Earl G. Noble

Exercise Improves Postischemic Cardiac Function in Males but Not Females Consequences of a Novel Sex-Specific Heat Shock Protein 70 Response

Exercise is a physiological inducer of the cardioprotective heat shock protein, Hsp70. The putative biological events involved in signaling this response exhibit sexual dimorphism. Thus, it was hypothesized that exercise-mediated induction of Hsp70 would demonstrate sex specificity. After treadmill running, male rats exhibited 2-fold greater levels of cardiac Hsp70 relative to the levels in gonadally intact female rats (P <0.001). Ovariectomized female rats exhibited exercise-mediated induction of Hsp70 similar to that observed for male rats, and estrogen treatment in these female rats reversed this effect (P <0.001). Attenuation of Hsp70 signaling by estrogen was non–receptor-mediated, possibly involving a cellular membrane–stabilizing mechanism of action. The physiological importance of this sex-specific hormone-mediated stress response is underscored by the disparity in functional adaptation in response to exercise between male rats and female rats. Exercise markedly improved postischemic left ventricular developed pressure, the maximal rate of contraction, and maximal rate of relaxation, and it reduced left ventricular end-diastolic pressure in male rats (P <0.001). No such benefit of exercise was observed in intact female rats. A causal role for Hsp70 in this sex-specific cardioprotective adaptation was indicated, inasmuch as ablation of Hsp70 induction with antisense oligonucleotides designed against Hsp70 transcripts attenuated improvement in the recovery of cardiac function in exercised male rats (P <0.05). Thus, the sex-specific hormone-mediated Hsp70 response to exercise results in cardioprotective adaptation, preferentially in male rats relative to female rats. These findings suggest that exercise may be more important for males than for females in defending against the effects of heart disease and offer a novel manner by which males may reduce the sex gap in susceptibility to adverse cardiac events.

Heat shock proteins and exercise: a primer.

Heat shock proteins (HSPs) are, in general, prosurvival molecules within the cellular environment, and the overexpression of even just 1 family of HSPs can lead to protection against and improvements after a variety of stressors. Not surprisingly, a fertile area of study has grown out of efforts to exploit the innate biologic behaviour of HSPs. Exercise, because of the inherent physiologic stresses associated with it, is but 1 stimulus that can result in a robust increase in various HSPs in several tissues, not the least of which happen to be the heart and skeletal muscle. The purpose of this review is to introduce the reader to the major HSP families, the control of their expression, and some of their biologic functions, specifically with respect to the influence of exercise. Moreover, as the first in a series of reviews from a common symposium, we will briefly introduce the concepts presented by the other authors, which include the effects of different exercise paradigms on skeletal muscle HSPs in the adult and aged systems, HSPs as regulators of inflammation, and the ion channel stabilizing effects of HSPs.

Regulation of survival gene hsp70

Rapid expression of the survival gene, inducible heat shock protein 70 (hsp70), is critical for mounting cytoprotection against severe cellular stress, like elevated temperature. Hsp70 protein chaperones the refolding of heat-denatured peptides to minimize proteolytic degradation as a part of an eukaryotically conserved phenomenon referred to as the heat shock response. The physiologic stress associated with exercise, which can include elevated temperature, mechanical damage, hypoxia, lowered pH, and reactive oxygen species generation, may promote protein unfolding, leading to hsp70 gene expression in skeletal myofibers. Although the pre-transcriptional activation of hsp70 gene expression has been thoroughly reviewed, discussion of downstream hsp70 gene regulation is less extensive. The purpose of this brief review was to examine all levels of hsp70 gene regulation in response to heat stress and exercise with a special focus on skeletal myofibers where data are available. In general, while heat stress represses bulk gene expression, hsp70 mRNA expression is enhanced. Post-transcriptionally, intronless hsp70 mRNA circumvents a host of decay pathways, as well as heat stress-repressed pre-mRNA splicing and nuclear export. Pre-translationally, hsp70 mRNA is excluded from stress granules and preferentially translated during heat stress-repressed global cap-dependent translation. Post-translationally, nascent Hsp70 protein is thermodynamically stable at elevated temperatures, allowing for the commencement of chaperoning activity early after synthesis to attenuate the heat shock response and protect against subsequent injury. This review demonstrates that hsp70 mRNA expression is closely coupled with functional protein translation.

Determinants of oxygen uptake kinetics in older humans following single-limb endurance exercise training.

We hypothesised that the observed acceleration in the kinetics of exercise on‐transient oxygen uptake (V̇O2) of five older humans (77 ± 7 years (mean ± S.D.) following 9 weeks of single‐leg endurance exercise training was due to adaptations at the level of the muscle cell. Prior to, and following training, subjects performed constant‐load single‐limb knee extension exercise. Following training V̇O2 kinetics (phase 2, τ) were accelerated in the trained leg (week 0, 92 ± 44 s; week 9, 48 ± 22 s) and unchanged in the untrained leg (week 0, 104 ± 43 s; week 9, 126 ± 35 s). The kinetics of mean blood velocity in the femoral artery were faster than the kinetics of V̇O2, but were unchanged in both the trained (week 0, 19 ± 10 s; week 9, 26 ± 11 s) and untrained leg (week 0, 20 ± 18 s; week 9, 18 ± 10 s). Maximal citrate synthase activity, measured from biopsies of the vastus lateralis muscle, increased (P < 0.05) in the trained leg (week 0, 6.7 ± 2.0 μmol (g wet wt)−1 min−1; week 9, 11.4 ± 3.6 μmol (g wet wt)−1 min−1) but was unchanged in the untrained leg (week 0, 5.9 ± 0.5 μmol (g wet wt)−1 min−1; week 9, 7.9 ± 1.9 μmol (g wet wt)−1 min−1). These data suggest that the acceleration of V̇O2 kinetics was due to an improved rate of O2 utilisation by the muscle, but was not a result of increased O2 delivery.

Estrogen attenuates HSP 72 expression in acutely exercised male rodents.

Abstract Estrogen has been shown to reduce post-exercise skeletal muscle damage. Exercise-induced muscle damage may be a factor in the elevated post-exercise expression of heat-shock proteins (HSPs). Thus, the present investigation was conducted in order to examine the influence of estrogen on post-exercise levels of HSP 72 and heat-shock cognate, HSC 73, in male and female rodents. Prior to an acute bout of treadmill running, male and female Sprague-Dawley rats received daily injections of either 40 μg · kg−1 of β-estradiol 3-benzoate or olive oil vehicle for 2 weeks. A two- to fourfold reduction in post-exercise HSP 72 content was observed in the heart, liver, lung and red and white vastus muscles of estradiol-treated males compared with their vehicle-injected counterparts (P < 0.05). Compared to the males, the females had significantly lower post-exercise HSP 72 levels which were not affected by estradiol supplementation. Moreover, estradiol administration in male rodents resulted in a HSP response similar to that of females following exercise. Thus, the results of the present investigation suggest that estrogen is the factor responsible for the observed differences in post-exercise HSP 72 levels between males and females.

Increased EMG of rat plantaris during locomotion following surgical removal of its synergists

A chronic EMG electrode implant system was used to determine recruitment patterns of rat plantaris, during treadmill locomotion, before and after surgical removal of its synergists, gastrocnemius and soleus. Bilateral synergist removal resulted in increased plantaris muscle weight and myofibrillar protein content of 59% and 44%, respectively, by 30 days following surgery. Evidence of increased plantaris EMG during treadmill walking (increases in amplitude and integral of EMG bursts to ca. 200% of pre-excision values) occurred at 15-30 days and decreased to non-significant levels at 35 days postsurgery. No corresponding alterations occurred in sham-operated controls. At 25 and 30 days, EMG of plantaris during locomotion was still submaximal, since burst amplitudes were 53% to 67% of those recorded during a dynamic grid-climbing task. The magnitude and time course of changes in EMG of overloaded plantaris during a standardized locomotor task, which reflect increased recruitment and rate of discharge of motor units, are consistent with the chronology of morphological and metabolic events previously described for this model.

Simple and complex carbohydrate-rich diets and muscle glycogen content of marathon runners

SummaryThe effects of simple-carbohydrate (CHO)- and complex-CHO-rich diets on skeletal muscle glycogen content were compared. Twenty male marathon runners were divided into four equal groups with reference to dietary consumption: depletion/simple, depletion/complex, non-depletion/simple, and nondepletion/complex. Subjects consumed either a low-CHO (15% energy [E] intake), or a mixed diet (50% CHO) for 3 days, immediately followed by a high-CHO diet (70% E intake) predominant in either simple-CHO or in complex-CHO (85% of total CHO intake) for another 3 days. Skeletal muscle biopsies and venous blood samples were obtained one day prior to the start of the low-CHO diet or mixed diet (PRE), and then again one day after the completion of the high-CHO diet (POST). The samples were analysed for skeletal muscle glycogen, serum free fatty acids (FFA), insulin, and lactate and blood glucose. Skeletal muscle glycogen content increased significantly (p<0.05) only in the nondepletion/simple group. When groups were combined, according to the type of CHO ingested and/or utilization of a depletion diet, significant increases were observed in glycogen content. Serum FFA decreased significantly (p<0.05) for the nondepletion/complex group only, while serum insulin, blood glucose, and serum lactate were not altered. It is concluded that significant increases in skeletal muscle glycogen content can be achieved with a diet high in simple-CHO or complex-CHO, with or without initial consumption of a low-CHO diet.

The role of resistance and aerobic exercise training on insulin sensitivity measures in STZ-induced Type 1 diabetic rodents.

UNLABELLED Individuals with Type 1 Diabetes Mellitus (T1DM) can develop insulin resistance. Regular exercise may improve insulin resistance partially through increased expression of skeletal muscle GLUT4 content. OBJECTIVE To examine if different exercise training modalities can alter glucose tolerance through changes in skeletal muscle GLUT4 content in T1DM rats. METHODS Fifty rats were divided into 5 groups; control, diabetic control, diabetic resistance exercised, and diabetic high and low intensity treadmill exercised. Diabetes was induced using multiple low dose Streptozotocin (20 mg/kg/day) injections and blood glucose concentrations were maintained moderately hyperglycemic through subcutaneous insulin pellets. Resistance trained rats climbed a ladder with incremental loads, while treadmill trained rats ran on a treadmill at 27 or 15 m/min, respectively, all for 6 weeks. RESULTS At weeks 3 and 6, area under the curve measurements following an intravenous glucose tolerance test (AUC-IVGTT) in all diabetic groups were higher than control rats (p<0.05). At 6 weeks, all exercise groups had significantly lower AUC-IVGTT values than diabetic control animals (p<0.05). Treadmill trained rats had the lowest insulin dose requirement of the T1DM rats and the greatest reduction in insulin dosage was evident in high intensity treadmill exercise. Concomitant with improvements in glucose handling improvements, tissue-specific elevations in GLUT4 content were demonstrated in both red and white portions of vastus lateralis and gastrocnemius muscles, suggesting that glucose handling capacity was altered in the skeletal muscle of exercised T1DM rats. CONCLUSIONS These results suggest that, while all exercise modalities can improve glucose tolerance, each mode leads to differential improvements in insulin requirements and protein content alterations.

Gender‐modulated endogenous baseline neuropeptide Y Y1‐receptor activation in the hindlimb of Sprague‐Dawley rats

This study examined the effect of neuropeptide Y Y1‐receptor blockade both alone, and in interaction with α1‐adrenoceptor antagonism, on basal hindlimb vascular conductance in male and female Sprague‐Dawley rats. Hindlimb vascular conductance was measured during infusion of BIBP3226 (Y1‐receptor antagonist; 100 μg kg−1), prazosin (α1‐receptor antagonist; 20 μg kg−1), and combined blockade. In males, vascular conductance increased 1.1 ± 0.3 μl min−1 mmHg−1 above baseline with BIBP3226, and 2.4 ± 0.4 μl min−1 mmHg−1 above baseline with prazosin (both P < 0.05). The increase in vascular conductance during combined blockade (5.1 ± 0.7 μl min−1 mmHg−1) was greater than the sum of the independent BIBP3226 and prazosin responses (P < 0.05). In females, basal hindlimb vascular conductance was unaffected by Y1‐receptor blockade. However, α1‐receptor blockade resulted in a 3.5 ± 0.6 μl min−1 mmHg−1 increase in vascular conductance above baseline, which was not different than the combined blockade condition. Males had greater skeletal muscle neuropeptide Y concentration (P < 0.05; ELISA) than females. Furthermore, compared with females, male skeletal muscle contained greater Y1‐receptor expression (P < 0.05; Western blot). It was concluded that, under baseline conditions, agonist and receptor‐based mechanisms for Y1‐receptor dependent control of vascular conductance in skeletal muscle was greater in male versus female rats.

Rapid in vivo whole body composition of rats using cone beam μCT

Precise, noninvasive analysis and quantification of in vivo body composition is essential for research involving longitudinal, small-animal disease models. We investigated the feasibility and precision of a rapid, flat-panel μCT scanner to report whole body adipose tissue volume (ATV), lean tissue volume (LTV), skeletal tissue volume (STV), and bone mineral content (BMC) in 25 postmortem female and 52 live male Sprague-Dawley rats. μCT images, acquired in three 90-mm segments and reconstructed with 308 μm of isotropic voxel spacing, formed contiguous image volumes of each entire rat specimen. Three signal-intensity thresholds (determined to be -186, 5, and 155 HU) were used to classify each voxel as adipose, lean, or skeletal tissue, respectively. Tissue masses from the volume fractions of ATV, LTV, and STV were calculated from assumed tissue densities of 0.95, 1.05, and 1.92 g/cm(-3), respectively. A CT-derived total mass was calculated for each rat and compared with the gravimetrically measured mass, which differed on average for the postmortem female and the live male group by 2.5 and 1.1%, respectively. To evaluate the accuracy of the CT-derived body composition technique, following the live male study excised muscle tissue in the lower right leg of all rats in group B were compared with the image-derived LT measurement of the same regional compartment and found to differ on average by 2.2%. Through repeated CT measurements of postmortem specimens, the whole body ATV, LTV, STV, and BMC measurement analysis gave a precision value of ±0.6, 1.9, 1.7, and 0.5% of the average value, respectively.