Hyposmotic challenge inhibits inward rectifying K+ channels in cerebral arterial smooth muscle cells

Pacemaking in the lymphatic system

Lymphatic collecting vessels exhibit spontaneous phasic contractions that are critical for lymph propulsion and tissue fluid homeostasis. This rhythmic activity is driven by action potentials conducted across the lymphatic muscle cell (LMC) layer to produce entrained contractions. The contraction frequency of a lymphatic collecting vessel displays exquisite mechanosensitivity, with a dynamic range from <1 to >20 contractions per minute. A myogenic pacemaker mechanism intrinsic to the LMCs was initially postulated to account for pressure-dependent chronotropy. Further interrogation into the cellular constituents of the lymphatic vessel wall identified non-muscle cell populations that shared some characteristics with interstitial cells of Cajal, which have pacemaker functions in the gastrointestinal and lower urinary tracts, thus raising the possibility of a non-muscle cell pacemaker. However, recent genetic knockout studies in mice support LMCs and a myogenic origin of the pacemaker activity. LMCs exhibit stochastic, but pressure-sensitive, sarcoplasmic reticulum calcium release (puffs and waves) from IP3R1 receptors, which couple to the calcium-activated chloride channel Anoctamin 1, causing depolarisation. The resulting electrical activity integrates across the highly coupled lymphatic muscle electrical syncytia through connexin 45 to modulate diastolic depolarisation. However, multiple other cation channels may also contribute to the ionic pacemaking cycle. Upon reaching threshold, a voltage-gated calcium channel-dependent action potential fires, resulting in a nearly synchronous calcium global calcium flash within the LMC layer to drive an entrained contraction. This review summarizes the key ion channels potentially responsible for the pressure-dependent chronotropy of lymphatic collecting vessels and various mechanisms of IP3R1 regulation that could contribute to frequency tuning.

Advancements in the study of inward rectifying potassium channels on vascular cells

Inward rectifier potassium channels (Kir channels) exist in a variety of cells and are involved in maintaining resting membrane potential and signal transduction in most cells, as well as connecting metabolism and membrane excitability of body cells. It is closely related to normal physiological functions of body and the occurrence and development of some diseases. Although the functional expression of Kir channels and their role in disease have been studied, they have not been fully elucidated. In this paper, the functional expression of Kir channels in vascular endothelial cells and smooth muscle cells and their changes in disease states were reviewed, especially the recent research progress of Kir channels in stem cells was introduced, in order to have a deeper understanding of Kir channels in vascular tissues and provide new ideas and directions for the treatment of related ion channel diseases.

Vascular mechanotransduction

This review aims to survey the current state of mechanotransduction in vascular smooth muscle cells (VSMCs) and endothelial cells (ECs), including their sensing of mechanical stimuli and transduction of mechanical signals that result in the acute functional modulation and longer-term transcriptomic and epigenetic regulation of blood vessels. The mechanosensors discussed include ion channels, plasma membrane-associated structures and receptors, and junction proteins. The mechanosignaling pathways presented include the cytoskeleton, integrins, extracellular matrix, and intracellular signaling molecules. These are followed by discussions on mechanical regulation of transcriptome and epigenetics, relevance of mechanotransduction to health and disease, and interactions between VSMCs and ECs. Throughout this review, we offer suggestions for specific topics that require further understanding. In the closing section on conclusions and perspectives, we summarize what is known and point out the need to treat the vasculature as a system, including not only VSMCs and ECs but also the extracellular matrix and other types of cells such as resident macrophages and pericytes, so that we can fully understand the physiology and pathophysiology of the blood vessel as a whole, thus enhancing the comprehension, diagnosis, treatment, and prevention of vascular diseases.

Genetic ablation of smooth muscle KIR2.1 is inconsequential to the function of mouse cerebral arteries

Cerebral blood flow is a finely tuned process dependent on coordinated changes in arterial tone. These changes are strongly tied to smooth muscle membrane potential and inwardly rectifying K⁺ (K_IR) channels are thought to be a key determinant. To elucidate the role of K_IR2.1 in cerebral arterial tone development, this study examined the electrical and functional properties of cells, vessels and living tissue from tamoxifen-induced smooth muscle cell (SMC)-specific K_IR2.1 knockout mice. Patch-clamp electrophysiology revealed a robust Ba²⁺-sensitive inwardly rectifying K⁺ current in cerebral arterial myocytes irrespective of K_IR2.1 knockout. Immunolabeling clarified that K_IR2.1 expression was low in SMCs while K_IR2.2 labeling was remarkably abundant at the membrane. In alignment with these observations, pressure myography revealed that the myogenic response and K⁺-induced dilation were intact in cerebral arteries post knockout. At the whole organ level, this translated to a maintenance of brain perfusion in SMC K_IR2.1^-/- mice, as assessed with arterial spin-labeling MRI. We confirmed these findings in superior epigastric arteries and implicated K_IR2.2 as more functionally relevant in SMCs. Together, these results suggest that subunits other than K_IR2.1 play a significant role in setting native current in SMCs and driving arterial tone.

Inward Rectifier Potassium Channels: Membrane Lipid-Dependent Mechanosensitive Gates in Brain Vascular Cells

Cerebral arteries contain two primary and interacting cell types, smooth muscle (SMCs) and endothelial cells (ECs), which are each capable of sensing particular hemodynamic forces to set basal tone and brain perfusion. These biomechanical stimuli help confer tone within arterial networks upon which local neurovascular stimuli function. Tone development is intimately tied to arterial membrane potential (VM) and changes in intracellular [Ca2+] driven by voltage-gated Ca2+ channels (VGCCs). Arterial VM is in turn set by the dynamic interplay among ion channel species, the strongly inward rectifying K+ (Kir) channel being of special interest. Kir2 channels possess a unique biophysical signature in that they strongly rectify, display negative slope conductance, respond to elevated extracellular K+ and are blocked by micromolar Ba2+. While functional Kir2 channels are expressed in both smooth muscle and endothelium, they lack classic regulatory control, thus are often viewed as a simple background conductance. Recent literature has provided new insight, with two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP2) and cholesterol, noted to (1) stabilize Kir2 channels in a preferred open or closed state, respectively, and (2) confer, in association with the cytoskeleton, caveolin-1 (Cav1) and syntrophin, hemodynamic sensitivity. It is these aspects of vascular Kir2 channels that will be the primary focus of this review.

Alzheimer's disease and cerebrovascular pathology alter brain endothelial inward rectifier potassium (KIR 2.1) channels

Background and Purpose

Inward rectifier potassium (K_IR) channels are key effectors of vasodilatation in neurovascular coupling (NVC). K_IR channels expressed in cerebral endothelial cells (ECs) have been confirmed as essential modulators of NVC. Alzheimer's disease (AD) and cerebrovascular disease (CVD) impact on EC‐K_IR channel function, but whether oxidative stress or inflammation explains this impairment remains elusive.

Experimental Approach

We evaluated K_IR channel function in intact and EC‐denuded pial arteries of wild‐type (WT) and transgenic mice overexpressing a mutated form of the human amyloid precursor protein (APP mice, recapitulating amyloid β‐induced oxidative stress seen in AD) or a constitutively active form of TGF‐β1 (TGF mice, recapitulating inflammation seen in cerebrovascular pathology). The benefits of antioxidant (catalase) or anti‐inflammatory (indomethacin) drugs also were investigated. Vascular and neuronal components of NVC were assessed in vivo.

Key Results

Our findings show that (i) K_IR channel‐mediated maximal vasodilatation in APP and TGF mice reaches only 37% and 10%, respectively, of the response seen in WT mice; (ii) K_IR channel dysfunction results from K_IR2.1 subunit impairment; (iii) about 50% of K⁺‐induced artery dilatation is mediated by EC‐K_IR channels; (iv) oxidative stress and inflammation impair K_IR channel function, which can be restored by antioxidant and anti‐inflammatory drugs; and (v) inflammation induces K_IR2.1 overexpression and impairs NVC in TGF mice.

Conclusion and Implications

Therapies targeting both oxidative stress and inflammation are necessary for full recovery of K_IR2.1 channel function in cerebrovascular pathology caused by AD and CVD.

What Evolutionary Evidence Implies About the Identity of the Mechanoelectrical Couplers in Vascular Smooth Muscle Cells

Loss of pressure-induced vasoconstriction increases susceptibility to renal and cerebral vascular injury. Favored paradigms underlying initiation of the response include transient receptor potential channels coupled to G protein-coupled receptors or integrins as transducers. Degenerin channels may also mediate the response. This review addresses the 1) evolutionary role of these molecules in mechanosensing, 2) limitations to identifying mechanosensitive molecules, and 3) paradigm shifting molecular model for a VSMC mechanosensor.

Clinical Importance of the Human Umbilical Artery Potassium Channels

Potassium (K⁺) channels are usually predominant in the membranes of vascular smooth muscle cells (SMCs). These channels play an important role in regulating the membrane potential and vessel contractility-a role that depends on the vascular bed. Thus, the activity of K⁺ channels represents one of the main mechanisms regulating the vascular tone in physiological and pathophysiological conditions. Briefly, the activation of K⁺ channels in SMC leads to hyperpolarization and vasorelaxation, while its inhibition induces depolarization and consequent vascular contraction. Currently, there are four different types of K⁺ channels described in SMCs: voltage-dependent K⁺ (K_V) channels, calcium-activated K⁺ (K_Ca) channels, inward rectifier K⁺ (Kir) channels, and 2-pore domain K⁺ (K_2P) channels. Due to the fundamental role of K⁺ channels in excitable cells, these channels are promising therapeutic targets in clinical practice. Therefore, this review discusses the basic properties of the various types of K⁺ channels, including structure, cellular mechanisms that regulate their activity, and new advances in the development of activators and blockers of these channels. The vascular functions of these channels will be discussed with a focus on vascular SMCs of the human umbilical artery. Then, the clinical importance of K⁺ channels in the treatment and prevention of cardiovascular diseases during pregnancy, such as gestational hypertension and preeclampsia, will be explored.

Prostanoids contribute to regulation of inwardly rectifying K+ channels in intrarenal arterial smooth muscle cells

AIM

The inward rectifier K⁺ (Kir) channels and prostanoids are important factors in regulating vascular tone, but the relationship between them has not been well studied. We aimed to study the involvement of prostanoids in regulating Kir activity in the rat intrarenal arteries (RIRAs).

MAIN METHODS

The vascular tone of isolated RIRAs was recorded with a wire myograph. The intracellular Ca²⁺ concentrations ([Ca²⁺]_i) and Kir currents were measured with a Ca²⁺-sensitive fluorescence probe and patch clamp, respectively, in the arterial smooth muscle cell (ASMC) freshly isolated from RIRAs. Kir2.1 expression in RIRAs was assayed by Western blotting.

KEY FINDINGS

At 0.03-1.0 mM, BaCl₂ (a specific Kir blocker) concentration-dependently contracted RIRAs and elevated [Ca²⁺]_i levels. Mild stimulations with various vasoconstrictors at low concentrations significantly potentiated RIRA contraction induced by Kir closure with BaCl₂. In both the quiescent and the stimulated RIRAs, cyclooxygenase inhibition and thromboxane-prostanoid receptor (TPR) antagonism depressed BaCl₂-induced RIRA contraction, while nitric oxide (NO) synthetase inhibition and endothelium-denudation enhanced the contraction. Kir2.1 expression was significantly more abundant in smaller RIRAs. Ba²⁺-sensitive Kir currents were depressed by TPR agonist U46619 while increased by NO donor sodium nitroprusside.

SIGNIFICANCE

The present results reveal that vasoconstrictor stimulation augments RIRA contraction induced by Kir closure with Ba⁺ and indicate that prostanoid synthesis followed by TPR activation is involved in the modulation of the myocyte Kir activity. This study suggests that prostanoid synthesis and TPR may be potential targets for dysfunctions in renal blood circulation.

KIR channels in the microvasculature: Regulatory properties and the lipid-hemodynamic environment

Basal tone and perfusion control is set in cerebral arteries by the sensing of pressure and flow, key hemodynamic stimuli. These forces establish a contractile foundation within arterial networks upon which local neurovascular stimuli operate. This fundamental process is intimately tied to arterial V_M and the rise in cytosolic [Ca²⁺] by the graded opening of voltage-operated Ca²⁺ channels. Arterial V_M is in turn controlled by a dynamic interaction among several resident ion channels, K_IR being one of particular significance. As the name suggests, K_IR displays strong inward rectification, retains a small outward component, potentiated by extracellular K⁺ and blocked by micromolar Ba²⁺. Cerebrovascular K_IR is unique from other K⁺ currents as it is present in both smooth muscle and endothelium yet lacking in classical regulatory modulation. Such observations have fostered the view that K_IR is nothing more than a background conductance, activated by extracellular K⁺ and which passively facilitates dilation. Recent work in cell model systems has; however, identified two membrane lipids, phosphatidylinositol 4,5-bisphosphate (PIP₂) and cholesterol, that interact with K_IR2.x, to stabilize the channel in the preferred open or silent state, respectively. Translating this unique form of regulation, recent studies have demonstrated that specific lipid-protein interactions enable unique K_IR populations to sense distinct hemodynamic stimuli and set basal tone. This review summarizes the current knowledge of vascular K_IR channels and how the lipid and hemodynamic impact their activity.

A stepwise approach to resolving small ionic currents in vascular tissue

Arterial membrane potential (V_m) is set by an active interplay among ion channels whose principal function is to set contractility through the gating of voltage-operated Ca²⁺ channels. To garner an understanding of this electrical parameter, the activity of each channel must be established under near-physiological conditions, a significant challenge given their small magnitude. The inward rectifying K⁺ (K_IR) channel is illustrative of the problem, as its outward "physiological" component is almost undetectable. This study describes a stepwise approach to dissect small ionic currents at physiological V_m using endothelial and smooth muscle cells freshly isolated from rat cerebral arteries. We highlight three critical steps, beginning with the voltage clamping of vascular cells bathed in physiological solutions while maintaining a giga-ohm seal. K_IR channels are then inhibited (micromolar Ba²⁺) so that a difference current can be created, once Ba²⁺ traces are corrected for the changing seal resistance and subtle instrument drift, pulling the reversal potential rightward. The latter is a new procedure and entails the alignment of whole cell current traces at a voltage where K_IR is silent and other channels exhibit limited activity. We subsequently introduced corrected and uncorrected currents into computer models of the arterial wall to show how these subtle adjustments markedly impact the importance of K_IR in V_m and arterial tone regulation. We argue that this refined approach can be used on an array of vascular ion channels to build a complete picture of how they dynamically interact to set arterial tone in key organs like the brain.NEW & NOTEWORTHY This work describes a stepwise approach to resolve small ionic currents involved in controlling V_m in resistance arteries. Using this new methodology, we particularly resolved the outward component of the K_IR current in native vascular cells, voltage clamped in near-physiological conditions. This novel approach can be applied to any other vascular currents and used to better interpret how vascular ion channels cooperate to control arterial tone.

Membrane Lipid-K IR 2.x Channel Interactions Enable Hemodynamic Sensing in Cerebral Arteries

Objective—

Inward rectifying K + (K IR ) channels are present in cerebral arterial smooth muscle and endothelial cells, a tandem arrangement suggestive of a dynamic yet undiscovered role for this channel. This study defined whether distinct pools of cerebral arterial K IR channels were uniquely modulated by membrane lipids and hemodynamic stimuli.

Approach and Results—

A Ba 2+ -sensitive K IR current was isolated in smooth muscle and endothelial cells of rat cerebral arteries; molecular analyses subsequently confirmed K IR 2.1/K IR 2.2 mRNA and protein expression in both cells. Patch-clamp electrophysiology next demonstrated that each population of K IR channels was sensitive to key membrane lipids and hemodynamic stimuli. In this regard, endothelial K IR was sensitive to phosphatidylinositol 4,5-bisphosphate content, with depletion impairing the ability of laminar shear stress to activate this channel pool. In contrast, smooth muscle K IR was sensitive to membrane cholesterol content, with sequestration blocking the ability of pressure to inhibit channel activity. The idea that membrane lipids help confer shear stress and pressure sensitivity of K IR channels was confirmed in intact arteries using myography. Virtual models integrating structural/electrical observations reconceptualized K IR as a dynamic regulator of membrane potential working in concert with other currents to set basal tone across a range of shear stresses and intravascular pressures.

Conclusions—

The data show for the first time that specific membrane lipid-K IR interactions enable unique channel populations to sense hemodynamic stimuli and drive vasomotor responses to set basal perfusion in the cerebral circulation.

An assessment of KIR channel function in human cerebral arteries

In the rodent cerebral circulation, inward rectifying K⁺ (K_IR) channels set resting tone and the distance over which electrical phenomena spread along the arterial wall. The present study sought to translate these observations into human cerebral arteries obtained from resected brain tissue. Computational modeling and a conduction assay first defined the impact of K_IR channels on electrical communication; patch-clamp electrophysiology, quantitative PCR, and immunohistochemistry then characterized K_IR2.x channel expression/activity. In keeping with rodent observations, computer modeling highlighted that K_IR blockade should constrict cerebral arteries and attenuate electrical communication if functionally expressed. Surprisingly, Ba²⁺ (a K_IR channel inhibitor) had no effect on human cerebral arterial tone or intercellular conduction. In alignment with these observations, immunohistochemistry and patch-clamp electrophysiology revealed minimal K_IR channel expression/activity in both smooth muscle and endothelial cells. This absence may be reflective of chronic stress as dysphormic neurons, leukocyte infiltrate, and glial fibrillary acidic protein expression was notable in the epileptic cortex. In closing, K_IR2.x channel expression is limited in human cerebral arteries from patients with epilepsy and thus has little impact on resting tone or the spread of vasomotor responses. NEW & NOTEWORTHY K_IR2.x channels are expressed in rodent cerebral arterial smooth muscle and endothelial cells. As they are critical to setting membrane potential and the distance signals conduct, we sought to translate this work into humans. Surprisingly, K_IR2.x channel activity/expression was limited in human cerebral arteries, a paucity tied to chronic brain stress in the epileptic cortex. Without substantive expression, K_IR2.x channels were unable to govern arterial tone or conduction.

Perivascular adipose tissue and the dynamic regulation of Kv 7 and Kir channels: Implications for resistant hypertension

Resistant hypertension is defined as high blood pressure that remains uncontrolled despite treatment with at least three antihypertensive drugs at adequate doses. Resistant hypertension is an increasingly common clinical problem in older age, obesity, diabetes, sleep apnea, and chronic kidney disease. Although the direct vasodilator minoxidil was introduced in the early 1970s, only recently has this drug been shown to be particularly effective in a subgroup of patients with treatment-resistant or uncontrolled hypertension. This pharmacological approach is interesting from a mechanistic perspective as minoxidil is the only clinically used K⁺ channel opener today, which targets a subclass of K⁺ channels, namely K_ATP channels in VSMCs. Beside K_ATP channels, two other classes of VSMC K⁺ channels could represent novel effective targets for treatment of resistant hypertension, namely K_v 7 (KCNQ) and inward rectifier potassium (K_ir 2.1) channels. Interestingly, these channels are unique among VSMC potassium channels. First, both have been implicated in the control of microvascular tone by perivascular adipose tissue. Second, they exhibit biophysical properties strongly controlled and regulated by membrane voltage, but not intracellular calcium. This review focuses on K_v 7 (K_v 7.1-5) and K_ir (K_ir 2.1) channels in VSMCs as potential novel drug targets for treatment of resistant hypertension, particularly in comorbid conditions such as obesity and metabolic syndrome.

Boosting the signal: Endothelial inward rectifier K+ channels

Endothelial cells express a diverse array of ion channels including members of the strong inward rectifier family composed of K_IR 2 subunits. These two-membrane spanning domain channels are modulated by their lipid environment, and exist in macromolecular signaling complexes with receptors, protein kinases and other ion channels. Inward rectifier K⁺ channel (K_IR ) currents display a region of negative slope conductance at membrane potentials positive to the K⁺ equilibrium potential that allows outward current through the channels to be activated by membrane hyperpolarization, permitting K_IR to amplify hyperpolarization induced by other K⁺ channels and ion transporters. Increases in extracellular K⁺ concentration activate K_IR allowing them to sense extracellular K⁺ concentration and transduce this change into membrane hyperpolarization. These properties position K_IR to participate in the mechanism of action of hyperpolarizing vasodilators and contribute to cell-cell conduction of hyperpolarization along the wall of microvessels. The expression of K_IR in capillaries in electrically active tissues may allow K_IR to sense extracellular K⁺ , contributing to functional hyperemia. Understanding the regulation of expression and function of microvascular endothelial K_IR will improve our understanding of the control of blood flow in the microcirculation in health and disease and may provide new targets for the development of therapeutics in the future.

The Conducted Vasomotor Response: Function, Biophysical Basis, and Pharmacological Control

Arterial tone is coordinated among vessel segments to optimize nutrient transport and organ function. Coordinated vasomotor activity is remarkable to observe and depends on stimuli, sparsely generated in tissue, eliciting electrical responses that conduct lengthwise among electrically coupled vascular cells. The conducted response is the focus of this topical review, and in this regard, the authors highlight literature that advances an appreciation of functional significance, cellular mechanisms, and biophysical principles. Of particular note, this review stresses that conduction is enabled by a defined pattern of charge movement along the arterial wall as set by three key parameters (tissue structure, gap junctional resistivity, and ion channel activity). The impact of disease on conduction is carefully discussed, as are potential strategies to restore this key biological response and, along with it, the match of blood flow delivery with tissue energetic demand.

Restoration of the response of the middle cerebral artery of the rat to acidosis in hyposmotic hyponatremia by the opener of large-conductance calcium sensitive potassium channels (BKCa)

Hyposmotic hyponatremia (the decrease of extracellular concentration of sodium ions from 145 to 121 mM and the decrease of hyposmolality from 300 to 250 mOsm/kg H₂O) impairs response of the middle cerebral artery (MCA) to acetylcholine and NO donor (S-nitroso-N-acetyl-DL-penicillamine). Since acidosis activates a similar intracellular signaling pathway, the present study was designed to verify the hypothesis that the response of the MCA to acidosis is impaired during acute hyposmotic hyponatremia due to abnormal NO-related signal transduction in vascular smooth muscle cells. Studies performed on isolated, cannulated, and pressurized rat MCA revealed that hyposmotic hyponatremia impaired the response of the MCA to acidosis and this was associated with hyposmolality rather than with decreased sodium ion concentration. Response to acidosis was restored by the BK_Ca but not by the K_ATP channel activator. Patch-clamp electrophysiology performed on myocytes freshly isolated from MCAs, demonstrated that hyposmotic hyponatremia does not affect BK_Ca currents but decreases the voltage-dependency of the activation of the BK_Ca channels in the presence of a specific opener of these channels. Our study suggests that reduced sensitivity of BK_Ca channels in the MCA to agonists results in the lack of response of this artery to acidosis during acute hyposmotic hyponatremia.

KIR channels tune electrical communication in cerebral arteries

The conducted vasomotor response reflects electrical communication in the arterial wall and the distance signals spread is regulated by three factors including resident ion channels. This study defined the role of inward-rectifying K⁺ channels (K_IR) in governing electrical communication along hamster cerebral arteries. Focal KCl application induced a vasoconstriction that conducted robustly, indicative of electrical communication among cells. Inhibiting dominant K⁺ conductances had no attenuating effect, the exception being Ba²⁺ blockade of K_IR. Electrophysiology and Q-PCR analysis of smooth muscle cells revealed a Ba²⁺-sensitive K_IR current comprised of K_IR2.1/2.2 subunits. This current was surprisingly small and when incorporated into a model, failed to account for the observed changes in conduction. We theorized a second population of K_IR channels exist and consistent with this idea, a robust Ba²⁺-sensitive K_IR2.1/2.2 current was observed in endothelial cells. When both K_IR currents were incorporated into, and then inhibited in our model, conduction decay was substantive, aligning with experiments. Enhanced decay was ascribed to the rightward shift in membrane potential and the increased feedback arising from voltage-dependent-K⁺ channels. In summary, this study shows that two K_IR populations work collaboratively to govern electrical communication and the spread of vasomotor responses along cerebral arteries.

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

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

Inhibition of vascular smooth muscle inward-rectifier K+ channels restores myogenic tone in mouse urinary bladder arterioles

Prolonged decreases in urinary bladder blood flow are linked to overactive and underactive bladder pathologies. However, the mechanisms regulating bladder vascular reactivity are largely unknown. To investigate these mechanisms, we examined myogenic and vasoactive properties of mouse bladder feed arterioles (BFAs). Unlike similar-sized arterioles from other vascular beds, BFAs failed to constrict in response to increases in intraluminal pressure (5-80 mmHg). Consistent with this lack of myogenic tone, arteriolar smooth muscle cell membrane potential was hyperpolarized (-72.8 ± 1.4 mV) at 20 mmHg and unaffected by increasing pressure to 80 mmHg (-74.3 ± 2.2 mV). In contrast, BFAs constricted to the thromboxane analog U-46619 (100 nM), the adrenergic agonist phenylephrine (10 µM), and KCl (60 mM). Inhibition of nitric oxide synthase or intermediate- and small-conductance Ca²⁺-activated K⁺ channels did not alter arteriolar diameter, indicating that the dilated state of BFAs is not attributable to overactive endothelium-dependent dilatory influences. Myocytes isolated from BFAs exhibited BaCl₂ (100 µM)-sensitive K⁺ currents consistent with strong inward-rectifier K⁺ (K_IR) channels. Notably, block of these K_IR channels "restored" pressure-induced constriction and membrane depolarization. This suggests that these channels, in part, account for hyperpolarization and associated absence of tone in BFAs. Furthermore, smooth muscle-specific knockout of K_IR2.1 caused significant myogenic tone to develop at physiological pressures. This suggests that 1) the regulation of vascular tone in the bladder is independent of pressure, insofar as pressure-induced depolarizing conductances cannot overcome K_IR2.1-mediated hyperpolarization; and 2) maintenance of bladder blood flow during bladder filling is likely controlled by neurohumoral influences.

Measuring T-Type Calcium Channel Currents in Isolated Vascular Smooth Muscle Cells

Patch clamp electrophysiology is a powerful tool that has been important in isolating and characterizing the ion channels that govern cellular excitability under physiological and pathophysiological conditions. The ability to enzymatically dissociate blood vessels and acutely isolate vascular smooth muscle cells has enabled the application of patch clamp electrophysiology to the identification of diverse voltage dependent ion channels that ultimately control vasoconstriction and vasodilation. Since intraluminal pressure results in depolarization of vascular smooth muscle, the channels that control the voltage dependent influx of extracellular calcium are of particular interest. This chapter describes methods for isolating smooth muscle cells from resistance vessels, and for recording, isolating, and characterizing voltage dependent calcium channel currents, using patch clamp electrophysiological and pharmacological protocols.

Potassium Channels in Regulation of Vascular Smooth Muscle Contraction and Growth

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

Equol increases cerebral blood flow in rats via activation of large-conductance Ca(2+)-activated K(+) channels in vascular smooth muscle cells

The present study was designed to investigate the effect of equol on cerebral blood flow and the underlying molecular mechanisms. The regional cerebral blood flow in parietal lobe of rats was measured by using a laser Doppler flowmetry. Isolated cerebral basilar artery and mesenteric artery rings from rats were used for vascular reactivity measurement with a multi wire myography system. Outward K(+) current in smooth muscle cells of cerebral basilar artery, large-conductance Ca(2+)-activated K(+) (BK) channel current in BK-HEK 293 cells stably expressing both human α (hSlo)- and β1-subunits, and hSlo channel current in hSlo-HEK 293 cells expressing only the α-subunit of BK channels were recorded with whole cell patch-clamp technique. The results showed that equol significantly increased regional cerebral blood flow in rats, and produced a concentration-dependent but endothelium-independent relaxation in rat cerebral basilar arteries. Both paxilline and iberiotoxin, two selective BK channel blockers, significantly inhibited equol-induced vasodilation in cerebral arteries. Outward K(+) currents in smooth muscle cells of cerebral basilar artery were increased by equol and fully reversed by washout or blockade of BK channels with iberiotoxin. Equol remarkably enhanced human BK current in BK-HEK 293 cells, but not hSlo current in hSlo-HEK 293 cells, and the increase was completely abolished by co-application of paxilline. Our findings provide the first information that equol selectively stimulates BK channel current by acting on its β1 subunit, which may in turn contribute to the equol-mediated vasodilation and cerebral blood flow increase.

Vascular inward rectifier K+ channels as external K+ sensors in the control of cerebral blood flow

For decades it has been known that external K(+) ions are rapid and potent vasodilators that increase CBF. Recent studies have implicated the local release of K(+) from astrocytic endfeet-which encase the entirety of the parenchymal vasculature-in the dynamic regulation of local CBF during NVC. It has been proposed that the activation of KIR channels in the vascular wall by external K(+) is a central component of these hyperemic responses; however, a number of significant gaps in our knowledge remain. Here, we explore the concept that vascular KIR channels are the major extracellular K(+) sensors in the control of CBF. We propose that K(+) is an ideal mediator of NVC, and discuss KIR channels as effectors that produce rapid hyperpolarization and robust vasodilation of cerebral arterioles. We provide evidence that KIR channels, of the KIR 2 subtype in particular, are present in both the endothelial and SM cells of parenchymal arterioles and propose that this dual positioning of KIR 2 channels increases the robustness of the vasodilation to external K(+), enables the endothelium to be actively engaged in NVC, and permits electrical signaling through the endothelial syncytium to promote upstream vasodilation to modulate CBF.

Calcium dynamics underlying the myogenic response of the renal afferent arteriole

The renal afferent arteriole reacts to an elevation in blood pressure with an increase in muscle tone and a decrease in luminal diameter. This effect, known as the myogenic response, is believed to stabilize glomerular filtration and to protect the glomerulus from systolic blood pressure increases, especially in hypertension. To study the mechanisms underlying the myogenic response, we developed a mathematical model of intracellular Ca(2+) signaling in an afferent arteriole smooth muscle cell. The model represents detailed transmembrane ionic transport, intracellular Ca(2+) dynamics, the kinetics of myosin light chain phosphorylation, and the mechanical behavior of the cell. It assumes that the myogenic response is initiated by pressure-induced changes in the activity of nonselective cation channels. Our model predicts spontaneous vasomotion at physiological luminal pressures and KCl- and diltiazem-induced diameter changes comparable to experimental findings. The time-periodic oscillations stem from the dynamic exchange of Ca(2+) between the cytosol and the sarcoplasmic reticulum, coupled to the stimulation of Ca(2+)-activated potassium (KCa) and chloride (ClCa) channels, and the modulation of voltage-activated L-type channels; blocking sarco/endoplasmic reticulum Ca(2+) pumps, ryanodine receptors (RyR), KCa, ClCa, or L-type channels abolishes these oscillations. Our results indicate that the profile of the myogenic response is also strongly dependent on the conductance of ClCa and L-type channels, as well as the activity of plasmalemmal Ca(2+) pumps. Furthermore, inhibition of KCa is not necessary to induce myogenic contraction. Lastly, our model suggests that the kinetic behavior of L-type channels results in myogenic kinetics that are substantially faster during constriction than during dilation, consistent with in vitro observations (Loutzenhiser R, Bidani A, Chilton L. Circ. Res. 90: 1316-1324, 2002).

Mechanisms underlying regional differences in the Ca2+ sensitivity of BK(Ca) current in arteriolar smooth muscle

Abstract  β1-Subunits enhance the gating properties of large-conductance Ca(2+)-activated K(+) channels (BKCa) formed by α-subunits. In arterial vascular smooth muscle cells (VSMCs), β1-subunits are vital in coupling SR-generated Ca(2+) sparks to BKCa activation, affecting contractility and blood pressure. Studies in cremaster and cerebral VSMCs show heterogeneity of BKCa activity due to apparent differences in the functional β1-subunit:α-subunit ratio. To define these differences, studies were conducted at the single-channel level while siRNA was used to manipulate specific subunit expression. β1 modulation of the α-subunit Ca(2+) sensitivity was studied using patch-clamp techniques. BKCa channel normalized open probability (NPo) versus membrane potential (Vm) curves were more left-shifted in cerebral versus cremaster VSMCs as cytoplasmic Ca(2+) was raised from 0.5 to 100 μm. Calculated V1/2 values of channel activation decreased from 72.0 ± 6.1 at 0.5 μm Ca(2+)i to -89 ± 9 mV at 100 μm Ca(2+)i in cerebral compared with 101 ± 10 to -63 ± 7 mV in cremaster VSMCs. Cremaster BKCa channels thus demonstrated an ∼2.5-fold weaker apparent Ca(2+) sensitivity such that at a value of Vm of -30 mV, a mean value of [Ca(2+)]i of 39 μm was required to open half of the channels in cremaster versus 16 μm [Ca(2+)]i in cerebral VSMCs. Further, shortened mean open and longer mean closed times were evident in BKCa channel events from cremaster VSMCs at either -30 or 30 mV at any given [Ca(2+)]. β1-Subunit-directed siRNA decreased both the apparent Ca(2+) sensitivity of BKCa in cerebral VSMCs and the appearance of spontaneous transient outward currents. The data are consistent with a higher ratio of β1-subunit:α-subunit of BKCa channels in cerebral compared with cremaster VSMCs. Functionally, this leads both to higher Ca(2+) sensitivity and NPo for BKCa channels in the cerebral vasculature relative to that of skeletal muscle.

Identification of L- and T-type Ca2+ channels in rat cerebral arteries: role in myogenic tone development

L-type Ca(2+) channels are broadly expressed in arterial smooth muscle cells, and their voltage-dependent properties are important in tone development. Recent studies have noted that these Ca(2+) channels are not singularly expressed in vascular tissue and that other subtypes are likely present. In this study, we ascertained which voltage-gated Ca(2+) channels are expressed in rat cerebral arterial smooth muscle and determined their contribution to the myogenic response. mRNA analysis revealed that the α(1)-subunit of L-type (Ca(v)1.2) and T-type (Ca(v)3.1 and Ca(v)3.2) Ca(2+) channels are present in isolated smooth muscle cells. Western blot analysis subsequently confirmed protein expression in whole arteries. With the use of patch clamp electrophysiology, nifedipine-sensitive and -insensitive Ba(2+) currents were isolated and each were shown to retain electrical characteristics consistent with L- and T-type Ca(2+) channels. The nifedipine-insensitive Ba(2+) current was blocked by mibefradil, kurtoxin, and efonidpine, T-type Ca(2+) channel inhibitors. Pressure myography revealed that L-type Ca(2+) channel inhibition reduced tone at 20 and 80 mmHg, with the greatest effect at high pressure when the vessel is depolarized. In comparison, the effect of T-type Ca(2+) channel blockade on myogenic tone was more limited, with their greatest effect at low pressure where vessels are hyperpolarized. Blood flow modeling revealed that the vasomotor responses induced by T-type Ca(2+) blockade could alter arterial flow by ∼20-50%. Overall, our findings indicate that L- and T-type Ca(2+) channels are expressed in cerebral arterial smooth muscle and can be electrically isolated from one another. Both conductances contribute to myogenic tone, although their overall contribution is unequal.

Impairment of neurovascular coupling in type 1 diabetes mellitus in rats is linked to PKC modulation of BK(Ca) and Kir channels

We hypothesized that chronic hyperglycemia has a detrimental effect on neurovascular coupling in the brain and that this may be linked to protein kinase C (PKC)-mediated phosphorylation. Therefore, in a rat model of streptozotocin-induced chronic type 1 diabetes mellitus (T1DM), and in nondiabetic (ND) controls, we monitored pial arteriole diameter changes during sciatic nerve stimulation and topical applications of the large-conductance Ca(2+)-operated K(+) channel (BK(Ca)) opener, NS-1619, or the K(+) inward rectifier (Kir) channel agonist, K(+). In the T1DM vs. ND rats, the dilatory response associated with sciatic nerve stimulation was decreased by ∼30%, whereas pial arteriolar dilations to NS-1619 and K(+) were largely suppressed. These responses were completely restored by the acute topical application of a PKC antagonist, calphostin C. Moreover, the suffusion of a PKC activator, phorbol 12,13-dibutyrate, in ND rats was able to reproduce the vascular reactivity impairments found in T1DM rats. Assay of PKC activity in brain samples from T1DM vs. ND rats revealed a significant gain in activity only in specimens harvested from the pial and superficial glia limitans tissue, but not in bulk cortical gray matter. Altogether, these findings suggest that the T1DM-associated impairment of neurovascular coupling may be mechanistically linked to a readily reversible PKC-mediated depression of BK(Ca) and Kir channel activity.

Development of an image-based system for measurement of membrane potential, intracellular Ca(2+) and contraction in arteriolar smooth muscle cells

OBJECTIVE

Changes in smooth muscle cell (SMC) membrane potential (Em) are critical to vasomotor responses. As a fluorescent indicator approach would lessen limitations of glass electrodes in contracting preparations, we aimed to develop a Forster (or fluorescence) resonance energy transfer (FRET)-based measurement for Em.

METHODS

The FRET pair used in this study (donor CC2-DMPE [excitation 405 nm] and acceptor DisBAC(4) (3)) provide rapid measurements at a sensitivity not achievable with many ratiometric indicators. The method also combined measurement of changes in Ca(2+) (i) using fluo-4 and excitation at 490 nm.

RESULTS

After establishing loading conditions, a linear relationship was demonstrated between Em and fluorescence signal in FRET dye-loaded HEK cells held under voltage clamp. Over the voltage range from -70 to +30 mV, slope (of FRET signal vs. voltage, m) = 0.49 ± 0.07, r(2)  = 0.96 ± 0.025. Similar data were obtained in cerebral artery SMCs, slope (m) = 0.30 ± 0.02, r(2)  = 0.98 ± 0.02. Change in FRET emission ratio over the holding potential of -70 to +30 mV was 41.7 ± 4.9% for HEK cells and 30.0 ± 2.3% for arterial SMCs. The FRET signal was also shown to be modulated by KCl-induced depolarization in a concentration-dependent manner. Further, in isolated arterial SMCs, KCl-induced depolarization (60 mM) measurements occurred with increased fluo-4 fluorescence emission (62 ± 9%) and contraction (-27 ± 4.2%).

CONCLUSIONS

The data support the FRET-based approach for measuring changes in Em in arterial SMCs. Further, image-based measurements of Em can be combined with analysis of temporal changes in Ca(2+) (i) and contraction.

Protein kinases modulate store-operated channels in pulmonary artery smooth muscle cells

Background

This study investigates whether protein kinase G (PKG), protein kinase A (PKA) and protein kinase C (PKC) are involved in the regulatory mechanisms of store-operated channel (SOC) in pulmonary arteries.

Methods

Pulmonary artery smooth muscle cells (PASMCs) were enzymatically dissociated from rat intralobar pulmonary arteries. Whole cell, cell-attached and inside-out patch-clamp electrophysiology were used to monitor SOCs in isolated PASMCs.

Results

Initially the Ca²⁺-ATPase inhibitor cyclopiazonic acid (CPA, 10 μM) initiated a whole cell current that was reduced by the SOC blocker SKF-96365 (10 μM). Subsequent work using both cell-attached and whole cell configurations revealed that the PKG and PKA inhibitors, KT5823 (3 μM) and H-89 (10 μM), also stimulated SOC activity; this augmentation was attenuated by the SOC blockers SKF-96365 (10 μM) and Ni²⁺ (0.1 mM). Finally using the inside-out configuration, the PKC activator phorbol 12-myristate 13-acetate (PMA, 10 μM) was confirmed to modestly stimulate SOC activity although this augmentation appeared to be more substantial following the application of 10 μM inositol 1,4,5-triphosphate (Ins(1,4,5)P₃).

Conclusions

SOC activity in PASMCs was stimulated by the inhibition of PKG and PKA and the activation of PKC. Our findings suggest that the SOC could be a substrate of these protein kinases, which therefore would regulate the intracellular concentration of calcium and pulmonary arteriopathy via SOC.

Baicalin, a flavonoid from Scutellaria baicalensis Georgi, activates large-conductance Ca2+-activated K+ channels via cyclic nucleotide-dependent protein kinases in mesenteric artery

Baicalin isolated from Scutellaria baicalensis is a traditional Chinese herbal medicine used for cardiovascular dysfunction. The ionic mechanism of the vasorelaxant effects of baicalin remains unclear. We investigated whether baicalin relaxes mesenteric arteries (MAs) via large-conductance Ca2+-activated K+ (BK(Ca)) channel activation and voltage-dependent Ca2+ channel (VDCC) inhibition. The contractility of MA was determined by dual wire myograph. BK(Ca) channels and VDCCs were measured using whole-cell recordings in single myocytes, enzymatically dispersed from rat MAs. Baicalin (10-100 microM) attenuated 80 mM KCl-contracted MA in a concentration-related manner. L-NAME (30 microM) and indomethacin (10 microM) little affected baicalin (100 microM)-induced vasorelaxations. Contractions induced by iberiotoxin (IbTX, 0.1 microM), Bay K8644 (0.1 microM) or PMA (10 microM) were abolished by baicalin 100 microM. In MA myocytes, baicalin (0.3-30 microM) enhanced BK(Ca) channel activity in a concentration-dependent manner. Increased BK(Ca) currents were abolished by IbTX (0.1 microM). Baicalin-mediated (30 microM) BK(Ca) current activation was significantly attenuated by an adenylate cyclase inhibitor (SQ 22536, 10 microM), a soluble guanylate cyclase inhibitor (ODQ, 10 microM), competitive antagonists of cAMP and cGMP (Rp-cAMP, 100 microM and Rp-cGMP, 100 microM), and cAMP- and cGMP-dependent protein kinase inhibitors (KT5720, 0.3 microM and KT5823, 0.3 microM). Perfusate with PMA (0.1 microM) abolished baicalin-enhanced BK(Ca) currents. Additionally, baicalin (0.3-30 microM) reduced the amplitude of VDCC currents in a concentration-dependent manner and abolished VDCC activator Bay K8644-enhanced (0.1 microM) currents. Baicalin produced MA relaxation by activating BK(Ca) and inhibiting VDCC channels by endothelium-independent mechanisms and by stimulating the cGMP/PKG and cAMP/PKA pathways.

Ca(v)1.2 splice variant with exon 9* is critical for regulation of cerebral artery diameter

L-type voltage-dependent Ca(2+) channels (VDCCs) are essential for numerous processes in the cardiovascular and nervous systems. Alternative splicing modulates proteomic composition of Ca(v)1.2 to generate functional variation between channel isoforms. Here, we describe expression and function of Ca(v)1.2 channels containing alternatively spliced exon 9* in cerebral artery myocytes. RT-PCR showed expression of Ca(v)1.2 splice variants both containing (alpha(1)C(9/9*/10)) and lacking (alpha(1)C(9/10)) exon 9* in intact rabbit and human cerebral arteries. With the use of laser capture microdissection and RT-PCR, expression of mRNA for both alpha(1)C(9/9*/10) and alpha(1)C(9/10) was demonstrated in isolated cerebral artery myocytes. Quantitative real-time PCR revealed significantly greater alpha(1)C(9/9*/10) expression relative to alpha(1)C(9/10) in intact rabbit cerebral arteries compared with cardiac tissue and cerebral cortex. To demonstrate a functional role for alpha(1)C(9/9*/10), smooth muscle of intact cerebral arteries was treated with antisense oligonucleotides targeting alpha(1)C(9/9*/10) (alpha(1)C(9/9*/10)-AS) or exon 9 (alpha(1)C-AS), expressed in all Ca(v)1.2 splice variants, by reversible permeabilization and organ cultured for 1-4 days. Treatment with alpha(1)C(9/9*/10)-AS reduced maximal constriction induced by elevated extracellular K(+) ([K(+)](o)) by approximately 75% compared with alpha(1)C(9/9*/10-)sense-treated arteries. Maximal constriction in response to the Ca(2+) ionophore ionomycin and [K(+)](o) EC(50) values were not altered by antisense treatment. Decreases in maximal [K(+)](o)-induced constriction were similar between alpha(1)C(9/9*/10)-AS and alpha(1)C-AS groups (22.7 + or - 9% and 25.6 + or - 4% constriction, respectively). We conclude that although cerebral artery myocytes express both alpha(1)C(9/9*/10) and alpha(1)C(9/10) VDCC splice variants, alpha(1)C(9/9*/10) is functionally dominant in the control of cerebral artery diameter.

Heterogeneity in function of small artery smooth muscle BKCa: involvement of the beta1-subunit

Arteriolar myogenic vasoconstriction occurs when increased stretch or membrane tension leads to smooth muscle cell depolarization and opening of voltage-gated Ca2+ channels. To prevent positive feedback and excessive pressure-induced vasoconstriction, studies in cerebral artery smooth muscle have suggested that activation of large conductance, Ca2+-activated K+ channels (BKCa) provides an opposing hyperpolarizing influence reducing Ca2+ channel activity. We have hypothesized that this mechanism may not equally apply to all vascular beds. To establish the existence of such heterogeneity in vascular reactivity, studies were performed on rat vascular smooth muscle (VSM) cells from cremaster muscle arterioles and cerebral arteries. Whole cell K+ currents were determined at pipette [Ca2+] of 100 nM or 5 microM in the presence and absence of the BKCa inhibitor, iberiotoxin (IBTX; 0.1 microM). Similar outward current densities were observed for the two cell preparations at the lower pipette Ca2+ levels. At 5 microM Ca2+, cremaster VSM showed a significantly (P < 0.05) lower current density compared to cerebral VSM (34.5 +/- 1.9 vs 45.5 +/- 1.7 pA pF(-1) at +70 mV). Studies with IBTX suggested that the differences in K+ conductance at 5 microM intracellular [Ca2+] were largely due to activity of BKCa. 17beta-Oestradiol (1 microM), reported to potentiate BKCa current via the channel's beta-subunit, caused a greater effect on whole cell K+ currents in cerebral vessel smooth muscle cells (SMCs) compared to those of cremaster muscle. In contrast, the alpha-subunit-selective BKCa opener, NS-1619 (20 microM), exerted a similar effect in both preparations. Spontaneously transient outward currents (STOCs) were more apparent (frequency and amplitude) and occurred at more negative membrane potentials in cerebral compared to cremaster SMCs. Also consistent with decreased STOC activity in cremaster SMCs was an absence of detectable Ca2+ sparks (0 of 76 cells) compared to that in cerebral SMCs (76 of 105 cells). Quantitative PCR showed decreased mRNA expression for the beta1 subunit and a decrease in the beta1:alpha ratio in cremaster arterioles compared to cerebral vessels. Similarly, cremaster arterioles showed a decrease in total BKCa protein and the beta1:alpha-subunit ratio. The data support vascular heterogeneity with respect to the activity of BKCa in terms of both beta-subunit regulation and interaction with SR-mediated Ca2+ signalling.

Rho-kinase-mediated suppression of KDR current in cerebral arteries requires an intact actin cytoskeleton

This study examined the role of the actin cytoskeleton in Rho-kinase-mediated suppression of the delayed-rectifier K(+) (K(DR)) current in cerebral arteries. Myocytes from rat cerebral arteries were enzymatically isolated, and whole cell K(DR) currents were monitored using conventional patch-clamp electrophysiology. At +40 mV, the K(DR) current averaged 19.8 +/- 1.6 pA/pF (mean +/- SE) and was potently inhibited by UTP (3 x 10(-5) M). This suppression was observed to depend on Rho signaling and was abolished by the Rho-kinase inhibitors H-1152 (3 x 10(-7) M) and Y-27632 (3 x 10(-5) M). Rho-kinase was also found to concomitantly facilitate actin polymerization in response to UTP. We therefore examined whether actin dynamics played a role in the ability of Rho-kinase to suppress K(DR) current and found that actin disruption using either cytochalasin D (1 x 10(-5) M) or latrunculin A (1 x 10(-8) M) prevented current modulation. Consistent with our electrophysiological observations, both Rho-kinase inhibition and actin disruption significantly attenuated UTP-induced depolarization and constriction of cerebral arteries. We propose that UTP initiates Rho-kinase-mediated remodeling of the actin cytoskeleton and consequently suppresses the K(DR) current, thereby facilitating the depolarization and constriction of cerebral arteries.

Inward rectifier K(+) currents and Kir2.1 expression in renal afferent and efferent arterioles

The afferent and efferent arterioles regulate the inflow and outflow resistance of the glomerulus, acting in concert to control the glomerular capillary pressure and glomerular filtration rate. The myocytes of these two vessels are remarkably different, especially regarding electromechanical coupling. This study investigated the expression and function of inward rectifier K(+) channels in these two vessels using perfused hydronephrotic rat kidneys and arterioles and myocytes isolated from normal rat kidneys. In afferent arterioles pre-constricted with angiotensin II, elevating [K(+)](0) from 5 to 15 mmol/L induced hyperpolarization (-27 +/- 2 to 41 +/- 3 mV) and vasodilation (6.6 +/- 0.9 to 13.1 +/- 0.6 microm). This manipulation also attenuated angiotensin II-induced Ca(2+) signaling, an effect blocked by 100 micromol/LBa(2+). By contrast, elevating [K(+)](o) did not alter angiotensin II-induced Ca2(+) signaling or vasoconstriction in efferent arterioles, even though a significant hyperpolarization was observed (from -30 +/- 1 to 37 +/- 3 mV, P = 0.003). Both vessels expressed mRNA for Kir2.1 and exhibited anti-Kir2.1 antibody labeling.Patch-clamp measurements revealed prominent inwardly rectifying and Ba(2+)-sensitive currents in afferent and efferent arteriolar myocytes. Our findings indicate that both arterioles express an inward rectifier K(+) current, but that modulation of this current alters responsiveness of only the a different arteriole. The expression of Kir in the efferent arteriole, a resistance vessel whose tone is not affected by membrane potential, is intriguing and may suggest a novel function of this channel in the renal microcirculation.

KIR channels function as electrical amplifiers in rat vascular smooth muscle

Strong inward rectifying K(+) (K(IR)) channels have been observed in vascular smooth muscle and can display negative slope conductance. In principle, this biophysical characteristic could enable K(IR) channels to 'amplify' responses initiated by other K(+) conductances. To test this, we have characterized the diversity of smooth muscle K(IR) properties in resistance arteries, confirmed the presence of negative slope conductance and then determined whether K(IR) inhibition alters the responsiveness of middle cerebral, coronary septal and third-order mesenteric arteries to K(+) channel activators. Our initial characterization revealed that smooth muscle K(IR) channels were highly expressed in cerebral and coronary, but not mesenteric arteries. These channels comprised K(IR)2.1 and 2.2 subunits and electrophysiological recordings demonstrated that they display negative slope conductance. Computational modelling predicted that a K(IR)-like current could amplify the hyperpolarization and dilatation initiated by a vascular K(+) conductance. This prediction was consistent with experimental observations which showed that 30 mum Ba(2+) attenuated the ability of K(+) channel activators to dilate cerebral and coronary arteries. This attenuation was absent in mesenteric arteries where smooth muscle K(IR) channels were poorly expressed. In summary, smooth muscle K(IR) expression varies among resistance arteries and when channel are expressed, their negative slope conductance amplifies responses initiated by smooth muscle and endothelial K(+) conductances. These findings highlight the fact that the subtle biophysical properties of K(IR) have a substantive, albeit indirect, role in enabling agonists to alter the electrical state of a multilayered artery.

Activation of BKCa channels via cyclic AMP- and cyclic GMP-dependent protein kinases by eugenosedin-A in rat basilar artery myocytes

BACKGROUND AND PURPOSE

The study investigated whether eugenosedin-A, a 5-hydroxytryptamine and alpha/beta adrenoceptor antagonist, enhanced delayed-rectifier potassium (K(DR))- or large-conductance Ca(2+)-activated potassium (BK(Ca))-channel activity in basilar artery myocytes through cyclic AMP/GMP-dependent and -independent protein kinases.

EXPERIMENTAL APPROACH

Cerebral smooth muscle cells (SMCs) were enzymatically dissociated from rat basilar arteries. Conventional whole cell, perforated and inside-out patch-clamp electrophysiology was used to monitor K(+)- and Ca(2+)-channel activities.

KEY RESULTS

Eugenosedin-A (1 microM) did not affect the K(DR) current but dramatically augmented BK(Ca) channel activity in a concentration-dependent manner. Increased BK(Ca) current was abolished by charybdotoxin (ChTX, 0.1 microM) or iberiotoxin (IbTX, 0.1 microM), but not affected by a small-conductance K(Ca) blocker (apamin, 100 microM). BK(Ca) current activation by eugenosedin-A was significantly inhibited by an adenylate cyclase inhibitor (SQ 22536, 10 microM), a soluble guanylate cyclase inhibitor (ODQ, 10 microM), competitive antagonists of cAMP and cGMP (Rp-cAMP, 100 microM and Rp-cGMP, 100 microM), and cAMP- and cGMP-dependent protein kinase inhibitors (KT5720, 0.3 microM and KT5823, 0.3 microM). Eugenosedin-A reversed the inhibition of BK(Ca) current induced by the protein kinase C activator, phorbol myristyl acetate (PMA, 0.1 microM). Eugenosedin-A also prevented BK(Ca) current inhibition induced by adding PMA, KT5720 and KT5823. Moreover, eugenosedin-A reduced the amplitude of voltage-dependent L-type Ca(2+) current (I(Ca,L)), but without modifying the voltage-dependence of the current.

CONCLUSIONS AND IMPLICATIONS

Eugenosedin-A enhanced BK(Ca) currents by stimulating the activity of cyclic nucleotide-dependent protein kinases. Physiologically, this activation would result in the closure of voltage-dependent calcium channels and thereby relax cerebral SMCs.