Epigenetic control of exercise adaptations in the equine athlete: Current evidence and future directions

Horses (Equus ferus caballus) have evolved over the past 300 years in response to man-made selection for particular athletic traits. Some of the selected traits were selected based on the size and horses' muscular power (eg Clydesdales), whereas other breeds were bred for peak running performance (eg Thoroughbred and Arabian). Although the physiological changes and some of the cellular adaptations responsible for athletic potential of horses have been identified, the molecular mechanisms are only just beginning to be comprehensively investigated. The purpose of this review was to outline and discuss the current understanding of the molecular mechanisms underpinning the athletic performance and cardiorespiratory fitness in athletic breeds of horses. A brief review of the biology of epigenetics is provided, including discussion on DNA methylation, histone modifications and small RNAs, followed by a summary and critical review of the current work on the exercise-induced epigenetic and transcriptional changes in horses. Important unanswered questions and currently unexplored areas that deserve attention are highlighted. Finally, a rationale for the analysis of epigenetic modifications in the context with exercise-related traits and ailments associated with athletic breeds of horses is outlined in order to help guide future research.

Comparison of the Effects of Endurance Training on Alternate Days and on Consecutive 4 Days Each Week for 8 Weeks on the Abundance of PGC-1α, CaMKII, NRF-1, mtTFA, and COXIV Proteins in Rat Skeletal Muscle

Qiu, J, Huang, L, Davie, AJ, and Zhou, S. Comparison of the effects of endurance training on alternate days and on consecutive 4 days each week for 8 weeks on the abundance of PGC-1α, CaMKII, NRF-1, mtTFA, and COXIV proteins in rat skeletal muscle. J Strength Cond Res 33(11): 3136-3144, 2019-The aim of this study was to compare the effects of 2 training protocols, training on alternate days (A) or on consecutive 4 days followed by 3 days of rest in each week (C) for 8 weeks, on selected proteins involved in the biogenesis and function of mitochondria in skeletal muscle. Eighty male Sprague Dawley rats were randomly allocated into 10 groups (n = 8 each), including Pre and Post control groups and A or C training groups with 8, 16, 24, and 32 training sessions, respectively. The vastus lateralis and soleus muscle samples were obtained 24 hours after the last training session, or at rest for the controls. The abundance of the proteins for PGC-1α, CaMKII, NRF-1, mtTFA, and COXIV was analyzed by Western blotting. Analysis of the results with 2-way ANOVA showed no significant effect and interaction (training protocol by duration) in abundance of the proteins by the 2 protocols. However, fold changes normalized to control showed significant increases COXIV of the soleus muscle at most time points in both A and C training as indicated by Kruskal-Wallis H tests. There were significant correlations found between the abundance of the measured proteins of the vastus lateralis. The findings suggest that the 2 training protocols with the same intensity and total volume of work would not make a significant difference in respect of the changes in the targeted proteins. Alternative regulatory factors and the responses in different types of muscles to the training programs need to be examined in future research.

The effects of moderate intensity training in a hypoxic environment on transcriptional responses in Thoroughbred horses

This study investigated the effects of six weeks of normobaric hypoxic training on transcriptional expression of the genes associated with mitochondrial and glycolytic activities in Thoroughbred horses. Eight horses were divided into two groups of four. They completed an identical incremental, moderate intensity training program, except that one group trained in a hypoxic chamber with 15% oxygen for 30 min on alternate days except Sundays (HT), while the other group trained in normal air (NC). Prior to and post training, heart rate and blood lactate were measured during an incremental treadmill test. Muscle biopsy samples were taken prior to and 24 h post the training period for qPCR analysis of mRNA changes in VEGF, PPARγ, HIF-1α, PGC-1α, COX4, AK3, LDH, PFK, PKm and SOD-2. No significant differences between the HT and NC were detected by independent-samples t-test with Bonferroni correction for multiple comparisons (P>0.05) in relative changes of mRNA abundance. There were no significant differences between groups for heart rate and blood lactate during the treadmill test. The outcomes indicated that this hypoxia training program did not cause a significant variation in basal level expression of the selected mRNAs in Thoroughbreds as compared with normoxic training.

Characteristics of titin in strength and power athletes

The purpose of this investigation was to identify characteristics of the muscle protein titin in different athletic populations with increased levels of strength and power relative to non-athletes. Subjects fell into one of four groups: (1) non-athletes (NA) ( n=5), (2) weightlifters (WL) (n=5), (3) powerlifters (PL) (n=5), (4) sprinters (S) (n=5). A one repetition maximum in the squat exercise was performed to assess strength. In addition, countermovement vertical jump trials were performed to assess power capabilities. Peak power (W(peak)) was calculated for the vertical jumps from force plate measurements. From gel electrophoresis analyses of muscle samples, titin-1 (T1) and titin-2 (T2) protein bands were identified, quantified and expressed relative to each other. In addition the relative mobility (R(f)) of T1 and T2 was determined as an estimate of molecular weight. The NA group [%T1=47.8 (5.1), %T2=52.2 (5.1), mean (SE)] had lower T1 and higher T2 percentages than WL [%T1=62.3 (6.6), %T2=37.7 (6.6)], PL [%T1=66.8 (5.0), %T2=33.2 (5.0)] and S [%T1=65.9 (4.9), %T2=34.1 (4.9)] groups (P< or =0.10, preliminary investigation into titin and exercise justifies more liberal alpha level). No significant differences were found in R(f) of T1 or T2 between the groups. This investigation has shown that there is a differential expression of titin protein bands in competitive athletes with increased levels of strength and power in comparison to untrained non-athletic individuals. Some relationships between titin characteristics and athletic performance were observed; however, no conclusions can be made based on these data as to the contribution of titin to strength or power capabilities.

Plasma lactate and uric acid responses to racing in pacing Standardbreds and relationships with performance

Relationships between racing performance and plasma lactate and uric acid concentrations after racing were investigated in pacing Standardbred racehorses. The study was undertaken in order to better understand factors that limit racing performance. Twenty horses were tested after races of 1760 m and 28 horses after races over 2160 m. Blood samples were taken 30-60 min before and 8 and 30 min after a race. There were no significant differences between the race distances for prerace and 8 min postrace plasma lactates. Significant low correlations were obtained for plasma lactate concentration 8 min postrace and the number of race wins (r = 0.29, P = 0.04), number of race placings (1st, 2nd or 3rd; r = 0.34, P = 0.02) and lifetime earnings (r = 0.29, P = 0.04). Net lactate was correlated with the number of race placings (r = 0.31, P = 0.03). There were no significant correlations between performance indices and plasma uric acid concentrations in races of 1760 m. For races over 2160 m, correlations were found between plasma uric acid concentration at 8 min postrace and the number of race wins (r = 0.37, P = 0.06, 95% confidence limits 0-0.65). As well, there was a significant correlation between uric acid concentration at 8 min postrace and lifetime earnings (r = 0.35, P = 0.07, 95% confidence limits 0.06-0.64). Only 10-15% of the variability in retrospective career performance in pacing Standardbreds can be explained by these metabolic markers of the muscle anaerobic response to racing. Plasma lactate and uric acid responses to maximal exercise are unlikely to be useful for evaluating racing performance in pacing Standardbreds in univariate analysis.

Metabolic responses to submaximal field exercise tests and relationships with racing performance in pacing Standardbreds

This study was undertaken to invesigate the application of submaximal field tests in assessment of performance. A submaximal field exercise test consisting of 2 bouts of pacing for 1600 m, with 5 min rest or walking between runs was performed on Standardbred racehorses at 2 training centres (A and B). Performance indices were compiled for each horse: number of race starts, number of race wins, number of race placings (1, 2 or 3), and lifetime earnings. Regression analysis was conducted to describe the relationship between plasma lactate concentrations and speed for tests one, 2 and pooled results. Using the regression equation, observed (measured) minus expected (predicted) (O-E) lactate concentrations for tests were calculated and plotted against performance indices to determine their relationship. For stables A and B the association between lactate and velocity for the 2 runs was best described by the exponential equations: y = 0.0014(e07306x) (r2 = 0.54, P<0.001) and y = 0.002(e06812x) (r2 = 0.42, P<0.001), respectively, where y is lactate concentration (mmol/l) and x is speed (m/s). For the pooled results for heat 1 and heat 2 for stables A and B the relationship between lactate and velocity was best described by an exponential equation y = 0.0016(e07117x) (r2 = 0.47, P<0.001). This study found no relationship between (O-E) and performance indices (number of race wins, number of race placings, lifetime earnings and average $/start) at stables A or B for test run 1, 2 or pooled lactates. At stable A, there was a significant association between V(La4) and log lifetime earnings (r = 0.51, P = 0.053) and log average $/start (r = 0.54, P = 0.036). There were no significant correlations at stable B. In conclusion a 2-step determination method of V(La4) is a possible means for studying limits to performance in pacing Standardbred racehorses.

The life cycle of haemogregarina bigemina (Adeleina: Haemogregarinidae) in South African hosts

Haemogregarina bigemina Laveran et Mesnil, 1901 was examined in marine fishes and the gnathiid isopod, Gnathia africana Barnard, 1914 in South Africa. Its development in fishes was similar to that described previously for this species. Gnathiids taken from fishes with H. bigemina, and prepared sequentially over 28 days post feeding (d.p.f.), contained stages of syzygy, immature and mature oocysts, sporozoites and merozoites of at least three types. Sporozoites, often five in number, formed from each oocyst from 9 d.p.f. First-generation merozoites appeared in small numbers at 11 d.p.f., arising from small, rounded meronts. Mature, second-generation merozoites appeared in large clusters within gut tissue at 18 d.p.f. They were presumed to arise from fan-shaped meronts, first observed at 11 d.p.f. Third-generation merozoites were the shortest, and resulted from binary fission of meronts, derived from second-generation merozoites. Gnathiids taken from sponges within rock pools contained only gamonts and immature oocysts. It is concluded that the development of H. bigemina in its arthropod host illustrates an affinity with Hemolivia and one species of Hepatozoon. However, the absence of sporokinctes and sporocysts also distances it from these genera, and from Karyolysus. Furthermore, H. bigemina produces fewer sporozoites than Cyrilia and Desseria, although, as in Desseria, Haemogregarina (sensu stricto) and Babesiosoma, post-sporogonic production of merozoites occurs in the invertebrate host. The presence of intraerythrocytic binary fission in its fish host means that H. bigemina is not a Desseria. Overall it most closely resembles Haemogregarina (sensu stricto) in its development, although the match is not exact.

Muscle activation of the knee extensors following high intensity endurance exercise in cyclists

This study was conducted to assess the effects in trained cyclists of exhausting endurance cycle exercise (CE) on maximal isometric force production, surface electromyogram (EMG) and activation deficit (AD) of the knee extensors. Ten male subjects made four isometric maximal voluntary contractions (MVC) of the knee extensor muscles immediately prior (pre), 10 min after (post) and 6 h after completion of CE. The CE consisted of 30 min of exercise on a stationary cycle ergometer at an intensity corresponding to 80% of maximal oxygen uptake (VO(2max)) followed by four x 60-s periods at 120% of VO(2max). Two MVC were performed with recording of surface EMG from the knee extensors, whilst an additional two MVC were completed with percutaneous electrical muscle stimulation (EMS; 25 pulses at 100 Hz with the maximal tolerable current) superimposed over the maximal voluntary contraction force (MVF) but without EMG (to avoid interference). The MVF, integrated EMG (iEMG), and AD [calculated as the difference between MVF and the electrically stimulated force (ESF) during the EMS contractions] were statistically analysed. The MVF was significantly reduced (P < 0.05) post and 6-h post compared to pre-CE level. The iEMG was significantly reduced (P < 0.05) post and 6 h post CE. The ESF was also reduced, whilst AD was significantly increased (P < 0.05) post and 6-h post CE compared to the pre CE. These results suggest that the level of exercise stress administered in this study was sufficient to impair the central and peripheral mechanisms of force generation in knee extensors for a period of 6-h. Athletes engaged in concurrent training (strength and endurance) should consider this effect in exercise programming.

Effects of muscle glycogen depletion on some metabolic and physiological responses to submaximal treadmill exercise

The aim of this study was to investigate the effects of reduced muscle glycogen concentration on some physiological and metabolic responses during moderate intensity treadmill exercise in horses. Six Thoroughbred geldings were randomly allocated to 2 treatments (protocols A and B) or control in a 3 x 3 replicated Latin square design. In protocol A, horses performed low intensity exercise while horses in protocol B performed short bursts of high intensity exercise. Protocol A was designed to induce glycogen depletion mainly of slow twitch muscle fibers while protocol B aimed to deplete mainly fast twitch muscle fibers. Horses in the control group did not undergo exercise prior to the exercise test. Five hours after glycogen depletion, horses performed treadmill exercise at 60% VO2max at a treadmill slope of 10% until fatigue (20-30 min). The induced glycogen depletion prior to exercise had no significant effect on plasma glucose, insulin, or lactate concentrations during the exercise test, and there was no effect on glycogen utilization rate, although respiratory exchange ratios were lower in the glycogen-depleted groups. The VO2, heart rate and central blood temperature did not vary significantly between the protocols A and B and control throughout the exercise test. It was concluded that 20-30% depletion of glycogen concentration in the middle gluteal muscle resulted in a shift towards fat metabolism, but does not significantly affect heart rate, oxygen uptake, or concentrations of plasma glucose and lactate during moderate intensity exercise.

Correlations between peak power output, muscular strength and cycle time trial performance in triathletes

OBJECTIVE

To examine the relationship between the peak power output (Wmax), peak oxygen uptake (VO2peak), lower limb muscular strength and cycling time (CT) during a short course triathlon race.

EXPERIMENTAL DESIGN

The study involved a cross-sectional analysis involving both physiological and biomechanical variables.

SETTING

Testing was performed at the exercise physiology and biomechanics laboratory, School of Exercise Science and Sport Management, Southern Cross University, Lismore, Australia.

PARTICIPANTS

Ten male triathletes who had been endurance cycle training for a minimum of 12 months prior to the commencement of the study.

MEASURES

Subjects completed a maximal incremental cycle test as well as a series of muscular function tests including a 6-s cycle test, a concentric isoinertial squat jump as well as an isokinetic leg extension test performed at velocities of 60 degrees (s-1, 120 degrees (s-1 and 180 degrees.s-1. In addition, each subject also participated in a triathlon race of distance 1.5 km swim, 40 km cycle and 10 km run.

RESULTS

A significant correlation existed between CT and absolute VO2 peak and Wmax. However, no significant correlations were found between the results of the muscular function tests and the incremental cycle test as well, as CT during the triathlon race.

CONCLUSIONS

Wmax and WDmax are useful variables in assessing cycle performance in triathletes. However, the importance of muscular strength of the lower limbs may be minimal in overall cycle performance during a short course triathlon race.

The effect of endurance exercise on muscle force generating capacity of the lower limbs

The purpose of this study was to investigate the recovery of muscle force generating capacity (FGC) of the lower limbs following a session of cycle exercise (CE). Fourteen male cyclists (mean +/- SD age 25 +/- 4 yrs and VO2max 65.8 +/- 5 ml x kg(-1)min(-1)) performed tests assessing lower limb muscle FGC at rest (pre-test), as well as 6 and 24 hrs following CE performed on a mechanically-braked cycle ergometer. The CE consisted of 30 min at a workload corresponding to the lactate (Dmax) threshold (+/-15 W), and four 60 s rides at 120% VO2max with one min rest between each ride. At the completion of the CE a 6 or 24 hr recovery period was initiated, after which, each subject's muscle FGC was measured. The analysis of lower limb muscle FGC included, (1) 6 s all-out cycle test; (2) a maximal isokinetic leg extension at 60, 120 and 180 degrees x s(-1); and (3) a maximal concentric squat jump. Statistical analysis showed that compared to pre-test levels, a significant reduction in both isokinetic peak torque at 60 degrees x s(-1) and isoinertial maximum force occurred after 6 hrs of recovery. Although not significant, reductions also occurred at 6 hrs of recovery in isokinetic peak torque at 120 and 180 degrees x s(-1), as well as maximum rate of force development (RFD) during the squat jumps. No significant differences were observed between isokinetic peak torque, maximum force or RFD pre-test and following the 24 hr recovery period, indicating these tests had returned to normal by this time. No significant differences were found between peak power (PP) during the 6 s cycle test, pre-test and following either 6 or 24 hrs of recovery. These findings confirm earlier research that maximal voluntary strength is reduced for at least 6 hours following exhaustive dynamic exercise. The reduction in muscle FGC should be considered when resistance training is scheduled after endurance exercise.

Significance of prolonged left ventricular wall motion abnormalities after exercise echocardiography following non-Q-wave acute myocardial infarction

Exercise echocardiography was used to assess myocardial ischemia after non-Q-wave acute myocardial infarction in 40 consecutive patients. Resting parasternal long- and short-axis views and apical 4- and 2-chamber views were recorded, digitized, and stored. A maximal symptom-limited exercise test was performed within 21 days (mean 17.7 +/- 3) using a cycle ergometer with continuous monitoring and the echocardiogram was repeated in the same views. Resting and exercise echocardiograms were then compared. Coronary angiography was performed in all patients within 21 days of exercise echocardiography. Stenosis in > or =50% of the lumen diameter was considered significant. Of the 40 patients studied, 29 (72%) had continuing angina and 11 (28%) had no angina. Eighteen patients (62%) with angina developed angina during exercise testing and 19 (65%) developed ST-segment depression. In patients without angina, 1 (9%) developed postexercise angina and 2 (18%) developed ST-segment depression. The mean wall motion score index after exercise increased from 1.2 +/- 0.3 to 1.8 +/- 0.4 in patients with continuing angina (p <0.001) and from 1.2 +/- 0.3 to 1.4 +/- 0.3 in patients without angina (p = NS). Prolonged wall motion abnormalities lasting >20 minutes persisted in > or =1 segment in 27 of 29 patients (93%) with angina or in 2 of 1 1 patients (18%) without angina (p <0.001). Patients with continued angina had predominantly 3-vessel coronary artery disease (22 of 29 [76%]) or 2-vessel disease (7 of 29 [24%]), and those without angina had 1-vessel disease (6 of 11 [55%]) or 2-vessel disease (4 of 11 [36%]). One patient had 3-vessel disease. The duration of wall motion abnormality demonstrated a significant relation to 2- and 3-vessel coronary artery disease (p <0.001). Thus, patients with non-Q-wave acute myocardial infarction had a high incidence of multivessel coronary disease not necessarily detected on routine exercise testing. There was also a significant incidence of prolonged wall motion abnormality.

Correlations between short-course triathlon performance and physiological variables determined in laboratory cycle and treadmill tests

The aim of this study was to investigate the relationships between physiological variables measured in graded cycling (CM) and treadmill running (RM) maximal tests and the performance of a short-course triathlon (1 km swim, 30 km cycling and 8 km running) in recreational triathletes. Ten male athletes with mean (+/-SD) age of 27.4(+/-5.7) years and maximal oxygen consumption (VO2peak) of 63.3(+/-8.95) mL.kg-1.min-1 participated in the study. The results showed that the VO2peak and VO2 at ventilatory threshold (VO2VT) determined in CM were significantly correlated to the overall and running times of the triathlon (r = -0.64 - 0.77, p < or = 0.05). The VO2peak and VO2VT determined in RM were also correlated to the running time when expressed as mL.kg-1.min-1 (r = -0.73, p < or = 0.05). The VT expressed as %VO2peak in both tests showed no significant correlations to triathlon performance. It appears that it is the absolute aerobic capacity of these athletes being critical to maintain a high exercise intensity in the triathlon, especially during the running segment. The mean heart rate (HR) measured in four subjects during the cycling and running segments of the triathlon was similar to the HR at VT determined in CM, which indicated that these athletes could maintain an exercise intensity close to the VT level during the competition. There was no significant correlation found between the swimming time and physiological variables measured in the laboratory cycling and running tests, which could be explained by the specific skills and economy of movement required in these exercises.

The effects of tonicity, glucose concentration and temperature of an oral rehydration solution on its absorption and elimination

Effects of different tonicities, glucose concentrations and temperatures of an oral rehydration solution (ORS) on its uptake and elimination in resting horses were studied. Fluid and electrolyte deficits similar to those occurring during prolonged exercise were induced by the administration of 1 mg/kg bwt of frusemide i.m., 3 h prior to the ORS. Fluid was administered via nasogastric tube at a volume equivalent to 4% bodyweight, which approximated diuretic induced losses. The uptake of fluid was evaluated by changes in haematocrit (PCV) and plasma total protein concentration (TP). Changes in electrolyte balance were studied by measurements of plasma and urinary electrolyte concentrations while changes in bodyweight, urine volume and faecal water content were used to estimate retention of the administered fluids. Changes in acid base status were assessed from venous blood bicarbonate values. Fluid tonicity had a major effect on the uptake and elimination of the ORS. The hypertonic fluid (628 mOsm/kg bwt) was less rapidly absorbed and resulted in more rapid fluid and electrolyte excretion than the isotonic (314 mOsm/kg bwt) and hypotonic (water) fluids. The inclusion of glucose did not enhance the absorption of the ORS, although fluids containing higher concentrations of electrolytes resulted in more rapid elimination of fluid in urine. There was a direct relationship between higher concentrations of sodium in the ORS, plasma sodium values and osmolality. Fluid temperature (5, 21 and 37 degrees C) had no demonstrable effect on absorption of the ORS and elimination of fluids post administration. We concluded that while glucose concentration and fluid temperature have minimal effects on fluid absorption and elimination, fluid tonicity was a key element in the uptake and elimination of orally administered fluid. These findings are likely to be of relevance when administering ORS in association with exercise.