beta-Adrenoceptor stimulation activates large-conductance Ca2+-activated K+ channels in smooth muscle cells from basilar artery of guinea pig

Ca2+-Activated K+ Channels and the Regulation of the Uteroplacental Circulation

Adequate uteroplacental blood supply is essential for the development and growth of the placenta and fetus during pregnancy. Aberrant uteroplacental perfusion is associated with pregnancy complications such as preeclampsia, fetal growth restriction (FGR), and gestational diabetes. The regulation of uteroplacental blood flow is thus vital to the well-being of the mother and fetus. Ca²⁺-activated K⁺ (K_Ca) channels of small, intermediate, and large conductance participate in setting and regulating the resting membrane potential of vascular smooth muscle cells (VSMCs) and endothelial cells (ECs) and play a critical role in controlling vascular tone and blood pressure. K_Ca channels are important mediators of estrogen/pregnancy-induced adaptive changes in the uteroplacental circulation. Activation of the channels hyperpolarizes uteroplacental VSMCs/ECs, leading to attenuated vascular tone, blunted vasopressor responses, and increased uteroplacental blood flow. However, the regulation of uteroplacental vascular function by K_Ca channels is compromised in pregnancy complications. This review intends to provide a comprehensive overview of roles of K_Ca channels in the regulation of the uteroplacental circulation under physiological and pathophysiological conditions.

Aging-induced down-regulation of Pka/Bkca pathway in rat cerebral arteries

The incidence of cerebrovascular diseases increases significantly with aging. This study aimed to test the hypothesis that aging may influence the protein kinase A (PKA)-dependent vasodilation via RyR/BKCa pathway in the middle cerebral arteries (MCA). Male Sprague-Dawley rats were randomly divided into control (4-6 month-old) and aged (24-month-old) groups. The functions of MCA and ion channel activities in smooth muscle cells were examined using myograph system and patch-clamp. Aging decreased the isoproterenol/forskolin-induced relaxation in the MCA. Large-conductance Ca2+-activated-K+ (BKCa) channel inhibitor, iberiotoxin, significantly attenuated the forskolin-induced vasodilatation and hyperpolarization in the young group, but not in the aged group. The amplitude and frequency of spontaneous transient outward currents (STOCs) were significantly decreased in the aged group. Single channel recording revealed that the mean open time of BKCa channels were decreased, while an increased mean closed time of BKCa channels were found in the aged group. The Ca2+/voltage sensitivity of the channels was decreased accompanied by reduced BKCa α and β1-subunit, the expression of RyR2, PKA-Cα and PKA-Cβ subunits were also declined in the aged group. Aging induced down-regulation of PKA/BKCa pathway in cerebral artery in rats. The results provides new information on further understanding in cerebrovascular diseases resulted from age-related cerebral vascular dysfunction.

Dynein Coordinates β2-Adrenoceptor-Mediated Relaxation in Normotensive and Hypertensive Rat Mesenteric Arteries

BACKGROUND

The voltage-gated potassium channel (Kv)7.4 and Kv7.5 channels contribute to the β-adrenoceptor-mediated vasodilatation. In arteries from hypertensive rodents, the Kv7.4 channel is downregulated and function attenuated, which contributes to the reduced β-adrenoceptor-mediated vasodilatation observed in these arteries. Recently, we showed that disruption of the microtubule network, with colchicine, or inhibition of the microtubule motor protein, dynein, with ciliobrevin D, enhanced the membrane abundance and function of Kv7.4 channels in rat mesenteric arteries. This study aimed to determine whether these pharmacological compounds can improve Kv7.4 function in third-order mesenteric arteries from the spontaneously hypertensive rat, thereby restoring the β-adrenoceptor-mediated vasodilatation.

METHODS

Wire and intravital myography was performed on normotensive and hypertensive male rat mesenteric arteries and immunostaining was performed on isolated smooth muscle cells from the same arteries.

RESULTS

Using wire and intravital microscopy, we show that ciliobrevin D enhanced the β-adrenoceptor-mediated vasodilatation by isoprenaline. This effect was inhibited partially by the Kv7 channel blocker linopirdine and was dependent on an increased functional contribution of the β2-adrenoceptor to the isoprenaline-mediated relaxation. In mesenteric arteries from the spontaneously hypertensive rat, ciliobrevin D and colchicine both improved the isoprenaline-mediated vasorelaxation and relaxation to the Kv7.2 -7.5 activator, ML213. Immunostaining confirmed ciliobrevin D enhanced the membrane abundance of Kv7.4. As well as an increase in the function of Kv7.4, the functional changes were associated with an increase in the contribution of β2-adrenoceptor following isoprenaline treatment. Immunostaining experiments showed ciliobrevin D prevented isoprenaline-mediated internalizationof the β2-adrenoceptor.

CONCLUSIONS

Overall, these data show that colchicine and ciliobrevin D can induce a β2-adrenoceptor-mediated vasodilatation in arteries from the spontaneously hypertensive rat as well as reinstating Kv7.4 channel function.

The Calcium Signaling Mechanisms in Arterial Smooth Muscle and Endothelial Cells

The contractile state of resistance arteries and arterioles is a crucial determinant of blood pressure and blood flow. Physiological regulation of arterial contractility requires constant communication between endothelial and smooth muscle cells. Various Ca²⁺ signals and Ca²⁺ -sensitive targets ensure dynamic control of intercellular communications in the vascular wall. The functional effect of a Ca²⁺ signal on arterial contractility depends on the type of Ca²⁺ -sensitive target engaged by that signal. Recent studies using advanced imaging methods have identified the spatiotemporal signatures of individual Ca²⁺ signals that control arterial and arteriolar contractility. Broadly speaking, intracellular Ca²⁺ is increased by ion channels and transporters on the plasma membrane and endoplasmic reticular membrane. Physiological roles for many vascular Ca²⁺ signals have already been confirmed, while further investigation is needed for other Ca²⁺ signals. This article focuses on endothelial and smooth muscle Ca²⁺ signaling mechanisms in resistance arteries and arterioles. We discuss the Ca²⁺ entry pathways at the plasma membrane, Ca²⁺ release signals from the intracellular stores, the functional and physiological relevance of Ca²⁺ signals, and their regulatory mechanisms. Finally, we describe the contribution of abnormal endothelial and smooth muscle Ca²⁺ signals to the pathogenesis of vascular disorders. © 2021 American Physiological Society. Compr Physiol 11:1831-1869, 2021.

Ion channels as effectors of cyclic nucleotide pathways: Functional relevance for arterial tone regulation

Numerous mediators and drugs regulate blood flow or arterial pressure by acting on vascular tone, involving cyclic nucleotide intracellular pathways. These signals lead to regulation of several cellular effectors, including ion channels that tune cell membrane potential, Ca²⁺ influx and vascular tone. The characterization of these vasocontrictive or vasodilating mechanisms has grown in complexity due to i) the variety of ion channels that are expressed in both vascular endothelial and smooth muscle cells, ii) the heterogeneity of responses among the various vascular beds, and iii) the number of molecular mechanisms involved in cyclic nucleotide signalling in health and disease. This review synthesizes key data from literature that highlight ion channels as physiologically relevant effectors of cyclic nucleotide pathways in the vasculature, including the characterization of the molecular mechanisms involved. In smooth muscle cells, cation influx or chloride efflux through ion channels are associated with vasoconstriction, whereas K⁺ efflux repolarizes the cell membrane potential and mediates vasodilatation. Both categories of ion currents are under the influence of cAMP and cGMP pathways. Evidence that some ion channels are influenced by CN signalling in endothelial cells will also be presented. Emphasis will also be put on recent data touching a variety of determinants such as phosphodiesterases, EPAC and kinase anchoring, that complicate or even challenge former paradigms.

Calcium- and voltage-gated BK channels in vascular smooth muscle

Ion channels in vascular smooth muscle regulate myogenic tone and vessel contractility. In particular, activation of calcium- and voltage-gated potassium channels of large conductance (BK channels) results in outward current that shifts the membrane potential toward more negative values, triggering a negative feed-back loop on depolarization-induced calcium influx and SM contraction. In this short review, we first present the molecular basis of vascular smooth muscle BK channels and the role of subunit composition and trafficking in the regulation of myogenic tone and vascular contractility. BK channel modulation by endogenous signaling molecules, and paracrine and endocrine mediators follows. Lastly, we describe the functional changes in smooth muscle BK channels that contribute to, or are triggered by, common physiological conditions and pathologies, including obesity, diabetes, and systemic hypertension.

Ventilatory and cerebrovascular regulation and integration at high-altitude

Ascent to high-altitude elicits compensatory physiological adaptations in order to improve oxygenation throughout the body. The brain is particularly vulnerable to the hypoxemia of terrestrial altitude exposure. Herein we review the ventilatory and cerebrovascular changes at altitude and how they are both implicated in the maintenance of oxygen delivery to the brain. Further, the interdependence of ventilation and cerebral blood flow at altitude is discussed. Following the acute hypoxic ventilatory response, acclimatization leads to progressive increases in ventilation, and a partial mitigation of hypoxemia. Simultaneously, cerebral blood flow increases during initial exposure to altitude when hypoxemia is the greatest. Following ventilatory acclimatization to altitude, and an increase in hemoglobin concentration-which both underscore improvements in arterial oxygen content over time at altitude-cerebral blood flow progressively decreases back to sea-level values. The complimentary nature of these responses (ventilatory, hematological and cerebral) lead to a tightly maintained cerebral oxygen delivery while at altitude. Despite this general maintenance of global cerebral oxygen delivery, the manner in which this occurs reflects integration of these physiological responses. Indeed, ventilation directly influences cerebral blood flow by determining the prevailing blood gas and acid/base stimuli at altitude, but cerebral blood flow may also influence ventilation by altering central chemoreceptor stimulation via central CO₂ washout. The causes and consequences of the integration of ventilatory and cerebral blood flow regulation at high altitude are outlined.

Microtubule Regulation of Kv7 Channels Orchestrates cAMP-Mediated Vasorelaxations in Rat Arterial Smooth Muscle

Microtubules can regulate GPCR (G protein-coupled receptor) signaling in various cell types. In vascular smooth muscle, activation of the β-adrenoceptor leads to production of cAMP to mediate a vasorelaxation. Little is known about the role of microtubules in smooth muscle, and given the importance of this pathway in vascular smooth muscle cells, we investigated the role of microtubule stability on β-adrenoceptor signaling in rat renal and mesenteric arteries. In isometric tension experiments, incubation with the microtubule inhibitors colchicine and nocodazole enhanced isoprenaline-mediated relaxations of renal and mesenteric arteries that the microtubule stabilizer, paclitaxel, prevented. Sharp microelectrode experiments showed that colchicine treatment caused increased hyperpolarization of mesenteric artery segments in response to isoprenaline. Application of the Kv7 channel blocker, XE991, attenuated the effect of colchicine on isoprenaline relaxations, whereas iberiotoxin-a BKCa channel blocker-had no effect. In addition, colchicine improved the relaxations to the Kv7.2 to 7.5 activator, S-1, in both renal and mesenteric artery segments compared with dimethyl sulfoxide incubation. We determined that increased mesenteric artery myocytes treated with colchicine showed increased Kv7.4 membrane expression, but Western blot analysis showed no change in total Kv7.4 protein. This study is the first to show microtubule disruption improves the β-adrenoceptor-mediated relaxations of mesenteric and renal arteries and determine this enhancement to be because of increased membrane expression of the Kv7 voltage-gated potassium channels.

Maternal high salt diet altered Adenosine-mediated vasodilatation via PKA/BK channel pathway in offspring rats

SCOPE

High salt (HS) diets are related to cardiovascular diseases, and prenatal HS was suggested to increase risks of coronary artery diseases in the offspring. This study tested the hypothesis that prenatal HS may influence Adenosine-induced vasodilatation via protein kinase A (PKA) pathway in coronary arteries.

METHODS AND RESULTS

Sprague-Dawley rats were fed with 8% salt diet for gestation, the control was fed with 0.3% salt diet. Coronary arteries from male adult offspring were tested for K⁺ channels and Adenosine signal pathways. Adenosine-mediated vasodilatation was reduced in coronary arteries in HS. There was no difference in gene expression of A_2A receptors between the two groups. After pretreatment with PKA inhibitor, vasodilatation to Adenosine was decreased to a smaller extent in HS than that in control. Forskolin (activator of adenylate cyclase)-mediated vasodilatation was decreased in HS. Iberiotoxin (large-conductance Ca²⁺ -activated K⁺ channel [BK channel] inhibitor) attenuated Forskolin-induced vasodilatation in control, not in HS group. Currents of BK channels decreased in coronary artery smooth muscle cells, and PKA-modulated BK channel functions were declined. Protein levels of BK β1 and PKA C-subunits in coronary arteries of HS offspring were reduced.

CONCLUSIONS

Prenatal HS diets altered Adenosine-mediated coronary artery vasodilatation in the offspring, which was linked to downregulation of cAMP/PKA/BK channel pathway.

Hypoxemia, oxygen content, and the regulation of cerebral blood flow

This review highlights the influence of oxygen (O2) availability on cerebral blood flow (CBF). Evidence for reductions in O2 content (CaO2 ) rather than arterial O2 tension (PaO2 ) as the chief regulator of cerebral vasodilation, with deoxyhemoglobin as the primary O2 sensor and upstream response effector, is discussed. We review in vitro and in vivo data to summarize the molecular mechanisms underpinning CBF responses during changes in CaO2 . We surmise that 1) during hypoxemic hypoxia in healthy humans (e.g., conditions of acute and chronic exposure to normobaric and hypobaric hypoxia), elevations in CBF compensate for reductions in CaO2 and thus maintain cerebral O2 delivery; 2) evidence from studies implementing iso- and hypervolumic hemodilution, anemia, and polycythemia indicate that CaO2 has an independent influence on CBF; however, the increase in CBF does not fully compensate for the lower CaO2 during hemodilution, and delivery is reduced; and 3) the mechanisms underpinning CBF regulation during changes in O2 content are multifactorial, involving deoxyhemoglobin-mediated release of nitric oxide metabolites and ATP, deoxyhemoglobin nitrite reductase activity, and the downstream interplay of several vasoactive factors including adenosine and epoxyeicosatrienoic acids. The emerging picture supports the role of deoxyhemoglobin (associated with changes in CaO2 ) as the primary biological regulator of CBF. The mechanisms for vasodilation therefore appear more robust during hypoxemic hypoxia than during changes in CaO2 via hemodilution. Clinical implications (e.g., disorders associated with anemia and polycythemia) and future study directions are considered.

Stretch-activated BK channel and heart function

The heart is an organ that is exposed to extreme dynamic mechanical stimuli. From birth till death, the heart indefinitely repeats periodic contraction and dilation, i.e., shortening and elongation of cardiomyocytes. Mechanical stretch elicits a change in heart rate and may cause arrhythmia if it is excessive. Thus, mechanosensitivity is crucial to heart function. The molecule that is substantially involved in mechanosensitivity is a stretch-activated ion channel. Among several ion channels believed to be activated by stretch in the heart, the stretch-activated KCa (SAKCA) channel, a member of the group of large conductance (Big Potassium, BK) channels, shows a mechanosensitive (MS) response to membrane stretch. As BK channels respond to voltage and intracellular calcium concentration with large conductance, they are considered to be involved in repolarization after depolarization. Some BK channels are known to be activated by stretch and are expressed in a number of cells, including human osteoblasts and guinea pig intestinal neurons. The SAKCA channel was found to be sensitive to stretch in the chick heart. Given that the cardiomyocyte is unremittingly exposed to contraction and dilation and that it generates action potential and its contractility is modulated by intracellular calcium concentration, the SAKCA channel, which is dependent voltage and calcium, may be involved in action potential generation. It was recently reported that a BK channel is involved in the modulation of heart rate in the mouse. Further studies regarding the role of MS BK channels, including SAKCA, in the modulation of heart rate and contractility are expected.

Hypoxic Pulmonary Vasoconstriction

It has been known for more than 60 years, and suspected for over 100, that alveolar hypoxia causes pulmonary vasoconstriction by means of mechanisms local to the lung. For the last 20 years, it has been clear that the essential sensor, transduction, and effector mechanisms responsible for hypoxic pulmonary vasoconstriction (HPV) reside in the pulmonary arterial smooth muscle cell. The main focus of this review is the cellular and molecular work performed to clarify these intrinsic mechanisms and to determine how they are facilitated and inhibited by the extrinsic influences of other cells. Because the interaction of intrinsic and extrinsic mechanisms is likely to shape expression of HPV in vivo, we relate results obtained in cells to HPV in more intact preparations, such as intact and isolated lungs and isolated pulmonary vessels. Finally, we evaluate evidence regarding the contribution of HPV to the physiological and pathophysiological processes involved in the transition from fetal to neonatal life, pulmonary gas exchange, high-altitude pulmonary edema, and pulmonary hypertension. Although understanding of HPV has advanced significantly, major areas of ignorance and uncertainty await resolution.

Exercício físico, receptores β-adrenérgicos e resposta vascular

O exercício aeróbio promove efeitos benéficos na prevenção e tratamento de doenças como hipertensão arterial, aterosclerose, insuficiência venosa e doença arterial periférica. Os receptores β-adrenérgicos estão presentes em várias células. No sistema cardiovascular, promovem inotropismo e cronotropismo positivo cardíaco e relaxamento vascular. Embora os efeitos do exercício tenham sido investigados em receptores cardíacos, estudos focados nos vasos são escassos e controversos. Esta revisão abordará os efeitos do exercício físico sobre os receptores β-adrenérgicos vasculares em modelos animais e humanos e os mecanismos celulares envolvidos na resposta relaxante. Em geral, os estudos mostram resultantes conflitantes, onde observam diminuição, aumento ou nenhum efeito do exercício físico sobre a resposta relaxante. Assim, os efeitos do exercício na sensibilidade β-adrenérgica vascular merecem maior atenção, e os resultados mostram que a área de fisiopatologia vascular é um campo aberto para a descoberta de novos compostos e avanços na prática clínica.

The effects of dopamine receptor agonists on BK Ca channels and signal transduction mechanism in corpus cavernosal smooth muscle cells

In this study, we investigated the effect of dopamine receptor agonists on potassium channels' activity and their signal transduction pathway in corporal smooth muscle cells. We used cultured human corporal smooth muscle cells. The whole cell and cell-attached configuration of the patch-clamp technique were used for electrophysiological recordings, and enzyme immunoassay was used for measuring cyclic AMP (cAMP) and cyclic GMP levels. Extracellular application of 10 microM dopamine and apomorphine significantly increased whole-cell K(+) currents by 283.5+/-55.7% (at +60 mV; n=12, P<0.001), 292.4+/-58.8.0% (at +60 mV; n=9, P<0.005), respectively. We confirmed that the increase in whole-cell currents was mainly due to activation of the tetraethylammonium-sensitive large conductance Ca(2+)-activated K(+) channels (BK(Ca) channels). Enzyme immunoassay indicated that dopamine and apomorphine stimulates cAMP levels in corporal smooth muscle cells in a concentration-dependent fashion. The activation of BK(Ca) channels by dopamine receptor agonists in corporal smooth muscle cells might be one of the mechanisms in inducing penile erection.

Involvement of protein kinase A in nitric oxide stimulating effect on a BK(Ca) channel of human dermal fibroblasts

We reported previously that a large-conductance Ca2+-activated K+ (BK(Ca)) channel constitutes a significant fraction of the K+ current in human dermal fibroblasts, and that nitric oxide (NO) increases the open-channel probability (NPo) of BK(Ca) channels via a soluble guanylate cyclase (sGC)/cyclic guanosine monophosphate (cGMP)/protein kinase G (PKG) pathway. The purpose of this study was to investigate whether the adenylate cyclase (AC)/cAMP/protein kinase A (PKA) pathway may also be involved in NO action on BK(Ca) channels in human dermal fibroblasts. Electrophysiological single-channel recordings were performed on fifth-passage cells of human penile skin cultures. KT5720 (specific PKA inhibitor) blocked the stimulatory effect of sodium nitroprusside (NO donor) on BK(Ca) channels. By contrast, forskolin (AC activator) or 8-bromo-cAMP (cell-permeable cAMP analog) did not increase the NPo of the channel. The PKA catalytic subunit (PKAcs) alone did not increase the NPo of the channel in cell-attached and inside-out patches, however, PKAcs with cGMP increased the NPo. In contrast, PKAcs with cGMP did not increase the NPo of BK(Ca) channels with 1H-[1,2,4]-oxadiazolo[4,3-a]quinoxalin-1-one pretreatment, and KT5720 pretreatment also blocked the stimulatory effect of 8-Br-cGMP. In conclusion, the present data suggest the involvement of PKA in the stimulatory effect of NO on the BK(Ca) channel in human dermal fibroblasts through cGMP.

RACK1 is a BKCa channel binding protein

The large conductance calcium-activated potassium channel, or BKCa channel, plays an important feedback role in a variety of physiological processes, including neurotransmitter release and smooth muscle contraction. Some reports have suggested that this channel forms a stable complex with regulators of its function, including several kinases and phosphatases. To further define such signaling complexes, we used the yeast two-hybrid system to screen a human aorta cDNA library for proteins that bind to the BKCa channel's intracellular, COOH-terminal “tail”. One of the interactors we identified is the protein receptor for activated C kinase 1 (RACK1). RACK1 is a member of the WD40 protein family, which also includes the G protein β-subunits. Consistent with an important role in BKCa-channel regulation, RACK1 has been shown to be a scaffolding protein that interacts with a wide variety of signaling molecules, including cSRC and PKC. We have confirmed the interaction between RACK1 and the BKCa channel biochemically with GST pull-down and coimmunoprecipitation experiments. We have observed some co-localization of RACK1 with the BKCa channel in vascular smooth muscle cells with immunocytochemical experiments, and we have found that RACK1 has effects on the BKCa channel's biophysical properties. Thus RACK1 binds to the BKCa channel and it may form part of a BKCa-channel regulatory complex in vascular smooth muscle.

Regulation of membrane excitability by intracellular pH (pHi) changers through Ca2+-activated K+ current (BK channel) in single smooth muscle cells from rabbit basilar artery

Employing microfluorometric system and patch clamp technique in rabbit basilar arterial myocytes, regulation mechanisms of vascular excitability were investigated by applying intracellular pH (pH(i)) changers such as sodium acetate (SA) and NH(4)Cl. Applications of caffeine produced transient phasic contractions in a reversible manner. These caffeine-induced contractions were significantly enhanced by SA and suppressed by NH(4)Cl. Intracellular Ca(2+) concentration ([Ca(2+)](i)) was monitored in a single isolated myocyte and based the ratio of fluorescence using Fura-2 AM (R (340/380)). SA (20 mM) increased and NH(4)Cl (20 mM) decreased R (340/380) by 0.2 +/- 0.03 and 0.1 +/- 0.02, respectively, in a reversible manner. Caffeine (10 mM) transiently increased R (340/380) by 0.9 +/- 0.07, and the ratio increment was significantly enhanced by SA and suppressed by NH(4)Cl, implying that SA and NH(4)Cl may affect [Ca(2+)](i) (p < 0.05). Accordingly, we studied the effects of SA and NH(4)Cl on Ca(2+)-activated K(+) current (IK(Ca)) under patch clamp technique. Caffeine produced transient outward current at holding potential (V (h)) of 0 mV, caffeine induced transient outward K(+) current, and the spontaneous transient outward currents were significantly enhanced by SA and suppressed by NH(4)Cl. In addition, IK(Ca) was significantly increased by acidotic condition when pH(i) was lowered by altering the NH(4)Cl gradient across the cell membrane. Finally, the effects of SA and NH(4)Cl on the membrane excitability and basal tension were studied: Under current clamp mode, resting membrane potential (RMP) was -28 +/- 2.3 mV in a single cell level and was depolarized by 13 +/- 2.4 mV with 2 mM tetraethylammonium (TEA). SA hyperpolarized and NH(4)Cl depolarized RMP by 10 +/- 1.9 and 16 +/- 4.7 mV, respectively. SA-induced hyperpolarization and relaxation of basal tension was significantly inhibited by TEA. These results suggest that SA and NH(4)Cl might regulate vascular tone by altering membrane excitability through modulation of [Ca(2+)](i) and Ca(2+)-activated K channels in rabbit basilar artery.

Direct modulation of Ca(2+)-activated K(+) current by H-89 in rabbit coronary arterial smooth muscle cells

The effects of H-89, a potent and selective inhibitor of protein kinase A (PKA) on Ca(2+)-activated K(+) (BK(Ca)) channels in coronary arterial smooth muscle cells were examined using a patch-clamp technique. In inside-out configuration, H-89 increased the NP(o) of the BK(Ca) channel, but it reduced the dwell time of BK(Ca) currents. In whole-cell configuration, H-89 markedly increased BK(Ca) currents in a concentration-dependent manner. The EC(50) was 0.470+/-0.0741 microM based on dwell time, 0.582+/-0.0691 microM based on the NP(o), and 0.519+/-0.0295 microM based on the whole-cell current, respectively. H-85, which is an inactive form of H-89, increased BK(Ca) currents, similar to the result of H-89. The other PKA inhibitors (Rp-8-CPT-cAMPs and KT 5720) and protein phosphatase inhibitor (okadaic acid, 1 microM) had little effect on BK(Ca) currents and did not significantly alter the stimulatory effects of 1 microM H-89. These findings suggest that H-89 increases the BK(Ca) current independently of PKA.

New insights into beta-adrenoceptors in smooth muscle: distribution of receptor subtypes and molecular mechanisms triggering muscle relaxation

1. The beta-adrenoceptor is currently classified into beta(1), beta(2) and beta(3) subtypes and all three subtypes are expressed in smooth muscle. Each beta-adrenoceptor subtype exhibits tissue-specific distribution patterns, which may be a determinant controlling the mechanical functions of corresponding smooth muscle. Airway and uterine smooth muscles abundantly express the beta(2)-adrenoceptor, the physiological significance of which is established as a fundamental regulator of the mechanical activities of these muscles. Recent pharmacomechanical and molecular approaches have revealed roles for the beta(3)-adrenoceptor in the gastrointestinal tract and urinary bladder smooth muscle. 2. The beta-adrenoceptor is a G(s)-protein-coupled receptor and its activation elevates smooth muscle cAMP. A substantial role for a cAMP-dependent mechanism(s) is generally believed to be the key trigger for eliciting beta-adrenoceptor-mediated relaxation of smooth muscle. Downstream effectors activated via a cAMP-dependent mechanism(s) include plasma membrane K(+) channels, such as the large-conductance, Ca(2+)-activated K(+) (MaxiK) channel. 3. Beta-Adrenoceptor-mediated relaxant mechanisms also include cAMP-independent signalling pathways. This view is supported by numerous pharmacological and electrophysiological lines of evidence. In airway smooth muscle, direct activation of the MaxiK channel by G(s)alpha is a mechanism by which stimulation of beta(2)-adrenoceptors elicits muscle relaxation independently of the elevation of cAMP. 4. The cAMP-independent mechanism(s) is also substantial in beta(3)-adrenoceptor-mediated relaxation of gastrointestinal tract smooth muscle. However, in the case of the beta(3)-adrenoceptor, a delayed rectified K(+) channel rather than the MaxiK channel seems to mediate, in part, cAMP-independent relaxant mechanisms. 5. In the present article, we review the distribution of beta-adrenoceptor subtypes in smooth muscle tissues and discuss the molecular mechanisms by which each subtype elicits muscle relaxation, focusing on the roles of cAMP and plasma membrane K(+) channels.

Ketamine blocks Ca2+-activated K+ channels in rabbit cerebral arterial smooth muscle cells

Although ketamine and Ca2+-activated K+ (KCa) channels have been implicated in the contractile activity regulation of cerebral arteries, no studies have addressed the specific interactions between ketamine and the KCa channels in cerebral arteries. The purpose of this study was to examine the direct effects of ketamine on KCa channel activities using the patch-clamp technique in single-cell preparations of rabbit middle cerebral arterial smooth muscle. We tested the hypothesis that ketamine modulates the KCa channel activity of the cerebral arterial smooth muscle cells of the rabbit. Vascular myocytes were isolated from rabbit middle cerebral arteries using enzymatic dissociation. Single KCa channel activities of smooth muscle cells from rabbit cerebral arteries were recorded using the patch-clamp technique. In the inside-out patches, ketamine in the micromolar range inhibited channel activity with a half-maximal inhibition of the ketamine concentration value of 83.8 +/- 12.9 microM. The Hill coefficient was 1.2 +/- 0.3. The slope conductance of the current-voltage relationship was 320.1 +/- 2.0 pS between 0 and +60 mV in the presence of ketamine and symmetrical 145 mM K+. Ketamine had little effect on either the voltage-dependency or open- and closed-time histograms of KCa channel. The present study clearly demonstrates that ketamine inhibits KCa channel activities in rabbit middle cerebral arterial smooth muscle cells. This inhibition of KCa channels may represent a mechanism for ketamine-induced cerebral vasoconstriction.

cGMP-dependent and not cAMP-dependent kinase is required for adenosine-induced dilation of intracerebral arterioles

Adenosine (ADO) is a potent cerebral vasodilator and has been proposed as a metabolic regulator of cerebral blood flow. However, the signal transduction pathway by which ADO causes vasodilation in cerebral microvessels is currently unknown. The current study was designed to investigate the role of cyclic nucleotides and cyclic nucleotide-dependent protein kinases in ADO-induced dilation of resistance-sized rat cerebral arterioles that develop spontaneous tone. Arterioles were cannulated and perfused intraluminally at constant flow (2 microl/min) and pressure (60 mm Hg). ADO (29.7 +/- 2.0%; 1 microM), CGS-21680 (16 +/- 4%, 1 microM), 8-bromo-cyclic guanosine monophosphate (8 Br-cGMP; 29.9 +/- 3.9%; 100 microM), sodium nitroprusside (SNP; 30.6 +/- 3.3%, 1 microM), cyclic guanine monophosphate-dependent protein kinase activator (Sp-8-pCPT-cGMPS, 25.9 +/- 4.2%; 10 microM), forskolin (30.5 +/- 5.9%; 0.1 microM), and pH 6.8 all produced large dilations. The selective cGMP-dependent protein kinase inhibitor, Rp-8-pCPT-cGMPS (10 microM), had no effect on resting diameter or reactivity to acidic pH, but significantly ( < 0.05) attenuated arteriolar dilations to ADO (59%, n = 8), CGS-21680 (60%, n = 4), SNP (62%, n = 3), 8 Br-cGMP (88%, n = 3), and Sp-8-pCPT-cGMPS (98%, n = 3). H8, the less-selective cyclic nucleotide-dependent protein kinase inhibitor, had similar effects as Rp-8-pCPT-cGMPS. Additionally, the inhibitor of the soluble guanylate cyclase, 1H-[1,24]oxadiazolo-[4,3-a]quinoxalin-1-one (ODQ), blocked the response to SNP (70% inhibition) and significantly inhibited the ADO response (43% inhibition). In contrast, inhibition of the cyclic ADO monophosphate (cAMP)-dependent protein kinase Rp-8-CPT-cAMPS had no effect on the ADO, SNP, or pH responses, but significantly blocked forskolin-induced vasodilation (53%). It is concluded that ADO-induced vasodilation in cerebral microvessels, at least in part, involves cGMP and cGMP-dependent protein kinase, but not cAMP or cAMP-dependent kinase. Our data therefore provides a new insight into mechanisms by which ADO invokes vasodilation in cerebral microvascular arterioles.

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.

Protein kinases: tuners of the BKCa channel in smooth muscle

Large-conductance, Ca(2+)-activated K(+) (BK(Ca)) channels in smooth muscle cells are unique because they integrate changes in both intracellular Ca(2+) and membrane potential. Protein kinases such as cAMP-dependent protein kinase, cGMP-dependent protein kinase and protein kinase C can affect tissue function by 'tuning' the apparent Ca(2+)- and/or voltage-sensitivity of the BK(Ca) channel to physiological changes in both Ca(2+) concentrations and membrane potential. However, despite the central importance of kinase-mediated modulation of BK(Ca) channels in different smooth muscle tissues, many key issues, including the sites and mechanisms of actions of protein kinases, remain unresolved. In this article, the role of protein kinases in the regulation of BK(Ca) channels is discussed.

Involvement of both G protein alphas and beta gamma subunits in beta-adrenergic stimulation of vascular L-type Ca(2+) channels

1. Previous data have shown that activation of beta(3)-adrenoceptors stimulates vascular L-type Ca(2+) channels through a G alphas-induced stimulation of the cyclic AMP/PKA pathway. The present study investigated whether beta-adrenergic stimulation also uses the G beta gamma/PI3K/PKC pathway to modulate L-type Ca(2+) channels in rat portal vein myocytes. 2. Peak Ba(2+) current (I(Ba)) measured using the whole-cell patch clamp method was maximally increased by application of 10 microm isoprenaline after blockade of beta(3)-adrenoceptors by 1 microM SR59230A. Under these conditions, the isoprenaline-induced stimulation of I(Ba) was reversed by ICI-118551 (a specific beta(2)-adrenoceptor antagonist) but not by atenolol (a specific beta(1)-adrenoceptor antagonist). The beta(2)-adrenoceptor agonist salbutamol increased I(Ba), an effect which was reversed by ICI-118551 whereas the beta(1)-adrenoceptor agonist dobutamine had no effect on I(Ba). 3. Application of PKA inhibitors (H-89 and Rp 8-Br-cyclic AMPs) or a PKC inhibitor (calphostin C) alone did not affect the beta(2)-adrenergic stimulation of I(Ba) whereas simultaneous application of both PKA and PKC inhibitors completely blocked this stimulation. 4. The beta(2)-adrenergic stimulation of L-type Ca(2+) channels was blocked by a pre-treatment with cholera toxin and by intracellular application of an anti-G alphas antibody (directed against the carboxyl terminus of G alphas). In the presence of H-89, intracellular infusion of an anti-Gss(com) antibody or a beta ARK(1) peptide as well as a pre-treatment with wortmannin (a PI3K inhibitor) blocked the beta(2)-adrenergic stimulation of I(Ba). 5. These results suggest that the beta(2)-adrenergic stimulation of vascular L-type Ca(2+) channels involves both G alphas and G beta gamma subunits which exert their stimulatory effects through PKA and PI3K/PKC pathways, respectively.

Involvement of BK(Ca) channels in the relaxation of detrusor muscle via beta-adrenoceptors

Detrusor muscle relaxes upon activation of beta-adrenoceptors on smooth muscle cells. However, the mechanism of relaxation following the stimulation of beta-adrenoceptors remains unclear. In order to clarify the mechanism, we investigated the involvement of ion channels in bladder relaxation. In guinea-pig isolated bladder strips precontracted by high-K(+), isoproterenol caused concentration-dependent relaxation. The relaxation caused by isoproterenol (1 microM) was larger in 30 mM K(+) than in 120 mM K(+) (54.2+/-8.0% and 18.2+/-4.1% of papaverine-induced relaxation, respectively, n=4). Iberiotoxin (100 nM) inhibited the isoproterenol-induced relaxation (vehicle 69.5+/-8.0% vs. iberiotoxin 24.9+/-6.2%, respectively, n=5). Whole-cell patch-clamp recording revealed that isoproterenol as well as forskolin increased the iberiotoxin-sensitive K(+) currents, and this increase was abolished by protein kinase inhibitor. These results suggest that the isoproterenol-induced relaxation of guinea-pig bladder smooth muscle is mainly mediated by facilitation of BK(Ca) channels subsequent to the activation of the cAMP/protein kinase A pathway.

High-calcium diet enhances vasorelaxation in nitric oxide-deficient hypertension

Because the effects of calcium supplementation on arterial tone in nitric oxide-deficient hypertension are unknown, we investigated the influence of elevating dietary calcium from 1.1 to 3.0% in Wistar rats treated with N(G)-nitro-L-arginine methyl ester (L-NAME; 20 mg. kg(-1). day(-1)) for 8 wk. A high-calcium diet attenuated the development of hypertension induced by L-NAME and abrogated the associated impairments of endothelium-independent mesenteric arterial relaxations to nitroprusside, isoproterenol, and cromakalim. Endothelium-dependent relaxations to acetylcholine during nitric oxide synthase inhibition in vitro were decreased in L-NAME rats and improved by calcium supplementation. The inhibition of cyclooxygenase by diclofenac augmented the responses to acetylcholine in L-NAME rats but not in calcium + L-NAME rats. When hyperpolarization of smooth muscle was prevented by KCl precontraction, the responses to acetylcholine during combined nitric oxide synthase and cyclooxygenase inhibition were similar in all groups. Furthermore, superoxide dismutase enhanced the acetylcholine-induced relaxations in L-NAME rats but not in calcium + L-NAME rats. In conclusion, calcium supplementation reduced blood pressure during chronic nitric oxide synthase inhibition and abrogated the associated impairments in endothelium-dependent and -independent arterial relaxation. The augmented vasorelaxation after increased calcium intake in L-NAME hypertension may be explained by enhanced hyperpolarization and increased sensitivity to nitric oxide in arterial smooth muscle and decreased vascular production of superoxide and vasoconstrictor prostanoids.

cAMP-dependent vasodilators cross-activate the cGMP-dependent protein kinase to stimulate BK(Ca) channel activity in coronary artery smooth muscle cells

cAMP-dependent vasodilators are used to treat a variety of cardiovascular disorders; however, the signal transduction pathways and effector mechanisms stimulated by these agents are not fully understood. In the present study we demonstrate that cAMP-stimulating agents enhance the activity of the large-conductance, calcium-activated potassium (BK(Ca)) channel in single myocytes from coronary arteries by "cross-activation" of the cGMP-dependent protein kinase (protein kinase G, PKG). Single-channel patch-clamp data revealed that 10 micromol/L isoproterenol, forskolin, or dopamine opens BK(Ca) channels in coronary myocytes and that this effect is attenuated by inhibitors of PKG (KT5823; Rp-8-pCPT-cGMPS), but not by inhibiting the cAMP-dependent protein kinase (protein kinase A, PKA). In addition, a membrane-permeable analog, CPT-cAMP, also opened BK(Ca) channels in these myocytes, and this effect was reversed by KT5823. Direct biochemical measurement confirmed that dopamine or forskolin stimulates PKG activity in coronary arteries but does not elevate cGMP. Finally, the stimulatory effect of cAMP on BK(Ca) channels was reconstituted in a cell-free, inside-out patch by addition of purified PKG activated by either cGMP or cAMP. In contrast, channel gating was unaffected by exposure to the purified catalytic subunit of PKA. In summary, findings from on-cell and cell-free patch-clamp experiments provide direct evidence that cAMP-dependent vasodilators open BK(Ca) channels in coronary myocytes by cross-activation of PKG (but not via PKA). Biochemical assay confirmed this cross-activation mechanism of cAMP action in these arteries. This signaling pathway is a novel mechanism for regulation of potassium channel activity in vascular smooth muscle and other cells.

A novel transduction mechanism mediating dopamine-induced vascular relaxation: opening of BKCa channels by cyclic AMP-induced stimulation of the cyclic GMP-dependent protein kinase

Dopamine dilates the coronary, renal and other vascular beds; however, the signaling pathway underlying this effect is unclear. In this study the signal-transduction process mediating dopamine-induced relaxation of porcine coronary arteries was investigated in isolated vessels and single arterial myocytes. Dopamine-induced relaxation of arteries was mediated through the DA- receptor and involved K+ efflux, and subsequent patch-clamp studies demonstrated that either dopamine or fenoldopam, a selective DA-1 agonist, increased the opening probability of the large-conductance, calcium- and voltage-activated K+ (BKCa) channel in coronary myocytes. Moreover, blockade of this channel by iberiotoxin prevented dopamine-induced coronary relaxation. Dopamine stimulation of BKCa channels was completely prevented by a DA-1-receptor antagonist, but was unaffected by propranolol. Furthermore, inhibiting adenylyl cyclase activity prevented stimulation of BKCa channel activity, whereas chlorophenylthio (CPT)-cyclic adenosine monophosphate (AMP), a membrane-permeable analog of cyclic AMP, mimicked the effects of dopamine. Interestingly, inhibiting the cyclic AMP-dependent protein kinase (PKA) did not affect the response to dopamine, whereas dopamine-induced channel activity was completely blocked by inhibiting the activity of the cyclic guanosine monophosphate (GMP)-dependent protein kinase (PKG). These findings demonstrate that activation of DA-1 receptors causes stimulation of BKCa channel activity by a mechanism involving cyclic AMP-dependent stimulation of PKG, but not PKA, and further suggest that this cross-reactivity mediates dopamine-induced coronary vasodilation.

Potassium channel-mediated vasorelaxation is impaired in experimental renal failure

Chronic renal failure is associated with increased cardiovascular morbidity and abnormal arterial tone, but the underlying pathophysiological mechanisms are poorly understood. Therefore, we studied the responses of isolated mesenteric arterial rings from Wistar-Kyoto rats in standard organ chambers 6 wk after subtotal (5/6) nephrectomy or sham operation. Subtotal nephrectomy resulted in a 1.7-fold elevation of plasma urea nitrogen, whereas blood pressure was not significantly affected. Endothelium-mediated relaxations of norepinephrine-precontracted rings to ACh were impaired in renal failure rats. The nitric oxide (NO) synthase inhibitor N(G)-nitro-L-arginine methyl ester inhibited relaxations to ACh more effectively in the renal failure group, whereas the cyclooxygenase inhibitor diclofenac did not significantly affect the response in either group. Inhibition of Ca(2+)-activated K(+) channels by charybdotoxin and apamin attenuated NO synthase- and cyclooxygenase-resistant relaxations to ACh in control but not renal failure rats and abolished the difference between these groups. Endothelium-independent relaxations to isoproterenol and cromakalim, vasodilators acting via beta-adrenoceptors and ATP-sensitive K(+) channels, respectively, were impaired in the renal failure group, whereas relaxations to the NO donor nitroprusside were similar in both groups. In conclusion, endothelium-mediated relaxation in renal failure rats was impaired in the absence and presence of NO synthase and cyclooxygenase inhibition but not with prevented smooth muscle hyperpolarization. Endothelium-independent relaxations to isoproterenol and cromakalim were also attenuated after 5/6 nephrectomy. These results suggest that impaired vasodilatation in experimental renal failure could be attributed to reduced relaxation via arterial K(+) channels.

Role of a novel KCa opener in regulating K+ channels of hypoxic human pulmonary vascular cells

Hypoxic pulmonary vasoconstriction (HPVC) is mediated, in part, via membrane depolarization and inhibition of K+ channels. We recently observed that the naturally occurring steroid dehydroepiandrosterone (DHEA) reversed and prevented HPVC in isolated perfused and ventilated ferret lungs. In the current study, we investigated the effects of DHEA on the major K+ channels of chronically hypoxic human pulmonary smooth-muscle cells (HPSMC). K+ channels were recorded by using the patch-clamp technique in whole-cell and single-channel configurations. Single-channel recordings were performed in inside-out and outside-out excised patches, and in intact HPSMC in cell-attached configuration. Using whole-cell current recording, chronic hypoxia decreased the high-amplitude, high-noise, and charybdotoxin-sensitive Ca2+-dependent K+ channels (KCa). DHEA reversed the effect of chronic hypoxia on KCa, but had no effect on the low-amplitude, low-noise, and 4-aminopyridine-sensitive delayed rectifying K+ channels. In the cell-attached configuration, chronic hypoxia caused a decrease in KCa sensitivity to membrane potential (Em). DHEA reversed the effect of hypoxia on KCa sensitivity to Em and caused a mean of 40-mV left shift in voltage-dependent activation of KCa. DHEA increased KCa activation from both sides of membrane patches of hypoxic HPSMC via a cyclic adenosine monophosphate- and cyclic guanosine monophosphate-independent pathway. We concluded that DHEA is a novel KCa opener of the human pulmonary vasculature.

Losartan and enalapril therapies enhance vasodilatation in the mesenteric artery of spontaneously hypertensive rats

We studied the effects of 10-week long enalapril and losartan treatments (4 and 15 mg kg(-1) day(-1), respectively) on mesenteric arterial function in vitro in spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY). The relaxations of noradrenaline-precontracted rings to acetylcholine, nitroprusside and cromakalim were similar in WKY and enalapril- and losartan-treated SHR, and more pronounced than in untreated SHR. The responses to acetylcholine were attenuated by N(G)-nitro-L-arginine methyl ester in WKY and drug-treated SHR, but were completely inhibited in untreated SHR. When hyperpolarization of smooth muscle was prevented by KCl-induced precontractions, no differences were found in the relaxations to acetylcholine and nitroprusside between the groups, and the dilatations to cromakalim were abolished. Moreover, in noradrenaline-precontracted rings of drug-treated SHR, the addition of tetraethylammonium attenuated the nitric oxide synthase and cyclooxygenase-resistant relaxations to acetylcholine and abolished the enhanced dilatations to nitroprusside. In conclusion, since the enhancement of vasorelaxation in enalapril- and losartan-treated SHR was abolished by conditions preventing hyperpolarization, the improved vasodilatation following these therapies could be attributed to enhanced vasodilatation via K+ channels in this model of hypertension.

Influence of chronic ethanol consumption on arterial tone in young and aged rats

The aim of this work was to evaluate the effects of long-term ethanol consumption on arterial responses in vitro in young and aged rats. Therefore, Wistar rats (ages 3 and 29 mo, respectively) were allocated to six groups: control-young, sucrose-young, ethanol-young, control-aged, sucrose-aged, and ethanol-aged. The ethanol-fed groups were given 25% ethanol by intragastric gavage three times a day 4 days a week. Responses of mesenteric arterial rings were examined in standard organ chambers after 5 treatment weeks. In norepinephrine-precontracted arterial rings, endothelium-dependent relaxations to acetylcholine, as well as endothelium-independent relaxations to isoproterenol, were attenuated in aged rats when compared with young controls. Relaxation responses to isoproterenol, but not to acetylcholine and nitroprusside, were clearly improved by ethanol treatment in both young and aged rats. The cyclooxygenase inhibitor diclofenac, which reduces the synthesis of dilating and constricting prostanoids, enhanced the relaxation to acetylcholine in all three aged rat groups but was without significant effect in the young rats. In the presence of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester the relaxation to acetylcholine in control and sucrose-fed aged rats was markedly reduced compared with control rats, whereas in the young controls and in both young and aged ethanol-exposed groups, distinct relaxations to higher concentrations of acetylcholine were still present. The endothelium-independent relaxations to cromakalim, a hyperpolarizing vasodilator acting via ATP-sensitive potassium channels, were also markedly augmented by ethanol feeding in both young and aged rats. In conclusion, ethanol consumption in both young and aged rats was associated with markedly improved arterial relaxations to isoproterenol and cromakalim, as well as clearly augmented relaxation to acetylcholine during inhibition of cyclooxygenase and nitric oxide synthase. These findings suggest that especially the potassium channel-related component of arterial relaxation was augmented by long-term ethanol exposure.

Force, membrane potential and cytoplasmic Ca2+ responses to cyclic nucleotides in rat anococcygeus muscle

Simultaneous recordings of membrane potential and force, and cytoplasmic calcium ([Ca2+]i) and force were made in rat anococcygeus to determine whether membrane hyperpolarisation plays a role in cyclic nucleotide-induced relaxation. In the presence of phenylephrine (0.2 microM), which evoked sustained contraction, an elevation in [Ca2+]i, and depolarisation, nitroprusside (5 microM) caused 96+/-3% relaxation, 77+/-3% decrease in suprabasal [Ca2+]i, and 16+/-2 mV hyperpolarisation. Forskolin (1 microM) caused 98+/-1% relaxation, 92+/-2% decrease in suprabasal [Ca2+]i, and 18+/-1 mV hyperpolarisation. These responses persisted in the presence of a variety of K+ channel blockers or in ouabain. The decrease in [Ca2+]i preceded the commencement of relaxation whereas the onset of hyperpolarisation lagged behind. Thus, cyclic nucleotide-mediated relaxation in rat anococcygeus is not dependent on hyperpolarisation mediated by the opening of K+ channels. Rather, it is suggested that the decrease in [Ca2+]i gives rise to hyperpolarisation, which reflects a decline in the Ca2+ dependent conductance(s) activated by phenylephrine.

K+ channel modulation in arterial smooth muscle

Potassium channels play an essential role in the membrane potential of arterial smooth muscle, and also in regulating contractile tone. Four types of K+ channel have been described in vascular smooth muscle: Voltage-activated K+ channels (Kv) are encoded by the Kv gene family, Ca(2+)-activated K+ channels (BKCa) are encoded by the slo gene, inward rectifiers (KIR) by Kir2.0, and ATP-sensitive K+ channels (KATP) by Kir6.0 and sulphonylurea receptor genes. In smooth muscle, the channel subunit genes reported to be expressed are: Kv1.0, Kv1.2, Kv1.4-1.6, Kv2.1, Kv9.3, Kv beta 1-beta 4, slo alpha and beta, Kir2.1, Kir6.2, and SUR1 and SUR2. Arterial K+ channels are modulated by physiological vasodilators, which increase K+ channel activity, and vasoconstrictors, which decrease it. Several vasodilators acting at receptors linked to cAMP-dependent protein kinase activate KATP channels. These include adenosine, calcitonin gene-related peptide, and beta-adrenoceptor agonists. beta-adrenoceptors can also activate BKCa and Kv channels. Several vasoconstrictors that activate protein kinase C inhibit KATP channels, and inhibition of BKCa and Kv channels through PKC has also been described. Activators of cGMP-dependent protein kinase, in particular NO, activate BKCa channels, and possibly KATP channels. Hypoxia leads to activation of KATP channels, and activation of BKCa channels has also been reported. Hypoxic pulmonary vasoconstriction involves inhibition of Kv channels. Vasodilation to increased external K+ involves KIR channels. Endothelium-derived hyperpolarizing factor activates K+ channels that are not yet clearly defined. Such K+ channel modulations, through their effects on membrane potential and contractile tone, make important contributions to the regulation of blood flow.

Potassium channels mediate dilatation of cerebral arterioles in response to arachidonate

We tested the hypothesis that cerebral vasodilatation in response to arachidonate is dependent on activation of cyclooxygenase and cytochrome P-450 pathways and formation of endogenous reactive oxygen species and is mediated by activation of potassium channels. The diameter of cerebral arterioles was measured using cranial windows in anesthetized rats. Under control conditions [baseline diameter = 45 +/- 1 micrometer (mean +/- SE)], arachidonate (1-100 microM) and papaverine (10-50 microM) produced concentration-dependent vasodilatation. Cerebral vasodilator responses to arachidonate, but not papaverine, were abolished during topical application of indomethacin (10 microM, an inhibitor of cyclooxygenase) or catalase (100 U/ml, which inactivates hydrogen peroxide). In contrast, clotrimazole (10 microM) and 17-ODYA (20 microM), inhibitors of cytochrome P-450 activity, had no effect on dilator responses of cerebral arterioles to arachidonate. Superoxide dismutase (SOD, 100 U/ml) had no effect on vasodilator responses to papaverine or lower concentrations of arachidonate, whereas dilator responses to 100 microM arachidonate were inhibited modestly (by 22%) by SOD. Similarly, deferoxamine (1 mM) partly inhibited dilator responses to 10 and 100 microM arachidonate (by approximately 30% at each concentration). Tetraethylammonium ion (1 mM) or iberiotoxin (50 nM), inhibitors of calcium-activated potassium channels, markedly inhibited vasodilatation in response to arachidonate (by 70-90%) but not papaverine. These findings suggest that dilatation of cerebral arterioles in response to arachidonate is mediated largely by endogenously formed reactive oxygen species, which are generated from cyclooxygenase activity, and activation of calcium-activated potassium channels. Thus activation of potassium channels appears to be a major mechanism of cerebral vasodilatation in response to reactive oxygen species produced endogenously.

Beta-adrenoceptor activation and PKA regulate delayed rectifier K+ channels of vascular smooth muscle cells

Macroscopic 4-aminopyridine (4-AP)-sensitive, delayed rectifier K+ current of vascular smooth muscle cells is increased during beta-adrenoceptor activation with isoproterenol via a signal transduction pathway involving adenylyl cyclase and cAMP-dependent protein kinase (PKA) (Aiello, E. A., M. P. Walsh, and W. C. Cole. Am. J. Physiol. 268 (Heart Circ. Physiol. 37): H926-H934, 1995.). In this study, we identified the single delayed rectifier K+ (KDR) channel(s) of rabbit portal vein myocytes affected by treatment with isoproterenol or the catalytic subunit of PKA. 4-AP-sensitive KDR channels of 15.3 +/- 0.6 pS (n = 5) and 14.8 +/- 0.6 pS (n = 5) conductance, respectively, were observed in inside-out (I-O) and cell-attached (C-A) membrane patches in symmetrical KCl recording conditions. The kinetics of activation (time constant of 10.7 +/- 3. 02 ms) and inactivation (fast and slow time constants of 0.3 and 2.5 s, respectively) of ensemble currents produced by these channels mimicked those reported for inactivating, 4-AP-sensitive whole cell KDR current of vascular myocytes. Under control conditions, the open probability (NPo) of KDR channels of C-A membrane patches at -40 mV was 0.014 +/- 0.005 (n = 8). Treatment with 1 microM isoproterenol caused a significant, approximately threefold increase in NPo to 0. 041 +/- 0.02 (P < 0.05). KDR channels of I-O patches exhibited rundown after approximately 5 min, which was not affected by ATP (5 mM) in the bath solution. Treatment with the purified catalytic subunit of PKA (50 nM; 5 mM ATP) restored KDR channel activity and caused NPo to increase from 0.011 +/- 0.003 to 0.138 +/- 0.03 (P < 0. 05; n = 11). These data indicate that small-conductance, 15-pS KDR channels are responsible for inactivating the macroscopic delayed rectifier K+ current of rabbit portal vein myocytes and that the activity of these channels is enhanced by a signal transduction mechanism involving beta-adrenoceptors and phosphorylation by PKA at a membrane potential consistent with that observed in the myocytes in situ.

The effect of subarachnoid hemorrhage on mechanisms of vasodilation mediated by cyclic adenosine monophosphate

OBJECT

This study was designed to determine whether subarachnoid hemorrhage (SAH) affects the function of the K+ channels responsible for relaxation of canine cerebral arteries in response to adenylate cyclase activation.

METHOD

The effect of K+ channel inhibitors on the arterial relaxation response to forskolin, a direct adenylate cyclase activator, was studied in rings of basilar arteries obtained from normal dogs and dogs in which SAH was induced (double-hemorrhage model). The levels of adenosine 3',5'-cyclic monophosphate (cAMP) were measured using the radioimmunoassay technique. In rings with the endothelium removed, relaxation induced by forskolin was not affected by SAH. The relaxation response to forskolin was reduced by charybdotoxin (10(-7) mol/L), a selective Ca++-activated K+ channel inhibitor, in normal arteries and arteries subjected to autologous blood injection. This inhibitory effect of charybdotoxin was significantly greater in arteries involved in SAH than in normal vessels. The relaxation response to forskolin was reduced by 4-aminopyridine (10(-3) mol/L), a delayed rectifier K+ channel inhibitor, only in arteries involved in SAH. In contrast, the relaxation response to forskolin was not affected by glyburide (10(-5) mol/L), an adenosine 5'-triphosphate-sensitive K+ channel inhibitor, in both normal and SAH arteries. Forskolin (3 x 10(-7) mol/L) produced an approximately 10-fold increase in levels of cAMP. The basal values and increased levels of cAMP detected after stimulation with forskolin were no different in normal arteries and those exposed to SAH.

CONCLUSIONS

These results demonstrate that formation of cAMP and the relaxation response to adenylate cyclase activation are not affected by SAH. However, in diseased arteries, K+ channels assume a more important role in the mediation of relaxation response to forskolin, indicating that SAH may change the mechanisms responsible for vasodilation induced by cAMP.

Reconstitution of beta-adrenergic modulation of large conductance, calcium-activated potassium (maxi-K) channels in Xenopus oocytes. Identification of the camp-dependent protein kinase phosphorylation site

The human large conductance, calcium-activated potassium (maxi-K) channel (alpha and beta subunits) and beta2-adrenergic receptor genes were coexpressed in Xenopus oocytes in order to study the mechanism of beta-adrenergic modulation of channel function. Isoproterenol and forskolin increased maxi-K potassium channel currents in voltage-clamped oocytes expressing the receptor and both channel subunits by 33 +/- 5% and 35 +/- 8%, respectively, without affecting current activation or inactivation. The percentage of stimulation by isoproterenol and forskolin was not different in oocytes coexpressing the alpha and beta subunits versus those expressing the only the alpha subunit, suggesting that the alpha subunit is the target for regulation. The stimulatory effect of isoproterenol was almost completely blocked by intracellular injection of the cyclic AMP dependent protein kinase (cAMP-PK) regulatory subunit, whereas injection of a cyclic GMP dependent protein kinase inhibitory peptide had little effect, indicating that cellular coupling of beta2-adrenergic receptors to maxi-K channels involves endogenous cAMP-PK. Mutation of one of several potential consensus cAMP-PK phosphorylation sites (serine 869) on the alpha subunit almost completely inhibited beta-adrenergic receptor/channel stimulatory coupling, whereas forskolin still stimulated currents moderately (16 +/- 4%). These data demonstrate that physiological coupling between beta2 receptors and maxi-K channels occurs by the cAMP-PK mediated phosphorylation of serine 869 on the alpha subunit on the channel.

Role of endothelium and K+ channels in dobutamine-induced relaxation in rat mesenteric artery

1. In order to examine the possible involvement of the endothelium and K+ channel activation in the relaxation induced by dobutamine, a beta 1-adrenoceptor agonist, in rat isolated mesenteric arteries, the effects of inhibitors of nitric oxide (NO) activity, blockers of K+ channels and high extracellular K+ were studied by measuring isometric tension in both endothelium-intact and -denuded arteries. 2. Dobutamine inhibited the phenylephrine (PE)-induced sustained tension with a pEC50 of 7.40 +/- 0.08 in endothelium-intact arteries. Removal of functional endothelium attenuated the effect of dobutamine. The relaxation induced by dobutamine was inhibited by the beta 1-adrenoceptor antagonist CGP 20712A (3 mumol/L) but not by the beta 2-adrenoceptor antagonist ICI 118,551 (3 mumol/L) in endothelium-denuded arteries. 3. Pretreatment with NG-nitro-L-arginine (L-NNA; 100 mumol/L) or methylene blue (3 mumol/L) induced a similar degree of inhibition of the dobutamine-induced relaxation in endothelium-intact arteries, while NG-nitro-D-arginine (100 mumol/L) and indomethacin (10 mumol/L) had no effect. In contrast, pretreatment with L-NNA (100 mumol/L) did not affect the relaxation induced by sodium nitroprusside (SNP) or forskolin. Methylene blue (3 mumol/L) inhibited the relaxant response to SNP. 4. Charybdotoxin (CTX; 100 nmol/L), iberiotoxin (IBX; 100 nmol/L) and tetraethylammonium ions (TEA+; 3 mmol/L) significantly reduced the dobutamine-induced relaxation. Tetrapentylammonium ions (TPA+; 5 mumol/L) markedly inhibited the relaxant effect of dobutamine. The pEC50 values for control and in the presence of TPA+ in endothelium-intact arteries were 7.35 +/- 0.11 and 6.14 +/- 0.17, respectively, and 6.35 +/- 0.09 and 5.87 +/- 0.17 for control and in the presence of TPA+ in endothelium-denuded arteries, respectively. In contrast, glibenclamide (3 mumol/L) was ineffective. At 5 mumol/L, TPA+ also inhibited the relaxation induced by forskolin. 5. The maximal relaxation of PE-contracted arteries induced by 3 mumol/L dobutamine was completely abolished in the 60 mmol/L K(+)-contracted arteries with and without endothelium, while dobutamine at a concentration greater than 3 mumol/L induced inhibition of the high-K+ response. 6. The present results indicate that endothelium, probably NO but not prostacyclin, was involved in the dobutamine-induced relaxation in rat mesenteric arteries. Activation of CTX-, IBX- and TPA(+)-sensitive K+ channels contributed towards the observed relaxation. Loss of the ability to relax the 60 mmol/L K(+)-contracted arteries suggests that endothelium-derived vasoactive factors affected by concentrations of dobutamine less than 3 mumol/L may also act through K+ channels in our preparations. Higher concentrations of dobutamine may have a direct, endothelium-independent relaxant effect.

Effects of calcium and potassium supplements on arterial tone in vitro in spontaneously hypertensive rats

1. Calcium and potassium intakes inversely correlate with blood pressure in experimental hypertension. Therefore, we examined the effects of calcium and potassium supplements alone and in combination on arterial tone in spontaneously hypertensive rats (SHR). Wistar-Kyoto (WKY) rats served as normotensive controls. Calcium and potassium contents in the control diet were both 1%, while those in supplemented chows were 3% and 3.5%, respectively. The sodium content of all diets was moderately elevated to 1.1%. 2. After 12 weeks of the study systolic blood pressures in SHR on high calcium and on high potassium diets were markedly lower (about 53 and 58 mmHg, respectively) than in hypertensive controls, while combined supplementation of these cations reduced blood pressure even further (about 69 mmHg). 3. Responses of mesenteric arterial rings in vitro were examined at the end of the study. Both high calcium and high potassium diets improved the impaired relaxation to acetylcholine (ACh) in SHR, while the combination of these supplements completely normalized this response. Cyclo-oxygenase inhibition by diclofenac augmented the relaxation to ACh in hypertensive controls but not in the other groups. Nevertheless, enhanced endothelium-mediated dilatation was still observed in the presence of diclofenac and the nitric oxide synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME) in all supplemented groups. Interestingly, additional blockade of Ca2+-activated K+ channels by tetraethylammonium abolished the improved relaxation to ACh in SHR on high calcium and on high potassium, but distinct responses were still observed in WKY rats and SHR on the combined supplement. 4. When hyperpolarization of smooth muscle was prevented by precontraction of the preparations with 50 mM KCl, only marginal differences were observed in the diclofenac and L-NAME-resistant relaxations to ACh between the study groups. Finally, endothelium-independent vasorelaxations of noradrenaline-precontracted rings to nitroprusside, isoprenaline and cromakalim were comparably augmented by all supplements. 5. In conclusion, the vascular mechanisms underlying the antihypertensive effect of high calcium and high potassium diets during moderately elevated sodium intake in SHR may involve enhanced arterial hyperpolarization, increased smooth muscle sensitivity to nitric oxide and decreased production of vasoconstrictor prostanoids. The administration of these cations in combination was more effective than either of them alone in reducing blood pressure and restoring arterial tone.

Beta-adrenoceptor-mediated relaxation inhibited by tetrapentylammonium ions in rat mesenteric artery

The aim of this study was to examine the contribution of K+ channel activation to beta-adrenoceptor-mediated relaxation in rat mesenteric arteries. Isoprenaline and fenoterol concentration-dependently relaxed the phenylephrine-preconstricted endothelium-intact arteries of the rat with EC50 values of 0.26 +/- 0.03 microM and 0.87 +/- 0.12 microM, respectively. Beta-adrenoceptor-mediated relaxation was significantly attenuated upon removal of endothelium. Tetrapentylammonium ions (TPA+) at low concentrations (1-5 microM) inhibited relaxations induced by beta-adrenoceptor agonists in arteries with and without endothelium, while glibenclamide (3 microM) had no effect. TPA+ (5 microM) inhibited isoprenaline-induced relaxation in the presence of either iberiotoxin (100 nM) or glibenclamide (3 microM). TPA+ did not alter forskolin-induced relaxation. In the presence of 60 mM extracellular K+, the relaxations induced by two agonists were reduced in endothelium-intact arteries and abolished in endothelium-denuded arteries. The present results suggest that the activation of TPA+-sensitive K+ channels contributes toward the relaxations mediated through beta- and beta2-adrenoceptor stimulation in rat mesenteric arteries.

Effects of nitric oxide donors, S-nitroso-L-cysteine and sodium nitroprusside, on the whole-cell and single channel currents in single myocytes of the guinea-pig proximal colon

1. The nature of the membrane channels underlying the membrane conductance changes induced by the nitric oxide (NO) donors, S-nitroso-L-cysteine (NOCys) and sodium nitroprusside (SNP) were investigated in single myocytes isolated from the circular muscle layer of the guinea-pig proximal colon, by use of standard whole-cell and single channel recording techniques. 2. Under voltage clamp, depolarizing steps from -60 mV elicited a rapidly-developing, little-inactivating outward K+ current (IK) at potentials positive to -40 mV (at 20-25 degrees C). The steady-state level (ISS) of this K current increased in amplitude as the step potential was made to more positive potentials. If the depolarizing steps were made from a holding potential of -80 mV an additional rapidly activating and inactivating outward K+ current was also elicited, superimposed on IK. 3. At 20-25 degrees C, NOCys (2.5 microM), SNP (100 microM) and 8-bromo-cyclic GMP (500 microM) increased the amplitude of ISS of IK elicited from a holding potential of -60 mV. In contrast, NOCys (2-5 microM) had little effect on ISS at 35 degrees C. Higher concentrations (> or = 5 microM at 20-25 degrees C and > or = 10 microM at 35 degrees C) of NOCys decreased the peak amplitude (I[Peak]) and ISS of IK in a concentration-dependent manner. This blockade of IK with NOCys was always associated with an increase of the holding current (IHold), due to the activation of a membrane conductance with a reversal potential between 0 and + 30 mV and which was reduced approximately 50% upon the addition of Cd2+ (1 mM). 4. NOCys (2.5 to 10 microM) or SNP (100 microM) increased the activity of large conductance Ca2+-activated (BK) K' channels in both cell-attached and excised inside-out patches, bathed in either a symmetrical high K+ (130 mM) or an asymmetrically K+ (6 mMout: 130 mMin) physiological saline. Increases in BK channel activity in NOCys (10 microM) or SNP (100 microM) were associated with an increase in the probability of BK channel opening (N.Po), and with a negative shift of the plots of ln(N.Po) against the patch potential, with little change in the slopes of these plots. In cell-attached patches, the increase in N.Po with NOCys was often associated with a decrease in the BK single channel conductance. 5. In both cell-attached and excised patches, NOCys (2.5 to 10 microM) also activated an additional population of channels which allowed inward current flow at potentials positive to EK. In excised inside-out patches bathed in asymmetrical K+ physiological saline, these single channel currents were 2-3 pA in amplitude at -30 mV and reversed in direction near + 10 mV, even if the NaCl (126 mM) concentration in the pipette solution had been replaced with an equimolar concentration of Na gluconate. 6. Under current clamp, NOCys (2.5 microM) and SNP (100 microM) had variable effects on the membrane potential of colonic myocytes, inducing either a small membrane hyperpolarization of <5 mV, or a slowly-developing membrane depolarization of about 5 mV. In contrast, NOCys (5 microM) produced a transient membrane hyperpolarization which was followed by a large depolarization of the membrane potential to positive potentials. The electrotonic potentials elicited in response to an injection of constant hyperpolarizing current (10 pA for 400 ms) were little changed during the NOCys (5 PM)-induced membrane hyperpolarization, but significantly reduced (to 61% of control) during the periods of membrane depolarization. 7. It was concluded that NOCys and SNP, directly increased the number of active BK channels in the membrane of colonic myocytes which leads to a small rapidly oscillating membrane hyperpolarization. The following rebound depolarization in NOCys arises from both the direct opening of a population of cationic channels and the blockade of voltage- and Ca-activated K+ conductances. Finally, the apamin-sensitive K+channels underlying the initial transient hyperpolarization recorded in the intact proximal colon, in response to nerve-released or directly-applied NO, have yet to be identified at the single channel or whole-cell current level.

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.

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.