Studies Of The Therapeutic Effects And Underlying Mechanisms Of Low-level Vagus Nerve Stimulation On Rats With Heart Failure

BackgroundHeart failure is a progressive disorder accompanied with maladaptive neurohormonal responses,in particular,an increased sympathetic tone and a reduction of vagal nerve activity.Extensive studies have shown that autonomic imbalance plays a detrimental role in the progression of heart failure and pharmacological therapy targeting on sympathetic overactivity is beneficial for improving cardiac remodeling and clinical outcomes in patients with HF.In recent years,vagus nerve stimulation（VNS）is a novel method to directly enhance the parasympathetic activity and the beneficial effects of VNS on heart failure were showed in some animal studies and clinical researches.However,the optimal stimulation paradigms were not determined and the underlying mechanisms were not fully understood.It is now well accepted that the altered calcium cycling is of significant relevance for the pathophysiology of heart failure and the natural autonomic function is essential for maintaining cellular Ca²⁺homeostasis.Besides,electrophysiological studies have shown that VNS has an anti-arrhythmic action against ventricular arrhythmias partially due to the improvement of calcium handling in ventricular myocytes.PurposeTo clarify the therapeutic effects of low-level vagus nerve stimulation on rats with heart failure and to explore the underlying mechanisms.MethodsThe experimental HF model was established by the ligation of left anterior descending coronary artery.Cardiac function was evaluated by echocardiography and the rats with left ventricular ejection fraction（LVEF）lower than 40%were defined as HF rats.Healthy male Sprague-Dawley rats were randomly assigned into Control group（sham-operated rats）,Sham group（HF rats with sham stimulation）,VNS group（HF rats with VNS）and LL-VNS group（HF rats with LL-VNS）.We stimulated the vagus nerve with two modes as common VNS（caused a reduction of baseline heart rate by 10%）and LL-VNS（stimulus intensity was set by 80%of threshold voltage）for 8 weeks.Heart rate variability（HRV）was evaluated by time-domain analysis and frequency-domain analysis continuously and dynamically.The plasma catecholamines including norepinephrine,epinephrine and dopamine were detected by using ELISA.Both HRV and plasma catecholamine levels were used to represent the autonomic balance.The extent of ventricular remodeling was evaluated by HE and Masson staining.Calcium-handling properties including sarcoplasmic reticulum Ca²⁺ATPase（SERCA2a）,Na⁺–Ca²⁺exchanger 1（NCX1）,phospholamban（PLB）,ryanodine receptor 2（RyR2）,CaMKII and its activated form were detected by Western-blot.The mRNA levels of SERCA2a,NCX1,PLB and RyR2were detected by real-time RCR.ResultsDuring the progression of HF,impaired autonomic balance characterized by the reduction of HRV and elevated plasma catecholamines was observed.After 8 weeks of treatment,both VNS and LL-VNS could significantly improve cardiac function which was indicated by the improvement of LVEF,LVESD and LVEDD.Meanwhile,VNS and LL-VNS normalized the changed HRV and plasma catecholamines levels comparably.Histologically,LL-VNS attenuated myocardial interstitial fibrosis and ventricular remodeling in rats with HF.Moreover,LL-VNS restored the protein and mRNA levels of SERCA2a,NCX1 and PLB whereas the expression of RyR2 as well as mRNA level was unaffected.The expression of CaMKII and its activated form were inhibited by LL-VNS,which lead to a reduction of PLB phosphorylation at Thr17.ConclusionOur study demonstrated that LL-VNS was an alternative approach to rebuild the autonomic balance and attenuate ventricular remodeling which could also improve cardiac function in rats with heart failure.Moreover,LL-VNS could rearrange the network of cardiac Ca²⁺handling properties,suggesting that the restoration of Ca²⁺handling function may be an underlying mechanism for LL-VNS to treat ischemic heart failure.