Cardiac Sympathetic Innervation and PGP 9.5 Expression by Cardiomyocytes in Rats After Myocardial Infarction. Effects of Central MR Blockade

Abstract

Central mechanisms involving aldosterone - mineralocorticoid receptor (MR) activation mediate the increase in sympathetic tone after myocardial infarction (MI). We hypothesized that an increase in cardiac sympathetic activity (CSA) post MI facilitates cardiac sympathetic axonal sprouting, and that central MR blockade attenuates CSA and reduces cardiac sympathetic hyperinnervation post MI. Western blotting and qRT-PCR were used to assess protein and gene expression, and fluorescent immunohistochemistry was used to study changes in sympathetic innervation. Tyrosine hydroxylase (TH) and Norepinephrine transporter protein content in the non-infarcted base of the heart remained unaltered. In contrast, protein gene product (PGP 9.5) protein was significantly increased 2 fold in the base of the heart, and 6 fold in the peri-infarct area at 1 wk post MI, and associated with increased ubiquitin expression. Cardiac myocytes rather than sympathetic axons were identified as the main source of elevated PGP 9.5 expression. At the infarct border sympathetic hyperinnervation was observed with a 4 fold increase in growth associated protein 43 (GAP 43), a 2 fold increase in TH and a 50% increase in PGP 9.5 positive fibers when compared to the epicardial side of the left ventricle in sham rats. Central infusion of the MR blocker eplerenone at 5 ug/day for 9 days post MI markedly attenuated the increase in TH, GAP 43 and PGP 9.5 nerve densities at the infarct border. Central MR blockade may attenuate sympathetic hyperinnervation by several mechanisms, including decreasing CSA post MI, or affecting expression or function of nerve growth factor protein. Marked PGP 9.5 expression occurs in cardiomyocytes early post MI, which may contribute to the increase in ubiquitin and the early cardiac remodeling post MI.