Effects of endurance training on resting and post-exercise cardiac autonomic control

Loss of resting bradycardia with detraining is associated with intrinsic heart rate changes

The mechanisms underlying the loss of resting bradycardia with detraining were studied in rats. The relative contribution of autonomic and non-autonomic mechanisms was studied in 26 male Wistar rats (180-220 g) randomly assigned to four groups: sedentary (S, N = 6), trained (T, N = 8), detrained for 1 week (D1, N = 6), and detrained for 2 weeks (D2, N = 6). T, D1 and D2 were treadmill trained 5 days/week for 60 min with a gradual increase towards 50% peak VO2. After the last training session, D1 and D2 were detrained for 1 and 2 weeks, respectively. The effect of the autonomic nervous system in causing training-induced resting bradycardia and in restoring heart rate (HR) to pre-exercise training level (PET) with detraining was examined indirectly after cardiac muscarinic and adrenergic receptor blockade. T rats significantly increased peak VO2 by 15 or 23.5% when compared to PET and S rats, respectively. Detraining reduced peak VO2 in both D1 and D2 rats by 22% compared to T rats, indicating loss of aerobic capacity. Resting HR was significantly lower in T and D1 rats than in S rats (313 +/- 6.67 and 321 +/- 6.01 vs 342 +/- 12.2 bpm) and was associated with a significantly decreased intrinsic HR (368 +/- 6.1 and 362 +/- 7.3 vs 390 +/- 8 bpm). Two weeks of detraining reversed the resting HR near PET (335 +/- 6.01 bpm) due to an increased intrinsic HR in D2 rats compared with T and D1 rats (376 +/- 8.8 bpm). The present study provides the first evidence of intrinsic HR-mediated loss of resting bradycardia with detraining in rats.

Cardiac vagal tone, exercise performance and the effect of respiratory training

Heart rate variability (HRV) at rest and heart rate recovery after exercise reflect cardiac vagal activity. The aim of this study was to determine whether increasing HRV during involuntary respiratory training induced by rebreathing air using a Hepburn heart and lung exerciser (HHALE) could, like exercise, improve vagal tone. Eighteen subjects (36-88 years) underwent a 6-week control period, then a 6-week training period with the HHALE following which half continued training for 6 weeks and half ceased training. Measurements were made of HRV, work at 60% predicted heart rate max for 15 min, heart rate recovery after exercise, resting blood pressure, heart rate, vital capacity and forced expiratory volume. After the first 6-week HHALE training, there was a significant increase of 13.2 +/- 5.7 nu in the high frequency peak of the power spectrum of HRV at rest, whereas, the low frequency peak decreased. Similarly, exercise performance showed a significant improvement of 0.031 +/- 0.012 J per heartbeat from a pre-training 0.128 +/- 0.022. Also, heart rate recovery after exercise significantly faster (drop in the first 20 s improving by 3.3 +/- 1.5 beats from a pre-training 12.9 +/- 1.6). The subgroup that continued training maintained or slightly improved these values. In those that ceased training the speed of heart rate recovery at the end of the exercise test returned to pre-trained levels, whereas, other responses were either maintained or decreased slightly. We conclude that training with the HHALE can, without additional exercise, increase cardiac vagal tone and exercise performance.

Heart rate variability in athletes and nonathletes at rest and during head-up tilt

The purpose of the present study was to determine if autonomic heart rate modulation, indicated by heart rate variability (HRV), differs during supine rest and head-up tilt (HUT) when sedentary and endurance-trained cyclists are compared. Eleven sedentary young men (S) and 10 trained cyclists (C) were studied. The volunteers were submitted to a dynamic ECG Holter to calculate HRV at rest and during a 70 masculine HUT. The major aerobic capacity of athletes was expressed by higher values of VO2 at anaerobic threshold and peak conditions (P < 0.05). At rest the athletes had lower heart rates (P < 0.05) and higher values in the time domain of HRV compared with controls (SD of normal RR interval, SDNN, medians): 59.1 ms (S) vs 89.9 ms (C), P < 0.05. During tilt athletes also had higher values in the time domain of HRV compared with controls (SDNN, medians): 55.7 ms (S) vs 69.7 ms (C), P < 0.05. No differences in power spectral components of HRV at rest or during HUT were detected between groups. Based on the analysis of data by the frequency domain method, we conclude that in athletes the resting bradycardia seems to be much more related to changes in intrinsic mechanisms than to modifications in autonomic control. Also, HUT caused comparable changes in sympathetic and parasympathetic modulation of the sinus node in both groups.

Swimming training increases cardiac vagal activity and induces cardiac hypertrophy in rats

The effect of swimming training (ST) on vagal and sympathetic cardiac effects was investigated in sedentary (S, N = 12) and trained (T, N = 12) male Wistar rats (200-220 g). ST consisted of 60-min swimming sessions 5 days/week for 8 weeks, with a 5% body weight load attached to the tail. The effect of the autonomic nervous system in generating training-induced resting bradycardia (RB) was examined indirectly after cardiac muscarinic and adrenergic receptor blockade. Cardiac hypertrophy was evaluated by cardiac weight and myocyte morphometry. Plasma catecholamine concentrations and citrate synthase activity in soleus muscle were also determined in both groups. Resting heart rate was significantly reduced in T rats (355 +/- 16 vs 330 +/- 20 bpm). RB was associated with a significantly increased cardiac vagal effect in T rats (103 +/- 25 vs 158 +/- 40 bpm), since the sympathetic cardiac effect and intrinsic heart rate were similar for the two groups. Likewise, no significant difference was observed for plasma catecholamine concentrations between S and T rats. In T rats, left ventricle weight (13%) and myocyte dimension (21%) were significantly increased, suggesting cardiac hypertrophy. Skeletal muscle citrate synthase activity was significantly increased by 52% in T rats, indicating endurance conditioning. These data suggest that RB induced by ST is mainly mediated parasympathetically and differs from other training modes, like running, that seems to mainly decrease intrinsic heart rate in rats. The increased cardiac vagal activity associated with ST is of clinical relevance, since both are related to increased life expectancy and prevention of cardiac events.

Decrease in heart rate variability with overtraining: assessment by the Poincare plot analysis

Numerous symptoms have been associated with the overtraining syndrome (OT), including changes in autonomic function. Heart rate variability (HRV) provides non-invasive data about the autonomic regulation of heart rate in real-life conditions. The aims of the study were to: (i) characterize the HRV profile of seven athletes (OA) diagnosed as suffering of OT, compared with eight healthy sedentary (C) and eight trained (T) subjects during supine rest and 60 degrees upright, and (ii) compare the traditional time- and frequency-domain analysis assessment of HRV with the non-linear Poincaré plot analysis. In the latter each R-R interval is plotted as a function of the previous one, and the standard deviations of the instantaneous (SD1) and long-term R-R interval variability are calculated. Total power was higher in T than in C and OA both in supine (1158 +/- 1137, 6092 +/- 3554 and 2970 +/- 2947 ms2 for C, T and OA, respectively) and in upright (640 +/- 499, 1814 +/- 806 and 1092 +/- 712 ms2 for C, T and OA, respectively; P<0.05) positions. In supine position, indicators of parasympathetic activity to the sinus node were higher in T compared with C and OA (high-frequency power: 419.1 +/- 381.2, 1105.3 +/- 781.4 and 463.7 +/- 715.8 ms2 for C, T and OA, respectively; P<0.05; SD1: 29.5 +/- 18.5, 75.2 +/- 17.2 and 37.6 +/- 27.5 for C, T and OA, respectively; P<0.05). OA had a marked predominance of sympathetic activity regardless of the position (LF/HF were 0.47 +/- 0.35, 0.47 +/- 0.50 and 3.96 +/- 5.71 in supine position for C, T and OA, respectively, and 2.09 +/- 2.17, 7.22 +/- 6.82 and 12.04 +/- 10.36 in upright position for C, T and OA, respectively). The changes in HRV indexes induced by the upright posture were greater in T than in OA. The shape of the Poincaré plots allowed the distinction between the three groups, with wide and narrow shapes in T and OA, respectively, compared with C. As Poincaré plot parameters are easy to compute and associated with the 'width' of the scatter gram, they corroborate the traditional time- and frequency-domain analysis. We suggest that they could be used to indicate fatigue and/or prevent OT.

Duration-controlled swimming exercise training induces cardiac hypertrophy in mice

Exercise training associated with robust conditioning can be useful for the study of molecular mechanisms underlying exercise-induced cardiac hypertrophy. A swimming apparatus is described to control training regimens in terms of duration, load, and frequency of exercise. Mice were submitted to 60- vs 90-min session/day, once vs twice a day, with 2 or 4% of the weight of the mouse or no workload attached to the tail, for 4 vs 6 weeks of exercise training. Blood pressure was unchanged in all groups while resting heart rate decreased in the trained groups (8-18%). Skeletal muscle citrate synthase activity, measured spectrophotometrically, increased (45-58%) only as a result of duration and frequency-controlled exercise training, indicating that endurance conditioning was obtained. In groups which received duration and endurance conditioning, cardiac weight (14-25%) and myocyte dimension (13-20%) increased. The best conditioning protocol to promote physiological hypertrophy, our primary goal in the present study, was 90 min, twice a day, 5 days a week for 4 weeks with no overload attached to the body. Thus, duration- and frequency-controlled exercise training in mice induces a significant conditioning response qualitatively similar to that observed in humans.

Influence of short-term endurance exercise training on heart rate variability

PURPOSE

To examine the influence of 2 wk (eight sessions) of endurance training on cardiac autonomic modulation, as measured by heart rate variability (HRV).

METHODS

Twenty-four males (mean age: 23.1 yr) were randomized to an exercise (EX; N = 12) or control group (CT; N = 12). EX trained for eight sessions (4x wk-1, 40 min, 80-85% HRreserve) on a cycle ergometer. ECG tracings were collected during 5 min of paced breathing (12 breaths x min-1 (PB)), 5 min of spontaneous breathing (SB1), 5 min of 70 degrees head-up tilt (TILT), and a second 5-min period of spontaneous breathing (SB2). Data were collected before (test 1), during (tests 2-4), and 48 h after (test 5) the 2-wk period. HRV was reported as the standard deviation of RR intervals, and as natural logarithm of the normalized units (NU) of high- and low-frequency power (lnHF and lnLF).

RESULTS

EX exhibited a significant increase in peak oxygen consumption (8%). During PB and TILT conditions, ANOVA revealed a group x time interaction such that EX exhibited lower lnLFNU and lnLF/lnHF during test 5 compared with test 1.

CONCLUSION

These data suggest that eight endurance exercise-training sessions performed over 2 wk enhance the relative vagal modulation of the heart during PB and TILT, but not during SB.

Effect of endurance exercise on autonomic control of heart rate

Long-term endurance training significantly influences how the autonomic nervous system controls heart function. Endurance training increases parasympathetic activity and decreases sympathetic activity in the human heart at rest. These two training-induced autonomic effects, coupled with a possible reduction in intrinsic heart rate, decrease resting heart rate. Long-term endurance training also decreases submaximal exercise heart rate by reducing sympathetic activity to the heart. Physiological ageing is associated with a reduction in parasympathetic control of the heart; this decline in parasympathetic activity can be reduced by regular endurance exercise. Some research has indicated that females have increased parasympathetic and decreased sympathetic control of heart rate. These gender-specific autonomic differences probably contribute to a decreased cardiovascular risk and increased longevity observed in females.

Enhanced neuronal nitric oxide synthase expression is central to cardiac vagal phenotype in exercise-trained mice

We investigated whether enhanced cardiac vagal responsiveness elicited by exercise training is dependent on neuronal nitric oxide synthase (NOS-1), since the NO-cGMP pathway facilitates acetylcholine release. Isolated atria with intact right vagal innervation were taken from male mice (18-22 weeks old) after a period of 10 weeks voluntary wheel-running (+EX, n = 27; peaked 9.8 +/- 0.6 km day(-1) at 5 weeks), and from mice housed in cages without wheels (-EX, n = 27). Immunostaining of whole atria for NOS-1 identified intrinsic neurones, all of which co-localized with choline acetyltransferase-positive ganglia. Western blot analysis confirmed that NOS-1 protein level was significantly greater in +EX compared to -EX atria (P < 0.05, unpaired t test). Basal heart rates (HR) were slower in +EX than in -EX atria (322 +/- 6 versus 360 +/- 7 beats min(-1); P < 0.05, unpaired t test) However, in +EX atria, HR responses to vagal stimulation (VNS, 3 and 5 Hz) were significantly enhanced compared to -EX atria (3 Hz, +EX: -76 +/- 8 beats min(-1) versus -EX: -62 +/- 7 beats min(-1); 5 Hz, +EX: -106 +/- 4 beats min(-1) versus -EX: -93 +/- 3 beats min(-1); P < 0.01, unpaired t test). Inhibition of NOS-1 with vinyl-L-N-5-(1-imino-3-butenyl)-L-ornithine (L-VNIO, 100 microM) or soluble guanylyl cyclase with 1H-[1, 2, 4]oxadiazolo[4, 3-a]quinoxalin-1-one (ODQ, 10 microM) abolished the difference in HR responses to VNS between +EX and -EX atria, and effects of L-VNIO were reversed by excess L-arginine (1 mM; P < 0.01, ANOVA). There were no differences between the HR responses to the bath-applied acetylcholine analogue carbamylcholine chloride in +EX and -EX atria (IC(50) concentrations were 5.9 +/- 0.4 microM (-EX) and 5.7 +/- 0.4 microM (+EX)), suggesting that the changes in vagal responsiveness resulted from presynaptic facilitation of neurotransmission. In conclusion, NOS-1 appears to be a key protein in generating the cardiac vagal gain of function elicited by exercise training.

Mortality, cardiac vagal control and physical training--what's the link?

There is little doubt that regular exercise results in increases in life expectancy and protects against adverse cardiac events in both healthy subjects and patients with cardiovascular disease. The mechanism of action of physical training remains unclear but a variety of evidence points towards an enhancement in cardiac vagal activity protecting against lethal arrhythmias. Just how physical training increases cardiac vagal activity is an area that is ill understood but plausible mechanisms include mediation via angiotensin II or NO. Further research is needed in this area. Exercise training is demanding and difficult, particularly for patients with cardiac disease. If the mechanism of increase in cardiac vagal activity with training can be determined it may be possible to use pharmacological approaches to mimic the effects of exercise with potentially beneficial effects.