Woodward, W. R. | bioRxiv Science

bioRxiv2023

Chronic nicotine exposure is associated with electrophysiological and sympathetic remodeling in the intact rabbit heart

Nicotine is the primary addictive component in tobacco products. Through its actions on the heart and autonomic nervous system, nicotine exposure is associated with electrophysiological changes and increased arrhythmia susceptibility. However, the underlying mechanisms are unclear. To address this, we treated rabbits with transdermal nicotine (NIC, 21 mg/day) or control (CT) patches for 28 days prior to performing dual optical mapping of transmembrane potential (RH237) and intracellular Ca2+ (Rhod-2 AM) in isolated hearts with intact sympathetic innervation. Sympathetic nerve stimulation (SNS) was performed at the 1st - 3rd thoracic vertebrae, and {beta}-adrenergic responsiveness was additionally evaluated as changes in heart rate (HR) following norepinephrine (NE) perfusion. Baseline ex vivo HR and SNS stimulation threshold were increased in NIC vs. CT (P = 0.004 and P = 0.003 respectively). Action potential duration alternans emerged at longer pacing cycle lengths (PCL) in NIC vs. CT at baseline (P = 0.002) and during SNS (P = 0.0003), with similar results obtained for Ca2+ transient alternans. SNS reduced the PCL at which alternans emerged in CT but not NIC hearts. NIC exposed hearts also tended to have slower and reduced HR responses to NE perfusion. While fibrosis was unaltered, NIC hearts had lower sympathetic nerve density (P = 0.03) but no difference in NE content vs. CT. These results suggest both sympathetic hypo-innervation of the myocardium and diminished {beta}-adrenergic responsiveness with NIC. This autonomic remodeling may underlie the increased risk of arrhythmias associated with nicotine exposure, which may be further exacerbated with continued long-term usage. NEW & NOTEWORTHYHere we show that chronic nicotine exposure was associated with increased heart rate, lower threshold for alternans and reduced sympathetic electrophysiological responses in the intact rabbit heart. We suggest that this was due to the sympathetic hypo-innervation of the myocardium and diminished {beta}- adrenergic responsiveness observed following nicotine treatment. Though these differences did not result in increased arrhythmia propensity in our study, we hypothesize that prolonged nicotine exposure may exacerbate this pro-arrhythmic remodeling.

physiology↗

bioRxiv2022

Chondroitin Sulfate Proteoglycan 4,6 sulfation regulates sympathetic nerve regeneration after myocardial infarction

Sympathetic denervation of the heart following myocardial infarction (MI) is sustained by chondroitin sulfate proteoglycans (CSPGs) in the cardiac scar, and denervation predicts risk of sudden cardiac death. Blocking CSPG signaling restores sympathetic axon outgrowth into the cardiac scar, decreasing arrhythmia susceptibility. Axon growth inhibition by CSPGs is thought to depend on the sulfation status of the glycosaminoglycans (CS-GAGs) attached to the core protein. Tandem sulfation of CS-GAGs at the 4th (4S) and 6th (6S) positions of n-acetyl-galactosamine inhibits outgrowth in several types of neurons within the central nervous system, but it is not known if sulfation is similarly critical during peripheral nerve regeneration. We asked if CSPG sulfation prevented sympathetic axon outgrowth. Sympathetic neurite outgrowth across purified CSPGs is restored in vitro by reducing 4S with the 4-sulfatase enzyme Arylsulfatase-B (ARSB). Additionally, we co-cultured cardiac scar tissue with sympathetic ganglia ex vivo and found that reducing 4S with ARSB restored axon outgrowth to control levels. We examined levels of the enzymes responsible for adding and removing sulfation to CS-GAGs by western blot to determine if they were altered in the left ventricle after MI. We found that CHST15 (4S dependent 6-sulfotransferase) was upregulated, and ARSB was downregulated after MI. Increased CHST15 combined with decreased ARSB suggests a mechanism for production and maintenance of sulfated CSPGs in the cardiac scar. We altered tandem sulfated 4S,6S CS-GAGs in vivo by transient siRNA knockdown of Chst15 and found that reducing 4S,6S restored Tyrosine Hydroxylase (TH) positive sympathetic nerve fibers in the cardiac scar after MI and reduced arrhythmias. Overall, our results suggest that modulating CSPG-sulfation after MI may be a therapeutic target to promote sympathetic nerve regeneration in the cardiac scar and reduce post-MI cardiac arrhythmias.

neuroscience↗