Negative-Feedback Loop Attenuates Hydrostatic Lung Edema via a cGMP-Dependent Regulation of Transient Receptor Potential Vanilloid 4

Abstract

Abstract-Although the formation of hydrostatic lung edema is generally attributed to imbalanced Starling forces, recent data show that lung endothelial cells respond to increased vascular pressure and may thus regulate vascular permeability and edema formation. In combining real-time optical imaging of the endothelial Ca 2ϩ concentration ([Ca 2ϩ ] i ) and NO production with filtration coefficient (K f ) measurements in the isolated perfused lung, we identified a series of endothelial responses that constitute a negative-feedback loop to protect the microvascular barrier. Elevation of lung microvascular pressure was shown to increase endothelial [Ca 2ϩ ] i via activation of transient receptor potential vanilloid 4 (TRPV4) channels. The endothelial [Ca 2ϩ ] i transient increased K f via activation of myosin light-chain kinase and simultaneously stimulated NO synthesis. In TRPV4 deficient mice, pressure-induced increases in endothelial [Ca 2ϩ ] i , NO synthesis, and lung wet/dry weight ratio were largely blocked. Endothelial NO formation limited the permeability increase by a cGMP-dependent attenuation of the pressure-induced [Ca 2ϩ ] i response. Inactivation of TRPV4 channels by cGMP was confirmed by whole-cell patch-clamp of pulmonary microvascular endothelial cells and intravital imaging of endothelial [Ca 2ϩ ] i . Hence, pressure-induced endothelial Ca 2ϩ influx via TRPV4 channels increases lung vascular permeability yet concomitantly activates an NO-mediated negative-feedback loop that protects the vascular barrier by a cGMP-dependent attenuation of the endothelial [Ca 2ϩ ] i response. The identification of this novel regulatory pathway gives rise to new treatment strategies, as demonstrated in vivo in rats with acute myocardial infarction in which inhibition of cGMP degradation by the phosphodiesterase 5 inhibitor sildenafil reduced hydrostatic lung edema. Key Words: pulmonary edema Ⅲ vascular permeability Ⅲ vascular endothelium Ⅲ phosphodiesterase type 5 inhibitor Ⅲ nitric oxide T he pathogenesis of hydrostatic lung edema has been attributed predominantly to an imbalance in Starling forces, ie, fluid extravasation attributable to an increased hydrostatic or reduced oncotic pressure gradient across the microvascular barrier. This classic view has been challenged by the findings of Parker and Ivey in isolated perfused rat lungs, which demonstrated an increase in lung filtration coefficient (K f ) following elevation of left atrial pressure (P LA ). 1 This increase was attenuated by the ␤-adrenergic agonist isoproterenol, indicating that the K f increase was not only caused by an enlarged vascular surface area but also resulted from an increase in vascular permeability that could be counteracted via the cAMP signaling pathway. The latter finding suggests that active endothelial responses may contribute critically to the formation of hydrostatic lung edema.By use of real-time fluorescence imaging techniques, we recently identified such endothelial responses to an acute elevation in hydrostatic pres...

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

“…These findings point to an interesting possibility that PKG phosphorylates the channel at a site that is different from protein kinase C or protein kinase A phosphorylation, and results in channel inhibition. Previous studies by Yin et al17 using Fura‐2 indicated that NO‐cGMP signaling lowers TRPV4‐induced increases in global Ca 2+ levels in lung venular capillary endothelium. Whether this effect of NO is because of a direct effect on single channel function of TRPV4 channels remains unknown.…”

“…NO can alter the activity of several ion channels by S ‐nitrosylation or activation of guanylyl cyclase‐protein kinase G (GC‐PKG) signaling 13, 14, 15, 16, 17. In cultured cells, S ‐nitrosylation of TRPV4 channels increased the channel function,18 and activation of GC‐PKG signaling reduced the channel activity 19.…”

Lung Endothelial Dysfunction in Congestive Heart Failure

“…Our experiments rule out downregulation of eNOS or lack of eNOS substrate which have been shown to underlie endothelial dysfunction in pulmonary arterial hypertension, 31,32 and consequently suggest that lung endothelial dysfunction in CHF may be attributable to impaired posttranslational regulation of eNOS. 17,22 failed to elicit a [Ca 2ϩ ] i increase in CHF lungs. TRPV4 and TRPV2 were both downregulated in CHF, seemingly supporting the notion that downregulation of these cation channels may underlie the loss of endothelial mechanosensitivity.…”

“…Ca 2ϩ concentration in cytosol ([Ca 2ϩ ] i ) and endosomal stores of lung endothelial cells was quantified by ratiometric imaging of fura-2 and fura-2FF, respectively. 17 The endothelial F-actin cytoskeleton was visualized by Alexa 568 phalloidin staining as described. 18 Immunofluorescence staining for ␣-smooth muscle actin in lung microvessels was performed as reported.…”

Update on vascular endothelial Ca2+signalling: A tale of ion channels, pumps and transporters

“…On the other hand, TRPV4-mediated Ca 2+ influx elicited by cyclic strain triggers cytoskeletal remodeling and movement necessary for cell realignment in the vasculature [292] . TRPV4 channels are extensively expressed in alveolar septal capillaries [163,313] , where they control lung endothelial permeability and barrier integrity in response to intravascular and airway pressures [314][315][316] . More specifically, TRPV4-gated Ca 2+ entry increases vascular permeability to such an extent that barrier disruption and alveolar flooding may occur [313] .…”