Second hand smoke exposure depresses excitability and SK channel function of cardiac vagal neurons in the nucleus ambiguus

Abstract

BACKGROUNDSecond hand smoke (SHS), a major indoor air pollutant, is a significant risk factor for cardiac dysfunction including arrhythmias and sudden cardiac death. Reduced heart rate variability (HRV), an index of autonomic function, has been implicated as one of the underlying mechanisms of these cardiac maladies. We have previously shown that exposure to air pollution in the form of iron‐soot significantly reduced HRV and spiking activity of cardiac vagal neurons in the nucleus ambiguus (NA), the final vagal cardiac output regulating HRV. Here, we sought to test the hypothesis that SHS exposure, at an environmentally relevant concentration that reduces HRV, depresses neuronal excitability of cardiac vagal neurons in the NA by altering potassium channel function.METHODSAdult male C57BL6/J mice underwent open chest surgeries to retrograde label cardiac vagal neurons by applying a fluorescent dye (DiI) to the sinoatrial node. After two weeks of recovery from surgery, mice were randomly assigned to either filtered air (FA) or SHS (3 mg/m3, 6 hr/d, 5 d/wk) for 4‐, 8‐ or 12‐weeks. Whole‐cell current clamp recordings in brainstem slices were performed on anatomically identified cardiac vagal neurons in the NA with fluorescent labeling.RESULTSFour weeks of SHS exposure was sufficient to increase action potential (AP) threshold and reduce spiking response to depolarizing current injections (p < 0.05) in cardiac vagal neurons. The minimal current required to evoke an AP is also significantly higher in SHS group. Perfusion with apamin (20 nM), an SK channel blocker, had no effect on AP threshold or on minimal current required to evoke an AP in either FA or SHS mice. However, apamin significantly increased neuronal spiking response to depolarization in both groups; this effect was significantly smaller in SHS animals (p < 0.05). These data suggest that SHS exposure reduces excitability of cardiac vagal neurons that is associated with attenuated SK channel modulation of spiking response. These data also suggest that SHS exposure may reduce dynamic spiking range of these cardiac vagal neurons, consistent with reduced cardiac vagal regulation of HRV. Resting membrane potential was not significantly changed until 12 weeks of SHS exposure (became more depolarized). These data suggest that there may be compensatory changes in potassium channel function to increase the neuronal excitability in response to extended SHS exposures.CONCLUSIONSWe conclude that environmentally relevant SHS exposure can reduce neuronal excitability of cardiac vagal neurons and contribute to reduced vagal regulation of HRV resulting in reduced HRV.Support or Funding InformationR01 ES025229; T32 HL086350