Acute hypoxia induces vasodilation and increases coronary blood flow by activating inward rectifier K(+) channels

Potassium Channels in the Transition from Fetal to the Neonatal Pulmonary Circulation

The transition from the fetal to the neonatal circulation includes dilatation of the pulmonary arteries (PA) and closure of the Ductus Arteriosus Botalli (DAB). The resting membrane potential and various potassium channel activities in smooth muscle cells (SMC) from fetal and neonatal PA and DAB obtained from the same species has not been systematically analyzed. The key issue addressed in this paper is how the resting membrane potential and the whole-cell potassium current (IK) change when PASMC or DABSMC are transitioned from hypoxia, reflecting the fetal state, to normoxia, reflecting the post-partal state. Patch-clamp measurements were employed to characterize whole-cell K⁺ channel activity in fetal and post-partal (newborn) PASMC and DABSMC. The main finding of this paper is that the SMC from both tissues use a similar set of K⁺ channels (voltage-dependent (Kv), calcium-sensitive (KCa), TASK-1 and probably also TASK-2 channels); however, their activity level depends on the cell type and the oxygen level. Furthermore, we provide the first evidence for pH-sensitive non-inactivating K⁺ current in newborn DABSMC and PASMC, suggesting physiologically relevant TASK-1 and TASK-2 channel activity, the latter particularly in the Ductus Arteriosus Botalli.

Hypoxia and metabolic inhibitors alter the intracellular ATP:ADP ratio and membrane potential in human coronary artery smooth muscle cells

ATP-sensitive potassium (K_ATP) channels couple cellular metabolism to excitability, making them ideal candidate sensors for hypoxic vasodilation. However, it is still unknown whether cellular nucleotide levels are affected sufficiently to activate vascular K_ATP channels during hypoxia. To address this fundamental issue, we measured changes in the intracellular ATP:ADP ratio using the biosensors Perceval/PercevalHR, and membrane potential using the fluorescent probe DiBAC₄(3) in human coronary artery smooth muscle cells (HCASMCs). ATP:ADP ratio was significantly reduced by exposure to hypoxia. Application of metabolic inhibitors for oxidative phosphorylation also reduced ATP:ADP ratio. Hyperpolarization caused by inhibiting oxidative phosphorylation was blocked by either 10 µM glibenclamide or 60 mM K⁺. Hyperpolarization caused by hypoxia was abolished by 60 mM K⁺ but not by individual K⁺ channel inhibitors. Taken together, these results suggest hypoxia causes hyperpolarization in part by modulating K⁺ channels in SMCs.

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.

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.

Role of ion channels in coronary microcirculation: a review of the literature

In normal coronary arteries, several different mechanisms of blood flow regulation exist, acting at different levels of the coronary tree: endothelial, nervous, myogenic and metabolic regulation. In addition, physiologic blood flow regulation is also dependent on the activity of several coronary ion channels, including ATP-dependent K(+) channels, voltage-gated K(+) channels and others. In this context, ion channels contribute by matching demands for homeostatic maintenance. They play a primary role in rapid response of both endothelium and vascular smooth muscle cells of larger and smaller arterial vessels of the coronary bed, leading to coronary vasodilation. Consequently, an alteration in ion channel function or expression could be directly involved in coronary vasomotion dysfunction.

Mechanisms of metabolic coronary flow regulation

Coronary blood flow is tightly adjusted to the oxygen requirements of the myocardium. The underlying control mechanisms keep coronary venous pO(2) at a rather constant level around 20mm Hg under a variety of physiological conditions. Because coronary flow may increase more than 5-fold during exercise without any signs of under- or overperfusion, coronary flow must be controlled, at least in part, in a feed forward manner. Likely metabolic factors contributing to feed forward control are carbon dioxide and reactive oxygen species. Adaptation of coronary flow to exercise under physiological conditions involves in addition to metabolic control feed forward neuronal and endothelium-dependent control. Under pathological conditions, e.g. vessel stenosis or anemia, or specific environmental conditions, e.g. high altitude exposure, cardiac oxygenation may become critical, especially if oxygen demand is increased during physical exercise. Under such conditions the fall of coronary pO(2) may directly result in opening of oxygen sensitive potassium or closure of calcium channels. Furthermore the fall of pO(2) results in the production of vasoactive metabolites, e.g. adenosine, nitric oxide or prostaglandins, and in proton accumulation. All of these adaptations support a reduction of coronary vessel resistance. This article is part of a Special Issue entitled "Coronoray Blood Flow".

Chronic hypoxia enhances 15-lipoxygenase-mediated vasorelaxation in rabbit arteries

15-Lipoxygenase (15-LO-1) metabolizes arachidonic acid (AA) to 11,12,15-trihydroxyeicosatrienoic acids (THETAs) and 15-hydroxy-11,12-epoxyeicosatrienoic acids (HEETA) that dilate rabbit arteries. Increased endothelial 15-LO-1 expression enhances arterial relaxations to agonists. We tested the effect of hypoxia on 15-LO-1 expression, THETA and HEETA synthesis, and relaxations in rabbit arteries. The incubation of rabbit aortic endothelial cells and isolated aortas in 0.7% O(2) increased 15-LO-1 expression. Rabbits were housed in a hypoxic atmosphere of 12% O(2) for 5 days. 15-LO-1 expression increased in the endothelium of the arteries of rabbits in 12% O(2) compared with room air. THETA and HEETA synthesis was also enhanced in aortas and mesenteric arteries. AA hyperpolarized the smooth muscle cells in indomethacin- and phenylephrine-treated mesenteric arteries of hypoxic rabbits from -29.4 +/- 1 to -50.1 +/- 3 mV. The hyperpolarization to AA was less in arteries of normoxic rabbits (from -26.0 +/- 2 to -37 +/- 2 mV). This AA-induced hyperpolarization was inhibited by the 15-LO inhibitor BW-755C. Nitric oxide and prostaglandin-independent maximum relaxations to acetylcholine (79.7 +/- 2%) and AA (38.3 +/- 4%) were enhanced in mesenteric arteries from hypoxic rabbits compared with the normoxic rabbits (49.7 +/- 6% and 19.9 +/- 2%, respectively). These relaxations were inhibited by BW-755C and nordihydroguaiaretic acid. Therefore, hypoxia increased the relaxations to agonists in the rabbit mesenteric arteries by enhancing endothelial 15-LO-1 expression and synthesis of the hyperpolarizing factors THETA and HEETA.