Aaron H. Bubolz

TRPV4-mediated endothelial Ca2+ influx and vasodilation in response to shear stress

The transient receptor potential vallinoid type 4 (TRPV4) channel has been implicated in the endothelial shear response and flow-mediated dilation, although the precise functions of this channel remain poorly understood. In the present study, we investigated the role of TRPV4 in shear stress-induced endothelial Ca(2+) entry and the potential link between this signaling response and relaxation of small resistance arteries. Using immunohistochemical analysis and RT-PCR, we detected strong expression of TRPV4 protein and mRNA in the endothelium in situ and endothelial cells freshly isolated from mouse small mesenteric arteries. The selective TRPV4 agonist GSK1016790A increased endothelial Ca(2+) and induced potent relaxation of small mesenteric arteries from wild-type (WT) but not TRPV4(-/-) mice. Luminal flow elicited endothelium-dependent relaxations that involved both nitric oxide and EDHFs. Both nitric oxide and EDHF components of flow-mediated relaxation were markedly reduced in TRPV4(-/-) mice compared with WT controls. Using a fura-2/Mn(2+) quenching assay, shear was observed to produce rapid Ca(2+) influx in endothelial cells, which was markedly inhibited by the TRPV4 channel blocker ruthenium red and TRPV4-specific short interfering RNA. Flow elicited a similar TRPV4-mediated Ca(2+) entry in HEK-293 cells transfected with TRPV4 channels but not in nontransfected cells. Collectively, these data indicate that TRPV4 may be a potential candidate of mechanosensitive channels in endothelial cells through which the shear stimulus is transduced into Ca(2+) signaling, leading to the release of endothelial relaxing factors and flow-mediated dilation of small resistance arteries.

H2O2 Is the Transferrable Factor Mediating Flow-Induced Dilation in Human Coronary Arterioles

Rationale: Endothelial derived hydrogen peroxide (H2O2) is a necessary component of the pathway regulating flow-mediated dilation (FMD) in human coronary arterioles (HCAs). However, H2O2 has never been shown to be the endothelium-dependent transferrable hyperpolarization factor (EDHF) in response to shear stress. Objective: We examined the hypothesis that H2O2 serves as the EDHF in HCAs to shear stress. Methods and Results: Two HCAs were cannulated in series (a donor intact vessel upstream and endothelium-denuded detector vessel downstream). Diameter changes to flow were examined in the absence and presence of polyethylene glycol catalase (PEG-CAT). The open state probability of large conductance Ca2+-activated K+ (BKCa) channels in smooth muscle cells downstream from the perfusate from an endothelium-intact arteriole was examined by patch clamping. In some experiments, a cyanogen bromide–activated resin column bound with CAT was used to remove H2O2 from the donor vessel. When flow proceeds from donor to detector, both vessels dilate (donor:68±7%; detector: 45±11%). With flow in the opposite direction, only the donor vessel dilates. PEG-CAT contacting only the detector vessel blocked FMD in that vessel (6±4%) but not in donor vessel (61±13%). Paxilline inhibited dilation of endothelium-denuded HCAs to H2O2. Effluent from donor vessels elicited K+ channel opening in an iberiotoxin- or PEG-CAT–sensitive fashion in cell-attached patches but had little effect on channel opening on inside-out patches. Vasodilation of detector vessels was diminished when exposed to effluent from CAT-column. Conclusions: Flow induced endothelial production of H2O2, which acts as the transferrable EDHF activating BKCa channels on the smooth muscle cells.

Transient Receptor Potential Vanilloid Type 4–Deficient Mice Exhibit Impaired Endothelium-Dependent Relaxation Induced by Acetylcholine In Vitro and In Vivo

Agonist-induced Ca2+ entry is important for the synthesis and release of vasoactive factors in endothelial cells. The transient receptor potential vanilloid type 4 (TRPV4) channel, a Ca2+-permeant cation channel, is expressed in endothelial cells and involved in the regulation of vascular tone. Here we investigated the role of TRPV4 channels in acetylcholine-induced vasodilation in vitro and in vivo using the TRPV4 knockout mouse model. The expression of TRPV4 mRNA and protein was detected in both conduit and resistance arteries from wild-type mice. In small mesenteric arteries from wild-type mice, the TRPV4 activator 4α-phorbol-12,13-didecanoate increased endothelial [Ca2+]i in situ, which was reversed by the TRPV4 blocker ruthenium red. In wild-type animals, acetylcholine dilated small mesenteric arteries that involved both NO and endothelium-derived hyperpolarizing factors. In TRPV4-deficient mice, the NO component of the relaxation was attenuated and the endothelium-derived hyperpolarizing factor component was largely eliminated. Compared with their wild-type littermates, TRPV4-deficient mice demonstrated a blunted endothelial Ca2+ response to acetylcholine in mesenteric arteries and reduced NO release in carotid arteries. Acetylcholine (5 mg/kg, IV) decreased blood pressure by 37.0±6.2 mm Hg in wild-type animals but only 16.6±2.7 mm Hg in knockout mice. We conclude that acetylcholine-induced endothelium-dependent vasodilation is reduced both in vitro and in vivo in TRPV4 knockout mice. These findings may provide novel insight into mechanisms of Ca2+ entry evoked by chemical agonists in endothelial cells.

Activation of endothelial TRPV4 channels mediates flow-induced dilation in human coronary arterioles: role of Ca2+ entry and mitochondrial ROS signaling.

In human coronary arterioles (HCAs) from patients with coronary artery disease, flow-induced dilation is mediated by a unique mechanism involving the release of H(2)O(2) from the mitochondria of endothelial cells (ECs). How flow activates ECs to elicit the mitochondrial release of H(2)O(2) remains unclear. Here, we examined the role of the transient receptor potential vanilloid type 4 (TRPV4) channel, a mechanosensitive Ca(2+)-permeable cation channel, in mediating ROS formation and flow-induced dilation in HCAs. Using RT-PCR, Western blot analysis, and immunohistochemical analysis, we detected the mRNA and protein expression of TRPV4 channels in ECs of HCAs and cultured human coronary artery ECs (HCAECs). In HCAECs, 4α-phorbol-12,13-didecanoate (4α-PDD), a selective TRPV4 agonist, markedly increased (via Ca(2+) influx) intracellular Ca(2+) concentration. In isolated HCAs, activation of TRPV4 channels by 4α-PDD resulted in a potent concentration-dependent dilation, and the dilation was inhibited by removal of the endothelium and by catalase, a H(2)O(2)-metabolizing enzyme. Fluorescence ROS assays showed that 4α-PDD increased the production of mitochondrial superoxide in HCAECs. 4α-PDD also enhanced the production of H(2)O(2) and superoxide in HCAs. Finally, we found that flow-induced dilation of HCAs was markedly inhibited by different TRPV4 antagonists and TRPV4-specific small interfering RNA. In conclusion, the endothelial TRPV4 channel is critically involved in flow-mediated dilation of HCAs. TRPV4-mediated Ca(2+) entry may be an important signaling event leading to the flow-induced release of mitochondrial ROS in HCAs. Elucidation of this novel TRPV4-ROS pathway may improve our understanding of the pathogenesis of coronary artery disease and/or other cardiovascular disorders.

Bradykinin-Induced Dilation of Human Coronary Arterioles Requires NADPH Oxidase–Derived Reactive Oxygen Species

Objective—Hydrogen peroxide (H2O2) is an endothelium-derived hyperpolarizing factor in human coronary arterioles (HCAs). H2O2 mediates bradykinin (BK)-induced vasodilation and reduces bioavailability of epoxyeicosatrienoic acids (EETs); however, the cellular and enzymatic source of H2O2 is unknown. Methods and Results—NADPH oxidase expression was determined by immunohistochemistry. Superoxide and H2O2 production was assayed in HCAs and human coronary artery endothelial cells (HCAECs) using dihydroethidium and dichlorodihydrofluorescein histofluorescence, respectively. Superoxide was quantified by HPLC separation of dihydroethidium products. Diameter changes of HCAs were measured by videomicroscopy. NADPH oxidase subunits Nox1, Nox2, Nox4, p22, p47, and p67 were each expressed in HCA endothelium. In HCAs or HCAECs incubated with dihydroethidium and dichlorodihydrofluorescein, BK induced superoxide and H2O2 formation, which was inhibited by gp91ds-tat or apocynin but not by gp91scram-tat or rotenone. HPLC analysis confirmed that BK specifically induced superoxide production. Gp91ds-tat reduced vasodilation to BK but not to papaverine. 14,15-EEZE (an EET antagonist) further reduced the residual dilation to BK in the presence of gp91ds-tat, but had no effect in the presence of gp91scram-tat, suggesting that NADPH oxidase–derived ROS modulate EET bioavailability. Conclusion—We conclude that endothelial NADPH oxidase is a functionally relevant source of H2O2 that mediates agonist-induced dilation in the human heart.