Endothelial Ion Channels and Cell-Cell Communication in the Microcirculation

Abstract

Endothelial cells in resistance arteries, arterioles, and capillaries express a diverse array of ion channels that contribute to Cell-Cell communication in the microcirculation. Endothelial cells are tightly electrically coupled to their neighboring endothelial cells by gap junctions allowing ion channel-induced changes in membrane potential to be conducted for considerable distances along the endothelial cell tube that lines arterioles and forms capillaries. In addition, endothelial cells may be electrically coupled to overlying smooth muscle cells in arterioles and to pericytes in capillaries via heterocellular gap junctions allowing electrical signals generated by endothelial cell ion channels to be transmitted to overlying mural cells to affect smooth muscle or pericyte contractile activity. Arteriolar endothelial cells express inositol 1,4,5 trisphosphate receptors (IP₃Rs) and transient receptor vanilloid family member 4 (TRPV4) channels that contribute to agonist-induced endothelial Ca²⁺ signals. These Ca²⁺ signals then activate intermediate and small conductance Ca²⁺-activated K⁺ (IK_Ca and SK_Ca) channels causing vasodilator-induced endothelial hyperpolarization. This hyperpolarization can be conducted along the endothelium via homocellular gap junctions and transmitted to overlying smooth muscle cells through heterocellular gap junctions to control the activity of voltage-gated Ca²⁺ channels and smooth muscle or pericyte contraction. The IK_Ca- and SK_Ca-induced hyperpolarization may be amplified by activation of inward rectifier K⁺ (K_IR) channels. Endothelial cell IP₃R- and TRPV4-mediated Ca²⁺ signals also control the production of endothelial cell vasodilator autacoids, such as NO, PGI₂, and epoxides of arachidonic acid contributing to control of overlying vascular smooth muscle contractile activity. Cerebral capillary endothelial cells lack IK_Ca and SK_Ca but express K_IR channels, IP₃R, TRPV4, and other Ca²⁺ permeable channels allowing capillary-to-arteriole signaling via hyperpolarization and Ca²⁺. This allows parenchymal cell signals to be detected in capillaries and signaled to upstream arterioles to control blood flow to capillaries by active parenchymal cells. Thus, endothelial cell ion channels importantly participate in several forms of Cell-Cell communication in the microcirculation that contribute to microcirculatory function and homeostasis.

Keywords: arterioles; blood flow; capillaries; endothelial cells; endothelial ion channels and cell-cell communication ion channels; functional hyperemia; pericytes; vascular smooth muscle cells.

Figure 1

Microvascular unit—schematic representation of hypothetical microvascular unit that consists of a feed arteriole that branches into a terminal arteriole. The terminal arteriole transitions into an initial capillary segment that is coated with contractile pericytes including at the initial branch points, as shown. As the capillaries progress, they lose coverage by contractile pericytes which are replaces by non-contractile pericytes (not shown in the figure for clarity). As shown, the capillaries are in close association with parenchymal cells. The capillaries then converge and eventually drain into venules, which are coated in pericytic smooth muscle cells that are morphologically distinct from arteriolar smooth muscle cells. Boxes in the figure depict regions corresponding to Figures 2–5 as shown. See text for details and references.

Figure 2

Arteriolar endothelial ion channels and cell-cell communication. Shown is a schematic representation of a longitudinal cross-section through one wall of an arteriole showing cross-sections of endothelial cells and overlying smooth muscle cells. Endothelial cells communicate with overlying smooth muscle cells at myoendothelial projections (MEPs) that pass through the internal elastic lamina to make contact with overlying smooth muscle cells, as shown. Gap junctions may form at MEPs to yield myoendothelial gap junctions (MEGJs) allowing endothelial cell hyperpolarization to be conducted to the smooth muscle cells, closing smooth muscle voltage-gated Ca²⁺ channels (VGCCs), decreasing [Ca²⁺]_in, and leading to vasodilation. At MEPs transient receptor potential vanilloid family member 4 (TRPV4), intermediate conductance Ca²⁺-activated K⁺ (IK_Ca) channels and inositol 1,4,5 trisphosphate receptors (IP₃R) are clustered forming signaling complexes to direct the endothelial cell responses to vasodilator agonists. In cerebral arteries/arterioles, transient receptor potential ankyrin family member 1 (TRPA1) is also in these complexes. Other ion channels, such as transient receptor potential C family member 3 (TRPC3) channels and small conductance Ca²⁺-activated K⁺ (SK_Ca) channels, may cluster elsewhere to form other signaling complexes. Endothelium-dependent vasodilators, such as acetyl choline, act on Gα_q-coupled receptors to activate phospholipase C-β (PLCβ) which hydrolyses membrane phosphatidylinositol 4,5-bisphosphate (PIP₂) forming IP₃ and diacylglycerol (DAG). The IP₃ activates IP₃Rs to release Ca²⁺ from the ER increasing [Ca²⁺]_in. The DAG activates membrane TRPC3 and TRPC6 channels which serve as receptor-operate channels conducting Na⁺ and Ca²⁺ into the cells. DAG also activates protein kinase C (PKC; not shown) which phosphorylates and activates TRPV4 channels. Calcium influx through TRPV4 channels acts on IP₃-sensitized IP₃R, amplifying Ca²⁺ influx through the TRPV4 channels. At MEP, the increase [Ca²⁺]_in results in activation of IK_Ca channels causing membrane hyperpolarization (−ΔV_m) that is transmitted through MEGJs to hyperpolarize overlying smooth muscle causing vasodilation. Transient receptor potential C family members 1 and 4 (TRPC1 and TRPC4), and ORAI1 channels are activated upon release of Ca²⁺ from the endoplasmic reticulum (ER) via IP₃Rs that is sensed by stromal interaction molecule 1 (STIM1) in the ER membrane. Inward rectifier K⁺ (K_IR) channels (likely K_IR2.1) are expressed and can be activated by membrane hyperpolarization to facilitate hyperpolarization induced by SK_Ca and IK_Ca channels. They also are activated by increases in extracellular K⁺ and involved with sensing shear stress. Shear stress also appears to activate TRPV4 channels, transient receptor potential polycystin family member 1 (TRPP1) channels, and PIEZO1 channels leading to increased endothelial [Ca²⁺]_in and vasodilation. In addition to activating SK_Ca and IK_Ca channels to produce hyperpolarization-induced vasodilation, increased endothelial cell [Ca²⁺]_in also results in production of endothelial cell vasodilator autacoids, such as nitric oxide (NO), prostacyclin (PGI₂), and epoxides of arachidonic acid (EETs) that cause arteriolar vasodilation. See text for additional information and references.

Figure 3

Capillaries as sensors of [K⁺]_out. Schematic of a capillary segment that transitions to a vessel invested with contractile pericytes (initial capillary segment) or smooth muscle cells (terminal arteriole). Active neurons release K⁺ during action potential repolarization. This increases [K⁺]_out which is sensed by K_IR2.1 channels (KIR). Movement of K⁺ out of activated K_IR2.1 channels results in membrane hyperpolarization (−ΔV_m). The local membrane hyperpolarization activates adjacent K_IR2.1 channels allowing this hyperpolarization to be conducted from the site of initiation through homocellular gap junctions from endothelial cell-to-endothelial cell back toward the initial capillary segment (invested with contractile pericytes) and the terminal arteriole (with smooth muscle cells). These contractile mural cells are coupled to underlying endothelial cells by heterocellular gap junctions which allow transmission of the hyperpolarization to the contractile cells. In smooth muscle cells (and presumably contractile pericytes), the hyperpolarization will deactivate voltage-gated Ca²⁺ channels, decreasing [Ca²⁺]_in and leading to smooth muscle (or pericyte) relaxation and vasodilation. Dilation of the terminal arteriole (or the initial capillary segment) will result in an increase in blood flow to the microvascular unit, directing blood flow to the site of increased neural activity.

Figure 4

Capillary Ca²⁺ signaling to control pericyte contraction. Schematic of a capillary segment invested with a contractile pericyte. Increased nerve activity will result in accumulation of extracellular K⁺ (as in Figure 2) and a yet unidentified mediator (like PGE₂, for example). The increase in [K⁺]_out will hyperpolarize the membrane (−ΔV_m) as in Figure 2. The mediator will activate Gα_q-coupled receptors, leading to hydrolysis of membrane phosphatidylinositol 4,5-bisphosphate (PIP₂) to form IP₃ and diacylglycerol (DAG). The IP₃ will activate IP₃R in the endoplasmic reticulum resulting in Ca²⁺ release. The increase in [Ca²⁺]_in, along with reduced membrane PIP₂, and DAG- and Ca²⁺ activation of protein kinase C (PKC) will activate membrane transient receptor potential vanilloid family member 4 (TRPV4) channels resulting in Ca²⁺ influx which will be bolstered by K_IR-mediated hyperpolarization. This will further increase local [Ca²⁺]_in resulting in additional Ca²⁺ release through IP₃R (Ca²⁺-induced-Ca²⁺ release; CICR). The resultant increase in [Ca²⁺]_in will activate eNOS, increasing production of NO. Nitric oxide will diffuse to the overlying contractile pericyte, activating guanylyl cyclase (GC) to form cGMP, which, in turn, will activate protein kinase G (PKG). Protein kinase G will phosphorylate a number of target proteins resulting decreased [Ca²⁺]_in in the pericyte and relaxation. Pericyte relaxation will cause local dilation of the capillary resulting in an increase in blood flow (red cell flux) to this active capillary segment. While the membrane hyperpolarization clearly can be transmitted from endothelial cell-to-endothelial cell as in Figure 2, it is not clear if Ca²⁺ and/or IP₃ can be transmitted through endothelial cell gap junctions in capillaries to promote cell-to-cell conduction of Ca²⁺ signals. See text for additional information and references.

Figure 5

TRPA1- and pannexin-mediated signaling in capillaries. Schematic of a capillary segment that transitions into the initial segment of the capillary coated with contractile pericytes or the terminal arteriole invested with smooth muscle cells. Increased neuron activity leads to increased accumulation of extracellular K⁺ (as in Figures 2, 3) and a yet unidentified mediator. The mediator activates TRPA1 channels in the membrane leading to an increase in [Ca²⁺]_in that then activates adjacent pannexin 1 (PANX1) channels which release ATP into the extracellular space. ATP binds to and activates P₂X receptors resulting in additional Ca²⁺ influx. The additional increase in [Ca²⁺]_in activates more PANX1 channels, propagating the Ca²⁺ signal from cell-to-cell. As shown in Figure 2, increased [K⁺]_out activates K_IR channels causing membrane hyperpolarization (−ΔV_m). This likely helps to maintain the electrochemical gradient for Ca²⁺ influx through TRPA1 and P₂X receptors. The hyperpolarization also will be conducted cell-to-cell via homocellular gap junctions. When the Ca²⁺ signal reaches endothelial cells in the initial segment of the capillary (coated with contractile pericytes) and also the endothelium in the terminal arteriole, SK_Ca and IK_Ca channels will be activated resulting in membrane hyperpolarization that is supported by hyperpolarization-induced activation of K_IR channels. The hyperpolarization will then be transmitted to overlying contractile mural cells via heterocellular gap junctions. In smooth muscle cells (and presumably contractile pericytes), the hyperpolarization will deactivate voltage-gated Ca²⁺ channels, decreasing [Ca²⁺]_in and leading to smooth muscle (or pericyte) relaxation and vasodilation. Dilation of the terminal arteriole (or the initial capillary segment) will result in an increase in blood flow to the microvascular unit, directing blood flow to the site of increased neural activity. See text for more information and references.