Autonomic Regulation of Heart Rate

A 3-lead ECG was recorded from male volunteers aged 18 to 30 years: 8 competitive endurance athletes and 10 sedentary age-matched (control) subjects. The heart rate was measured before and after complete autonomic blockade (achieved by intravenous injection of 0.04 mg/kg atropine and 0.2 mg/kg propranolol followed by top-up doses). After complete autonomic blockade, 7.5 mg ivabradine was administered orally, and the change in heart rate was recorded and used as a measure of the involvement of I_f in pacemaking. Ten-week-old C57BL/6J mice were trained by swimming for 60 minutes twice daily for 28 days.^{5 (link)} miR, mRNA, and protein expression in sinus node biopsies was measured by next-generation sequencing, quantitative real-time reverse transcription polymerase chain reaction (qPCR), Western blot, and high-resolution mass spectrometry. Computational predictions, luciferase reporter gene assays, and in vitro overexpression studies were used to identify miRs and transcription factors capable of regulating expression. The role of a candidate miR in the training-induced bradycardia was tested in vivo by administering an appropriate cholesterol-conjugated anti-miR.^{6 (link)} ECG recording, in vitro tissue electrophysiology, Western blot, sinus node cell isolation, and whole-cell patch clamp were used to characterize the mice and study HCN4 and I_f remodeling. Statistically significant differences were determined using an appropriate test; P<0.05 was regarded as significant. In figures, bar charts show means±SEM. Further details of methods are available in the Online Data Supplement.