Cardiac morphological and electrophysiological changes induced by sustained, high-intensity endurance training in large animal experimental models

Despite the well-known benefits of regular exercise, there is a growing body of evidence suggesting that elite athletes who engage in intense training beyond a certain threshold are more susceptible to harmful ventricular cardiac arrhythmias, and in some cases, even sudden cardiac death, although the mechanisms behind this remain unclear. To gain a better understanding of cardiac remodelling in response to chronic vigorous exercise, this study explores the effects of such exercise on cardiac structure and electrophysiology in new rabbit and dog models of the athlete's heart. In the first experiments, rabbits and dogs were divided into sedentary (SED), exercised (EX) subjected to 16 weeks of chronic treadmill exercise, and a testosterone-treated group in dogs (DOP). In the second experiments, a more intense training protocol for dogs was introduced, and changes between trained (TRN) dogs and their sedentary (SED) counterparts were studied. Various tests, including echocardiography and electrocardiograms, were conducted, along with assessing proarrhythmic sensitivity and autonomic responses in conscious dogs. In vitro studies, such as electrophysiological measurements, immunocytochemistry, and histopathological analysis, were carried out after heart removal. Results showed that EX animals, both rabbits and dogs, displayed left ventricular enlargement and bradycardia, indicating an increased vagal tone. EX and DOP dogs showed a lower response to the parasympatholytic agent atropine and more pronounced QTc interval lengthening after dofetilide challenge compared to the SED group. No significant morphological or functional changes were observed in dogs treated with steroids. In conscious trained dogs, ECG recordings indicated bradycardia, prolonged QTc intervals, and increased QT interval variability, reflecting elevated repolarization dispersion. At the cellular level, prolonged action potential duration and reduced magnitude of the transient outward potassium current were observed in the left ventricular myocytes of trained dogs. Left ventricular fibrosis and increased HCN4 protein expression were also noted. Our findings provide in vivo, cellular electrophysiological, and molecular biological evidence for the enhanced susceptibility to ventricular arrhythmias in the large animal model of chronic exercise. The sustained 4-month training regimen resulted in echocardiographic changes that are consistent with the morphology of the hearts of endurance-trained human athletes. These animal models hold promise for further investigations into the cardiovascular effects of competitive training

Acetylcholine Reduces L-Type Calcium Current without Major Changes in Repolarization of Canine and Human Purkinje and Ventricular Tissue

Vagal nerve stimulation (VNS) holds a strong basis as a potentially effective treatment modality for chronic heart failure, which explains why a multicenter VNS study in heart failure with reduced ejection fraction is ongoing. However, more detailed information is required on the effect of acetylcholine (ACh) on repolarization in Purkinje and ventricular cardiac preparations to identify the advantages, risks, and underlying cellular mechanisms of VNS. Here, we studied the effect of ACh on the action potential (AP) of canine Purkinje fibers (PFs) and several human ventricular preparations. In addition, we characterized the effects of ACh on the L-type Ca2+ current (I-CaL) and AP of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and performed computer simulations to explain the observed effects. Using microelectrode recordings, we found a small but significant AP prolongation in canine PFs. In the human myocardium, ACh slightly prolonged the AP in the midmyocardium but resulted in minor AP shortening in subepicardial tissue. Perforated patch-clamp experiments on hiPSC-CMs demonstrated that 5 mu M ACh caused an approximate to 15% decrease in I-CaL density without changes in gating properties. Using dynamic clamp, we found that under blocked K+ currents, 5 mu M ACh resulted in an approximate to 23% decrease in AP duration at 90% of repolarization in hiPSC-CMs. Computer simulations using the O'Hara-Rudy human ventricular cell model revealed that the overall effect of ACh on AP duration is a tight interplay between the ACh-induced reduction in I-CaL and ACh-induced changes in K+ currents. In conclusion, ACh results in minor changes in AP repolarization and duration of canine PFs and human ventricular myocardium due to the concomitant inhibition of inward I-CaL and outward K+ currents, which limits changes in net repolarizing current and thus prevents major changes in AP repolarization

Fokozott aritmiaérzékenységgel társuló kamrai szívizom-remodelling vizsgálata sportszív nagyállatmodellben

A közelmúltban több tanulmány is rámutatott arra, hogy az intenzív sporttevékenység szívritmuszavarok kialakulására hajlamosíthat és akár hirtelen szívhalálhoz is vezethet, azonban a háttérben húzódó elektrofiziológiai mechanizmusok kevéssé ismertek. Munkánk során a tartós állóképességi edzés indukálta szívizom-átépülést (ún. „remodelling”) és aritmiaérzékenységet vizsgáltuk kutya sportszív-modellben. Beagle-kutyákat véletlenszerűen „kontroll” és „edzett” csoportokba soroltunk (n=12/csoport), ez utóbbi egy 4 hónapos intenzív edzésprogramban vett részt. A szívizom-remodellációt és a ritmuszavar-érzékenységet számos in vivo és in vitro technikával vizsgáltuk (elektrokardiográfia, echokardiográfia, közvetlen kamrai „burst” ingerlés, patch-clamp, immunocitokémia, szövettani vizsgálatok). Az edzésprogram hatására nőtt a szeptális falvastagság (8,1±0,2 mm vs. 7,4±0,2 mm; p<0,05), a bal kamrai végdisztolés átmérő (32,0±0,7 mm vs. 30,4±0,7 mm; p<0,05) és a bal kamrai tömegindex (125,8±4,3 g/m2 vs. 97,7±6,4 g/m2 ; p<0,05). Megnyúlt kamrai repolarizációt figyeltünk meg in vivo és in vitro körülmények között (QTc: 237,1±3,4 ms vs. 213,6±2,8 ms; APD90: 472,8±29,6 ms vs. 370,1±32,7 ms; p<0,05), amelyek a repolarizáció emelkedett rövid távú variabilitásával társultak. Az edzett állatok szívéből izolált bal kamrai szívizomsejtek tranziens kifelé irányuló K+-áramának amplitúdója csökkent (6,4±0,5 pA/pF vs. 8,8±0,9 pA/ pF, 50 mV; p<0,05), mindemellett a bal kamrai fibrózis mértéke, illetve a HCN4-fehérje expressziója is emelkedett. Fokozott ektópiás aktivitást és aritmiaérzékenységet figyeltünk meg az edzett állatokban. Modellünkben egyértelmű strukturális és elektrofiziológiai szívizom-átalakulás jelentkezett. Szívmorfológiai megfigyeléseink összhangban állnak a humán állóképességi élsportolókban megfigyeltekkel. Tanulmányunkban a repolarizáció diszperziójának fokozódása, a fibrózis emelkedett mértéke és a HCN4-túlexpresszió a modell aritmiák iránti fokozott érzékenységét jelzik

Long-term endurance training-induced cardiac adaptation in new rabbit and dog animal models of the human athlete’s heart

Sudden cardiac death in athletes is rare and most often unexpectable. For a better understanding of cardiac re- modeling, this study presents the effects of chronic vigor- ous exercise on cardiac structure and electrophysiology in new rabbit and dog athlete’s heart models. Rabbits and dogs were randomized into sedentary (’Sed’), exer- cised (subjected to 16 weeks chronic treadmill exercise (’Ex’) groups, and a testosterone-treated (’Dop’) group in dogs. Echocardiography and electrocardiogram were performed. Proarrhythmic sensitivity and autonomic re- sponses were tested in conscious dogs. ‘Ex’ animals exhibited left ventricular enlargement with bradycardia (mean RR in ‘Ex’ vs. ‘Sed’ rabbits: 335 ± 15 vs. 288 ± 19 ms, p ≤ 0.05, and in ‘Dop’ vs. ‘Ex’ vs. ‘Sed’ dogs: 718 ± 6 vs. 638 ± 38 vs. 599 ± 49 ms) accompanied by an increase of heart rate variability in both species (e.g. SD RR in ‘Ex’ vs. ‘Sed’ rabbits: 3.4 ± 0.9 vs. 1.4 ± 0.1 ms, p ≤ 0.05, and in ‘Dop’ vs. ‘Ex’ vs. ‘Sed’ dogs: 156 ± 59 vs. 163 ± 44 vs. 111 ± 49 ms) indicating an increased vagal tone. A lower response to parasym- patholytic agent atropine and more pronounced QT c in- terval lengthening after dofetilide challenge were found in ’Ex’ and ’Dop’ dogs compared to the ‘Sed’ group. No morphological and functional changes were found after chronic steroid treatment in dogs. The structural-functional findings share more similarities with human athlete’s heart. Slight repolarization sensitivity in the exercised dogs may indicate an increased risk of arrhythmias in athletes under different circumstances. These animal models might be useful for the further investigations of the cardiovascular effects of competitive training