TASK-1 Channels: Functional Role in Arterial Smooth Muscle Cells

The change in the diameter of small arteries and arterioles is a key mechanism for regulating the resistance of the vascular bed and blood pressure and blood flow in organs and tissues. The tone of arterial smooth muscle cells (SMC) depends on the level of membrane potential (MP), which, in turn, is determined by the balance of depolarizing and hyperpolarizing currents. The main hyperpolarizing current of SMC is the outward potassium current. Activation and opening of potassium channels counteract depolarization and inhibit calcium entry into the cell and contraction. Thus, potassium channels play an anticonstrictor role in the arteries. TASK-1 channels, members of the two-pore domain potassium channel family (K2P), have relatively recently been described in the vasculature. It is known that TASK-1 channels mediate outward potassium leakage current in arterial SMC. In addition, TASK-1 channels are regulated by a number of stimuli: their activity augments with an increase of extracellular pH, decreases in hypoxia, and can also change under the influence of inhalation/local anesthetics and vasoactive substances. TASK-1 channels play an important role in the regulation of arterial tone in pulmonary circulation; their dysfunction is one of the causes of arterial pulmonary hypertension development. In systemic arteries of adult animals, the influence of TASK-1 channels under normal pH is small or absent, but it can manifest itself under conditions of extracellular alkalosis. In addition, the anticontractile role of TASK-1 channels is more pronounced at the early stages of postnatal development. This review outlines the current understanding of the functional role and regulation of TASK-1 channels in the vascular system.


INTRODUCTION
Potassium channels of smooth muscle cells (SMC) play a key role in the regulation of arterial tone [1]. They mediate the outward potassium current, which leads to hyperpolarization of the SMC membrane and, consequently, to a decrease in the activity of voltage-gated calcium channels and vasodilation. Thus, the activation of potassium channels counteracts the SMC contraction in the arteries. There are five functional types of potassium channels in arterial SMC: voltage-gated potassium channels (K v ), large conductance calcium-activated potassium channels (BK Ca ), inward-rectifier potassium channels (K ir ), ATP-sensitive potassium channels (K ATP ), and two-pore domain potassium channels (K 2P ) [1,2]. K 2P channels are the least studied type of potassium channels in the vascular system.
In recent years, one of the members of K 2P channels, TASK-1 (the weak inward-rectifying K + (TWIK)-related acid-sensitive K + channel), has been of particular interest to researchers [2]. Expression of the TASK-1 channel has been demonstrated in vessels of various organs [3][4][5][6]. The development of some cardiovascular disorders is associated with dysfunction of TASK-1 channels [7][8][9][10], which determines the relevance of studying the functional role of these channels in arteries. The purpose of this review is to summarize the current concepts of the vasomotor role of TASK-1 channels in the arteries of the pulmonary and systemic circulation as well as to consider the regulatory mechanisms of the activity of TASK-1 channels by substances that cause constriction and dilatation of blood vessels.

POTASSIUM CHANNELS OF ARTERIAL SMOOTH MUSCLE: KEY PARTICIPANTS IN THE REGULATION OF VASCULAR TONE
In this section, we briefly characterize the functional role and specific features of the regulation of potassium channels present in the arterial SMC.
Inward-rectifying potassium channels (K ir ) owe their name to their ability to conduct inward potassium current at the MP values above the equilibrium potential for potassium ions (E K ). The SMC resting potential is lower than E K and ranges from -50 to -60 mV [3,19,20]. Under these conditions, K ir channels mediate outward potassium current, but its amplitude is small, since the channel pore is blocked from the cytoplasmic side by Mg 2+ and Ca 2+ ions as well as by polyamines [21,22]. K ir channels mediate relaxation of arterial smooth muscle in response to a moderate increase in the external concentration of potassium (up to 15-20 mM), which can occur, for example, with an increase in neuronal activity in local areas of the brain [23]. Such increase in the extracellular potassium concentration leads to a shift in the equilibrium potassium potential in the electropositive direction, which results in the maximization of amplitude of the outward K ir -mediated current at a membrane potential value close to the SMC resting potential. This results in hyperpolarization and relaxation of SMC, dilation of the vessel, and, consequently, an increase in blood flow to actively working organs. ATP-sensitive potassium channels (K ATP ) are one of the subfamilies of the K ir family. The current through K ATP does not have the property of inward rectification but is regulated by intracellular ATP [24,25]. With a lack of ATP, and, consequently, an increase in the ADP/ATP ratio, the channel is activated [26,27]. The functional role of K ATP channels in the SMC of blood vessels is especially pronounced when metabolic processes in the surrounding tissue are enhanced (for example, in skeletal muscle during intense contractions). Under these conditions, vascular SMC are deficient in ATP, which contributes to the opening of K ATP channels, hyperpolarization, and relaxation [24]. In addition, the role of K ATP channels in the regulation of vascular tone is extremely significant under such pathological conditions as ischemia, hypoxia, and endotoxemia [24,25].
Two-pore domain potassium channels (K 2P ) have been relatively recently found in the vascular bed [28]. Unlike other families of potassium channels, the K 2P channel is formed not by four but only by two poreforming α-subunits (Fig. 1). Each subunit consists of four transmembrane segments (S1-S4). Between the first and second, as well as the third and fourth segments, there are loops that form the pore of the channel. Thus, there are two pore-forming domains per channel subunit (P-loops), hence the name of the channel, K 2P . K 2P channels are an extensive family of potassium channels that includes six structural and functional classes [2]. Under basal conditions, K 2P channels mediate outward potassium current, and the probability of their opening depends little on the voltage difference across the membrane [2,29], which allows us to call them leak channels. Along with this, different classes of K 2P channels are regulated by pH, O 2 , phospholipids, anesthetics, and mechanical stimuli (membrane stretching) [2].
TASK channels are a class of acid-sensitive K 2P channels that include a number of members: TASK-1, TASK-2, TASK-3, TASK-4, and TASK-5 channels [2]. The content of the TASK-1 channel in the vascular system is the highest compared to other members of TASK channels as well as compared to other classes of K 2P channels [3][4][5]30].

TASK-1: MEMBER OF ACID-SENSITIVE K 2P CHANNEL CLASS
Like other members of the K 2P channel family, TASK-1 is a leak channel [29]. In accordance with this, as follows from the current-voltage characteristic of the TASK-1 channel, the dependence of the amplitude of the current mediated by it on the membrane potential is close to linear (the property of the inward rectification is weakly pronounced) [29,31]. A distinctive feature of the class of TASK channels is their sensitivity to extracellular pH: acidosis leads to inhibi- tion, while alkalosis leads to activation of these channels [32]. This property of TASK-1 channels was demonstrated using the patch-clamp method both when channel was expressed in oocytes [31,32] and in isolated SMC of pulmonary arteries [5,28,33]. In both cases, acidification of the extracellular solution to pH values below 7.3-7.4 caused a decrease, while alkalization to pH values above 7.3-7.4 caused an increase in the amplitude of the TASK-1-mediated current. Under conditions of normal physiological pH 7.3-7.4, the amplitude of the TASK-1-mediated current had an intermediate value, i.e., the TASK-1 channel can also be functionally active under normal pH conditions. The sensitivity of the TASK-1 channel to acidification is attributed to a specific region in the channel structure. The amino acid residues of histidine (H98, part of P1) and aspartic acid (D204, part of P2) form the H98-D204-H98-D204 ring at the outer mouth of the ion-selective channel filter [34]. Protonation of this region with a decrease in extracellular pH leads to suppression of the TASK-1-mediated current [34,35].
Potassium channels of most families are characterized by interaction with specific regulatory subunits that modulate the biophysical properties of the channel. For example, the regulatory β subunit of the BK Ca channel increases its sensitivity to calcium [36]. The interaction of the K v 7.4 channel with the regulatory KCNE4 subunit leads to a decrease in the channel activation threshold [37]. Regulatory SUR subunits (SUR-sulfonylurea receptor) impart K ATP channels' sensitivity to nucleotides and other regulatory molecules [26,27]. For the TASK-1 channel, the presence of regulatory subunits has not yet been shown. Nevertheless, it is known that TASK-1 channels interact with a number of proteins that regulate their location in the cell. It has been shown that the interaction with the 14-3-3 protein promotes the incorporation of the channel into the plasma membrane [38], while the interaction with the COPI proteins and syntaxin-8, on the contrary, leads to a decrease in the number of TASK-1 channels in the plasma membrane of the cell as a result of their endocytosis or retention in intracellular compartments (endoplasmic reticulum or Golgi apparatus) [38,39].

PHARMACOLOGICAL APPROACHES TO STUDY THE FUNCTIONAL ROLE
OF TASK-1 CHANNELS The generally accepted approach to identifying the effect of various types of potassium channels on the level of the MP and contractile activity is the use of their blockers and/or activators. Early studies used such substances as anandamide, methanandamide, and bupivacaine as TASK-1 channel blockers [30,40]. However, there is some evidence supporting the effect of these substances on ion channels other than TASK-1, with effective concentration ranges for "side effect" and blockade of TASK-1 channels overlapping significantly. For example, anandamide promoted the development of vasodilation in the rat coronary artery by activating BK Ca channels [41]. In isolated rat aortic SMC, anandamide and methanandamide caused a decrease in the amplitude of the current mediated by K v channels [42]. It is important to note that researchers associate these effects with the direct effect of anandamide and methanandamide on potassium channels, regardless of the activation of cannabinoid receptors. The local anesthetic bupivacaine reduced the amplitude of the BK Ca -mediated current in isolated human umbilical artery SMC [43]. On the other hand, it has been shown that some inhalation anesthetics, such as isoflurane and halothane, can increase the amplitude of the TASK-1-mediated current [44]. In this regard, in some studies, the functional role of TASK-1 channels was evaluated using these sub- Pore Pore Pore stances [40]. At the same time, it has been shown that halothane and isoflurane can inhibit calcium current in SMC of the coronary arteries [45]. Thus, all the substances discussed above are not selective for TASK-1 channels, so the data obtained with their use should be interpreted with caution. It was shown relatively recently that AVE1231 (or A293) can block TASK-1 channels [46]. AVE1231 was originally developed as a K v 1.5 channel blocker and was considered an antiarrhythmic drug [47,48]. However, it was later found that AVE1231 blocks TASK-1 channels more effectively than K v 1.5 [46]. The activity of TASK-1 channels was assessed using the patchclamp method in the oocyte expression system of the clawed frog Xenopus laevis [46]. The half-maximal inhibition concentration (IC50) value of TASK-1 channels was 0.22 μM, while the IC50 for K v 1.5 was 9.50 μM, i.e., AVE1231 was 43 times more effective for TASK-1 channels. This made it possible to study the vasomotor role of TASK-1 channels with this relatively selective blocker. It is important that AVE1231 is able to increase the contractile responses of isolated arteries under conditions of blockade of K v 1.5 channels, i.e., by selectively influencing TASK-1 activity [3].

Contribution of TASK-1 Channels to the Regulation of Arterial Tone of the Pulmonary Circulation
Expression of the pore-forming subunit of the TASK-1 channel was found in SMC of the pulmonary arteries of rats [5,30], rabbits [28], and humans [5,33]. Accordingly, the current mediated by TASK-1 channels can be detected in SMC of the pulmonary arteries, and the anticonstrictor effect of these channels in the pulmonary circulation has been demonstrated both in experiments on isolated arteries and at the systemic level.
A number of studies have shown that TASK-1 channels have a hyperpolarizing effect in the arteries of the pulmonary circulation [5,33,49]. Suppressing the expression of TASK-1 channels by small interfering RNAs led to a decrease in TASK-1-mediated current and to depolarization of the membrane of isolated human pulmonary artery SMCs [33]. Similarly, the blockade of TASK-1 channels with AVE1231 and mutation of the Kcnk3 gene associated with impaired functioning of TASK-1 led to a decrease in the SMC MP in rat pulmonary arteries [5,49].
The addition of anandamide and bupivacaine caused an increase in the tone of the small pulmonary arteries of the rat [30]. Incubation of isolated segments of rat pulmonary arteries with the more selective blocker of TASK-1 channel AVE1231 enhanced vascular contractile responses to the thromboxane A2 receptor agonist U46619 [5]. Thus, nor-mally, TASK-1 channels help to maintain the resting potential of SMC in the pulmonary arteries, counteracting the development of contraction.
Changes in the MP of SMC in the pulmonary arteries with an increase and decrease in extracellular pH are also associated with the activity of TASK-1 channels. Thus, alkalization of the extracellular solution to pH 8.4 caused hyperpolarization, while acidification to pH 6.4 caused depolarization of SMC preparations of rat pulmonary arteries [30]. In these experiments, the combined blockade of the K v , BK Ca , K ATP , and K ir channels did not abolish the shifts of the SMC MP during acidification and alkalization, which indicates the key role of TASK-1 in these changes. In addition, incubation with anandamide or bupivacaine in a solution with pH 8.4 led to a shift in the MP of SMC to the level characteristic of pH 6.4.
The above-described changes in the MP of SMC of the pulmonary arteries in response to changes in extracellular pH correlate with the data of in vivo experiments: it has been shown that acidosis is accompanied by an increase and alkalosis, on the contrary, by a decrease in arterial pressure in the pulmonary circulation in pigs and rabbits [50,51].
Pulmonary arteries, unlike systemic ones, contract in response to hypoxia, which is important for gas exchange in the lungs. As a result of this constriction, blood is directed to more oxygenated areas of the lungs, which provides an optimal ratio between the perfusion of the lungs with blood and their ventilation with air. Since hypoxia depolarizes SMC of the pulmonary arteries [52], and TASK-1 channels make a significant contribution to maintaining the resting potential of these cells, it is logical to assume that hypoxia-induced pulmonary vasoconstriction is associated with suppression of the functioning of TASK-1 channels. Indeed, hypoxia led to a decrease in the amplitude of the TASK-1-mediated current in the SMC of human pulmonary arteries [33,53]. There are reasons to believe that this reaction is realized with the participation of nonreceptor Src-tyrosine kinase (Srckinase). It has been shown that, under normoxic conditions, TASK-1 channels interact with active (phosphorylated at the Tyr419 site) Src kinase, while hypoxia reduces the activity of Src kinase, causes dephosphorylation of the TASK-1 channel, and reduces the number of TASK-1 channels interacting with Src kinase. [53]. These changes were accompanied by a decrease in the amplitude of the TASK-1mediated current, and, consequently, SMC depolarization [53]. Accordingly, inhibition of Src kinase significantly increased pulmonary vascular resistance in mice [53]. The mechanism of suppression of Src kinase activity under the action of hypoxia is not fully understood; it may be associated with a decrease in the influence of reactive oxygen species (ROS), which, as is known, are able to activate Src kinase directly or indirectly by suppressing the activity of tyrosine phos- phatase [54]. Experiments on SMC of human pulmonary arteries have shown that short-term exposure to hypoxia (for 5-10 min) is accompanied by a decrease in ROS production [55]. Assuming that ROS produced in arterial SMC maintain Src kinase in an active state during normoxia, a decrease in their production under hypoxia can reduce the activity of Src kinase and, consequently, TASK-1 channels. The mechanism described above requires experimental confirmation, but the association between hypoxia-induced pulmonary vasoconstriction and suppression of the functioning of TASK-1 channels is beyond doubt. It is important to note that the sensitivity of TASK-1 channels to hypoxia is not directly related to tissue acidification that may accompany hypoxia. An increase in vascular resistance is the main cause of the development of arterial pulmonary hypertension, a serious disease characterized by increased pressure in the pulmonary circulation. Since TASK-1 channels normally have a pronounced anticonstrictor effect in the pulmonary arteries [5,33], it is not surprising that the disruption of their functioning can serve as one of the mechanisms of the pathogenesis of pulmonary hypertension. Indeed, a decrease in the amplitude of the TASK-1-mediated current and depolarization of the SMC membrane of the pulmonary arteries have been shown in rats with monocrotaline-induced pulmonary hypertension [5]. The content of TASK-1 channel pore-forming subunit protein in the lung tissue of these rats was reduced compared to that in the control group [5]. Moreover, chronic administration of the TASK-1 channel blocker AVE1231 to healthy rats for 4 weeks led to the appearance of a number of signs of the development of pulmonary hypertension: hypertrophy of the muscular layer of the pulmonary arteries and an increase in systolic pressure in the right ventricle [5].
To date, 12 mutations in the Kcnk3 gene encoding the pore-forming subunit of the TASK-1 channel have been described in patients with familial and idiopathic forms of pulmonary arterial hypertension [7][8][9][10]. The electrophysiological characteristics of TASK-1 channels altered as a result of these mutations were studied in cellular expression systems using the patch-clamp method. All studied mutations led to disruption of the functioning of the TASK-1 channel, which manifested itself in a decrease in the amplitude of the current mediated by it [7,8]. Similarly, a decrease in the content of mRNA and protein of the pore-forming subunit of the TASK-1 channel, as well as a significant decrease in the amplitude of the current sensitive to the TASK-1 channel blocker AVE1231, was demonstrated in SMC isolated from the pulmonary arteries of people with idiopathic and familial forms of pulmonary arterial hypertension [5]. Finally, a strain of rats with a mutation in the Kcnk3 gene, which leads to dysfunction of the TASK-1 channel and the development of pulmonary hypertension, was created and characterized relatively recently [49]. Based on the above, we can conclude that the dysfunction of the TASK-1 channel is one of the causes of the development of arterial pulmonary hypertension in humans and experimental animals.
It is noteworthy that mice are not a suitable object for studying the functional role of TASK-1 channels in the pulmonary circulation. It was found that the electrophysiological characteristics of SMC and the contractile responses of the pulmonary arteries of Kcnk3 knockout mice to various vasoconstrictors were not changed compared to the responses of the arteries of wild-type mice [56]. In addition, such mice are not prone to the development of arterial pulmonary hypertension [57]. Apparently, the vasomotor role of TASK-1 channels in mice is small. Indeed, the amplitude of the TASK-1-mediated current in the SMC of the pulmonary arteries in mice is five times lower than in rats [56]. Perhaps, the leading anticonstrictor role in the arteries of the pulmonary circulation in mice play a member of another class of K 2P channels, TWIK-2. This assumption is supported by the fact that knockout of the Kcnk6 gene encoding the TWIK-2 channel in mice led to the appearance of signs of arterial pulmonary hypertension: an increase in systolic pressure in the right ventricle, a thickening of the muscular layer of the pulmonary vessels, and an increase in the contractile responses of the pulmonary arteries to the thromboxane A2 receptor agonist U46619 [58].
Thus, the key role of TASK-1 channels in the regulation of arterial tone of the pulmonary circulation is beyond doubt. The main functions of the TASK-1 channels in the pulmonary arteries are listed in Fig. 2. TASK-1 channels help maintain the resting potential of SMC in the pulmonary arteries, preventing activation of voltage-gated calcium channels and vasoconstriction. In addition, TASK-1 channels mediate changes in pulmonary vascular smooth muscle MP in response to acidification and alkalization of the extracellular environment. The contraction of pulmonary vessels in response to hypoxia is implemented in part by suppressing the activity of the TASK-1 channel. Finally, disruption of the functioning of TASK-1 channels is one of the causes of the development of arterial pulmonary hypertension.

Contribution of TASK-1 Channels to the Regulation of the Arterial Tone of the Systemic Circulation
The role of TASK-1 channels in the SMC of the arteries of the systemic circulation has been studied to a much lesser extent as compared to the pulmonary arteries. It has been shown that TASK-1 channels are expressed in the aorta [40], middle cerebral artery [4], mesenteric arteries [30], and saphenous artery of rats [3]. At the same time, there is reason to believe that the contribution of TASK-1 channels to the regulation of vascular tone in the pulmonary and systemic circulation is not the same. Thus, the level of systemic arterial pressure in rats with a mutation suppressing the activ-ity of the TASK-1 channel did not differ from that in control animals [49]. At the same time, blood pressure in the pulmonary circulation was significantly higher in rats with the TASK-1 channel mutation [49]. Intravenous administration of AVE1231 also had no effect on systemic arterial pressure in mature rats [3] and pigs [59]. Thus, in vivo experiments did not reveal a functional role of TASK-1 channels in the regulation of systemic arterial pressure in contrast to the pressure in the pulmonary circulation.
In vitro blockade of TASK-1 channels by AVE1231 did not change the contractile responses of small arteries in the brain, heart [60], or skin [3] of mature rats.
The role of potassium channels, including TASK-1, in the regulation of systemic artery tone may change during postnatal ontogenesis [61]. In our experiments, AVE1231 significantly increased the basal tone and contractile responses of the saphenous artery in 1-2 week old rats but not in adult animals [3]. In accordance with this, the blockade of TASK-1 channels was accompanied by a more pronounced depolarization of SMC in the arteries of young rats than in the arteries of adult rats [3]. In addition, the smooth muscle of the saphenous artery of rat pups was characterized by a significantly higher content of mRNA and protein of the TASK-1 channel pore-forming subunit [3]. Finally, intravenous administration of AVE1231 resulted in an increase of systemic arterial pressure in pups but not in adult rats [3]. Thus, the anticonstrictor role of TASK-1 channels in systemic arteries decreases as the organism matures.
In the systemic arteries of an adult organism, the functional role of TASK-1 channels can manifest itself under certain conditions, for example, under the action of TASK-1 channel activating substances or when the extracellular environment is alkalized. Thus, halothane, which is capable to activate TASK-1 channels, caused hyperpolarization of SMCs in the rat aorta, and inhibition of TASK-1 channels with methanandamide eliminated the effect of halothane [40]. Similarly, increasing the pH of the extracellular solution from 7.4 to 8.4 resulted in SMC hyperpolarization in the rat's mesenteric artery [30]. To prove the involvement of TASK-1 channels in this reaction, the authors used anandamide and bupivacaine. Against the background of high pH (8.4), these substances caused SMC depolarization, bringing the MP value closer to the level observed at pH 6.4 [30]. According to our data, blockade of TASK-1 channels with AVE1231 under conditions of alkalization of the extracellular solution to pH 7.75 causes an increase in the tone of the interlobar arteries of the kidney up to 20% of the maximum contraction force [62]. It should be noted that an increase in blood pH causes a decrease in renal blood flow [63]. The pronounced anticontractile effect of TASK-1 channels at alkaline extracellular pH can attenuate the decrease in renal blood flow and, thereby, increase the ability of the kidney to compensate for systemic alkalosis. Thus, the functional contribution of TASK-1 channels in systemic arteries under normal pH conditions in adult animals is small or absent (Fig. 2). However, TASK-1 channels may exhibit an anticonstrictor role at the early stages of postnatal development, as well as in conditions of extracellular alkalosis, at least in some regions of the systemic circulation (Fig. 2).

REGULATION OF TASK-1 CHANNEL ACTIVITY BY INTRACELLULAR SIGNALING PATHWAYS
The interaction of potassium channels with participants of intracellular signaling cascades, in particular, phosphorylation by protein kinases, is an important mechanism for regulating their activity [1]. The effect of protein kinases on the activity of the main types of potassium channels (K v , BK Ca , K ir , and K ATP ) has a number of regularities. Thus, protein kinase A (PKA) and protein kinase G (PKG), activated under the action of vasodilators (NO, prostacyclin, β-adrenergic agonists, etc.), increase the activity of potassium channels, which helps to relax arterial SMC [1]. Conversely, substances that cause vasoconstriction (agonists of α1-adrenergic receptors, thromboxane A2 receptors, etc.) contribute to the activation of protein kinase C (PKC) or Rho-kinase (Rho-associated protein kinase ROCK), which inhibit the activity of potassium channels, leading to depolarization and contraction [1]. Next, we will consider how the activity of TASK-1 channels changes under the influence of vasoconstrictors and vasodilators.

Vasoconstrictors and TASK-1 Channels
Endothelin-1 secreted by arterial endothelium makes a significant contribution to the development of arterial pulmonary hypertension [64]. According to the literature, endothelin-1 can affect the activity of TASK-1 channels through different mechanisms.
In isolated SMC of human and mouse pulmonary arteries, as well as in rat cardiomyocytes, endothelin-1 inhibited TASK-1 channels by activating ET A receptors (endothelin receptor type A) associated with the G q -protein (GTP-binding protein with α q subunit) [63,64]. This results in triggering the signaling cascade of phospholipase C (PLC)-phosphatidylinositol 4,5-bisphosphate (PIP 2 )-diacylglycerol (DAG)-PKC, the participants of which are able to influence the activity of TASK-1.
In rat cardiomyocytes, the decrease in TASK-1mediated current was abolished by the PLC but not by PKC inhibitor [66]. On the other hand, the amplitude of the current mediated by TASK-1 channels in the expression system of Xenopus laevis oocytes increased under the action of PIP 2 [67]. Thus, PIP 2 has a positive effect on the activity of the TASK-1 channel. According to some authors, it may be necessary for the constitutive activity of TASK-1 channels [68]. Suppression of TASK-1-mediated current under the action of endothelin-1 may be associated with a decrease in the amount of PIP 2 as a result of its hydrolysis by PLC.
Several studies have shown that the decrease in the activity of TASK-1 channels under the action of endothelin-1 is associated with the activation of PKC. Thus, the addition of PKC inhibitors eliminated the endothelin-induced suppression of TASK-1-mediated current, while the DAG analog, on the contrary, suppressed TASK-1-mediated current in SMC isolated from human and mouse pulmonary arteries, which indicates a negative effect of PKC on TASK-1 channels [65]. In accordance with this, the content of the phosphorylated TASK-1 channel significantly increased under the action of endothelin-1 [65]. On HEK (cell line derived from human embryonic kidney) and PC12 (cell line derived from rat adrenal medulla pheochromocytoma) cell cultures, PKC activation has been shown to lead to internalization of TASK-1 channels [67,68]. It is important to note that suppression of the TASK-1-mediated current by PKC took several minutes, which is consistent with the time course of the endocytosis process. The suppression of the functioning of TASK-1 channels under the influence of endothelin-1 is probably associated with their phosphorylation by PKC, which, in turn, leads to the movement of the channel from the plasma membrane into intracellular compartments.
Another group of researchers showed that the suppression of the functioning of TASK-1 channels under the action of endothelin-1 in SMC of human pulmonary arteries is mediated not by PKC but by Rhokinase. The Rho kinase inhibitor Y-27632 attenuated endothelin-mediated suppression of current through TASK-1 channels [71]. The molecular mechanisms of this phenomenon were studied on clawed frog oocytes. It turned out that endothelin-1-induced suppression of TASK-1 channels is mediated by both ET A and ET B receptors (endothelin receptor type B) and is not associated with PKC activation [71]. In addition, Rho kinase phosphorylation sites (Ser-393 and Ser-336) were found in the structure of the TASK-1 channel.
Such difference in the mechanisms of endothelinmediated suppression of the function of TASK-1 channels may be due to different states of pulmonary SMC used in the experiments: freshly isolated [65] or cultured [71]. It is known that the cultivation of SMC leads to their dedifferentiation from the contractile to the synthetic (or proliferative) phenotype, which is accompanied by a change in the expression of many proteins that regulate contraction [72]. The contribution of PKC is high in contractile SMC and decreases during cultivation [73]. On the contrary, the contribution of Rho kinase is higher in SMC of the proliferative phenotype; data on high levels of expression and activity of Rho kinase in the SMC of the arteries of new-born rats when smooth muscle still retains a number of signs of the proliferative phenotype support this assumption [72,73].
In fairness, it should be noted that there are literature data on the positive effect of PKC on the activity of TASK-1 channels. An agonist of P2Y 11 receptors (associated with G q protein) activated TASK-1-mediated current in mouse aortic SMC [76]. The authors suggested a key role of PKC in this process since phorbol ester (PKC activator) caused an increase in the amplitude of TASK-1-mediated current against the background of blockade of other types of potassium channels (K v, BK Ca , K ir , and K ATP ), and the PKC Go 6850 inhibitor suppressed the effect of phorbol ester. However, as mentioned above, mice are not the most suitable object for such studies due to the low activity of TASK-1 channels in arterial SMC [56]. Therefore, the results of this work should be interpreted with caution.
Thus, the results of most studies indicate a positive effect of PIP 2 and a negative effect of PKC and Rhokinase on the activity of TASK-1 channels in arterial SMC under the action of vasoconstrictors (Fig. 3).

Vasodilators and TASK-1 Channels
Prostacyclin secreted by endothelial cells causes relaxation of the smooth muscle of the arteries. SMC receptors to prostacyclin are coupled to G s protein (GTP-binding protein with α s subunit). Activation of these receptors triggers the adenylate cyclase (AC)cyclic adenosine monophosphate (cAMP)-PKA signaling cascade. Treprostinil, a stable prostacyclin analog, has been shown to activate TASK-1 channels in SMC of human pulmonary arteries, causing PKAdependent TASK-1 phosphorylation [33].
Like prostacyclin receptors, SMC β-adrenergic receptors are coupled to the G s protein. Thus, activation of β-adrenergic receptors also leads to vasodilation through the signaling cascade described above [77]. It has been shown that hyperpolarization and relaxation of the rat pulmonary artery induced by the β-adrenoceptor agonist isoprenaline decrease when the activity of TASK-1 channels is suppressed by anandamide or when the pH of the solution is reduced to 6.4 [78]. This indicates that TASK-1 channels may be involved in SMC relaxation upon activation of β-adrenergic receptors.
Another important vasodilator secreted by the vascular endothelium is nitric oxide (NO). Penetrating into SMC, NO interacts with soluble guanylate cyclase (sGC), which leads to an increase in the amount of cyclic guanosine monophosphate (cGMP) and activation of PKG. PKG-dependent relaxation of arterial SMCs is due to phosphorylation of a number of targets, including potassium channels [79].
The sGC activator riocigulate has been shown to increase the TASK-1-mediated current in the tsA201 cell expression system (modification of human embryonic kidney cells) [80], which suggests a positive effect of the NO-sGC-cGMP-PKG signaling pathway on TASK-1 channels. In SMC of the rat middle cerebral artery, NO and cGMP analogue also increased the amplitude of the outward potassium current [4]. The authors found that the mRNA content of the TASK-1 channel in this artery is quite high and also demonstrated the presence of PKG phosphorylation sites in the structure of the TASK-1 channel. Therefore, they suggested the participation of Fig. 3. Regulation of the activity of TASK-1 channels under the action of vasoconstrictors and vasodilators. Designations: ACadenylate cyclase; ATP-adenosine triphosphate; cAMP-cyclic adenosine monophosphate; cGMP-cyclic guanosine monophosphate; DAG-diacylglycerol; GTP-guanosine triphosphate; IP3-inositol triphosphate; NO-nitric oxide; PIP2-phosphatidylinositol 4,5-bisphosphate; PKA-protein kinase A; P-protein kinase C; PKG-protein kinase G; PLC-phospholipase C; RhoA-small GTP-binding protein A, Rho kinase activator; sGC-soluble guanylate cyclase; TASK-acid-sensitive K 2P channels. these channels in the response to NO and cGMP analogue. However, the addition of tetraethylammonium, to which TASK-1 channels are insensitive, abolished the effects of NO and the cGMP analog. As a result, it was concluded that relaxation of the rat middle cerebral artery in response to NO is not associated with the activation of TASK-1 channels [4]. However, this does not exclude the effect of NO on the activity of TASK-1 channels in the arteries of other rat organs or in the arteries of other mammals. Thus, TASK-1 channels in arteries are characterized by activation by PKA-dependent phosphorylation under the action of G s -coupled receptor agonists, such as prostacyclin and β-adrenergic receptor agonists (Fig. 3). As for the signaling cascade triggered by NO, both its positive effect on the activity of TASK-1 channels and the absence of the effect are shown. Obviously, this issue requires further research (Fig. 3).
TASK-1 channels play an important role in the regulation of vascular tone, while their contribution in the arteries of the pulmonary and systemic circulation is different. In the pulmonary arteries, TASK-1 channels exhibit a pronounced anticonstrictor role at normal pH levels and normoxia, and impaired functioning of these channels may be one of the causes of the development of a dangerous disease, arterial pulmonary hypertension. In systemic arteries, the functional contribution of TASK-1 under normal pH conditions in adult animals is small but is pronounced in the period of early postnatal ontogenesis. However, under conditions of extracellular alkalosis, TASK-1 channels exhibit an anticonstrictor role in both pulmonary and systemic arteries. The effects of a number of vasoactive substances on vascular tone are associated with changes in the activity of TASK-1 channels. As a rule, vasodilators contribute to the activation, while vasoconstrictors, on the contrary, suppress the activity of the TASK-1 channels of SMC, which is similar to the features of the regulation of the activity of other families of potassium channels.