Tolerance/intolerance to exercise training in HF animals.
Training times were significantly lower in HF+EX-inT animals, compared to HF+EX-T rats (29.2 ± 10.1 vs. 100.0 ± 13.1 % change, HF+EX-inT vs. HF+EX-T, respectively) (Fig. 1A). EX tolerance versus intolerance in HF rats was 61% (n=16) and 39% (n=10), respectively (Fig. 1B). In addition, EX tolerance/intolerance was not related to the initial degree of cardiac dysfunction since all groups exhibited similar cardiac dimensions. Indeed, before the beginning of the protocol (2 weeks post-HF surgery) all animals display no statistical differences on left ventricle end-diastolic diameter (LVEDD) (7.0 ± 0.3 vs. 6.7 ± 1.1 vs. 8.1 ± 0.2 mm), LV end-systolic diameter (LVESD) (3.8 ± 0.4 vs. 3.3 ± 0.5 vs. 3.8 ± 0.2 mm), LVED volume (LVEDV) (281.9 ± 21.9 vs. 355.7 ± 24.7 vs. 342.0 ± 19.7 µl), LVESV (67.7 ± 9.9 vs. 64.2 ± 9.2 vs. 68.9 ± 10.4 µl), LV ejection fraction (LVEF) (75.3 ± 4.4 vs. 79.8 ± 2.8 vs. 82.3 ± 1.5 %) or LV fractional shortening (LVFS) (45.6 ± 3.9 vs. 50.5 ± 3.1 vs. 53.1 ± 1.8) (HF+Sed vs. HF+EX-T vs. HF+EX-inT, respectively).
Baseline physiological parameters
Baseline physiological parameters at 8th weeks post-HF surgery are displayed in Table 1 and Fig. 1. HF+EX tolerant rats showed a significant increase of LVESD (4.7 ± 0.4 vs. 3.6 ± 0.3 mm, HF+EX-T vs. HF+Sed rats, respectively) (Fig. 1E) and LVESV (110.9 ± 18.6 vs. 60.6 ± 13.6 µl, HF+EX-T vs. HF+Sed rats, respectively), compared to HF+Sed rats (Fig. 1H). In addition, LVFS was significantly decreased in HF+EX-T and HF+EX-inT animals compared to HF+Sed rats (44.5 ± 2.9 and 45.1 ± 4.1 vs. 55.2 ± 2.5 %, HF+EX-T and HF+EX-inT vs. HF+Sed rats, respectively) (Fig. 1F), while LVEF was significantly different between HF+EX-T vs. HF+Sed rats (73.3 ± 2.8 vs. 83.9 ± 2.3 %, HF+EX-T vs. HF+Sed rats, respectively) (Fig. 1I). HF+EX-T and HF+EX-inT rats showed no significant differences in LVEDD, LVESD, LVESD, LVESV, LVFS and LVEF (Fig. 1). No significant changes on LVEDD and LVEDV were found between all groups (Fig. 1D and G, respectively).
HF+EX-inT rats showed a significant increase of cardiac hypertrophy compared to HF+EX-T animals (Table 1). HF+EX-T showed an increase in the soleus muscle-to-body weight (soleus/BW) ratio compared to HF+Sed animals (p<0.05) (Table 1). HF-EX-inT rats showed a slight but not significant increase in soleus/BW compared to HF+Sed animals (Table 1). EX did not change cardiac hypertrophy in HF+EX-T and HF+EX-inT animals compared to HF+Sed rats (Table 1).
Resting respiratory parameters (in normoxia) are shown in Table 2. No significant changes were found in VT amplitude or Rf. Accordingly, no changes in respiratory cycle duration or peak flows were found between groups (Table 2).
Cardiac baseline parameters including SV, SW, LVESP, dP/dtmax, dP/dtmin, dV/dtmax, dV/dtmin, TauW were not different between groups (Table 3).
Cardiac sympathetic tone and arrhythmia incidence
HF+EX-T rats showed a smaller cardiac bradycardic response to propranolol compared to HF+Sed rats and HF+EX-inT (-59.0 ± 8.8 vs. -92.9 ± 11.3 and -99.9 ± 9.8 ΔHR, HF+EX-T vs. HF+EX-inT and HF+Sed). In contrast, HF+EX-inT rats showed similar chronotropic response to propranolol compared to HF+Sed rats (-99.9 ± 9.8 vs. 92.9 ± 11.3 ΔHR, HF+EX-inT vs. HF+Sed rats, respectively) (Fig. 2A and B).
Arrhythmia incidence was significantly reduced in HF+EX-T rats (31.4 ± 19.8 vs. 196.0 ± 84.8 events/hour, HF+EX-T vs. HF+Sed). HF+EX-inT rats showed no change in arrhythmia incidence compared to HF+Sed animals (Fig. 2C and D).
Cardiac hemodynamic function
Cardiac function parameters are shown in Figure 3. HF+EX-T animals not showed significant differences in diastolic cardiac function compared to HF+Sed rats (Fig. 3A left and B). However, HF+EX-inT evidenced a worsening of diastolic function compared to HF+EX-T and HF+Sed group (V0: 289.5 ± 21.3 vs. 254.2 ± 19.4 and 237.8 ± 6.6 µl, HF+EX-inT vs. HF+EX-T and HF+Sed rats, respectively) (Fig. 3A left and B). Nevertheless, LVEDP and ESPVR were improved in HF+EX-T rats compared to HF+Sed and HF+EX-inT animals (LVEDP: 3.6 ± 0.3 vs. 5.6 ± 0.2 and 5.9 ± 0.8 mmHg, HF+EX-T vs. HF+Sed and HF+EX-inT, respectively) (ESPVR: 0.8 ± 0.1 vs. 0.5 ± 0.1 and 0.5 ± 0.1 mmHg/µl, HF+EX-T vs. HF+Sed and HF+EX-inT rats, respectively) (Fig. 3C and D). Systolic function in HF+EX-inT rats were indistinguishable compared to the ones obtained in HF+Sed (Fig. 3A right and D). No differences in maximum isovolumetric pressures were found between groups (Fig. 3E).
Cardiac responses to chemoreflex activation
Our previous work showed that chemoreflex activation increased cardiac arrhythmias and promoted deterioration in cardiac function in volume-overloaded HF rats8. Accordingly, we tested the effects of EX on arrhythmia incidence during stimulation of central and peripheral chemoreflexes. Central chemoreflex activation with acute hypercapnia elicits cardiac arrhythmias in HF+Sed and HF+EX-inT rats to a similar extent (14.4 ± 5.5 vs 17.0 ± 3.8 evens/10 min, respectively). Notably, the chemoreflex-induced cardiac arrhythmogenesis was blunted in HF+EX-T rats. HF+EX-T animals showed a ~3-fold reduction in arrhythmia incidence compared to HF+Sed animals (5.6 ± 2.1 vs. 14.4 ± 5.5 evens/10 min, HF+EX-T vs. HF+Sed, respectively) (Fig. 4A and B).
Peripheral chemoreflex activation with acute hypoxia did not trigger an increase in cardiac arrhythmias in HF+Sed rats nor in HF+EX-T rats (9.8 ± 4.7 events/10 min, HF+EX-T rats) (Fig. 4C). Indeed, during the hypoxic challenge both HF+Sed and HF+EX-T animals displayed an increase Rf without changes in intraventricular pressure and/or EKG integrity (Fig. 4C, left panel). In contrast, peripheral chemoreflex activation induced a marked increase in cardiac arrhythmias and related mortality in HF+EX-inT rats (Fig. 4C). Within a minute of hypoxic stimulation, arrhythmic events begin to appear and were accompanied by decreases in intraventricular pressures (Fig. 4C, right panel). Peripheral chemoreflex activation led to mortality in 40% of HF+EX-inT, but did not have a similar effect in the other experimental groups (Fig. 4D).
Peripheral and central chemoreflex gain.
Chemoreflex gain assessed before the onset of the EX-program (2 weeks post-HF surgery) showed no statistical differences in either peripheral or central chemoreflex gain between groups (HVR: 2.6 ± 0.3 vs. 3.6 ± 0.3 vs. 2.5 ± 0.4 ΔVE/ΔFiCO2%; and HCVR: 2.3 ± 0.6 vs. 2.9 ± 0.6 vs. 2.5 ± 0.5 ΔVE/ΔFiCO2%, HF+Sed vs. HF+EX-T vs. HF+EX-inT, respectively).
Responses to central and peripheral chemoreflex stimulation were evaluated by the ventilatory response to hypercapnia (FiCO2 7%) and hypoxia (FiO2 10%), respectively (Fig. 5A). HF+EX-T rats had significantly lower central chemoreflex gain compared to HF+Sed (3.1 ± 0.8 vs. 6.4 ± 0.4 ΔVE/ΔFiCO2%, HF+EX-T vs. HF+Sed, respectively) (Fig. 5A and B), without differences compared to HF+EX-inT animals (Fig. 5B). In contrast, HCVR in HF+EX-inT was similar to that in HF+Sed (Fig. 5B).
Peripheral chemoreflex gain in HF+EX-T not showed significant differences compared to HF+Sed rats (Fig. 5A and C). While, HF+EX-inT rats evidenced a significantly lower HVR than HF+EX-T and HF+Sed animals (2.9 ± 0.7 vs. 5.6 ± 0.2 and 4.8 ± 0.2 ΔVE/ΔFiO2%, HF+EX-inT vs. HF+EX-T and HF+Sed) (Fig. 5A and C). In addition, HF+EX-inT animals showed significant differences in Rf, Ve, Te, Ttot, PiF and PeF responses to hypoxia compared to HF+EX-T and HF+Sed animals (Fig. 5 and Table 2). VT was significantly reduced in HF+EX-inT rats compared to the HF+EX-T animals (Table 2). In addition, Ti was significantly higher in HF+EX-inT rats compared to HF+Sed animals (Table 2).
Carotid body ablation (CBA) and EX tolerance in HF rats.
The effects of CBA on training times are displayed in Fig. 6. Total training time was reduced by ~50% in HF+EX-T rats after CBA compared to previous EX times obtained before ablation of carotid body chemoreceptors (Fig. 6A). Accordingly, CBA results in a significant reduction in the ventilatory response to hypoxia in HF+EX-T (50.3 ± 1.4 vs. 33.1 ± 3.7 ml/min/100g, before vs. after CBA in HF+EX-T; Fig. 6B and C).