Voltage-dependent K(+) current in capillary endothelial cells isolated from guinea pig heart

Cell-To-Cell Communication in the Resistance Vasculature

The arterial vasculature can be divided into large conduit arteries, intermediate contractile arteries, resistance arteries, arterioles, and capillaries. Resistance arteries and arterioles primarily function to control systemic blood pressure. The resistance arteries are composed of a layer of endothelial cells oriented parallel to the direction of blood flow, which are separated by a matrix layer termed the internal elastic lamina from several layers of smooth muscle cells oriented perpendicular to the direction of blood flow. Cells within the vessel walls communicate in a homocellular and heterocellular fashion to govern luminal diameter, arterial resistance, and blood pressure. At rest, potassium currents govern the basal state of endothelial and smooth muscle cells. Multiple stimuli can elicit rises in intracellular calcium levels in either endothelial cells or smooth muscle cells, sourced from intracellular stores such as the endoplasmic reticulum or the extracellular space. In general, activation of endothelial cells results in the production of a vasodilatory signal, usually in the form of nitric oxide or endothelial-derived hyperpolarization. Conversely, activation of smooth muscle cells results in a vasoconstriction response through smooth muscle cell contraction. © 2022 American Physiological Society. Compr Physiol 12: 1-35, 2022.

Contribution of KV1.5 Channel to Hydrogen Peroxide-Induced Human Arteriolar Dilation and Its Modulation by Coronary Artery Disease

RATIONALE

Hydrogen peroxide (H₂O₂) regulates vascular tone in the human microcirculation under physiological and pathophysiological conditions. It dilates arterioles by activating large-conductance Ca²⁺-activated K⁺ channels in subjects with coronary artery disease (CAD), but its mechanisms of action in subjects without CAD (non-CAD) when compared with those with CAD remain unknown.

OBJECTIVE

We hypothesize that H₂O₂-elicited dilation involves different K⁺ channels in non-CAD versus CAD, resulting in an altered capacity for vasodilation during disease.

METHODS AND RESULTS

H₂O₂ induced endothelium-independent vasodilation in non-CAD adipose arterioles, which was reduced by paxilline, a large-conductance Ca²⁺-activated K⁺ channel blocker, and by 4-aminopyridine, a voltage-gated K⁺ (K_V) channel blocker. Assays of mRNA transcripts, protein expression, and subcellular localization revealed that K_V1.5 is the major K_V1 channel expressed in vascular smooth muscle cells and is abundantly localized on the plasma membrane. The selective K_V1.5 blocker diphenylphosphine oxide-1 and the K_V1.3/1.5 blocker 5-(4-phenylbutoxy)psoralen reduced H₂O₂-elicited dilation to a similar extent as 4-aminopyridine, but the selective K_V1.3 blocker phenoxyalkoxypsoralen-1 was without effect. In arterioles from CAD subjects, H₂O₂-induced dilation was significantly reduced, and this dilation was inhibited by paxilline but not by 4-aminopyridine, diphenylphosphine oxide-1, or 5-(4-phenylbutoxy)psoralen. K_V1.5 cell membrane localization and diphenylphosphine oxide-1-sensitive K⁺ currents were markedly reduced in isolated vascular smooth muscle cells from CAD arterioles, although mRNA or total cellular protein expression was largely unchanged.

CONCLUSIONS

In human arterioles, H₂O₂-induced dilation is impaired in CAD, which is associated with a transition from a combined large-conductance Ca²⁺-activated K⁺- and K_V (K_V1.5)-mediated vasodilation toward a large-conductance Ca²⁺-activated K⁺-predominant mechanism of dilation. Loss of K_V1.5 vasomotor function may play an important role in microvascular dysfunction in CAD or other vascular diseases.

Role of vascular potassium channels in the regulation of renal hemodynamics

K+ conductance is a major determinant of membrane potential ( Vm) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by Vm through the action of voltage-operated Ca2+ channels (VOCC) in VSMC. Increased K+ conductance leads to hyperpolarization and vasodilation, while inactivation of K+ channels causes depolarization and vasoconstriction. K+ channels in EC indirectly participate in the control of vascular tone by several mechanisms, e.g., release of nitric oxide and endothelium-derived hyperpolarizing factor. In the kidney, a change in the activity of one or more classes of K+ channels will lead to a change in hemodynamic resistance and therefore of renal blood flow and glomerular filtration pressure. Through these effects, the activity of renal vascular K+ channels influences renal salt and water excretion, fluid homeostasis, and ultimately blood pressure. Four main classes of K+ channels [calcium activated (KCa), inward rectifier (Kir), voltage activated (KV), and ATP sensitive (KATP)] are found in the renal vasculature. Several in vitro experiments have suggested a role for individual classes of K+ channels in the regulation of renal vascular function. Results from in vivo experiments are sparse. We discuss the role of the different classes of renal vascular K+ channels and their possible role in the integrated function of the renal microvasculature. Since several pathological conditions, among them hypertension, are associated with alterations in K+ channel function, the role of renal vascular K+ channels in the control of salt and water excretion deserves attention.

Kaempferol stimulates large conductance Ca2+ -activated K+ (BKCa) channels in human umbilical vein endothelial cells via a cAMP/PKA-dependent pathway

BACKGROUND AND PURPOSE

Kaempferol has been shown to possess a vasodilator effect but its mechanism of action remains unclear. In this study, experiments were carried out to study the effect of kaempferol on K+ channels in endothelial cells.

EXPERIMENTAL APPROACH

K+ channel activities in human umbilical vein endothelial cells (HUVECs) were studied by conventional whole cell and cell-attached patch-clamp electrophysiology.

KEY RESULTS

Kaempferol stimulated an outward-rectifying current in HUVECs in a dose-dependent manner with an EC50 value of 2.5+/-0.02 microM. This kaempferol-induced current was abolished by large conductance Ca2+ -activated K+ (BKCa) channel blockers, such as iberiotoxin (IbTX) and charybdotoxin (ChTX), whereas the small conductance Ca2+ -activated K+ (SKCa) channel blocker, apamin, and the voltage-dependent K+ (KV) channel blocker, 4-aminopyridine, had no effect. Cell-attached patches demonstrated that kaempferol increased the open probability of BkCa channels in HUVECs. Clamping intracellular Ca2+ did not prevent kaempferol-induced increases in outward current. In addition, the kaempferol-induced current was diminished by the adenylyl cyclase inhibitor SQ22536, the cAMP antagonist Rp-8-Br-cAMP and the PKA inhibitor KT5720, but was not affected by the guanylyl cyclase inhibitor ODQ, the cGMP antagonist Rp-8-Br-cGMP and the PKG inhibitor KT5823. The activation of BKCa channels by kaempferol caused membrane hyperpolarization of HUVECs.

CONCLUSION AND IMPLICATIONS

These results demonstrate that kaempferol activates the opening of BKCa channels in HUVECs via a cAMP/PKA-dependent pathway, resulting in membrane hyperpolarization. This mechanism may partly account for the vasodilator effects of kaempferol.

Potassium channels in the peripheral microcirculation

Vascular smooth muscle (VSM) cells, endothelial cells (EC), and pericytes that form the walls of vessels in the microcirculation express a diverse array of ion channels that play an important role in the function of these cells and the microcirculation in both health and disease. This brief review focuses on the K+ channels expressed in smooth muscle and endothelial cells in arterioles. Microvascular VSM cells express at least four different classes of K+ channels, including inward-rectifier K+ channels (Kin), ATP-sensitive K+ channels (KATP), voltage-gated K+ channels (Kv), and large conductance Ca2+-activated K+ channels (BKCa). VSM KIR participate in dilation induced by elevated extracellular K+ and may also be activated by C-type natriuretic peptide, a putative endothelium-derived hyperpolarizing factor (EDHF). Vasodilators acting through cAMP or cGMP signaling pathways in VSM may open KATP, Kv, and BKCa, causing membrane hyperpolarization and vasodilation. VSMBKc. may also be activated by epoxides of arachidonic acid (EETs) identified as EDHF in some systems. Conversely, vasoconstrictors may close KATP, Kv, and BKCa through protein kinase C, Rho-kinase, or c-Src pathways and contribute to VSM depolarization and vasoconstriction. At the same time Kv and BKCa act in a negative feedback manner to limit depolarization and prevent vasospasm. Microvascular EC express at least 5 classes of K+ channels, including small (sKCa) and intermediate(IKCa) conductance Ca2+-activated K+ channels, Kin, KATP, and Kv. Both sK and IK are opened by endothelium-dependent vasodilators that increase EC intracellular Ca2+ to cause membrane hyper-polarization that may be conducted through myoendothelial gap junctions to hyperpolarize and relax arteriolar VSM. KIR may serve to amplify sKCa- and IKCa-induced hyperpolarization and allow active transmission of hyperpolarization along EC through gap junctions. EC KIR channels may also be opened by elevated extracellular K+ and participate in K+-induced vasodilation. EC KATP channels may be activated by vasodilators as in VSM. Kv channels may provide a negative feedback mechanism to limit depolarization in some endothelial cells.

Freshly isolated bovine coronary endothelial cells do not express the BK Ca channel gene

Recent reports have suggested that different types of Ca(2+)-activated K(+) channels may be selectively expressed either in the vascular endothelial cells (ECs) or smooth muscle cells (SMCs) of a single artery. In this study, we directly compared mRNA, protein and functional expression of the high-conductance Ca(2+)-activated K(+) (BK(Ca)) channel between freshly isolated ECs and SMCs from bovine coronary arteries. Fresh ECs and SMCs were enzymatically isolated, and their separation verified by immunofluorescent detection of alpha-actin and platelet/endothelium cell adhesion molecule (PECAM) proteins, respectively. Subsequently, studies using a sequence-specific antibody directed against the pore-forming alpha-subunit of the BK(Ca) channel only detected its expression in the SMCs, whereas PECAM-positive ECs were devoid of the alpha-subunit protein. Additionally, multicell RT-PCR performed using cDNA derived from either SMCs or ECs only detected mRNA encoding the BK(Ca) alpha-subunit in the SMCs. Finally, whole-cell recordings of outward K(+) current detected a prominent iberiotoxin-sensitive BK(Ca) current in SMCs that was absent in ECs, and the BK(Ca) channel opener NS 1619 only enhanced K(+) current in the SMCs. Thus, bovine coronary SMCs densely express BK(Ca) channels whereas adjacent ECs in the same artery appear to lack the expression of the BK(Ca) channel gene. These findings indicate a cell-specific distribution of Ca(2+)-activated K(+) channels in SMCs and ECs from a single arterial site.

Modeling diverse range of potassium channels with Brownian dynamics

Using the experimentally determined KcsA structure as a template, we propose a plausible explanation for the diversity of potassium channels seen in nature. A simplified model of KcsA is constructed from its atomic resolution structure by smoothing out the protein-water boundary and representing the atoms forming the channel protein as a homogeneous, low dielectric medium. The properties of the simplified and atomic-detail models, deduced from electrostatic calculations and Brownian dynamics simulations, are shown to be qualitatively similar. We then study how the current flowing across the simplified model channel changes as the shape of the intrapore region is modified. This is achieved by increasing the radius of the intracellular pore systematically from 1.5 to 5 A while leaving the dimensions of the selectivity filter and inner chamber unaltered. The strengths of the dipoles located near the entrances of the channel, the carbonyl groups lining the selectivity filter, and the helix macrodipoles are kept constant. The channel conductance increases steadily as the radius of the intracellular pore is increased. The rate-limiting step for both the outward and inward current is the time it takes for an ion to cross the residual energy barrier located in the intrapore region. The current-voltage relationship obtained with symmetrical solutions is linear when the applied potential is less than approximately 100 mV but deviates slightly from Ohm's law at higher applied potentials. The nonlinearity in the current-voltage curve becomes less pronounced as the radius of the intracellular pore is increased. When the strengths of the dipoles near the intracellular entrance are reduced, the channel shows a pronounced inward rectification. Finally, the conductance exhibits the saturation property observed experimentally. We discuss the implications of these findings on the transport of ions across the potassium channels and membrane channels in general.

Cellular target of voltage and calcium-dependent K(+) channel blockers involved in EDHF-mediated responses in rat superior mesenteric artery

1. We have investigated the cellular target of K(+) channel blockers responsible for the inhibition of the EDHF-mediated relaxation in the rat mesenteric artery by studying their effects on tension, smooth muscle cell (SMC) membrane potential and endothelial cell Ca(2+) signal ([Ca(2+)](endo)). 2. In arteries contracted with prostaglandin F(2 alpha) (2.5 - 10 microM), relaxation evoked by ACh (0.01 - 3 microM) was abolished by a combination of charybdotoxin (ChTX, 0.1 microM) plus apamin (Apa, 0.1 microM) and was inhibited by 68+/-6% (n=6) by 4-aminopyridine (4-AP, 5 mM). 3. ACh(0.001 - 3 microM) increased [Ca(2+)](endo) and hyperpolarized SMCs with the same potency, the pD(2) values were equal to 7.2+/-0.08 (n=4) and 7.2+/-0.07 (n=9), respectively. SMCs hyperpolarization to ACh (1 microM) was abolished by high K(+) solution or by ChTX/Apa. It was decreased by 66+/-5% (n=6) by 4-AP. 4. The increase in [Ca(2+)](endo) evoked by ACh (1 microM) was insensitive to ChTX/Apa but was depressed by 58+/-16% (n=6) and 27+/-4% (n=7) by raising external K(+) concentration and by 4-AP, respectively. 5. The effect of 4-AP on [Ca(2+)](endo) was not affected by increasing external K(+) concentration. In Ca-free/EGTA solution, the transient increase in [Ca(2+)](endo) evoked by ACh (1 microM) was abolished by thapsigargin (1 microM) and was decreased by 75+/-7% (n=5) by 4-AP. 6. These results show that inhibition of EDHF-evoked responses by 4-AP may be attributed to a decrease in the Ca(2+) release activated by ACh in endothelial cells. The abolition of SMCs hyperpolarization to ACh by ChTX/Apa is not related to an interaction with the [Ca(2+)](endo).

Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca(2+) signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca(2+)-entry pathways or stabilize the driving force for Ca(2+) influx through these pathways. These Ca(2+)-entry pathways comprise agonist-activated nonselective Ca(2+)-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca(2+) channels or capacitative Ca(2+) entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca(2+) entry is mainly controlled by large-conductance Ca(2+)-dependent BK(Ca) channels (slo), inwardly rectifying K(+) channels (Kir2.1), and at least two types of Cl( -) channels, i.e., the Ca(2+)-activated Cl(-) channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca(2+) signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.