1. Introduction

Appropriate local control of blood flow through resistance arteries is critical to the functioning of tissues and organs, and to regulation of blood pressure. Lying at the interface between the blood and smooth muscle cells of the vessel wall, the endothelium plays a vital role in this dynamic process by transducing diverse chemical and mechanical stimuli into

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and eproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

coordinated changes in arterial diameter. Endothelial cells respond to vasodilatory stimuli by releasing diffusible mediators such as nitric oxide (NO) and prostacyclin (PGI2) and by initiating membrane hyperpolarization that spreads to surrounding smooth muscle cells via myoendothelial gap junctions (MEGJs) to inhibit contractility, a mechanism termed endothelium-dependent hyperpolarization (EDH) [1–3]. NO and PGI2 are local mediators that diffuse to surrounding smooth muscle cells to cause relaxation. Once initiated, EDH spreads to surrounding smooth muscle cells to affect relaxation but conduction of hyperpolarization longitudinally through the endothelial layer means that EDH also contributes to coordination of changes in blood flow in multiple arterial segments within a vascular bed [4]. Thus, the ability of a stimulus to engage diffusible mediators versus EDH determines whether it's effects on arterial diameter and thus blood flow, are restricted to the local area or can impact blood flow at the level of the vascular bed.

eliminated [14]. Exposure of the endothelium to TRPV4 agonists and/or acetylcholine increased the activity of these discrete Ca2+ signals which were linked to activation of both IKCa and small conductance (SKCa) Ca2+-activated K+ channels, effects which were absent in arteries from mice lacking TRPV4 [14]. In rat cremaster arterioles, clustering of TRPV4-mediated sparklets in endothelial projections was linked exclusively to activation of IKCa channels [16] and in mouse small pulmonary arteries, shear stress-stimulated TRPV4 activity was linked to NO production [17]. Larger endothelial sparklets mediated by simultaneous opening of two TRPA1 and leading to activation of IKCa channels were shown to underlie dilation to reactive oxygen species in rat cerebral arteries [18]. We will now discuss how grouping of Ca2+ signaling and effector proteins into microdomains allows dynamic, stimulus-specific Ca2+ events which determine the recruit-

The Endothelium: The Vascular Information Exchange http://dx.doi.org/10.5772/intechopen.79897 45

In vivo, endothelial sensing of laminar shear stress, the tangential frictional force exerted by blood flowing across the cell surface, plays a dominant role in acute modulation of vascular tone and therefore, tissue perfusion [19–21]. In the majority of resistance arteries, increases in blood flow stimulate endothelium-dependent relaxation of surrounding smooth muscle cells and increase arterial diameter, a response termed flow-mediated dilatation. Flow also influences gene expression and structural remodeling with areas of disturbed flow and reduced shear stress is associated with development of atherosclerotic plaques [22]. Measurement of acute responses to increases in shear stress is the most widely used clinical index of endothelial function and vascular health with attenuation of flow-induced dilation associated with increased risk of cardiovascular diseases [23, 24]. Indeed, reductions in shear stress are a likely mechanism by which endothelial function is altered with inactivity, an effect which can be

In animals and humans, acute shear stress-induced vasodilation can be mediated by both NO and EDH [27–31]. Although endothelial cells express both SKCa and IKCa channels, data from isolated arteries indicate that it is SKCa channels that play a predominate role in mediating the EDH component of this response. Deletion of SKCa but not IKCa channels impaired both NO and EDH-mediated dilation to shear stress stimulation in mouse isolated carotid arteries [32]. In rat isolated perfused mesenteric beds, shear stress-induced modulation of sympathetic vasoconstriction was prevented by both the NOS inhibitor L-NG-nitroarginine methyl ester (L-NAME) and apamin, a selective blocker of SKCa channels, but not by the IKCa channel inhibitor TRAM-34 [33]. Similarly, shear stress-evoked dilation of mouse isolated coronary

Mechanotransduction, the conversion of increases in shear stress into changes in arterial diameter, is reliant upon rises in endothelial [Ca2+]i mediated by Ca2+ entry. In vitro studies have identified multiple candidates as potential endothelial mechanosensors including

ment of effectors thus the degree to which blood flow is impacted.

2. Stimulus-specific endothelial Ca2+ signaling

overcome by exercise interventions [25, 26].

arteries was inhibited by apamin [34] and L-NAME [35].

2.1. Shear stress

Increases in endothelial intracellular Ca2+ concentration ([Ca2+]i) drive these vasodilator pathways; NO is synthesized from L-arginine by the Ca2+-calmodulin-dependent enzyme NO synthase (NOS) [3], PGI2 is generated by the actions of cyclooxygenase on arachidonic acid released by the actions of Ca2+-dependent phospholipase A2 on membrane phospholipids [5, 6], and opening of Ca2+-activated K+ (KCa) channels causes hyperpolarization [3, 7]. Global changes in endothelial [Ca2+]i have been widely studied [8, 9], but development of new technologies such as high-speed, high-resolution confocal Ca2+ imaging and generation of transgenic mice expressing genetically encoded Ca2+ indicators has allowed resolution of a wide range of transient Ca2+ events within endothelial cells of intact arteries to provide a growing body of support for the concept of stimulus-specific engagement of effectors underpinned by spatially and temporally discrete Ca2+ signaling patterns that occur independently of changes in bulk endothelial [Ca2+]i [10–13].

Pulsars [10] and wavelets [11] are spontaneous, short-lived, (<0.03 s duration) spatially fixed Ca2+ events originating from distinct clusters of inositol 1,4,5-trisphosphate (InsP3) receptors on the membrane of endoplasmic reticulum (ER). First identified in mouse mesenteric artery and hamster skeletal muscle arteriolar endothelial cells, these events predominantly occur close to endothelial projections that abut or form MEGJs with smooth muscle cells [10, 11] and exert a basal vasodilator influence through activation of intermediate conductance (IKCa) Ca2+-activated K+ channels. Their dependence on InsP3 provides a mechanism by which pulsars are linked to and regulated by G protein-coupled receptor (GPCR) signaling. Elevation of InsP3 by endothelium-dependent vasodilators [10] or by flux of InsP3 from smooth muscle cells following stimulation of α1-adrenoceptors [11] increases pulsar size and/or frequency through recruitment of new sites and a reduction in the interval between pulsars at a given site. In porcine coronary arteries, InsP3-dependent Ca2+ events similar to pulsars propagate into longer lasting Ca2+ waves (>8 s duration) facilitated by the longitudinal arrangement of ER/InsP3 receptors to promote directional Ca2+-induced Ca2+ release along the endothelial cell axis and are associated with activation of both NOS and KCa channels [12].

Sparklets are generated by spatially restricted Ca2+ influx through members of the transient receptor potential (TRP) ion channel family [14, 15]. Sparklets were first identified in mouse mesenteric arteries under experimental conditions in which InsP3-mediated pulsars were eliminated [14]. Exposure of the endothelium to TRPV4 agonists and/or acetylcholine increased the activity of these discrete Ca2+ signals which were linked to activation of both IKCa and small conductance (SKCa) Ca2+-activated K+ channels, effects which were absent in arteries from mice lacking TRPV4 [14]. In rat cremaster arterioles, clustering of TRPV4-mediated sparklets in endothelial projections was linked exclusively to activation of IKCa channels [16] and in mouse small pulmonary arteries, shear stress-stimulated TRPV4 activity was linked to NO production [17]. Larger endothelial sparklets mediated by simultaneous opening of two TRPA1 and leading to activation of IKCa channels were shown to underlie dilation to reactive oxygen species in rat cerebral arteries [18]. We will now discuss how grouping of Ca2+ signaling and effector proteins into microdomains allows dynamic, stimulus-specific Ca2+ events which determine the recruitment of effectors thus the degree to which blood flow is impacted.
