Resistance arteries are comprised of a single endothelial layer surrounded by one or more smooth muscle layers. The internal elastic lamina (IEL) is a layer of collagen and elastin separating these two cell types. The thickness of the IEL was thought to preclude direct contact between endothelium and smooth muscle. Work over the last decade, however, have revealed the presence of “holes” in the IEL, regions devoid of elastin (Sandow et al., 2002, 2006, 2009; Ledoux et al., 2008b). These regions contain thin endothelial projections that extend through the IEL and make contact with the overlying smooth muscle (Sandow et al., 2002, 2006, 2009). While the process by which they are formed remains elusive, endothelial projections appear to retain structures such as endoplasmic reticulum (ER), caveoli, and trafficking vesicles. More importantly, the proteins essential to controlling resistance vessel tone are preserved. These proteins will be discussed below.

Gap junctions are comprised of two docking hemichannels (connexons) that enable the movement of charge (anions and cations) and small metabolites/molecules among neighboring cells (Revel and Karnovsky, 1967). Each connexon is an oligomer of six connexin (Cx) subunits (Caspar et al., 1977; Makowski et al., 1977), each of which possess four hydrophobic membrane-spanning domains, two conserved extracellular domains and three variable intracellular domains. Connexins retain distinct molecular properties and varying connexon composition alters the specific permeability of gap junction channels (Bruzzone et al., 1996; Willecke et al., 2002; Saez et al., 2005). This is exemplified by the ability of Cx40 to enable passive diffusion of IP₃ a key second messenger (Sneyd et al., 1998; Kanaporis et al., 2011). Among the 21 members of the Cx family, Cx37, Cx40, Cx43, and Cx45 are typically observed in vascular cells (Little et al., 1995; Li and Simard, 2001; Hill et al., 2002). Immunohistochemical evidence suggests that Cx expression in the endothelium is substantively higher than in the smooth muscle (Sandow and Hill, 2000; Sandow et al., 2003). Consistent with this view, coupling resistance among endothelial cells (1.5–3.0 MΩ) (Lidington et al., 2000) was 30 fold lower than among smooth muscle cells (Yamamoto et al., 2001). Interestingly, myoendothelial coupling is orders of magnitude greater than smooth muscle cells (>1800 MΩ) (Yamamoto et al., 2001). This high resistivity is in agreement with the immunohistochemical evidence demonstrating few Cx37 and Cx40 expressed in IEL “holes” (Sandow et al., 2006). Although MEGJs are present in endothelial projections passing through the IEL, not all IEL holes possess endothelial projections. Indeed, as vessel size increases, the incidence of MEGJs appears to decrease (Sandow and Hill, 2000; Sandow et al., 2009) indicative of myoendothelial feedback playing a greater role in small resistance arterioles. As these MEGJs are sparsely distributed, the channels stimulated by transiting second messengers must be very close to MEGJs.

The three isoforms of IP₃R (i.e., IP₃R1, IP₃R2, IP₃R3) are widely expressed and uniquely distributing in a range of cells. In whole mesenteric arteries, all 3 isoforms have been detected, with IP₃R1 and IP₃R2 appearing to be heavily expressed in endothelial cells (Ledoux et al., 2008b; Sandow et al., 2009). These receptors are important in vascular tone development, as they are involved in regulating intracellular [Ca²⁺]. IP₃ binds to the IP₃Rs and lowers the affinity of the stimulatory site for Ca²⁺, thereby promoting channel opening and release of Ca²⁺ (Bootman et al., 1995; Chalmers et al., 2007). In the presence of IP₃, these receptors are activated by intracellular [Ca²⁺] of ~300 nM. Functional studies demonstrate that IP₃Rs on the ER play an important role in myoendothelial feedback as impairing ER Ca²⁺ mobilization and inhibition of IP₃Rs augmented agonist-induced contraction (Nausch et al., 2012; Tran et al., 2012). The original model for myoendothelial feedback required the flux of second messengers across the MEGJs from the contracting smooth muscle. Given that MEGJ communication is minimal, bulk diffusion of Ca²⁺ alone is unlikely to elevate endothelial [Ca²⁺] (Dora et al., 1997; Kansui et al., 2008). If IP₃ were to cross the MEGJs to elicit a change in endothelial [Ca²⁺], the IP₃Rs would have to localize near the myoendothelial contact site in order to elicit a response. Past immunohistochemistry studies support the view that a close spatial relationship between IP₃Rs and MEGJ proteins (i.e., Cx37 and Cx40) does indeed exist (Ledoux et al., 2008b; Sandow et al., 2009; Nausch et al., 2012; Tran et al., 2012). Localization of IP₃Rs within the endothelial projections place these receptors in an ideal position to respond when a small quanta of IP₃ crosses the MEGJs from contracting smooth muscle. Subsequent release of Ca²⁺ from the ER causes a discrete rise in endothelial [Ca²⁺]. In order for a discrete rise in [Ca²⁺] to influence global [Ca²⁺], that Ca²⁺ must be able to affect neighboring Ca²⁺ sensitive ion channels.

The likely candidates for discrete activation by Ca²⁺ are the calcium activated K⁺ channels. Within this family of channels, the SK and IK channels are purported to be the most important in terms of myoendothelial feedback. To date, three members of the SK channel family have been identified (i.e., K_Ca2.1–2.3). Due to high degree of similarity with other SK channels, the previously identified IK or K_Ca3.1 channel is often viewed as the fourth member of the SK family. Both K_Ca3.1 and K_Ca2.3 channels are predominantly expressed in the endothelial cells (Nilius and Droogmans, 2001; Taylor et al., 2003; Sandow et al., 2006). Both K_Ca2.3 and K_Ca3.1 channels lack voltage sensitivity (Ledoux et al., 2008a); they are instead gated by nanomolar intracellular [Ca²⁺] (i.e., EC₅₀ 300–500 nM) via coupling of calmodulin to the carboxy-terminus acting as Ca²⁺ sensor (Bond et al., 1999; Schumacher et al., 2001). In order to be involved in myoendothelial feedback, these channels must be localized within endothelial projections where the discrete ER Ca²⁺ release occurs which is also near the MEGJ. In fact, immunohistochemistry has repeatedly shown K_Ca3.1 channels are expressed in close proximity to MEGJs (Sandow and Hill, 2000; Sandow et al., 2002, 2004, 2006; Haddock et al., 2006; Dora et al., 2008; Tran et al., 2012). However, the K_Ca2.3 channels appear to be more diffusely distributed (Sandow and Hill, 2000; Sandow et al., 2002, 2006, 2009). Further support for the K_Ca3.1 channel was the functional evidence showing TRAM34, a K_Ca3.1 channel blocker, but not apamin, a K_Ca2.x channel blocker, inhibit myoendothelial feedback (Nausch et al., 2012; Tran et al., 2012). Thus, the K_Ca3.1 channel appears to be localized within the endothelial projection where it can be involved in myoendothelial feedback. Activation of endothelial K_Ca3.1 channels leads to hyperpolarization and mediates relaxation via transmission of hyperpolarizing current through MEGJs.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.