Ionic mechanisms mediating the myogenic response in newborn porcine cerebral arteries

Kv2.1 Channels Prevent Vasomotion and Safeguard Myogenic Reactivity in Rat Small Superior Cerebellar Arteries

Vascular smooth muscle voltage-gated potassium (Kv) channels have been proposed to contribute to myogenic autoregulation. Surprisingly, in initial experiments, we observed that the Kv2 channel inhibitor stromatoxin induced vasomotion without affecting myogenic tone. Thus, we tested the hypothesis that Kv2 channels contribute to myogenic autoregulation by fine-tuning the myogenic response. Expression of Kv2 channel mRNA was determined using real-time PCR and 'multiplex' single-cell RT-PCR. Potassium currents were measured using the patch-clamp technique. Contractile responses of intact arteries were studied using isobaric myography. Expression of Kv2.1 but not Kv2.2 channels was detected in intact rat superior cerebellar arteries and in single smooth muscle cells. Stromatoxin, a high-affinity inhibitor of Kv2 channels, reduced smooth muscle Kv currents by 61% at saturating concentrations (EC50 36 nmol/L). Further, stromatoxin (10-100 nmol/L) induced pronounced vasomotion in 48% of the vessels studied. In vessels not exhibiting vasomotion, stromatoxin did not affect myogenic reactivity. Notably, in vessels exhibiting stromatoxin-induced vasomotion, pressure increases evoked two effects: First, they facilitated the occurrence of random vasodilations and/or vasoconstrictions, disturbing the myogenic response (24% of the vessels). Second, they modified the vasomotion by decreasing its amplitude and increasing its frequency, thereby destabilizing myogenic tone (76% of the vessels). Our study demonstrates that (i) Kv2.1 channels are the predominantly expressed Kv channels in smooth muscle cells of rat superior cerebellar arteries, and (ii) Kv2.1 channels provide a novel type of negative feedback mechanism in myogenic autoregulation by preventing vasomotion and thereby safeguarding the myogenic response.

Differential Functional Contribution of BK Channel Subunits to Aldosterone-Induced Channel Activation in Vascular Smooth Muscle and Eventual Cerebral Artery Dilation

Calcium/voltage-activated potassium channels (BK) control smooth muscle (SM) tone and cerebral artery diameter. They include channel-forming α and regulatory β₁ subunits, the latter being highly expressed in SM. Both subunits participate in steroid-induced modification of BK activity: β₁ provides recognition for estradiol and cholanes, resulting in BK potentiation, whereas α suffices for BK inhibition by cholesterol or pregnenolone. Aldosterone can modify cerebral artery function independently of its effects outside the brain, yet BK involvement in aldosterone's cerebrovascular action and identification of channel subunits, possibly involved in steroid action, remains uninvestigated. Using microscale thermophoresis, we demonstrated that each subunit type presents two recognition sites for aldosterone: at 0.3 and ≥10 µM for α and at 0.3-1 µM and ≥100 µM for β₁. Next, we probed aldosterone on SM BK activity and diameter of middle cerebral artery (MCA) isolated from β₁^-/- vs. wt mice. Data showed that β₁ leftward-shifted aldosterone-induced BK activation, rendering EC₅₀~3 μM and EC_MAX ≥ 10 μM, at which BK activity increased by 20%. At similar concentrations, aldosterone mildly yet significantly dilated MCA independently of circulating and endothelial factors. Lastly, aldosterone-induced MCA dilation was lost in β₁^-/- mice. Therefore, β₁ enables BK activation and MCA dilation by low µM aldosterone.

Interspecies and regional variability of alcohol action on large cerebral arteries: regulation by KCNMB1 proteins

Alcohol intake leading to blood ethanol concentrations (BEC) ≥ legal intoxication modifies brain blood flow with increases in some regions and decreases in others. Brain regions receive blood from the Willis' circle branches: anterior, middle (MCA) and posterior cerebral (PCA), and basilar (BA) arteries. Rats and mice have been used to identify the targets mediating ethanol-induced effects on cerebral arteries, with conclusions being freely interchanged, albeit data were obtained in different species/arterial branches. We tested whether ethanol action on cerebral arteries differed between male rat and mouse and/or across different brain regions and identified the targets of alcohol action. In both species and all Willis' circle branches, ethanol evoked reversible and concentration-dependent constriction (EC50s ≈ 37-86 mM; below lethal BEC in alcohol-naïve humans). Although showing similar constriction to depolarization, both species displayed differential responses to ethanol: in mice, MCA constriction was highly sensitive to the presence/absence of the endothelium, whereas in rat PCA was significantly more sensitive to ethanol than its mouse counterpart. In the rat, but not the mouse, BA was more ethanol sensitive than other branches. Both interspecies and regional variability were ameliorated by endothelium. Selective large conductance (BK) channel block in de-endothelialized vessels demonstrated that these channels were the effectors of alcohol-induced cerebral artery constriction across regions and species. Variabilities in alcohol actions did not fully matched KCNMB1 expression across vessels. However, immunofluorescence data from KCNMB1-/- mouse arteries electroporated with KCNMB1-coding cDNA demonstrate that KCNMB1 proteins, which regulate smooth muscle (SM) BK channel function and vasodilation, regulate interspecies and regional variability of brain artery responses to alcohol.

Oxidative Regulation of Vascular Cav1.2 Channels Triggers Vascular Dysfunction in Hypertension-Related Disorders

Blood pressure is determined by cardiac output and peripheral vascular resistance. The L-type voltage-gated Ca²⁺ (Ca_v1.2) channel in small arteries and arterioles plays an essential role in regulating Ca²⁺ influx, vascular resistance, and blood pressure. Hypertension and preeclampsia are characterized by high blood pressure. In addition, diabetes has a high prevalence of hypertension. The etiology of these disorders remains elusive, involving the complex interplay of environmental and genetic factors. Common to these disorders are oxidative stress and vascular dysfunction. Reactive oxygen species (ROS) derived from NADPH oxidases (NOXs) and mitochondria are primary sources of vascular oxidative stress, whereas dysfunction of the Ca_v1.2 channel confers increased vascular resistance in hypertension. This review will discuss the importance of ROS derived from NOXs and mitochondria in regulating vascular Ca_v1.2 and potential roles of ROS-mediated Ca_v1.2 dysfunction in aberrant vascular function in hypertension, diabetes, and preeclampsia.

Regulation of Coronary Blood Flow

The heart is uniquely responsible for providing its own blood supply through the coronary circulation. Regulation of coronary blood flow is quite complex and, after over 100 years of dedicated research, is understood to be dictated through multiple mechanisms that include extravascular compressive forces (tissue pressure), coronary perfusion pressure, myogenic, local metabolic, endothelial as well as neural and hormonal influences. While each of these determinants can have profound influence over myocardial perfusion, largely through effects on end-effector ion channels, these mechanisms collectively modulate coronary vascular resistance and act to ensure that the myocardial requirements for oxygen and substrates are adequately provided by the coronary circulation. The purpose of this series of Comprehensive Physiology is to highlight current knowledge regarding the physiologic regulation of coronary blood flow, with emphasis on functional anatomy and the interplay between the physical and biological determinants of myocardial oxygen delivery. © 2017 American Physiological Society. Compr Physiol 7:321-382, 2017.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Hydrogen sulfide activates Ca²⁺ sparks to induce cerebral arteriole dilatation

Hydrogen sulfide (H₂S) is a gaseous vasodilator produced by endothelial cells. Mechanisms by which H₂S induces vasodilatation are unclear. We tested the hypothesis that H₂S dilates cerebral arterioles by modulating local and global intracellular Ca²⁺ signals in smooth muscle cells. High-speed confocal imaging revealed that Na₂S, an H₂S donor, increased Ca²⁺ spark frequency ∼1.43-fold and decreased global intracellular Ca²⁺ concentration ([Ca²⁺]i) by ∼37 nM in smooth muscle cells of intact piglet cerebral arterioles. In contrast, H₂S did not alter Ca²⁺ wave frequency. In voltage-clamped (-40 mV) cells, H₂S increased the frequency of iberiotoxin-sensitive, Ca²⁺ spark-induced transient Ca²⁺-activated K⁺ (KCa) currents ∼1.83-fold, but did not alter the amplitude of these events. H₂S did not alter the activity of single KCa channels recorded in the absence of Ca²⁺ sparks in arteriole smooth muscle cells. H₂S increased SR Ca²⁺ load ([Ca²⁺]SR), measured as caffeine (10 and 20mM)-induced [Ca²⁺]i transients, ∼1.5-fold. H₂S hyperpolarized (by ∼18 mV) and dilated pressurized (40 mmHg) cerebral arterioles. Iberiotoxin, a KCa channel blocker, reduced H₂S-induced hyperpolarization by ∼51%. Iberiotoxin and ryanodine, a ryanodine receptor channel inhibitor, reduced H₂S-induced vasodilatation by ∼38 and ∼37%, respectively. In summary, our data indicate that H₂S elevates [Ca²⁺]SR, leading to Ca²⁺ spark activation in cerebral arteriole smooth muscle cells. The subsequent elevation in transient KCa current frequency leads to membrane hyperpolarization, a reduction in global [Ca²⁺]i and vasodilatation.

Contribution of voltage-dependent K+ and Ca2+ channels to coronary pressure-flow autoregulation

The mechanisms responsible for coronary pressure-flow autoregulation, a critical physiologic phenomenon that maintains coronary blood flow relatively constant in the presence of changes in perfusion pressure, remain poorly understood. This investigation tested the hypothesis that voltage-sensitive K(+) (K(V)) and Ca(2+) (Ca(V)1.2) channels play a critical role in coronary pressure-flow autoregulation in vivo. Experiments were performed in open-chest, anesthetized Ossabaw swine during step changes in coronary perfusion pressure (CPP) from 40 to 140 mmHg before and during inhibition of K(V) channels with 4-aminopyridine (4AP, 0.3 mM, ic) or Ca(V)1.2 channels with diltiazem (10 μg/min, ic). 4AP significantly decreased vasodilatory responses to H(2)O(2) (0.3-10 μM, ic) and coronary flow at CPPs = 60-140 mmHg. This decrease in coronary flow was associated with diminished ventricular contractile function (dP/dT) and myocardial oxygen consumption. However, the overall sensitivity to changes in CPP from 60 to 100 mmHg (i.e. autoregulatory gain; Gc) was unaltered by 4-AP administration (Gc = 0.46 ± 0.11 control vs. 0.46 ± 0.06 4-AP). In contrast, inhibition of Ca(V)1.2 channels progressively increased coronary blood flow at CPPs > 80 mmHg and substantially diminished coronary Gc to -0.20 ± 0.11 (P < 0.01), with no effect on contractile function or oxygen consumption. Taken together, these findings demonstrate that (1) K(V) channels tonically contribute to the control of microvascular resistance over a wide range of CPPs, but do not contribute to coronary responses to changes in pressure; (2) progressive activation of Ca(V)1.2 channels with increases in CPP represents a critical mechanism of coronary pressure-flow autoregulation.

Stimulation of the adenosine A3 receptor reverses vascular hyporeactivity after hemorrhagic shock in rats

AIM

To investigate whether adenosine A(3) receptors (A(3)AR) stimulation restore vascular reactivity after hemorrhagic shock through a ryanodine receptor (RyR)-mediated and large conductance calcium-activated potassium (BK(Ca)) channel-dependent pathway.

METHODS

Rat hemorrhagic shock model (40 mmHg) and vascular smooth muscle cell (VSMC) hypoxic model were used. The expression of A(3)AR was determined by Western blot and RT-PCR. The effect of A(3)AR stimulation on RyR-mediated Ca(2+) release in VSMCs was analyzed by the Fura-3/AM loading Ca(2+) imaging. The modulation of vascular reactivity to norepinephrine (NE) by A(3)AR stimulation was monitored by an isolated organ tension instrument.

RESULTS

Decrease of A(3)AR expression is consistent with the loss of vasoreactivity to NE in hemorrhagic shock rats. The stimulation of A(3)AR with a selective agonist, IB-MECA, could partly but significantly restore the vasoreactivity in the rats, and this restorative effect could be counteracted by MRS1523, a selective A(3)AR antagonist. In hypoxic VSMCs, RyR activation by caffeine significantly evoked the rise of [Ca(2+)] compared with the control cells, a phenomenon closely associated with the development of vascular hyporeactivity in hemorrhagic shock rats. The stimulation of A(3)AR with IB-MECA significantly blocked this over activation of RyR-mediated Ca(2+) release. RyR activation by caffeine and BK(Ca) channel activation by NS1619 attenuated the restoration of vasoreactivity to NE resulting from A(3)AR stimulation by IB-MECA after hemorrhagic shock; this attenuation effect could be antagonized by a selective BK(Ca) channel blocker.

CONCLUSION

These findings suggest that A(3)AR is involved in the modulation of vasoreactivity after hemorrhagic shock and that stimulation of A(3)AR can restore the decreased vasoreactivity to NE through a RyR-mediated, BK(Ca) channel-dependent signal pathway.

Aging and muscle fiber type alter K+ channel contributions to the myogenic response in skeletal muscle arterioles

Aging diminishes myogenic tone in arterioles from skeletal muscle. Recent evidence indicates that both large-conductance Ca2+-activated (BKCa) and voltage-dependent (KV) K+ channels mediate negative feedback control of the myogenic response. Thus we tested the hypothesis that aging increases the contributions of KV and BKCa channels to myogenic regulation of vascular tone. Because myogenic responsiveness differs between oxidative and glycolytic muscles, we predicted that KV and BKCa channel contributions to myogenic responsiveness vary with fiber type. Myogenic responses of first-order arterioles from the gastrocnemius and soleus muscles of 4- and 24-mo-old Fischer 344 rats were evaluated in the presence and absence of 4-aminopyridine (5 mM) or iberiotoxin (30 nM), inhibitors of KV and BKCa, respectively. 4-Aminopyridine enhanced myogenic tone with aging and normalized age-related differences in both muscle types. By contrast, iberiotoxin eliminated age-related differences in soleus arterioles and had no effect in gastrocnemius vessels. KV1.5 is an integral component of KV channels in vascular smooth muscle; therefore, we determined the relative protein expression of KV1.5, as well as BKCa, in soleus and gastrocnemius arterioles. Immunoblot analysis revealed no differences in KV1.5 protein with aging or between variant fiber types, whereas BKCa protein levels declined with age in arterioles from both muscle groups. Collectively, these results suggest that the contribution of BKCa to myogenic regulation of vascular tone changes with age in soleus muscle arterioles, whereas increased KV channel expression and negative feedback regulation of myogenic tone increases with advancing age in arterioles from both oxidative and glycolytic muscles.

Vascular control in humans: focus on the coronary microcirculation

Myocardial perfusion is regulated by a variety of factors that influence arteriolar vasomotor tone. An understanding of the physiological and pathophysiological factors that modulate coronary blood flow provides the basis for the judicious use of medications for the treatment of patients with coronary artery disease. Vasomotor properties of the coronary circulation vary among species. This review highlights the results of recent studies that examine the mechanisms by which the human coronary microcirculation is regulated in normal and disease states, focusing on diabetes. Multiple pathways responsible for myogenic constriction and flow-mediated dilation in human coronary arterioles are addressed. The important role of endothelium-derived hyperpolarizing factors, their interactions in mediating dilation, as well as speculation regarding the clinical significance are emphasized. Unique properties of coronary arterioles in human vs. other species are discussed.

Astrocyte-derived CO is a diffusible messenger that mediates glutamate-induced cerebral arteriolar dilation by activating smooth muscle Cell KCa channels

Astrocyte signals can modulate arteriolar tone, contributing to regulation of cerebral blood flow, but specific intercellular communication mechanisms are unclear. Here we used isolated cerebral arteriole myocytes, astrocytes, and brain slices to investigate whether carbon monoxide (CO) generated by the enzyme heme oxygenase (HO) acts as an astrocyte-to-myocyte gasotransmitter in the brain. Glutamate stimulated CO production by astrocytes with intact HO-2, but not those genetically deficient in HO-2. Glutamate activated transient K(Ca) currents and single K(Ca) channels in myocytes that were in contact with astrocytes, but did not affect K(Ca) channel activity in myocytes that were alone. Pretreatment of astrocytes with chromium mesoporphyrin (CrMP), a HO inhibitor, or genetic ablation of HO-2 prevented glutamate-induced activation of myocyte transient K(Ca) currents and K(Ca) channels. Glutamate decreased arteriole myocyte intracellular Ca2+ concentration and dilated brain slice arterioles and this decrease and dilation were blocked by CrMP. Brain slice arteriole dilation to glutamate was also blocked by L-2-alpha aminoadipic acid, a selective astrocyte toxin, and paxilline, a K(Ca) channel blocker. These data indicate that an astrocytic signal, notably HO-2-derived CO, is used by glutamate to stimulate arteriole myocyte K(Ca) channels and dilate cerebral arterioles. Our study explains the astrocyte and HO dependence of glutamatergic functional hyperemia observed in the newborn cerebrovascular circulation in vivo.

Postnatal maturation attenuates pressure-evoked myogenic tone and stretch-induced increases in Ca2+ in rat cerebral arteries

Although postnatal maturation potently modulates agonist-induced cerebrovascular contractility, its effects on the mechanisms mediating cerebrovascular myogenic tone remain poorly understood. Because the regulation of calcium influx and myofilament calcium sensitivity change markedly during early postnatal life, the present study tested the general hypothesis that early postnatal maturation increases the pressure sensitivity of cerebrovascular myogenic tone via age-dependent enhancement of pressure-induced calcium mobilization and myofilament calcium sensitivity. Pressure-induced myogenic tone and changes in artery wall intracellular calcium concentrations ([Ca(2+)](i)) were measured simultaneously in endothelium-denuded, fura-2-loaded middle cerebral arteries (MCA) from pup [postnatal day 14 (P14)] and adult (6-mo-old) Sprague-Dawley rats. Increases in pressure from 20 to 80 mmHg enhanced myogenic tone in MCA from both pups and adults although the normalized magnitudes of these increases were significantly greater in pup than adult MCA. At each pressure step, vascular wall [Ca(2+)](i) was also significantly greater in pup than in adult MCA. Nifedipine significantly attenuated pressure-evoked constrictions in pup MCA and essentially eliminated all responses to pressure in the adult MCA. Both pup and adult MCA exhibited pressure-dependent increases in calcium sensitivity, as estimated by changes in the ratio of pressure-induced myogenic tone to wall [Ca(2+)](i). However, there were no differences in the magnitudes of these increases between pup and adult MCA. The results support the view that regardless of postnatal age, changes in both calcium influx and myofilament calcium sensitivity contribute to the regulation of cerebral artery myogenic tone. The greater cerebral myogenic response in P14 compared with adult MCA appears to be due to greater pressure-induced increases in [Ca(2+)](i), rather than enhanced augmentation of myofilament calcium sensitivity.

Stretch-activated conductances in smooth muscles

The excitability of smooth muscle cells is regulated, in part, by stretch-activated ion channels in the plasma membrane. The response to stretch of a particular muscle or organ is tuned to specific functional needs by the types of ion channels expressed. Mechanosensitive ionic conductances that yield either inward or outward currents have been observed in and characterized in studies of smooth muscles. In vascular muscles, the dominant response to stretch is muscle contraction (the myogenic response). This chapter proposes several mechanisms for the myogenic response; one of these hypotheses involves stretch-dependent activation of nonselective cation channels. The inward current resulting from an activation of these channels causes plasma membrane depolarization, activation of voltage-gated Ca(2+) channels, Ca(2+) entry, and excitation-contraction coupling. Thus, increasing the vascular pressure and distension of blood vessels cause responsive vasoconstriction. Other conductances are also proposed as participants in the myogenic response, and progress characterizing the inward current channels responsive to stretch is summarized. Outward currents responding to muscle stretch are also present in smooth muscles. For example, expression of stretch-sensitive two-pore domain K(+) (K2P) channels has been reported in visceral smooth muscles. These organs resist contraction on filling and provide a reservoir function. Stretch-dependent outward current channels are hypothesized to help stabilize membrane potential until it becomes desirable to empty the stored contents. Mechanosensitive conductances participate in the integrated responses of smooth muscle tissues. The chapter summarizes the class of channels found in smooth muscles.

Characteristics of myogenic tone in the rat ophthalmic artery

The pressure-induced constriction in the rat ophthalmic artery was characterized. Ophthalmic arteries were isolated, cannulated in an arteriograph, and pressurized. Arteries developed 25% constriction at 70 mmHg of intraluminal pressure. Arteries maintained almost similar diameter over the range of pressures 50–210 mmHg, and forced dilatation was observed at pressures >210 mmHg. Denudation of endothelium increased the sensitivity of arteries to pressure-induced constriction, and significantly higher myogenic tone was observed in the pressure range of 10–100 mmHg. Indomethacin and cyclooxygenase-2 inhibition by SC-236 decreased myogenic tone, whereas cyclooxygenase-1 inhibition by SC-560 potentiated myogenic tone in a lower concentration range and decreased at a higher concentration. Pressure-induced constriction was completely blocked by 1 μM nifedipine. Phospholipase C inhibition by 6 μM U-73122 decreased myogenic tone by 39%, whereas PKC inhibitor GF-109203X (3 μM) had no effect. Constriction to phenylephrine was significantly decreased by U-73122 (1 μM) and GF-109203X (3 μM) at an intraluminal pressure of 10 mmHg. Rho-kinase inhibition by Y-27632 (30 μM) and HA-1077 (30 μM) decreased myogenic tone by 75% and 73%, respectively, and 1 μM Y-27632 significantly decreased myogenic tone developed in response to graded increases in pressure. These results suggest that rat ophthalmic artery has an efficient pressure-dependent autoregulatory function that is modulated by endothelium. Contribution of phospholipase C-activation to myogenic tone is minimal, whereas Rho-kinase activation plays a predominant role in the myogenic reactivity in this artery.

KCa channel insensitivity to Ca2+ sparks underlies fractional uncoupling in newborn cerebral artery smooth muscle cells

In smooth muscle cells, localized intracellular Ca2+ transients, termed "Ca2+ sparks," activate several large-conductance Ca2+-activated K+ (KCa) channels, resulting in a transient KCa current. In some smooth muscle cell types, a significant proportion of Ca2+ sparks do not activate KCa channels. The goal of this study was to explore mechanisms that underlie fractional Ca2+ spark-KCa channel coupling. We investigated whether membrane depolarization or ryanodine-sensitive Ca2+ release (RyR) channel activation modulates coupling in newborn (1- to 3-day-old) porcine cerebral artery myocytes. At steady membrane potentials of -40, 0, and +40 mV, mean transient KCa current frequency was approximately 0.18, 0.43, and 0.26 Hz and KCa channel activity [number of KCa channels activated by Ca2+ sparksxopen probability of KCa channels at peak of Ca2+ sparks (NPo)] at the transient KCa current peak was approximately 4, 12, and 24, respectively. Depolarization between -40 and +40 mV increased KCa channel sensitivity to Ca2+ sparks and elevated the percentage of Ca2+ sparks that activated a transient KCa current from 59 to 86%. In a Ca2+-free bath solution or in diltiazem, a voltage-dependent Ca2+ channel blocker, steady membrane depolarization between -40 and +40 mV increased transient KCa current frequency up to approximately 1.6-fold. In contrast, caffeine (10 microM), an RyR channel activator, increased mean transient KCa current frequency but did not alter Ca2+ spark-KCa channel coupling. These data indicate that coupling is increased by mechanisms that elevate KCa channel sensitivity to Ca2+ sparks, but not by RyR channel activation. Overall, KCa channel insensitivity to Ca2+ sparks is a prominent factor underlying fractional Ca2+ spark uncoupling in newborn cerebral artery myocytes.

Essential role for smooth muscle BK channels in alcohol-induced cerebrovascular constriction

Binge drinking is associated with increased risk for cerebrovascular spasm and stroke. Acute exposure to ethanol at concentrations obtained during binge drinking constricts cerebral arteries in several species, including humans, but the mechanisms underlying this action are largely unknown. In a rodent model, we used fluorescence microscopy, patch-clamp electrophysiology, and pharmacological studies in intact cerebral arteries to pinpoint the molecular effectors of ethanol cerebrovascular constriction. Clinically relevant concentrations of ethanol elevated wall intracellular Ca(2+) concentration and caused a reversible constriction of cerebral arteries (EC(50) = 27 mM; E(max) = 100 mM) that depended on voltage-gated Ca(2+) entry into myocytes. However, ethanol did not directly increase voltage-dependent Ca(2+) currents in isolated myocytes. Constriction occurred because of an ethanol reduction in the frequency (-53%) and amplitude (-32%) of transient Ca(2+)-activated K(+) (BK) currents. Ethanol inhibition of BK transients was caused by a reduction in Ca(2+) spark frequency (-49%), a subsarcolemmal Ca(2+) signal that evokes the BK transients, and a direct inhibition of BK channel steady-state activity (-44%). In contrast, ethanol failed to modify Ca(2+) waves, a major vasoconstrictor mechanism. Selective block of BK channels largely prevented ethanol constriction in pressurized arteries. This study pinpoints the Ca(2+) spark/BK channel negative-feedback mechanism as the primary effector of ethanol vasoconstriction.