No abstract

This study examined whether HSP70 could bind to and protect against thermal inactivation of SERCA1a, the SERCA isoform expressed in adult fast-twitch skeletal muscle. Sarcoplasmic reticulum vesicles prepared from rat gastrocnemius muscle were incubated with purified HSP70 at both 37 and 41°C for either 30, 60, or 120 min. Maximal SERCA1a activity (mol/g protein/min) in the absence of HSP70 was reduced progressively with time, with greater reductions occurring at 41°C compared with 37°C. HSP70 protected against thermal inactivation of SERCA1a activity at 37°C but not at 41°C and only at 30 and 60 min but not at 120 min. HSP70 also protected against reductions in binding capacity for fluorescein isothiocyanate, a fluorescent probe that binds to Lys 515 in the nucleotide binding domain of SERCA, at 30 and 60 min but not at 120 min, an effect that was independent of temperature. HEK-293 cells were cotransfected with cDNAs encoding rabbit SERCA1a and human HSP-EYFP and subjected to 40°C for 1 h. Immunohistochemistry revealed nearly complete co-localization of SERCA1a with HSP70 under these conditions. Co-immunoprecipitation showed physical interaction between HSP70 and SERCA1a under all thermal conditions both in vitro and in HEK-293 cells. Modeling showed that the fluorescein isothiocyanate-binding site of intact SERCA1a in the E2 form lies in its close proximity to a potential interaction site between SERCA1a and HSP70. These results indicate that HSP70 can bind to SERCA1a and, depending on the severity of heat stress, protect SERCA1a function by stabilizing the nucleotide binding domain. Sarco(endo)plasmic reticulum Ca2ϩ -ATPases (SERCAs) 1 are 110-kDa integral membrane proteins that catalyze the ATPdependent transport of Ca 2ϩ from the cytosol to the lumen of the sarco(endo)plasmic reticulum (SR) (1). Two SERCA isoforms predominate in adult striated muscle, namely SERCA1a and SERCA2a. SERCA1a accounts for more than 99% of the SERCA isoforms expressed in adult fast-twitch skeletal muscle, whereas SERCA2a is the major isoform expressed in heart and slow-twitch skeletal muscle (2). In muscle, SERCAs perform at least two crucial functions obligatory for precise control of intracellular Ca 2ϩ . In conjunction with plasma membrane Ca 2ϩ -ATPases, SERCAs are responsible for setting resting cytoplasmic Ca 2ϩ concentrations, and during repetitive muscle contractions, they induce muscle relaxation through the rapid sequestration of large Ca 2ϩ loads from the cytoplasm into the lumen of the SR, thereby determining the size of the SR Ca 2ϩ

In brief: To assess the effects of rapid weight reduction, four university wrestlers decreased their body weight by 8% over a four-day period by food and liquid intake reductions. Significant decreases in muscle glycogen concentration and dynamic strength, but not aerobic or anaerobic capacity, accompanied weight loss. A three-hour rehydration period did not improve glycogen levels or strength performance. These results suggest that rapid weight reduction may impair wrestling performance.

We hypothesized that near-infrared spectroscopy (NIRS) measures of hemoglobin and/or myoglobin O2 saturation (IR-SO2) in the vascular bed of exercising muscle would parallel changes in femoral venous O2 saturation (SfvO2) at the onset of leg-kicking exercise in humans. Six healthy subjects performed transitions from rest to 48 +/- 3 (SE)-W two-legged kicking exercise while breathing 14, 21, or 70% inspired O2. IR-SO2 was measured over the vastus lateralis muscle continuously during all tests, and femoral venous and radial artery blood samples were drawn simultaneously during rest and during 5 min of exercise. In all gas-breathing conditions, there was a rapid decrease in both IR-SO2 and SfvO2 at the onset of moderate-intensity leg-kicking exercise. Although SfvO2 remained at low levels throughout exercise, IR-SO2 increased significantly after the first minute of exercise in both normoxia and hyperoxia. Contrary to the hypothesis, these data show that NIRS does not provide a reliable estimate of hemoglobin and/or O2 saturation as reflected by direct femoral vein sampling.