acetylpenicillamine-induced relaxation. 5. The indomethacin plus L-NAME resistant component of the relaxation to BK was significantly attenuated by the K+ channel blocker tetrabutylammonium (TBA, 1 mM). However, it was not affected by other K+ channel blockers such as apamin (10 microM), 4-aminopyridine (100 microM), glibenclamide (10 microM), tetraethylammonium (10 mM) and charybdotoxin (50 nM). 6. In the presence of indomethacin plus L-NAME, the relaxation produced by BK was not affected by the phospholipase A2 inhibitor, quinacrine (10 microM) or by the inhibitor of cytochrome P450, SKF 525a (10 microM). Another cytochrome P450 inhibitor, clotrimazole (10 microM) which also inhibits K+ channels, inhibited the relaxation to BK. 7. These results show that BK induces endothelium-dependent relaxation in human small omental arteries via multiple mechanisms involving nitric oxide, cyclo-oxygenase derived prostanoid(s) and another factor (probably an endothelium-derived hyperpolarizing factor). They indicate that nitric oxide and cyclo-oxygenase derivative(s) can substitute for each other in producing relaxation and that the third component is not a metabolite of arachidonic acid, formed through the cytochrome P-450 pathway, in these arteries.

Acetylcholine-induced K+ currents in smooth muscle cells of intact rat small arteries

1. The mechanism of the sustained acetylcholine-induced endothelium-dependent hyperpolarization (EDH) in intact rat small mesenteric arteries prestimulated with noradrenaline (10(-6) M) was investigated by means of the single microelectrode voltage-clamp method. 2. The vascular smooth muscle cells (VSMCs) in this preparation are poorly or even not coupled for the reasons that: (1) the mean input resistance Rlnp of the clamped vascular smooth muscle increases from 120 M omega under control conditions to 440 M omega after application of K+ channel blocking drugs, (2) the voltage relaxation after injection of hyperpolarizing currents has a monoexponential time course and is linearly dependent on Rlnp, and (3) voltage steps induced by current-clamp steps are not transferred to locations in the vascular musculature 120 microns apart from the current injecting microelectrode. 3. Sustained (> 5 min) application of ACh (10(-5) M) hyperpolarized the VSMCs by induction of a hyperpolarizing current. This effect was completely blocked by the inhibitor of the nitric oxide (NO) synthase L-NAME (10(-3) M) but not by the inhibitor of the soluble guanylate cyclase (sGCl) Methylene Blue (MB, 10(-4) M). 4. Application of the NO donor sodium nitroprusside (SNP, 10(-6) M) for more than 5 min mimicked the induction of the endothelium-dependent hyperpolarizing current in vessels with destroyed endothelium. The reversal potential of this current is dependent on the extracellular K+ concentration. The effect of SNP could also not be blocked by MB. 5. The blockers of ATP-dependent and Ca(2+)-dependent K+ channels, glibenclamide (Glb, 10(-5) M) and charybdotoxin (CTX, 5 x 10(-8) M), respectively, blocked a hyperpolarizing current in the VSMCs similar to the ACh- or SNP-induced current. 6. The isolated application of either Glb or CTX did not block the activation of the hyperpolarizing current by SNP. Only the combined administration of Glb and CTX blocked the SNP-induced current completely. 7. Our results suggest that in rat small mesenteric artery, ACh hyperpolarizes the VSMCs tonically by activating both ATP- and Ca(2+)-dependent K+ currents, only via release of NO from the endothelium without need for activation of the sGCl.

Evidence that mechanisms dependent and independent of nitric oxide mediate endothelium-dependent relaxation to bradykinin in human small resistance-like coronary arteries

1. The effects of the nitric oxide (NO) synthase inhibitor, NG-nitro-L-arginine (L-NOARG), the NO scavenger, oxyhaemoglobin (HbO) and high extracellular K+ upon endothelium-dependent relaxation to bradykinin were investigated in human isolated small coronary arteries. 2. Endothelium-dependent relaxations to bradykinin were compared in vessels contracted to approximately 50% of their maximum contraction to 124 mM KCl Krebs solution, regardless of treatments, with the thromboxane A2 mimetic, U46619 and acetylcholine. All relaxations were expressed as percentage reversal of the initial level of active force. 3. L-NOARG (100 microM) caused a small but significant, 12% (P < 0.01), decrease in the maximum relaxation (Rmax: 91.5 +/- 5.4%) to bradykinin but did not significantly affect the sensitivity (pEC50: 8.08 +/- 0.17). Increasing the concentration of L-NOARG to 300 microM had no further effect on the pEC50 or Rmax to bradykinin. HbO (20 microM) and a combination of HbO (20 microM) and L-NOARG (100 microM) reduced Rmax to bradykinin by 58% (P < 0.05) and 54% (P < 0.05), respectively. HbO (20 microM) and L-NOARG (100 microM, combined but not HbO (20 microM) alone, caused a significant 11 fold (P < 0.05) decrease in sensitivity to bradykinin. HbO (20 microM) decreased the sensitivity to the endothelium-independent NO donor, S-nitroso-N-acetylpenicillamine (SNAP), approximately 17 fold (P < 0.05). 4. Raising the extracellular concentration of K+ isotonically to 30 mM, reduced the Rmax to bradykinin from 96.6 +/- 3.1% to 43.9 +/- 10.1% (P < 0.01) with no significant change in sensitivity. A combination of HbO, L-NOARG and high K+ (30 mM) abolished the response to bradykinin. High K+ did not change either the sensitivity or maximum relaxation to SNAP. 5. In conclusion, L-NOARG does not completely inhibit endothelial cell NO synthesis in human isolated small coronary arteries. By comparison, HbO appeared to block all the effects of NO in this tissue and revealed that most of the relaxation to bradykinin was due to NO. The non-NO -dependent relaxation to bradykinin in the human isolated small coronary arteries appeared to be mediated by a K(+)-sensitive vasodilator mechanism, possibly endothelium-derived hyperpolarizing factor (EDHF).

Actions of neurotransmitters and other messengers on Ca2+ channels and K+ channels in smooth muscle cells

Ion channels play key roles in determining smooth muscle tone by setting the membrane potential and allowing Ca2+ influx. Perhaps not surprisingly, therefore, they also provide targets for neurotransmitters and other messengers that act on smooth muscle. Application of patch-clamp and molecular biology techniques and the use of selective pharmacology has started to provide a wealth of information on the ion channel systems of smooth muscle cells, revealing complexity and functional significance. Reviewed are the actions of messengers (e.g., noradrenaline, acetylcholine, endothelin, angiotensin II, neuropeptide Y, 5-hydroxytryptamine, histamine, adenosine, calcitonin gene-related peptide, substance P, prostacyclin, nitric oxide and oxygen) on specific types of ion channel in smooth muscle, the L-type calcium channel, and the large conductance Ca(2+)-activated, ATP-sensitive, delayed rectifier and apamin-sensitive K+ 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.

Endothelium-derived factors and hyperpolarization of the carotid artery of the guinea-pig

1. Transmembrane potentials were recorded from isolated carotid arteries of the guinea-pig superfused with modified Krebs-Ringer bicarbonate solution. Smooth muscle cells were impaled from the adventitial side with intracellular glass microelectrodes filled with KCl (30-80 M omega). 2. Acetylcholine (1 microM) in the presence of inhibitors of nitric oxide synthase, (N omega-nitro-L-arginine (L-NOARG) 100 microM) and cyclo-oxygenase, (indomethacin 5 microM) induced an endothelium-dependent hyperpolarization (-18.9 +/- 1.6 mV, n = 15). 3. In the presence of these two inhibitors, S-nitroso-L-glutathione (10 microM), sodium nitroprusside (10 microM), 3-morpholinosydnonimine (SIN-1, 10 microM) and iloprost (0.1 microM) induced endothelium-independent hyperpolarizations of the smooth muscle cells (respectively: -16.0 +/- 2.3, -16.3 +/- 3.4, -12.8 +/- 2.0 and -14.5 +/- 1.5 mV, n = 4-6). 4. The addition of glibenclamide (1 microM) did not influence the acetylcholine-induced L-NOARG/ indomethacin-resistant hyperpolarization (-18.0 +/- 1.8 mV, n = 10). In contrast, the responses induced by S-nitroso-L-glutathione, sodium nitroprusside, SIN-1 and iloprost were abolished (changes in membrane potential: -0.8 +/- 1.1, 1.3 +/- 3.9, 4.5 +/- 4.6 and 0.3 +/- 0.8 mV respectively, n = 4-5). 5. In the presence of NO synthase and cyclo-oxygenase inhibitors, charybdotoxin (0.1 microM) or apamin (0.5 microM) did not influence the hyperpolarization produced by acetylcholine. However, in the presence of the combination of charybdotoxin and apamin, the acetylcholine-induced L-NOARG/indomethacin-resistant hyperpolarization was converted to a depolarization (4.4 +/- 1.2 mV, n = 20) while the endothelium-independent hyperpolarizations induced by S-nitroso-L-glutathione, sodium nitroprusside, SIN-1 and iloprost were not affected significantly (respectively: -20.4 +/- 3.4, -22.5 +/- 4.9, -14.5 +/- 4.7 and -14.5 +/- 0.5 mV, n = 4-5). 6. In the presence of the combination of charybdotoxin and apamin and in the absence of L-NOARG and indomethacin, acetylcholine induced a hyperpolarization (-19.5 +/- 3.7 mV, n = 4). This hyperpolarization induced by acetylcholine was not affected by the addition of indomethacin (-18.3 +/- 4.6 mV, n = 3). In the presence of the combination of charybdotoxin, apamin and L-NOARG (in the absence of indomethacin), acetylcholine, in 5 out of 7 vessels, still produced hyperpolarization which was not significantly smaller (-9.1 +/- 5.6 mV, n = 7) than the one observed in the absence of L-NOARG. 7. These findings suggest that, in the guinea-pig isolated carotid artery, the endothelium-independent hyperpolarizations induced by NO donors and iloprost involve the opening of KATP channels while the acetylcholine-induced endothelium-dependent hyperpolarization (resistant to the inhibition of NO-synthase and cyclo-oxygenase) involves the opening of Ca(2+)-activated potassium channel(s). Furthermore, in this tissue, acetylcholine induces the simultaneous release of various factors from endothelial origin: hyperpolarizing factors (NO, endothelium derived hyperpolarizing factor (EDHF) and prostaglandins) and possibly a depolarizing factor.