The cholinergic anti-inflammatory pathway and heart rate variability with special reference to dialysis and renal denervation

Abstract

Background: The Cholinergic Anti-inflammatory Pathway (CAP) is a nerve-mediated circuit through which the vagus nerve regulates the activity of the inflammatory immune response. Injury, ischemia or infection activate the afferent vagus. In the medulla oblongata. a response is subsequently returned via the efferent vagus, referred to as the inflammatory reflex. The signal is propagated to the spleen where specialized T-cells (ChAT) synthesize acetylcholine that binds to α-7-nicotinic acetylcholine receptors on macrophages leading to a down-regulation of inflammatory cytokines. CAP is modified with drugs or by vagus nerve stimulation (VNS) in animal models of sepsis, rheumatoid arthritis (RA) and inflammatory bowel disease (IBD). Clinical VNS pilot studies have demonstrated reduced inflammation and improvement in clinical symptoms in RA and IBD. Chronic kidney disease (CKD), characterized by autonomic dysfunction (AD), is linked to chronic inflammation and associated with excessive morbidity and mortality mainly from cardiovascular disease. The aims of this thesis were to study CAP and VNS in dialysis patients, and to investigate if renal denervation (RDN), in patients with resistant hypertension, could influence CAP. A final objective was to identify the optimal length of ECG for heart rate variability (HRV), a noninvasive tool to assess the cardiac autonomic nervous system, and correlates to inflammatory markers.

Methods and subjects: In study I-III we analyzed LPS stimulated cytokine levels ex vivo and the effect of GTS-21, a cholinergic analogue. Study I. Twenty dialysis patients and 8 healthy controls provided blood samples which were analyzed in the LPS-model. ECG was recorded for HRV in some patients and in all controls. Study II. Ten patients scheduled for RDN and 4 disease controls (DC) for elective coronary angiography (CA) were included. Bloods were drawn and ECG recorded before procedures, 1 day after, and in RND patients at follow-up after 3 and 6 months. Study III. Twelve dialysis patients underwent VNS treatment with a minimally invasive oscillating device before dialysis for 4 weeks. Bloods were drawn and ECG was recorded at baseline, after 2 and 4 weeks of treatment, and at 8- and 12-weeks follow-up. Study IV. The database HeartBEAT contains inflammatory markers and nocturnal ECG recordings from 318 subjects at baseline and 301 at follow-up. The data was used to analyze correlations between inflammatory markers, HRV indices, and different lengths of ECG recording.

Results and conclusion: In study I we found that the LPS-induced cytokine response was stronger in patients, where reduced HRV confirmed autonomic dysfunvtion, than in controls. Adding GTS-21 decreased cytokines in both groups, suggesting that dialysis patients also have a functional CAP. Study II showed that RDN had a strong effect on inflammatory markers 1 day after RDN, which disappeared during follow-up. With VNS in hemodialysis patients (study III) trends in cytokine response appeared but did not reach statistical significance. There is a negative correlation between inflammatory markers and HRV indices, but CRP alone is not a reliable predictor of autonomic dysfunction. ECG recording length of 60 minutes secured reliable HRV analysis. Outside of the protocol, in study III, we found that 3 out of 4 diabetics could reduce insulin doses with 25 % during the study suggesting a potential benefit regarding insulin resistance. Furthermore GTS-21 strengthened the cytokine response in study III suggesting a potential additional capacity for VNS. Dialysis and CKD patients may be suitable for non-invasive VNS with daily or prolonged treatment in future trials. It appears as longer ECG recordings may be useful, but HRV as prognostic tool for anti-inflammatory interventions require further studies.