Reimagining Electromechanical Coupling in the Control of Arterial Tone and Capillary Blood Flow

Mohammed A. El-Lakany, The University of Western Ontario

Abstract

Efficient tissue perfusion relies on precise O₂ demand-supply matching, whereby vascular responses to signals originating in tissues, arteries or capillaries optimize flow delivery. Electrical charges across membranes of vascular smooth muscles define their contractility, termed electromechanical coupling, novel control of which is interrogated herein in two parts: First, Ca_V3.1 channels and tone development, and second, capillary-to-arteriole conduction mediating O₂ signals in the microvasculature. The arterial myogenic response involves smooth muscle contraction to intraluminal pressure, wherein Ca²⁺ influx through voltage-gated Ca²⁺ channels, primarily L-type (Ca_V1.2), is pivotal. Other Ca²⁺ channels expectedly dominate when Ca_V1.2 activity drops at hyperpolarized membrane potentials. Using mesenteric arteries from C57BL/6 and Ca_V3.1^-/- mice, we demonstrate that Ca_V3.1 channels facilitate myogenic tone development at hyperpolarized voltages through IP₃R-mediated Ca²⁺ waves. This mechanism is linked to systemic blood pressure regulation, emphasizing Ca_V3.1 as a therapeutic target. Capillary-to-arteriole conducted signalling partly fosters O₂ demand-supply matching, importantly in skeletal muscle where RBC-released ATP during O₂ desaturation theoretically hyperpolarizes capillary endothelial cells (cECs). In vivo imaging in anesthetized C57BL/6 mice monitored capillary hemodynamics and RBC O₂ saturation (SO₂) in exteriorized skeletal muscle (EDL), situated overtop a gas chamber, in which a drop in O₂ reduces capillary RBC SO₂ and augments RBC supply rate. Findings postulate that P2X₄ receptors (P2X₄Rs), and ATP-sensitive K⁺ channels (K_ATP) mediate a novel pathway for conducted O₂ signalling, evidenced by diminished capillary hemodynamic responses to low O₂ following EDL incubation with selective blockers of K_ATP (glyburide) or P2X₄R (5-BDBD). Immunofluorescence and patch-clamp electrophysiology confirmed functional expression of K_ir6.1 subunits and P2X₄Rs in EDL capillaries, revealing K⁺ currents in cECs that are pinacidil-activated, glyburide-sensitive, and ATP-γ-S-activated, 5-BDBD-sensitive. Speculatively, K_ATP inhibition and demand-supply decoupling constitutes a possible mechanism for the cardiac side effects of sulfonylureas, supported by computer modelling of human cardiac tissue. Results dismissed the involvement of other K⁺ channels, P1 receptors, or nitric oxide bioactivity in observed responses. Nevertheless, endothelial nitric oxide synthase bioactivity maintains baseline RBC flux for adequate oxygenation. Together, this work establishes novel molecular mechanisms underlying vascular responses to pressure and O₂, offering insights into therapeutic strategies for blood pressure control and tissue oxygenation.