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Dexamethasone treatment of post-MI rats attenuates sympathetic innervation of the infarct region

¹Département de Physiologie, Université de Montréal; and ²Montreal Heart Institute, Montreal, Quebec, Canada

Submitted 21 June 2007 ; accepted in final form 28 September 2007

Sympathetic fiber innervation of the damaged region following injury represents a conserved event of wound healing. The present study tested the hypothesis that impaired scar healing in post-myocardial infarction (post-MI) rats was associated with a reduction of sympathetic fibers innervating the infarct region. In 1-wk post-MI rats, neurofilament-M-immunoreactive fibers (1,116 ± 250 µm²/mm²) were detected innervating the infarct region and observed in close proximity to a modest number of endothelial nitric oxide synthase-immunoreactive scar-residing vessels. Dexamethasone (Dex) treatment (6 days) of post-MI rats led to a significant reduction of scar weight (Dex + MI 38 ± 4 mg vs. MI 63 ± 2 mg) and a disproportionate nonsignificant decrease of scar surface area (Dex + MI 0.54 ± 0.06 cm² vs. MI 0.68 ± 0.06 cm²). In Dex-treated post-MI rats, the density of neurofilament-M-immunoreactive fibers (125 ± 47 µm²/mm²) innervating the infarct region was significantly reduced and associated with a decreased expression of nerve growth factor (NGF) mRNA (Dex + MI 0.80 ± 0.07 vs. MI 1.11 ± 0.08; P < 0.05 vs. MI). Previous studies have demonstrated that scar myofibroblasts synthesize NGF and may represent a cellular target of Dex. The exposure of 1st passage scar myofibroblasts to Dex led to a dose-dependent suppression of [³H]thymidine uptake and a concomitant attenuation of NGF mRNA expression (untreated 3.47 ± 0.35 vs. Dex treated 2.28 ± 0.40; P < 0.05 vs. untreated). Thus the present study has demonstrated that impaired scar healing in Dex-treated post-MI rats was associated with a reduction of neurofilament-M-immunoreactive fibers innervating the infarct region. The attenuation of scar myofibroblast proliferation and NGF mRNA expression may represent underlying mechanisms contributing to the diminished neural response in the infarct region of Dex-treated post-MI rats.

dexamethasone; neural remodeling; myocardial infarction; myofibroblasts; nerve growth factor

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