Evidence that Sympathetic Activity is Increased Selectively to the Heart in a Mouse Model of Hypertrophic Cardiomyopathy

Abstract

Hypertrophic cardiomyopathy (HCM), the most common genetic heart disease, is characterized by cardiac hypertrophy, fibrosis, and increased risk of arrhythmias and sudden death, even in the absence of heart failure. We have studied young (<20 wks) transgenic mice with cardiac‐specific expression of mutated alpha‐tropomyosin (Glu180Gly), an established model of HCM, to investigate autonomic mechanisms. In early experiments, abnormal heart rate and blood pressure responses to β‐adrenergic receptor blockade and ganglionic blockade indicated high cardiac accompanied by low vasomotor sympathetic tone in HCM vs. wild‐type (WT) mice. The aim of this study was to further characterize regional sympathetic activity in HCM vs. WT littermates by measuring expression/activity of tyrosine hydroxylase (TH, rate‐limiting enzyme for norepinephrine (NE) synthesis) and the norepinephrine transporter (NET) in sympathetic ganglia (stellate, superior cervical, celiac/mesenteric), and expression of β1 receptors in left ventricle (LV). Results (below) are expressed relative to WT (n=4 male mice each group). TH protein expression (Western blot) (standardized to neuronal content with PGP9.5) was significantly higher in stellate (cardiac) ganglia in HCM mice (HCM 162±12 vs. WT 100±7; p<0.05), but not significantly different in superior cervical (HCM 82±4 vs. WT 100±14; p>0.05) or celiac‐mesenteric (HCM 95±7 vs. WT 100±4; p>0.05) ganglia. TH/PGP9.5 protein expression was also higher in the LV of HCM (135±15) vs. WT (100±1) mice (p<0.05). LV β1 receptor protein expression was significantly lower in HCM (60±1) vs. WT (100±4) mice (p<0.05), consistent with increased local NE levels. NET protein (immunofluorescence) was lower in stellate ganglia from HCM mice vs. WT (n=3 mice per group, 3 sections/mouse, p<0.05). NET activity was assessed by measuring the rate of uptake of a fluorescent NET substrate (Molecular Devices) into individual sympathetic neurons isolated from stellate and superior cervical ganglia. NET activity measured in stellate neurons was significantly lower in HCM (4.5±0.3 a.u.*min−1, n=115 neurons, from 4 mice) vs. WT (5.3±0.4 a.u.*min−1, n=98 neurons, from 4 mice). NET activity in superior cervical ganglia did not differ between genotypes. In conclusion, increased TH expression (enhanced ‐NE synthesis) and decreased NET expression/activity (reduced NE reuptake) in stellate ganglia are consistent with increased cardiac sympathetic activity in HCM, whereas the lack of differences between HCM and WT in other sympathetic ganglia are consistent with normal sympathetic activity to non‐cardiac target organs in HCM.Support or Funding InformationNIH F32HL140880