Low intravascular pressure activates endothelial cell TRPV4 channels, local Ca ²⁺ events, and IK _Ca channels, reducing arteriolar tone

Abstract

Endothelial cell (EC) Ca 2+ -activated K channels (SK Ca and IK Ca channels) generate hyperpolarization that passes to the adjacent smooth muscle cells causing vasodilation. IK Ca channels focused within EC projections toward the smooth muscle cells are activated by spontaneous Ca 2+ events (Ca 2+ puffs/pulsars). We now show that transient receptor potential, vanilloid 4 channels (TRPV4 channels) also cluster within this microdomain and are selectively activated at low intravascular pressure. In arterioles pressurized to 80 mmHg, ECs generated low-frequency (∼2 min −1 ) inositol 1,4,5-trisphosphate receptor-based Ca 2+ events. Decreasing intraluminal pressure below 50 mmHg increased the frequency of EC Ca 2+ events twofold to threefold, an effect blocked with the TRPV4 antagonist RN1734. These discrete events represent both TRPV4-sparklet-and nonsparklet-evoked Ca 2+ increases, which on occasion led to intracellular Ca 2+ waves. The concurrent vasodilation associated with increases in Ca 2+ event frequency was inhibited, and basal myogenic tone was increased, by either RN1734 or TRAM-34 (IK Ca channel blocker), but not by apamin (SK Ca channel blocker). These data show that intraluminal pressure influences an endothelial microdomain inversely to alter Ca 2+ event frequency; at low pressures the consequence is activation of EC IK Ca channels and vasodilation, reducing the myogenic tone that underpins tissue blood-flow autoregulation.endothelial cell calcium | cremaster arterioles | mesenteric arteries C a 2+ -activated K + (K Ca ) channels in arteriolar endothelial cells (ECs) are activated by intrinsic spontaneous or receptormediated Ca 2+ events, each leading to hyperpolarization of smooth muscle cells (SMCs) and vasodilation independent of nitric oxide or prostacyclin-the endothelium-dependent hyperpolarization (EDH) response. This hyperpolarization spreads both radially and longitudinally through the vascular wall via patent gap junctions to evoke local and conducted dilation, and it is central to cardiovascular function (1, 2).EDH is the predominant endothelium-dependent mechanism in smaller "resistance" arteries and arterioles. The underlying hyperpolarization is generated by two subtypes of K Ca channels found in the EC, but not SMC, membrane, the small (SK Ca ,K Ca 2.3) and intermediate (IK Ca, K Ca 3.1) conductance forms that may be activated independently of each other (3). The physiological importance of independent activation is apparent from studies with K Ca 3.1-deficient mice in which the mean blood pressure is raised by ∼7 mmHg, but further elevated by disrupting both K Ca channels (4). In mesenteric resistance arteries, IK Ca channels are focused within EC projections through the internal elastic lamina (IEL) termed myoendothelial junctions (MEJs). MEJs can contain gap junctions (MEGJs) coupling ECs to SMCs, and EDH can spread by direct electrical coupling and/or a diffusible factor (5, 6). The IK Ca channels enriched within MEJs can be activated by spontaneous inositol 1,4,5-trisphosphate (IP 3 ...

Nitric Oxide–Dependent Feedback Loop Regulates Transient Receptor Potential Vanilloid 4 (TRPV4) Channel Cooperativity and Endothelial Function in Small Pulmonary Arteries

“…In small MAs, the majority (≈60%) of TRPV4 sparklet sites occurred at endothelial projections to SMCs or MEPs,5 where IK channels were also localized 1, 5. In small PAs, however, a small percentage of sparklet sites (25±3%; n=5 PAs) occurred at the MEPs.…”

“…Previous studies of endothelial Ca 2+ signals have demonstrated that localized Ca 2+ signals predominantly activate IK and SK channels to cause vasodilation in small systemic arteries including mesenteric, cremaster, and cerebral arteries 1, 2, 3, 4, 56. In small PAs, TRPV4 sparklets preferentially activated eNOS to cause vasodilation.…”

“…Alexa 633 hydrazide cannot penetrate into the cells, but preferentially binds to elastin‐containing internal elastic lamina (IEL), thereby illuminating the IEL1, 30 for 15 minutes following Ca 2+ imaging experiment. The images were then acquired in the fields of view from which Ca 2+ signals were recorded, using excitation wavelengths of 640 nm (for IEL staining) and 488 nm (for fluo‐4), and capturing the emitted fluorescence using 685/40‐ and 525/36‐nm band‐pass filters, respectively.…”

“…Recent studies in systemic arteries—including mesenteric, cremaster, and cerebral arteries—have demonstrated that endothelium‐dependent vasodilation is controlled by local, spatially restricted increases in endothelial Ca 2+ while the whole‐cell Ca 2+ level remains unchanged 1, 2, 3, 4, 5. In contrast to systemic arteries, pulmonary arteries (PAs) comprise a low‐pressure, high‐flow circulation.…”

“…Whether a specific local Ca 2+ signal can activate eNOS has not been elucidated. In this study, we explored the possibility that local, unitary Ca 2+ influx through TRPV4 channels activates eNOS in small PAs, a signaling mechanism different from small mesenteric/cremaster/cerebral arteries 1, 4, 7…”