500 Late-Breaking: Heart Rate During Exercise Is Positively Correlated with State IV Mitochondrial Respiration in the Equine Skeletal Muscle

Mitochondria are critical for oxidative phosphorylation in skeletal muscle, especially in athletic species such as the horse. Mitochondrial respiration increases with physical exercise, but the relationship between mitochondrial respiration and cardiovascular functions are not well described in the horse. The objective of this study was to determine if there is a relationship between heart rate (HR) during and after submaximal exercise tests (SETs) and skeletal muscle mitochondrial respiration in polo ponies. We hypothesized that horses with greater maximum HR and average HR during the exercise tests would have greater mitochondrial respiration in skeletal muscle. Twelve fit polo ponies (14.8 ± 1.7 years old, 10 mares and 2 geldings) were equipped with Polar equine heart rate monitors (Polar Electro Inc., Lake Success, NY) and underwent 26-minute SETs designed to mimic a polo chukker followed by a 30-minute recovery period. Muscle biopsy samples from the semitendinosus muscle were taken 2 weeks prior to the SET to determine mitochondrial oxygen consumption using the Oroboros O2k high-resolution respirometer (Oroboros Instruments, Innsbruck, Austria). Data were analyzed using the PROC CORR procedure (SAS Inst. Inc., Cary, NC). Correlations were considered strong at r > 0.6 and significant at P < 0.05. Maximum HR during SET and state IV respiration were positively correlated (P = 0.02, r = 0.68). Average HR during SET and state IV respiration were also positively correlated (P = 0.01, r = 0.72). However, correlations between maximum and average HR and state III respiration were not as strong (P ≥ 0.05, r < 0.6). These data suggest that state IV mitochondrial respiration (proton leak) in equine skeletal muscle may impact cardiac responses to high-intensity exercise. Horses with higher HR during exercise may have less efficient oxidative phosphorylation, resulting in earlier fatigue and/or greater formation of reactive oxygen species resulting from proton leak.

Abstract P307: An Exquisite Cardiac Performance And Protection Package Emerges In Mouse With Cardiac-specific Overexpression Of Adenylyl Cyclase Type 8

Young adult (three-month old) mice with cardiac-specific overexpression of AC type 8 (TGAC8) have a markedly elevated heart rate and markedly enhanced ejection fraction around the clock, mimicking cardiac responses of sympathetic autonomic input during acute exercise on a chronic base, but the TGAC8 mice do not exhibit heart failure or increased mortality up to about a year. Using this incessant young TGAC8 mouse as an ideal model to elucidate a high grade cardiac “Performance and Protection Package” (PPP) in response to chronic cardiac stress, we hypothesized that the TGAC8 heart adjusts to the chronical stress by reprogramming itself at multiple omics scales to code the exquisite cardiac PPP. To this end, we compared three-month old TGAC8 mice and their wildtype (WT) littermates using multiple omics analyses, from transcriptome to proteome, to phosphoproteome. Compared to WT, the phosphorylation level of most proteins was increased in the TGAC8 mice, including transcription factors (TF), kinases and phosphatases that are important in regulating transcription and phosphorylation. Among the 191 TFs identified in phosphoproteome, 91 were increased significantly, in line with the general upregulation in transcriptome. Many important stress response signaling pathways were enriched from phosphoproteome. Wherein, three protein quality control pathways, PI3K/AKT signaling, ERK/MAPK signaling and ubiquitination, were consistently enriched across the three omics scales. Most components in PI3K/AKT signaling pathway were upregulated, especially at protein and phosphorylation levels. Consistently, PI3K/AKT substrate phosphopeptides were increased, and downstream effects and functions of PI3K/AKT signaling were activated, including energy metabolism, protein synthesis, cell growth, cardiovascular functions, and NF-kB mediated functions. In summary, profiling the transcriptome, proteome and phosphoproteome of the TGAC8 heart unraveled the mechanism that controls its PPP. The cardiac overexpression of AC8 activates the AC8-cAMP-PKA axis to increase phosphorylation; widespread phosphorylation of the TFs promotes the transcription of numerous additional molecules that also regulate phosphorylation to reprogram the heart. Thereby, the TGAC8 mouse upregulates many stress response signaling pathways to activate the exquisite cardiac PPP.

Cardiac and behavioural responses to hypoxia and warming in free-swimming gilthead seabream (Sparus aurata)

Gilthead seabream were equipped with intraperitoneal biologging tags to investigate cardiac responses to hypoxia and warming, comparing when fish were either swimming freely in a tank with conspecifics or confined to individual respirometers. After tag implantation under anaesthesia, heart rate (fH) required 60 hours to recover to a stable value in a holding tank. Subsequently, when undisturbed under control conditions (normoxia, 21°C), mean fH was always significantly lower in the tank than respirometers. In progressive hypoxia (100 - 15% oxygen saturation), mean fH in the tank was significantly lower than respirometers at oxygen levels until 40%, with significant bradycardia in both holding conditions below this. Simultaneous logging of tri-axial body acceleration revealed that spontaneous activity, inferred as the variance of external acceleration (VARm), was low and invariant in hypoxia. Warming (21 to 31°C) caused progressive tachycardia with no differences in fH between holding conditions. Mean VARm was, however, significantly higher in the tank during warming, with a positive relationship between VARm and fH across all temperatures. Therefore, spontaneous activity contributed to raising fH of fish in the tank during warming. Mean fH in respirometers had a highly significant linear relationship with mean rates of oxygen uptake, considering data from hypoxia and warming together. The high fH of confined seabream indicates that respirometry techniques may bias estimates of metabolic traits in some fishes, and that biologging on free-swimming fishes will provide more reliable insight into cardiac and behavioural responses to environmental stressors by fishes in their natural environment.

Insulin Signaling In the Heart

Insulin receptors are highly expressed in the heart and vasculature. Insulin signaling regulates cardiac growth, survival, substrate uptake, utilization and mitochondrial metabolism. Insulin signaling modulates the cardiac responses to physiological and pathological stressors. Altered insulin signaling in the heart may contribute to the pathophysiology of ventricular remodeling and heart failure progression. Myocardial insulin signaling adapts rapidly to changes in the systemic metabolic milieu. What may initially represent an adaptation to protect the heart from carbo-toxicity, may contribute to amplifying the risk of heart failure in obesity and diabetes. This review article presents the multiple roles of insulin signaling in cardiac physiology and pathology and discusses the potential therapeutic consequences of modulating myocardial insulin signaling.

Development of a Classical Conditioning Task for Humans Examining Phasic Heart Rate Responses to Signaled Appetitive Stimuli: A Pilot Study

Cardiac responses to appetitive stimuli have been studied as indices of motivational states and attentional processes, the former being associated with cardiac acceleration and latter deceleration. Very few studies have examined heart rate changes in appetitive classical conditioning in humans. The current study describes the development and pilot testing of a classical conditioning task to assess cardiac responses to appetitive stimuli and cues that reliably precede them. Data from 18 adults were examined. They were shown initially neutral visual stimuli (putative CS) on a computer screen followed by pictures of high-caloric food (US). Phasic cardiac deceleration to food images was observed, consistent with an orienting response to motivationally significant stimuli. Similar responses were observed to non-appetitive stimuli when they were preceded by the cue associated with the food images, suggesting that attentional processes were engaged by conditioned stimuli. These autonomic changes provide significant information about classical conditioning effects in humans.

Research on sablefish ( Anoplopoma fimbria ) suggests that limited capacity to increase heart function leaves hypoxic fish susceptible to heat waves

Studies of heart function and metabolism have been used to predict the impact of global warming on fish survival and distribution, and their susceptibility to acute and chronic temperature increases. Yet, despite the fact that hypoxia and high temperatures often co-occur, only one study has examined the effects of hypoxia on fish thermal tolerance, and the consequences of hypoxia for fish cardiac responses to acute warming have not been investigated. We report that sablefish ( Anoplopoma fimbria ) did not increase heart rate or cardiac output when warmed while hypoxic, and that this response was associated with reductions in maximum O 2 consumption and thermal tolerance (CT max ) of 66% and approximately 3°C, respectively. Further, acclimation to hypoxia for four to six months did not substantially alter the sablefish's temperature-dependent physiological responses or improve its CT max . These results provide novel, and compelling, evidence that hypoxia can impair the cardiac and metabolic response to increased temperatures in fish, and suggest that some coastal species may be more vulnerable to climate change-related heat waves than previously thought. Further, they support research showing that cross-tolerance and physiological plasticity in fish following hypoxia acclimation are limited.

Linking metabolic dysfunction with cardiovascular diseases: Brn-3b/POU4F2 transcription factor in cardiometabolic tissues in health and disease

Metabolic and cardiovascular diseases are highly prevalent and chronic conditions that are closely linked by complex molecular and pathological changes. Such adverse effects often arise from changes in the expression of genes that control essential cellular functions, but the factors that drive such effects are not fully understood. Since tissue-specific transcription factors control the expression of multiple genes, which affect cell fate under different conditions, then identifying such regulators can provide valuable insight into the molecular basis of such diseases. This review explores emerging evidence that supports novel and important roles for the POU4F2/Brn-3b transcription factor (TF) in controlling cellular genes that regulate cardiometabolic function. Brn-3b is expressed in insulin-responsive metabolic tissues (e.g. skeletal muscle and adipose tissue) and is important for normal function because constitutive Brn-3b-knockout (KO) mice develop profound metabolic dysfunction (hyperglycaemia; insulin resistance). Brn-3b is highly expressed in the developing hearts, with lower levels in adult hearts. However, Brn-3b is re-expressed in adult cardiomyocytes following haemodynamic stress or injury and is necessary for adaptive cardiac responses, particularly in male hearts, because male Brn-3b KO mice develop adverse remodelling and reduced cardiac function. As a TF, Brn-3b regulates the expression of multiple target genes, including GLUT4, GSK3β, sonic hedgehog (SHH), cyclin D1 and CDK4, which have known functions in controlling metabolic processes but also participate in cardiac responses to stress or injury. Therefore, loss of Brn-3b and the resultant alterations in the expression of such genes could potentially provide the link between metabolic dysfunctions with adverse cardiovascular responses, which is seen in Brn-3b KO mutants. Since the loss of Brn-3b is associated with obesity, type II diabetes (T2DM) and altered cardiac responses to stress, this regulator may provide a new and important link for understanding how pathological changes arise in such endemic diseases.