Effects of exercise training on contractile function in myocardial trabeculae after ischemia-reperfusion

Preventive training does not interfere with mRNA-encoding myosin and collagen expression during pulmonary arterial hypertension

To gain insight on the impact of preventive exercise during pulmonary arterial hypertension (PAH), we evaluated the gene expression of myosins and gene-encoding proteins associated with the extracellular matrix remodeling of right hypertrophied ventricles. We used 32 male Wistar rats, separated in four groups: Sedentary Control (S, n = 8); Control with Training (T, n = 8); Sedentary with Pulmonary Arterial Hypertension (SPAH, n = 8); and Pulmonary Arterial Hypertension with Training (TPAH, n = 8). All rats underwent a two-week adaptation period; T and TPAH group rats then proceeded to an eight-week training period on a treadmill. At the beginning of the 11th week, S and T groups received an intraperitoneal injection of saline, and SPAH and TPAH groups received an injection of monocrotaline (60 mg/kg). Rats in the T and TPAH groups then continued with the training protocol until the 13th week. We assessed exercise capacity, echocardiography analysis, Fulton's index, cross-sectional areas of cardiomyocytes, collagen content and types, and fractal dimension (FD). Transcript abundance of myosins and extracellular matrix genes were estimated through reverse transcription-quantitative PCR (RT-qPCR). When compared to the SPAH group, the TPAH group showed increases in functional capacity and pulmonary artery acceleration time/pulmonary ejection time ratio and decreases in Fulton's index and cross-sectional areas of myocyte cells. However, preventive exercise did not induce alterations in col1a1 and myh7 gene expression. Our findings demonstrate that preventive exercise improved functional capacity, reduced cardiac hypertrophy, and attenuated PH development without interfering in mRNA-encoding myosin and collagen expression during PAH.

Intermittent cardiac overload results in adaptive hypertrophy and provides protection against left ventricular acute pressure overload insult

The present study aimed to test whether a chronic intermittent workload could induce an adaptive cardiac phenotype Chronic intermittent workload induced features of adaptive hypertrophy This was paralleled by protection against acute pressure overload insult The heart may adapt favourably to balanced demands, regardless of the nature of the stimuli. The present study aimed to test whether submitting the healthy heart to intermittent and tolerable amounts of workload, independently of its nature, could result in an adaptive cardiac phenotype. Male Wistar rats were subjected to treadmill running (Ex) (n = 20), intermittent cardiac overload with dobutamine (ITO) (2 mg kg(-1) , s.c.; n = 20) or placebo administration (Cont) (n = 20) for 5 days week(-1) for 8 weeks. Animals were then killed for histological and biochemical analysis or subjected to left ventricular haemodynamic evaluation under baseline conditions, in response to isovolumetric contractions and to sustained LV acute pressure overload (35% increase in peak systolic pressure maintained for 2 h). Baseline cardiac function was enhanced only in Ex, whereas the response to isovolumetric heartbeats was improved in both ITO and Ex. By contrast to the Cont group, in which rats developed diastolic dysfunction with sustained acute pressure overload, ITO and Ex showed increased tolerance to this stress test. Both ITO and Ex developed cardiomyocyte hypertrophy without fibrosis, no overexpression of osteopontin-1 or β-myosin heavy chain, and increased expression of sarcoplasmic reticulum Ca(2+) protein. Regarding hypertrophic pathways, ITO and Ex showed activation of the protein kinase B/mammalian target of rapamycin pathway but not calcineurin. Mitochondrial complex IV and V activities were also increased in ITO and Ex. Chronic submission to controlled intermittent cardiac overload, independently of its nature, results in an adaptive cardiac phenotype. Features of the cardiac overload, such as the duration and magnitude of the stimuli, may play a role in the development of an adaptive or maladaptive phenotype.

Effect of T3 treatment on the response to ischemia–reperfusion of heart preparations from sedentary and trained rats

We investigated whether swim training modifies the effect of T(3) treatment on rat heart response to ischemia-reperfusion. Homogenates of Langendorff preparations perfused for 25 min after 20-min ischemia were used for biochemical determinations and isolation of mitochondrial fractions. Oxidative damage and antioxidant levels of homogenates, O(2) consumption and H(2)O(2) release rates, oxidative damage, and susceptibility to Ca(2+)-induced swelling of mitochondria were determined. During reperfusion, hyperthyroid hearts displayed significant tachycardia and low inotropic recovery. This pattern was improved by training, which also attenuated tissue oxidative damage and glutathione depletion. Similar training effects were shown in euthyroid preparations. Moreover, training reduced mitochondrial H(2)O(2) production and oxidative damage in hyperthyroid and euthyroid hearts and susceptibility to Ca(2+)-induced swelling only in the hyperthyroid ones. Rates of mitochondrial O(2) consumption were not different in sedentary and trained hyperthyroid rats. However, determination of the oxidative capacity suggested that, in the sedentary rats, O(2) consumption was conditioned by oxidative damage mitochondria have suffered, whereas in trained rats, it was due to changes in mitochondrial characteristics. The above results suggest that moderate training is able to reduce hyperthyroid heart susceptibility to oxidative damage and dysfunction modifying mitochondrial population.

Reduced susceptibility to ventricular tachyarrhythmias in rats selectively bred for high aerobic capacity

Reperfusion after a brief period of cardiac ischemia can lead to potentially lethal arrhythmias. Human epidemiological studies and experimental work with animals indicate that regular physical activity is associated with reductions in cardiovascular disease (CVD) risk factors and sudden cardiac death. Similarly, artificial selection of rats for high aerobic treadmill-running capacity (high-capacity runners; HCR) has been shown to reduce CVD risk factors relative to rats selected as low-capacity runners (LCR). Therefore, we tested the hypothesis that HCR, relative to LCR rats, would be less susceptible to ischemia-reperfusion-mediated ventricular tachyarrhythmias. To test this hypothesis, we measured the susceptibility to ventricular tachyarrhythmias produced by 3 min of occlusion and reperfusion of the left main coronary artery in conscious LCR and HCR rats. Results document a significantly lower incidence of ventricular tachyarrhythmias in HCR (3 of 11, 27.3%) relative to LCR (6 of 7, 85.6%) rats. The decreased susceptibility to tachyarrhythmias in HCR rats was associated with a reduced cardiac metabolic demand during ischemia (lower rate-pressure product and ST segment elevation) as well as a wider range for the autonomic control of heart rate. The HCR and LCR represent a unique substrate for evaluation of the mechanisms underlying ischemia-mediated cardiac arrhythmogenesis.

Multiple isoforms of myosin light chain 1 in pig diaphragm slow fibers: correlation with maximal shortening velocity and force generation

Pig diaphragm slow fibers exhibit heterogeneity in myosin light chain 1 (MLC1) isoform expression, with many expressing fast-type MLC1 (MLC1F), as well as two isoforms of slow-type MLC1 (MLC1Sa and MLC1Sb). The goal of this study was to test if there is a relationship between MLC1 isoform expression and contractile properties among these fibers. Maximal shortening velocity (V(max)) and maximal isometric force generation, normalized with fiber cross-sectional area (P(o)/CSA), were measured in single fibers. V(max) was inversely related to the relative level of MLC1Sa. The level of MLC1Sa was reciprocally related to the levels of MLC1Sb and of MLC1F among individual fibers. Fibers expressing MLC1Sa and in which MLC1Sb was not detected generated greater P(o)/CSA, compared to fibers expressing MLC1Sb and not MLC1Sa. The results indicate a complex pattern of MLC1 isoform expression among pig diaphragm slow fibers and suggest that shortening velocity and force generation are modulated, in these fibers, by the MLC1 isoform composition.