Transient Receptor Potential Channel 4 Encodes a Vascular Permeability Defect and High-Frequency Ca(2+) Transients in Severe Pulmonary Arterial Hypertension

The canonical transient receptor potential channel 4 (TRPC4) comprises an endothelial store-operated Ca(2+) entry channel, and TRPC4 inactivation confers a survival benefit in pulmonary arterial hypertension (PAH). Endothelial Ca(2+) signals mediated by TRPC4 enhance vascular permeability in vitro, but the contribution of TRPC4-dependent Ca(2+) signals to the regulation of endothelial permeability in PAH is poorly understood. We tested the hypothesis that TRPC4 increases vascular permeability and alters the frequency of endothelial Ca(2+) transients in PAH. We measured permeability in isolated lungs, and found that TRPC4 exaggerated permeability responses to thapsigargin in Sugen/hypoxia-treated PAH rats. We compared endothelial Ca(2+) activity of wild-type with TRPC4-knockout rats using confocal microscopy, and evaluated how Ca(2+) signals were influenced in response to thapsigargin and sequential treatment with acetylcholine. We found that thapsigargin-stimulated Ca(2+) signals were increased in PAH, and recovered by TRPC4 inactivation. Store depletion revealed bimodal Ca(2+) responses to acetylcholine, with both short- and long-duration populations. Our results show that TRPC4 underlies an exaggerated endothelial permeability response in PAH. Furthermore, TRPC4 increased the frequency of endothelial Ca(2+) transients in severe PAH, suggesting that TRPC4 provides a Ca(2+) source associated with endothelial dysfunction in the pathophysiology of PAH. This phenomenon represents a new facet of the etiology of PAH, and may contribute to PAH vasculopathy by enabling inflammatory mediator flux across the endothelial barrier.