Potential Roles of Exercise in Regulating Myoendothelial Feedback in Resistance Arteries

Park, Young-Min; Hong, Kwang-Seok; Young-Min Park; Kwang-Seok Hong

doi:10.15857/ksep.2026.00052

Exerc Sci > Volume 35(2); 2026 > Article

Park and Hong: Potential Roles of Exercise in Regulating Myoendothelial Feedback in Resistance Arteries

Review Article

Exerc Sci. 2026; 35(2): 155-165.

Published online: May 7, 2026

DOI: https://doi.org/10.15857/ksep.2026.00052

Potential Roles of Exercise in Regulating Myoendothelial Feedback in Resistance Arteries

Young-Min Park PhD^1,²

, Kwang-Seok Hong PhD^3,^4,

¹Division of Health and Kinesiology, Incheon National University, Incheon, Korea

²Sport Science Institute & Sports Functional Disability Institute, Incheon National University, Incheon, Korea

³Department of Physical Education, College of Education, Chung-Ang University, Seoul, Korea

⁴Physical Education Laboratory, Chung-Ang University, Seoul, Korea

Corresponding author: Kwang-Seok Hong Tel +82-2-820-5764 Fax +82-2-812-2729 E-mail kshong@cau.ac.kr

*This work was supported by the Incheon National University (2024-0395).

Received Feb 9, 2026 Revised Mar 20, 2026 Accepted Mar 24, 2026

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/4.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

PURPOSE

Resistance arteries rely on tightly coordinated signaling between vascular smooth muscle cells (SMCs) and endothelial cells (ECs) to regulate vascular tone and ensure appropriate tissue perfusion. Myoendothelial feedback via gap junctions is crucial in limiting excessive vasoconstriction triggered by myogenic and sympathetic stimuli. This review synthesizes current knowledge on second messenger-dependent SMC-to-EC communication and explores how exercise training modulates these heterocellular signaling pathways to influence myoendothelial feedback.

METHODS

Relevant literature was systematically retrieved from major scientific databases (e.g., PubMed and Web of Science), with a primary focus on studies examining vascular myoendothelial feedback regulation and the effect of exercise training on SMC-EC heterocellular communication.

RESULTS

SMC activation by adrenergic or purinergic stimuli generates intracellular Ca²⁺ and IP₃ signals that are transmitted to adjacent ECs via myoendothelial gap junctions. These signals elicit localized endothelial Ca²⁺ events, stimulate nitric oxide production, activate endothelial IK_Ca and SK_Ca channels, and induce endothelium-dependent hyperpolarization, collectively suppressing SMC contraction. Ca²⁺ diffusion and IP₃ transfer contribute to this negative feedback, with growing evidence supporting C²⁺/IP₃-dependent signaling as a dominant mechanism during physiological sympathetic activation. Exercise training enhances endothelial C²⁺ signaling capacity, improves gap junctional communication, and augments downstream vasodilatory responses.

CONCLUSIONS

Myoendothelial feedback represents a critical protective mechanism against excessive vasoconstriction in resistance arteries. Exercise training reinforces this negative feedback by strengthening endothelial signaling and heterocellular communication, thereby promoting vascular homeostasis during physiological stress.

Keywords: Ca²⁺, Endothelium-dependent hyperpolarization (EDH), Heterocellular communication, Inositol 1,4,5-trisphosphate (IP₃), Myoendothelial gap junctions

INTRODUCTION

In the integrated cardiovascular system, resistance arteries regulate blood flow distribution and blood pressure [1]. Small arteries possess an intrinsic ability to tune their diameter in response to physiological changes - such as fluctuations in intravascular pressure - to control blood flow appropriately [2]. Additionally, perivascular sympathetic and sensory nerves play a crucial role in modulating peripheral vascular resistance by releasing neurotransmitters into the vessel wall [3]. Endothelial cells (ECs) and smooth muscle cells (SMCs) are the primary cellular components of resistance arteries and coordinate intracellular and intercellular signaling pathways to maintain hemodynamic homeostasis. Disruption of this coordinated signaling can contribute to the development of cardiovascular disorders.

For instance, myogenic vasoconstriction of cerebral arteriolar myocytes is restrained by Ca²⁺ release events from ryanodine receptors (RyRs) in the sarcoplasmic reticulum of SMCs, termed “Ca²⁺ sparks”. These signaling events stimulate large-conductance Ca²⁺-sensitive K⁺ (BK_Ca) channels, which limit Ca²⁺ influx via inhibition of voltage-dependent Ca²⁺ channels (VDCCs) [4,5]. This intracellular negative feedback prevents exaggerated vasoconstriction, cerebral ischemia, and neuronal hypoxia [5,6]. However, some vascular beds, including the skeletal muscle microcirculation, do not possess this intracellular signaling mechanism [7,8] and rely more heavily on intercellular communication between SMCs and ECs to govern vascular reactivity.

The functional integration of SMCs and ECs is critical for controlling blood flow/pressure and limiting excessive contractility; this is mediated by direct cell-to-cell contact and myoendothelial gap junctions (MEGJs) [9,10]. Initial contractile signaling of SMCs evoked by perivascular nerve activation is transmitted to ECs via MEGJs, where it is subsequently attenuated by endothelial-derived vasodilatory pathways. This feedback regulation in myogenically active and sympathetically innervated SMCs plays a vital role in preventing the total cessation of blood flow and maintaining vascular patency.

Exercise training is characterized by repetitive increases in blood flow and intraluminal shear stress. These hemodynamic stimuli act as potent physiological signals that enhance endothelial vasodilatory function [11-13]. In resistance arteries supplying skeletal muscle, exercise training improves endothelium-dependent vasodilation and attenuates excessive vasoconstrictor responses [14,15], suggesting enhanced EC-mediated negative feedback on SMC contractility. Specifically, exercise training increases endothelial Ca²⁺ signaling and reinforces Ca²⁺-dependent activation of endothelial small- and intermediate-conductance Ca²⁺-sensitive potassium channels (SK_Ca and IK_Ca) [16,17]. These adaptations strengthen endothelium-dependent hyperpolarization (EDH) and the diffusion of EDH factors (EDHF) through MEGJs to SMCs, ultimately improving vasodilatory function.

These mechanisms are particularly relevant during exercise, a state defined by markedly elevated sympathetic nerve activity. During physical activity, α1-adrenergic SMC contractile signaling must be precisely counterbalanced by EC-derived vasodilatory mechanisms (including NO- and EDH-dependent pathways) to preserve tissue perfusion. Although direct evidence linking exercise training to altered SMC-derived signals (e.g., Ca²⁺ and IP₃) transfer to ECs through MEGJs is limited, several lines of evidence support this possibility. Given that SMC-derived Ca²⁺ and IP₃ have been identified as critical messengers initiating endothelial Ca²⁺ release and subsequent NO- or EDH-mediated suppression of vasoconstriction [6,18,19], exercise-induced enhancement of endothelial signaling capacity may increase SMC-to-EC heterocellular myoendothelial feedback. This, in turn, contributes to proper blood flow regulation and vascular protection against excessive vasoconstriction. Therefore, this literature review discusses heterocellular communication between SMCs and ECs in resistance arteries, conceptualized as myoendothelial feedback mechanisms, and explores the specific mechanisms by which exercise training modulates myoendothelial negative feedback regulation. A schematic overview summarizing the proposed mechanisms of SMC-EC heterocellular communication and the potential modulatory effects of exercise training on myoendothelial feedback regulation is presented in Fig. 1.

Fig. 1.

Schematic illustration of myoendothelial feedback mechanisms and the modulatory effects of exercise training in resistance arteries. Sympathetic or agonist-induced α₁-adrenergic activation of smooth muscle cells (SMCs) generates Ca²⁺ and IP₃ signals that are transmitted to adjacent endothelial cells (ECs) via myoendothelial gap junctions (MEGJs). These signals evoke localized endothelial Ca²⁺ elevations, activating nitric oxide (NO) production and endothelium-dependent hyperpolarization (EDH) through IK_Ca/SK_Ca channels, which collectively suppress SMC contraction. Exercise training is potentially proposed to enhance endothelial Ca²⁺ signaling capacity, gap junctional communication, and downstream vasodilatory pathways. These adaptations strengthen myoendothelial negative feedback, contributing to improved vascular tone regulation and protection against excessive vasoconstriction.

1. Initiators for myoendothelial feedback mechanism

The sympathetic nervous system regulates vascular tone by activating adrenergic and purinergic receptors in SMCs [20]. Propagation of action potentials along the efferent axon triggers Ca²⁺ influx in the depolarized presynaptic membrane, stimulating the release of neurotransmitters such as norepinephrine and adenosine triphosphate (ATP) from sympathetic varicosities [21]. Once released, norepinephrine binds to postsynaptic α1-adrenergic receptors (α1ARs) on SMCs. As α1ARs belong to the G_q protein-coupled receptor (GPCR) subfamily, the binding of norepinephrine to α1ARs induces G_q protein-mediated activation of phospholipase C (PLC) activation, yielding diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃) through the metabolism of the membrane phospholipid, phosphatidylinositol 4,5-bisphosphate (PIP₂). DAG is linked to the activation of transient receptor potential canonical 6 (TRPC6) channels and facilitates Ca²⁺ entry through a PKC-independent mechanism [22,23]. The released IP₃ leads to intracellular Ca²⁺ release from IP₃ receptors (IP₃Rs) in the sarcoplasmic reticulum of SMCs [24].

Additionally, the neurotransmitter ATP activates two types of purinergic receptors: P2X and P2Y [25]. Specifically, P2X₁ ionotropic receptors are primarily expressed in the SMCs of resistance arteries [26,27]. ATP-mediated activation of P2X₁ receptors evokes Ca²⁺ and/or Na⁺ influx, causing rapid and transient depolarization [28]. When the excitatory junction potential is sufficiently large, sympathetic vasoconstriction is mediated by the activation of VDCCs in SMCs [29-31]. According to previous studies, adrenergic and purinergic receptor activation results in distinguishable Ca²⁺ events in SMCs: 1) P2X₁-mediated rapid and transient Ca²⁺ entry [26,32] and 2) IP₃-mediated slow-onset Ca²⁺ waves [29,33]. These distinct Ca²⁺ events are associated with SMC contraction [30,34]. The extracellular and intracellular Ca²⁺ signals and IP₃ generation in SMCs not only contribute to sympathetic vasoconstriction but are also involved in SMC-to-EC heterocellular communication. Thus, neurally released transmitters required for endogenous agonist-dependent activation of GPCRs in SMCs and the subsequent production of second messengers (Ca²⁺ and IP₃) serve as primary initiators of myoendothelial feedback regulation.

2. Ca²⁺-dependent SMC-EC communication

The functional cooperation of SMC and EC in resistance arteries is essential for appropriate vasomotor reactivity. Vascular smooth muscle tone is modulated by endothelium-derived relaxing or hyperpolarizing factors [35]. Phenylephrine (PE), a specific α1AR agonist, induces SMC depolarization, which is subsequently conducted to adjacent ECs [36]. Moreover, the second messengers Ca²⁺ and IP₃ are permeable through MEGJs [37,38], implying that SMCs and ECs are functionally coupled through these gap junctions [39].

Accordingly, Dora et al. [40] first demonstrated that SMC α1AR activation or membrane depolarization-induced rises in EC Ca²⁺ signals result from the diffusion of SMC Ca²⁺ through MEGJs. In hamster cheek pouch arterioles, the application of PE or KCl augmented Ca²⁺ concentrations in ECs to levels similar to those evoked by acetylcholine (ACh), an agonist of endothelial muscarinic receptors, but these levels were markedly reduced in the presence of nifedipine, a VDCC antagonist. This result suggests that the secondary increase in the EC Ca²⁺ signal following PE or KCl stimulation originates from Ca²⁺ influx through SMC VDCCs. Furthermore, Dora et al. [40] observed enhanced SMC contractile responses to PE or KCl in the presence of L-NAME, a global inhibitor of nitric oxide synthesis (NOS), consistent with previous studies [41,42]. Collectively, these results suggest that Ca²⁺ transfer from SMCs to ECs via MEGJs stimulates nitric oxide (NO) production, thereby suppressing PE- or KCl-mediated vasoconstriction.

Theoretically, both IP₃ and Ca²⁺ in SMCs are potential candidates mediating SMC-EC heterocellular communication through MEGJs. However, due to technical limitations in tracking the real-time movement of SMC IP₃ and Ca²⁺, identifying a predominant diffusing molecule for myoendothelial feedback regulation remains challenging. While SMC α1AR stimulation with PE increases both IP₃ and Ca²⁺ levels, KCl-dependent depolarization in SMCs induces Ca²⁺ elevations exclusively via the opening of VDCCs. Given that the change in EC Ca²⁺ concentration following PE stimulation closely mirrors that elicited by KCl, several studies suggest that Ca²⁺, rather than IP₃, may be the primary molecule diffusing from SMC to ECs [40,43].

Beyond NO-mediated myoendothelial feedback [40], subsequent studies investigated novel regulatory mechanisms following the passage of Ca²⁺ from SMCs to ECs. In these studies, SMC α1AR activation with PE resulted in simultaneous Ca²⁺ elevations in both cell types and subsequent vasorelaxation following PE-induced vasoconstriction [44]. Pretreatment with L-NAME increased agonist-dependent vasocontraction in rat mesentery arteries two-fold; remarkably, the subsequent application of apamin (an SK_Ca channel antagonist) further increased PE-induced vasoconstriction by more than three-fold. Because PE does not directly affect EC Ca²⁺ signaling and acts exclusively on SMCs [40,44-46], these findings suggest that SMC α1AR-mediated vasoconstriction is regulated via negative feedback from EC-dependent signaling, specifically NO production and K⁺ efflux-dependent hyperpolarization.

Indeed, the elevation of intracellular EC Ca²⁺ contributes to EDH through the activation of IK_Ca and SK_Ca channels [35,47,48]. These channels are exclusively expressed in ECs and serve as essential components of myoendothelial feedback regulation of vasoconstriction [49]. These K⁺ channels are voltage-insensitive and are activated by nanomolar increases in intracellular Ca²⁺ (e.g., EC₅₀ of 300-500 nM) through Ca²⁺-calmodulin binding at the channel carboxyl terminus [50,51]. Consequently, elevations in endothelial Ca²⁺ concentration promote EDH. This hyperpolarization, transmitted to SMCs via MEGJs, hyperpolarizes the SMC membrane and leads to vasodilation by inhibiting VDCCs. Taken together, the evidence suggests that SMC α1AR stimulation increases intracellular Ca²⁺ signals through the direct movement of SMC-derived Ca²⁺ and Ca²⁺-induced Ca²⁺ release from the endoplasmic reticulum (ER), which activates K⁺ channels to trigger an EDH-mediated negative feedback mechanism.

Regarding intercellular communication from SMC to EC, various pharmacological agents have been employed to uncouple MEGJ, including palmitoleic acid, 18α- or 18β-glycyrrhetinic acid (18α or 18βGA), carbenoxolone, and 1-heptanol [19,43,52]. Secondary increases in EC Ca²⁺ signals and EDH/NO production following SMC α1AR activation with PE or membrane depolarization with KCl were markedly diminished after pre-incubation with these gap junction uncouplers [19,43,52]. These observations further support the movement of SMC-derived second messengers toward ECs. However, caution is warranted because these uncouplers have been reported to exert non-specific side effects directly on ECs [53,54].

Given the lack of highly specific uncoupling agents, inherent difficulties remain in studying the interchange of second messengers between SMCs and ECs. To address this, Isakson et al. [52,55] developed an intriguing approach: a vascular cell co-culture system composed of monolayers of ECs and SMCs. Although vasomotor reactivity cannot be directly examined in this system, the SMC-EC co-culture model forms MEGJs similar to those found in intact resistance arteries, facilitates the measurement of Ca²⁺ signaling in both cell types, and allows for the selective pharmacological stimulation of SMCs and ECs [52,55]. Consistently, an increase in EC Ca²⁺ signaling was monitored after SMC stimulation with PE, which was significantly attenuated by 18αGA in the coculture system [52,55]. Furthermore, PE-induced SMC stimulation still resulted in a secondary rise in EC Ca²⁺ concentration even when IP₃R-mediated Ca²⁺ release in ECs was completely inhibited by thapsigargin, an inhibitor of the SERCA Ca²⁺ pump in the ER. These results from culture studies support findings from vascular studies indicating that Ca²⁺ diffusion from SMCs to ECs plays a critical role in myoendothelial feedback [6,40,43,56].

Technological advances in optical Ca²⁺ imaging techniques and sensitive detectors (e.g., fluo-4 and GCaMP2 biosensors) have enabled investigators to examine Ca²⁺ dynamics involving heterocellular communication. An innovative study in skeletal muscle resistance arteries found that the opening of SMC VDCCs with BayK8644 (a selective VDCC agonist) elicited elementary and localized EC Ca²⁺ transient events, termed vascular endothelial Ca²⁺ transients (VECTors) [6]. These events are distinct from IP₃R-dependent local Ca²⁺ events, such as Ca²⁺ pulsars or wavelets in ECs [18,46].

Under IP₃R inhibition with heparin in both SMCs and ECs, either direct or indirect VDCC-driven Ca²⁺ entry driven by BayK8644, membrane depolarization (using 4-aminopyridine: an antagonist of voltage-dependent K⁺ channels), or PE-induced signals transmitted through MEGJs activated IKCa channels predominantly expressed at myoendothelial projections (MEPs). Functional studies confirmed this, as BayK8644 or PE-induced vasoconstriction was followed by vasodilation toward or beyond baseline, a response that was substantially attenuated by the IK_Ca blocker TRAM-34. Garland and Bagher [6] provided the first optical evidence of Ca²⁺ movement from SMCs to ECs and suggested that SMC depolarization evokes local and transient Ca²⁺ events in ECs to activate negative feedback regulation, thereby suppressing excessive vasoconstriction.

3. Putative effects of exercise training on Ca²⁺-dependent SMC-EC negative feedback regulation

Exercise training is characterized by repetitive increases in blood flow and intraluminal shear stress, which constitute potent physiological stimuli for endothelial adaptation and the enhancement of Ca²⁺-dependent signaling pathways [11]. In resistance arteries supplying skeletal muscle, exercise training consistently improves endothelium-dependent vasodilation and attenuates exaggerated vasoconstrictor responsiveness, suggesting that exercise enhances endothelial restraint on SMC contractility [14]. These functional adaptations are particularly relevant to Ca²⁺-dependent myoendothelial feedback regulation, in which SMC-derived Ca²⁺ signals transferred through MEGJs evoke endothelial Ca²⁺ elevations that activate NO and EDH pathways to suppress excessive vasoconstriction.

Exercise training has been shown to enhance endothelial Ca²⁺ signaling capacity and to potentiate Ca²⁺-dependent activation of endothelial IK_Ca and SK_Ca channels, which are critical effectors of EDH-mediated vasodilation [16,17]. Because IK_Ca and SK_Ca channels are activated by sub-micromolar increases in endothelial intracellular Ca²⁺, exercise-induced improvements in endothelial Ca²⁺ handling are postulated to amplify the sensitivity and gain of EDH-dependent negative feedback on SMC membrane potential and Ca²⁺ influx. Indeed, in conditions such as aging and metabolic disease, exercise training restores EDH-mediated vasodilatory function by normalizing signaling through endothelial Ca²⁺-sensitive potassium channels, supporting the concept that exercise training enhances Ca²⁺-dependent endothelial restraint of SMC tone [16,17].

During exercise, sympathetic nerve activity is markedly elevated, yet blood flow to active skeletal muscle is preserved through functional sympatholysis, a process requiring precise local counterbalancing of α1-adrenergic SMC contractile signaling by endothelial vasodilatory mechanisms [57,58]. Exercise training enhances sympatholysis in both humans and experimental models, largely through NO- and EDH-dependent pathways [59,60]. Given that SMC-derived Ca²⁺ signals transmitted via MEGJs have been identified as initiators of this feedback [6,18], these findings provide indirect support for the hypothesis that exercise training strengthens Ca²⁺-dependent SMC-EC heterocellular feedback regulation.

Collectively, current evidence supports the concept that exercise training enhances Ca²⁺-dependent SMC-EC heterocellular communication primarily by augmenting endothelial Ca²⁺ signaling capacity, EDH/NO effector pathways, and potentially MEGJ coupling efficiency. These adaptations increase the effectiveness of myoendothelial negative feedback regulation, thereby protecting resistance arteries from excessive sympathetic vasoconstriction and ensuring adequate tissue perfusion during conditions of heightened neural and myogenic drive.

4. IP₃-dependent SMC-EC communication

Although the concept of SMC Ca²⁺ diffusion-mediated myoendothelial feedback regulation was first introduced and has been continually suggested in small arteries [6,40,56], SMC IP₃ is also considered a key determinant of SMC-EC intercellular communication [18,19,46,52,61]. IP₃ is generated by phospholipase C (PLC)-induced cleavage of PIP₂ once GPCRs are activated. Initial studies found that the blockade of PLC with U-73122 or the inhibition of IP₃-induced Ca²⁺ release from the ER with 2-aminoethoxydiphenylborate (2-APB) suppressed the increase in EC Ca²⁺ concentration following PE stimulation [62]. In contrast, a high extracellular K⁺ solution used to induce membrane depolarization augmented only SMC Ca²⁺ signals through VDCC-mediated Ca²⁺ entry without altering EC Ca²⁺ levels; this implies that agonist-induced SMC-to-EC communication may be mediated by IP₃ [62].

Isakson et al. [52] also attempted to examine the influence of SMC-derived IP₃ on EC Ca²⁺ signals using an SMC-EC co-culture model and pharmacological approaches. Even when Ca²⁺ signals in ECs were selectively buffered by BAPTA (a cell-permeable Ca²⁺ chelator) treatment, a secondary rise in EC Ca²⁺ concentration was not abolished following PE-dependent SMC stimulation. When SMC and EC monolayers were loaded with BAPTA and xestospongin C (an IP₃R inhibitor), respectively, SMC α1AR activation with PE did not alter Ca²⁺ signals in either cell type. These data support the movement of SMC-derived IP₃ toward ECs [52].

Research has revealed a novel signaling mechanism in which IP₃ generated by SMC α1AR stimulation induces eNOS phosphorylation at serine 1177. This phosphorylation event occurs specifically at MEPs and is notably absent in the global EC population [63]. MEJ eNOS phosphorylation following PE treatment was significantly inhibited by pre-treatment with 18αGA, a gap junction uncoupler, or Gö-6983, a pan-PKC inhibitor. Furthermore, the loading of thapsigargin, xestospongin C, or BAPTA consistently reduced eNOS phosphorylation at the MEJ after PE treatment.

These findings suggest that IP₃ traveling from SMCs to ECs triggers IP₃R-mediated Ca²⁺ release from the ER, which locally activates the PKC-Akt-eNOS pathway at MEPs [63]. This mechanism was further confirmed by observations that SMC α1AR activation with PE increased cyclic guanosine monophosphate (cGMP) levels within the SMCs [63]. Collectively, the transfer of SMC IP₃ to ECs appears to attenuate SMC α1AR activation-induced vasoconstriction by generating NO in ECs and subsequently increasing cGMP in the adjacent SMCs.

The discovery of elementary and spatially restricted Ca²⁺ events has broadened the understanding of myoendothelial feedback mechanisms in intact resistance arteries. Ledoux et al. [50] identified localized Ca²⁺ release events from the ER at MEPs, termed “Ca²⁺ pulsars”, in en face mesentery artery preparations. These events were demonstrated to be distinct from RyR-mediated Ca²⁺ sparks in SMCs and IP₃-mediated Ca²⁺ puffs observed in Xenopus oocytes [50]. This local increase in EC Ca²⁺ signaling is sufficient to activate IK_Ca and/or SK_Ca channels and subsequently induce EDH [64].

Research by Tran et al. [46] further elucidated that these fundamental Ca²⁺ signals participate in SMC-to-EC intercellular communication. Using confocal Ca²⁺ imaging, they observed IP₃R-dependent Ca²⁺ pulsars (referred to as “Ca²⁺ wavelets”) occurring in response to PE application. Furthermore, PE-induced SMC depolarization and vasoconstriction were enhanced in the presence of TRAM-34 and apamin (IK_Ca and SK_Ca channel blockers, respectively). Interrupting IP₃R-mediated Ca²⁺ release in ECs via luminal infusion of BAPTA-AM, thapsigargin, 2-APB, or xestospongin C increased agonist-induced vasoconstriction in pressurized retractor muscle feed arteries. These results indicate that the diffusion of IP₃ across MEGJs following PE-induced α1AR activation evokes Ca²⁺ pulsars/wavelets and EDH at MEPs, thereby attenuating agonist-induced vasoconstriction.

While many studies rely on pharmacological application (e.g., PE) to explore EC Ca²⁺ signaling response to SMC activation, electrical field stimulation of sympathetic nerves has confirmed that unitary Ca²⁺ pulsar events are increased by endogenous sympathetic nerve activation [18]. Sympathetic nerve activation releases norepinephrine and ATP, activating SMC α1AR and P2X₁ receptors, respectively [20]. While an α1AR antagonist (prazosin) or PLC inhibitor (U-73122) significantly diminished EC Ca²⁺ signaling in response to electrical stimulation of sympathetic nerves, a P2X₁ receptor antagonist (αβ-meATP) had a negligible effect [18]. Additionally, pre-treatment with the VDCC blocker nifedipine did not suppress rises in EC Ca²⁺ pulsar events following sympathetic nerve stimulation. These data support the concept that IP₃, rather than Ca²⁺, plays a vital role in increasing EC Ca²⁺ signaling after SMC α1AR activation by nerve-released norepinephrine [18]. Prior studies indicate that SMCs communicate with ECs via MEGJs to limit agonist/nerve-evoked vasoconstriction in an IP₃ flux-dependent manner.

Some previous investigations have suggested that direct diffusion of SMC Ca²⁺ to ECs mediates the myoendothelial feedback of SMC α1AR activation-induced vasoconstriction [40,56,65]. However, other studies [18,19,46,52,61-63] indicate that the diffusion of SMC-derived IP₃ via MEGJs is essential for SMC-EC heterocellular communication. This is supported by evidence that SMC Ca²⁺ depolarization or VDCC opening, which elevates SMC Ca²⁺ but not IP₃, fails to alter EC Ca²⁺ signaling, whereas PLC inhibition significantly abrogates the secondary EC Ca²⁺ responses to PE or nerve stimulation. In line with these observations, Ca²⁺ diffusion-independent myoendothelial feedback is further supported by the distinct kinetic properties of IP₃ [66]. IP₃ was discovered in 1983 and has been considered a “global” or highly mobile cellular messenger because it can diffuse significantly faster than Ca²⁺ (280 vs. 38 μm/sec), making it suitable for rapid, long-distance biological signaling [67]. In contrast, free Ca²⁺ exhibits slower mobility because its interactions with Ca²⁺-binding proteins (e.g., calmodulin) effectively increase its molecular weight and reduce the diffusion rate [68].

While IP₃ is a strong candidate for SMC to EC communication, research by Dickinson et al. [69] has recently suggested that its diffusion rate may vary significantly based on the density of functionally inactive IP₃Rs in mammalian cells, which can act as molecular buffers [69]. For instance, while IP₃ diffusion is relatively fast in cardiomyocytes with low IP₃R density, it appears diminished in Purkinje cells where IP₃R density is high [69]. Furthermore, an IP₃ diffusion rate of 280 μm/sec was first discovered in Xenopus oocytes [68], and neuroblastoma cells displayed a much slower diffusion coefficient of IP₃ (5–10 μm/sec, approximately 30-fold slower) [69]. As IP₃ mobilization varies across different cell types, further research on myoendothelial feedback mechanisms is required to explore IP₃R buffering capacity and IP₃ diffusion rates in SMCs and ECs, particularly at MEPs. In addition, the impact of IP₃ on SMC-EC heterocellular communication has been debated because IP₃ may be easily metabolized by type-I 5-phosphatases [70]. However, prior studies have demonstrated that IP₃ is sufficiently stable to survive and diffuse to neighboring cells through gap junctions [71,72]. Collectively, SMC-EC communication for myoendothelial feedback predominantly occurs in an IP₃-dependent manner.

Unitary, spatially restricted Ca²⁺ influx events through endothelial transient receptor potential vanilloid 4 (TRPV4) channels, termed “TRPV4 Ca²⁺ sparklets,” regulate endothelium-dependent vasodilation in resistance arteries [73]. These TRPV4 sparklets activate nearby IK_Ca and SK_Ca channels at MEPs to cause membrane hyperpolarization and subsequent vasodilation. While endothelial TRPV4 channels are known to be crucial sources of localized Ca²⁺ influx in resistance arteries, recent findings have reported that these Ca²⁺-selective cation channels are also involved in heterocellular communication and myoendothelial feedback [19]. Specifically, research has revealed that IP₃ generated from SMC GPCR activation stimulates endothelial TRPV4 channels.

In the presence of cyclopiazonic acid (CPA, a SERCA pump inhibitor) to eliminate IP₃R-mediated Ca²⁺ events, the application of GPCR agonists, including PE or U46619 (a thromboxane A2 receptor agonist), significantly increased TRPV4 Ca²⁺ sparklet activity predominantly at MEPs in intact systemic arteries. These events were markedly diminished by PLC inhibitors (U-73122) or gap junction uncouplers (18β-GA and 1-heptanol). While TRPV4 sparklet activity remained unaltered by VDCC activators (BayK 8644) or inhibitors (nifedipine), the photorelease of IP₃ using ultraviolet light augmented TRPV4 sparklet activity. This indicates that SMC-derived IP₃, rather than Ca²⁺, serves as the endogenous/physiological activator stimulating endothelial TRPV4 channels expressed at MEPs. These findings are further supported by previous reports that the binding of IP₃ to the ankyrin repeat domain of TRPV4 channels increases channel activity [74].

Furthermore, the pharmacological or genetic ablation of TRPV4 channels, the use of gap junction uncoupling agents, endothelium denudation, or IK_Ca/SK_Ca channel inhibition profoundly abolished vasodilation following PE-induced vasoconstriction. PE injection into global TRPV4 knockout mice also substantially elevated mean arterial pressure. Taken together, these findings suggest that heterocellular communication between SMC IP₃ and EC TRPV4 channels contributes significantly to preventing excessive vasoconstriction and the development of hypertension.

5. Putative effects of exercise on IP₃-dependent SMC-EC negative feedback regulation

Exercise training has not yet been shown directly to augment SMC-derived IP₃ transfer across MEGJs in intact resistance arteries; specifically, quantitative studies comparing IP₃ flux from SMC to EC via MEGJs before and after exercise training are currently lacking. However, there is a high likelihood that regular exercise can strengthen IP₃-dependent myoendothelial negative feedback that restrains excessive vasoconstriction. Repetitive exercise bouts may increase MEGJ communication capacity by altering the abundance or distribution of connexins (e.g., Cx37/ Cx40/Cx43) in ways that facilitate heterocellular signaling at MEJs [75,76]. In parallel, enhanced endothelial IP₃R-dependent Ca²⁺ release is postulated to promote vasodilator restraint of tone [77].

Consistent with this, exercise training could conceivably contribute to sensitizing endothelial Ca²⁺-dependent effector pathways that are typically engaged by GPCR-PLC-IP₃ signaling [78]. Since IP₃-dependent myoendothelial feedback relies on SMC GPCR activation, IP₃ diffusion to ECs, and EC Ca²⁺ release-mediated activation of NO and/or EDH pathways [77], an exercise-induced enhancement in endothelial Ca²⁺ responsiveness provides a robust biological mechanism. Through this mechanism, the same quantity of SMC-derived IP₃ could presumably elicit larger EC effector output (NO/EDH), thereby enhancing negative feedback against α₁-adrenergic vasoconstriction.

TRPV4 sparklet activity and downstream K_Ca2.3 activation provide a potent hyperpolarizing brake on SMC contraction [79]. Importantly, research has shown that exercise training restores impaired EDH/EDHF-mediated vasodilation in aged arteries via the TRPV4-K_Ca2.3 signaling complex. This indicates that exercise training improves the integrity or coupling efficiency of this endothelial microdomain, which is central to local vasodilator feedback [16]. In resistance-artery paradigms of heterocellular signaling, SMC-derived IP₃ acts as an endothelial activator capable of engaging TRPV4-dependent microdomain Ca²⁺ influx at myoendothelial projections (MEPs). This links the SMC IP₃-EC TRPV4 sparklets-IK/SK_Ca channel activation-EDH sequence, which subsequently spreads to SMCs as a negative-feedback loop that limits excessive vasoconstriction and hypertension [19,74].

Therefore, even without direct measurements of increased SMC-to-EC IP₃ transfer following exercise training, existing evidence supports the interpretation that exercise can amplify the endothelial component of IP₃-dependent myoendothelial feedback by: 1) increasing gap-junctional signaling capacity through shear-sensitive connexin regulation; 2) enhancing IP₃R-dependent endothelial Ca²⁺ release and NO signaling; and 3) improving TRPV4-K_Ca2.3 microdomain function that translates local Ca²⁺ events into EDH-mediated suppression of vasoconstriction.

CONCLUSIONS

Myoendothelial feedback mechanisms represent a fundamental regulatory system that prevents resistance arteries from exaggerated vasoconstriction induced by myogenic activity and sympathetic nerve stimulation. Second messengers generated in SMCs, specifically Ca²⁺ and IP₃, are transmitted to ECs through MEGJs, where they initiate localized endothelial Ca²⁺ signaling and activate NO and EDH(F)-dependent vasodilation. These heterocellular interactions ensure precise, site-specific regulation of vascular tone. Meanwhile, exercise training emerges as a potent physiological stimulus capable of enhancing these feedback processes. Exercise training reinforces myoendothelial negative feedback on SMC contractility. This effect is likely mediated by improvements in endothelial Ca²⁺ handling, preservation of IP₃-dependent signaling micro-domains, and enhanced gap junctional coupling. Such adaptations may be especially important under conditions of elevated sympathetic activity, contributing to improved blood flow regulation and vascular protection. Further experimental studies are required to directly elucidate exercise-induced alterations in SMC-EC heterocellular communication.

Notes

CONFLICT OF INTEREST

All authors declare that they have no conflicts of interest with the contents of this study.

AUTHOR CONTRIBUTIONS

Conceptualization: YM Park, KS Hong; Data curation: KS Hong; Formal analysis: KS Hong; Funding acquisition: YM Park, KS Hong; Methodology: YM Park, KS Hong; Project administration: KS Hong; Visualization: YM Park, KS Hong; Writing original draft: YM Park, KS Hong; Writing-review & editing: YM Park, KS Hong.

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