Ca2+ channels and pulmonary endothelial permeability:: Insights from study of intact lung and chronic pulmonary hypertension

Phenotypic heterogeneity in pulmonary vascular endothelial cells extends to regulation of endothelial permeability, a process which often depends upon Ca(2+) entry from the extracellular space. Scanning electron microscopy of vascular corrosion casts has documented distinct patterns of barrier disruption. Store depletion and activation of Ca(2+) entry via canonical transient potential channels (TRPC1 and TRPC4) disrupts the barrier in extraalveolar vessels. In contrast, numerous other models of acute lung injury, including high vascular pressure- or epoxyeicosatrienoic acid-induced injury, specifically disrupt the alveolar septal barrier. This review discusses Ca(2+) permeant channels which potentially could be involved in regulation of barrier integrity in the alveolar septal compartment: transient receptor potential channels, cyclic nucleotide gated channels, purinergic (P2X) channels, and T-type voltage gated channels. The evidence for the vanilloid transient receptor potential channel TRPV4 in regulating septal barrier function is discussed. Adaptations in barrier function in chronic pulmonary hypertension are reviewed, notably the loss of a store depletion-dependent permeability response in the intact lung. Finally, the authors propose that since specific disruption of the alveolar septal barrier will have deleterious functional consequences, such as alveolar flooding and impairment of gas exchange, identification of specific molecular targets for Ca(2 +) entry-dependent regulation of barrier function in this compartment is needed.