Short-term mechanisms activated by NGF to induce pulmonary arterial hyperreactivity

We have previously shown a pathophysiological role for NGF in pulmonary hypertension, particularly in pulmonary arterial (PA) hyperreactivity. Our experiments have suggested different mechanisms activated by NGF depending on a short- or a long-term PA treatment. The aim of the present study was to determine the mechanisms activated by a short-term NGF treatment (1 h) to induce PA hyperreactivity. Contractions of rat PA were induced ex vivo by endothelin-1 (ET-1, 10-12-10-6 M) in the absence or presence of NGF (100 ng/mL, 1 h), with or without K252a (TrkA kinase inhibitor, 300 nM) or Y–27632 (ROCK inhibitor, 10 μM). NGF–induced phosphorylation of myosin phosphatase target subunit 1 (MYPT1) was evaluated in vitro by Western blotting in primary PA smooth muscle cells (PASMC) from both rat and human origin. Basal and ET–1–induced (10-7 M) calcium responses were evaluated in vitro in PASMC using calcium imaging (Fura2, Fluo-4), in the absence or presence of NGF (100 ng/mL, 1 h). Calcium sources involved in NGF-dependent increase in calcium levels and response to ET-1 were determined using Thapsigargin and Cyclopiazonic acid (SERCA inhibitors, 1 or 10 μM, respectively) or verapamil and Nicardipine (calcium L–Type channels blocker, 2 μM). Ex vivo, NGF (1 h) induced rat PA hyperreactivity to ET-1 in a TrkA- and ROCK-dependent manner. In vitro, in both rat and human PASMC, NGF (1 h) enhanced basal calcium levels through activation of calcium L-type channels. NGF also enhanced ET–1–induced increase in intracellular calcium levels, with endoplasmic reticulum (ER) being the main calcium source contributing to these effects. Finally, in both rat and human cells, NGF also induced phosphorylation of MYPT1 in a TrkA- and ROCK-dependent manner. In conclusion, our results show that NGF induces PA hyperreactivity to ET-1 through activation of its TrkA receptor, leading to both increased calcium intracellular levels and increased ER calcium loading. NGF also activates ROCK-dependent calcium sensitization mechanisms. Such mechanisms may therefore contribute to NGF-induced PA hyperreactivity in pulmonary hypertension.