Reductions in sarcoplasmic reticulum Ca2+ ATPase activity in rat skeletal muscles of different fibre composition with ischemia and reperfusion

To investigate the significance of fibre type and the duration of ischemia on changes in sarcoplasmic reticulum Ca2+ ATPase activity (SR Ca2+ ATPase), blood flow was occluded to the rat hind limb for 1, 2, or 3 h (n = 10 per group) and the soleus and extensor digitorum longus (EDL) muscles were examined following 2 h of reperfusion. When compared with the contralateral control muscles, calcium-dependent (total tau basal) SR Ca2+ ATPase activity in soleus was reduced (p < 0.05) to 75.9% by 1 h of ischemia and 2 h of reperfusion (13.1 +/- 0.6 vs. 9.95 +/- 0.85 mumol.mg-1 wet weight.min-1; X +/- SE) with no further reduction (p > 0.05) observed at either 2 h (9.75 +/- 0.57) or 3 h (9.40 +/- 0.64) of ischemia and 2 h of reperfusion. For the EDL muscles, SR Ca2+ ATPase activity with 2 h reperfusion was not reduced (p > 0.05) with 1 h of ischemia (80.4 +/- 3.0 vs. 70.7 +/- 2.9 mumol.mg-1 wet weight.min-1) but was reduced (66.7 +/- 2.3 mumol.mg-1 wet weight.min-1; p < 0.05) in the 2-h ischemia group, with further reductions (53.2 +/- 3.4 mumol.mg-1 wet weight.min-1; p < 0.05) in the 3-h ischemia group. No changes (p > 0.05) in basal or SR Mg2+ ATPase were found for either muscle group with ischemia and reperfusion, regardless of the duration of ischemia. When these results are interpreted in the context of the increases in SR Ca2+ ATPase activity that occur with ischemia, it appears that two components are involved in the reductions in SR Ca2+ ATPase activity noted during reperfusion: one that reduces the SR Ca2+ ATPase activity to below normal and one that simply reverses the ischemic-induced increase in SR Ca2+ ATPase activity. The former component appears to be more pronounced in the EDL muscle.

Early metabolic adaptations of rabbit fast-twitch muscle to chronic low-frequency stimulation

To investigate early adaptive responses to chronic low-frequency stimulation (CLFS), rabbit tibialis anterior (TA) muscles were continuously stimulated at 10 Hz for 8 days, allowed to rest for 1 h, and then subjected to a 15-min fatigue test at 10-Hz stimulation. The contralateral TA muscles which had not been exposed to CLFS, served as controls during the fatigue test. Compared to the controls, the initial tension output of the 8-day prestimulated muscles was reduced by 25%. However, these muscles maintained higher tensions during the fatigue test than the controls. Citrate synthase activity, an indicator of aerobic-oxidative capacity, was only slightly elevated (40%) in the 8-day stimulated muscles. Unlike the controls, the prestimulated muscles failed to produce potentiation during the fatigue test. Control muscles responded to the fatigue test with pronounced reductions in contents of adenosine 5'-triphosphate (ATP), phosphocreatine (PCr), and glycogen, as well as with large increases in contents of inosine monophosphate (IMP), inorganic phosphate (Pi), creatine (Cr), and lactate. Under the same conditions contents of ATP, PCr, Cr, glycogen, lactate, Pi, and IMP were unaltered in the 8-day prestimulated muscles. These findings demonstrated that CLFS for 8 days elicited pronounced alterations in energy metabolism and contractile properties. These adaptive changes occurred prior to fibre type transitions and substantial increases in aerobic-oxidative potential.

Metabolic and contractile responses of fast and slow twitch rat skeletal muscles to ischemia and reperfusion

The purpose of this study was to investigate the significance of fiber type and the effects of the duration of ischemia on metabolic and contractile function of skeletal muscle. Under anesthesia, the distal tendons of the fast twitch extensor digitorum longus (EDL) and slow twitch soleus (SOL) muscles of the right hindlimb of female Wistar rats (250 to 300 gm) were connected to force transducers. Rats were assigned to group 1, 1 hour of ischemia; group 2, 2 hours of ischemia; or group 3, 3 hours of ischemia (n = 10 for each group). After ischemia, muscles were assessed for 2 hours of reperfusion. In both muscles, isometric twitch (Pt) and tetanus (Po) and 11 metabolic parameters were measured and compared with controls. After 1, 2, or 3 hours of ischemia Pt and Po were significantly (p < 0.05) lower than preischemic values. After 2 hours of reperfusion, forces and metabolic parameters of group 1 recovered to preischemic levels. However, contractile function of either muscle failed to recover fully after 2 hours of ischemia and 2 hours of reperfusion (SOL: Pt = 43.7 +/- 12 percent of initial; EDL: Pt = 32.2 +/- 9.2 percent) or after 3 hours of ischemia and 2 hours of reperfusion (SOL: Pt = 26.8 +/- 11 percent of initial; EDL: Pt = 19.3 +/- 6.8 percent). Although ADP and AMP recovered to preischemic levels in both muscles after 2 hours of ischemia and 2 hours of reperfusion, ATP recovered to just 70 percent in the soleus muscles (13.4 +/- 1.7 mmol/kg dry weight) and 60 percent in the extensor digitorum longus muscles (17.93 +/- 4.1 mmol/kg dry weight). After 3 hours of ischemia and 2 hours of reperfusion, ATP was further significantly (p < 0.05) decreased in the soleus muscles (48 percent initial) but not in the extensor digitorum longus muscles. Significant partial correlation coefficients (p < 0.005) were obtained between ATP levels and Pt (SOL: r = 0.757; EDL: r = 0.619) or Po (SOL: r = 0.810; EDL: r = 0.759). For this rat hindlimb model, we conclude that both fiber type and the duration of ischemia significantly affect metabolic and contractile function.

Fluid and electrolyte hormonal responses to exercise and acute plasma volume expansion

To investigate the effect of acute graded increases in plasma volume (PV) on fluid and regulatory hormone levels, eight untrained men (peak aerobic power 45.2 +/- 2.2 ml.kg-1.min-1) performed prolonged cycle exercise (46 +/- 4% maximal aerobic power on three occasions, namely, with no PV expansion (Con) and after 14% (Low) and 21% (High) expansions, respectively. The exercise plasma levels of aldosterone (Aldo), arginine vasopressin (AVP), and atrial natriuretic peptide (ANP) were all altered by acute PV increases. A pronounced blunting (P < 0.05) of the Aldo response during exercise was observed, the magnitude of which was directly related to the amount of hypervolemia (Con < Low < High). At 120 min of exercise, Aldo concentrations were 660 +/- 71, 490 +/- 85, and 365 +/- 78 pg/ml for Con, Low, and High conditions, respectively. In contrast, the lower AVP and the higher ANP observed during exercise appeared to be due to the effect of PV expansion on resting concentrations. Because osmolality did not vary among conditions, the results indicate that PV represents an important primary stimulus in the response of Aldo to exercise. The lower exercise blood concentrations of both epinephrine and norepinephrine observed with PV expansion would suggest that a lower sympathetic drive may be implicated at least in the lower Aldo responses.

Metabolic and contractile responses of fast- and slow-twitch rat skeletal muscles to ischemia

Complete occlusion of blood flow to rat hind limb by tourniquet was used to study the effects of total ischemia for 1, 2, and 3 h on contractile function and metabolic behaviour of two muscles composed predominantly of either fast-twitch (extensor digitorum longus, EDL) or slow-twitch (soleus, SOL) fibres. Percent loss in twitch force (Pt) was greater (p < 0.05) in SOL than EDL during the first 45 min of ischemia. Following 1 h of ischemia, ATP concentration was lower (p < 0.05) than in the contralateral control (20.8 +/- 2.0 vs. 26.4 +/- 1.5 mmol/kg dry weight). Thereafter, the decline in ATP was greater, with approximately 95% depleted by 3 h of ischemia (1.46 +/- 0.46 mmol/kg dry weight). The effect of ischemia on ATP levels in the SOL was similar to ATP levels in the EDL, 1 h of ischemia also resulted in a large decrement in PCr, from 50.1 +/- 2.9 to 11.7 +/- 2.4 mmol/kg dry weight, and a large increase in lactate, from 25.0 +/- 3.0 to 114 +/- 10 mmol/kg dry weight. As ischemia was prolonged, only lactate was increased (p < 0.05) both at 2 h (171 +/- 12 mmol/kg dry weight) and 3 h (208 +/- 5.4 mmol/kg dry weight). Similar trends were found for SOL. By 3 h of ischemia, glycogen was depleted (p < 0.05) by 88% in EDL and 92% in SOL, respectively. These results support the hypothesis that both high energy phosphate transfer and anerobic glycolysis are of major importance in defending ATP hemostasis, particularly during the 1st h of ischemia, and that the resulting metabolic disturbances are responsible for the large fatigability observed. The mechanisms underlying the greater resistance to fatigue observed for the SOL compared with the EDL during the earlier period of ischemia remain uncertain.

Ischemia-induced alterations in sarcoplasmic reticulum Ca(2+)-ATPase activity in rat soleus and EDL muscles

To investigate the time-dependent effects of ischemia, as modified by muscle fiber type composition, on sarcoplasmic reticulum (SR) function, Ca(2+)-ATPase activity (total minus basal) was measured in homogenates prepared from samples obtained from rat soleus and extensor digitorum longus (EDL) muscle of ischemic and contralateral controls. Ischemia was induced by occlusion of blood flow to one hindlimb for periods of 1, 2, and 3 h (n = 10 per group). In EDL, maximal Ca(2+)-ATPase activity (expressed in mumol.g wet wt-1.min-1) was higher (P < 0.05) in ischemic than in control at 1 h (80 +/- 10 vs. 56.5 +/- 5.3) and increased progressively with ischemia at both 2 h (88 +/- 4.6 vs. 53.1 +/- 2.8) and 3 h (116 +/- 3.8 vs. 67.8 +/- 3.2). In contrast, in soleus, increases (P < 0.05) in Ca(2+)-ATPase activity with ischemia were observed at 2 h (19.2 +/- 0.86 vs. 14.0 +/- 0.56) and 3 h (19.9 +/- 1.4 vs. 12.4 +/- 0.62) but not at 1 h (10.7 +/- 1.5 vs. 10.0 +/- 0.83). In both EDL and soleus, basal Mg(2+)-ATPase was unchanged with ischemia. On the basis of these findings, it can be concluded that ischemia results in an increase in the maximal SR Ca(2+)-ATPase activity but that the time course of the change is dependent on the fiber type composition of the muscle.

Effects of training duration on substrate turnover and oxidation during exercise

Adaptations in fat and carbohydrates metabolism after a prolonged endurance training program were examined using stable isotope tracers of glucose ([6,6-2H2]glucose), glycerol ([2H5]glycerol), and palmitate ([2H2]palmitate). Active, but untrained, males exercised on a cycle for 2 h/day [60% pretraining peak O2 consumption (VO2peak) = 44.3 +/- 2.4 ml.kg-1.min-1] for a total of 31 days. Three cycle tests (90 min at 60% pretraining VO2peak) were administered before training (PRE) and after 5 (5D) and 31 (31D) days of training. Exercise increased the rate of glucose production (Ra) and utilization (Rd) as well as the rate of lipolysis (glycerol Ra) and free fatty acid turnover (FFARa/Rd). At 5D, training induced a 10% (P < 0.05) increase in total fat oxidation because of an increase in intramuscular triglyceride oxidation (+63%, P < 0.05) and a decreased glycogen oxidation (-16%, P < 0.05). At 31D, total fat oxidation during exercise increased a further 58% (P < 0.01). The pattern of fat utilization during exercise at 31D showed a reduced reliance on plasma FFA oxidation (FFA Rd) and a greater dependence on oxidation of intramuscular triglyceride, which increased more than twofold (P < 0.001). In addition, glucose Ra and Rd were reduced at all time points during exercise at 31D compared with PRE and 5D. We conclude that long-term training induces a progressive increase in fat utilization mediated by a greater oxidation of fats from intramuscular sources and a reduction in glucose oxidation. Initial changes are present as early as 5D and occur before increases in muscle maximal mitochondrial enzyme activity.

Effects of tissue fractionation on exercise-induced alterations in SR function in rat gastrocnemius muscle

Because studies into exercise-induced alterations in sarcoplasmic reticulum (SR) Ca2+ sequestration have produced conflicting reports, we have hypothesized that the differences in SR Ca(2+)-adenosinetriphosphatase (ATPase) activity and Ca2+ uptake in SR fractions observed in different studies are due to different SR isolation techniques. To investigate this possibility, rat white and red gastrocnemius muscles from control and run animals were studied by using two conventional isolation techniques to obtain a crude microsomal fraction and an isolated SR vesicle (SRV) fraction. Indexes of CM and SRV function were compared with measurements from whole muscle homogenate. Treadmill running to exhaustion did not alter SR protein yields, percent SR extraction, or basal or Ca(2+)-ATPase purification in either fraction. Ca(2+)-activated ATPase activity was not altered by exercise in any of the fractions examined, but Ca2+ uptake was reduced in the homogenates (9.48 +/- 1.4 to 6.90 +/- 0.8 nmol . mg-1.min-1) and SRV fractions (84.0 +/- 11.5 to 50.7 +/- 14.0 nmol . mg-1.min-1) from the red gastrocnemius at free Ca2+ concentrations of 600-700 nM. These data indicate that reductions in SR Ca2+ uptake are dissociated from changes in Ca(2+)-ATPase in vitro and occur only in a specific population of vesicles. The mechanisms underlying these alterations are not known but may involve a reduction in the number of Ca(2+)-ATPase enzymes or a selective sedimentation of damaged vesicles in the SRV fraction.

Increments in skeletal muscle GLUT-1 and GLUT-4 after endurance training in humans

We investigated the time course of training-induced changes in the expression of GLUT-1 and GLUT-4 in human skeletal muscle. Seven healthy males trained for 2 h/day (approximately 60% pretraining VO2peak) for 31 days (31D). Muscle biopsies were obtained before training (PRE) and after 5 (5D) and 31 days (31D) of training. Training resulted in progressive increases in muscle GLUT-4 with increasing training duration (PRE<5D<31D; P<0.01). Muscle GLUT-1 content was also increased (P<0.05) after training; however, the increase was not observed until 31D (131%). Increases in muscle hexokinase (HK) activity were complete by 5D (P<0.01). Muscle malate dehydrogenase activity was not elevated after 5D of training but was increased (+35%; P<0.01) at 31D. Results from this study show that increases in both GLUT-4 and HK represent early training-induced adaptations to prolonged exercise training. As training progresses, further increases in GLUT-4, but not HK, occur in conjunction with an increase in muscle mitochondrial potential and GLUT-1.

Faster femoral artery blood velocity kinetics at the onset of exercise following short-term training

OBJECTIVE

The hypothesis that the adaptation to endurance exercise training included a faster increase in blood flow at the onset of exercise was tested in 12 healthy young men who endurance-trained (ET) 2 h/day, for 10 days at 65% VO2 peak on a cycle ergometer, and in 11 non-training control (C) subjects.

METHODS

Blood flow was estimated from changes in femoral artery mean blood velocity (MBV) by pulsed Doppler. Beat-by-beat changes in cardiac output (CO) and mean arterial pressure (MAP) were obtained by impedance cardiography and a Finapres finger cuff, respectively. MBV, MAP and CO were measured at rest and during 5 min of dynamic knee extension exercise. Both legs worked alternately with 2 s raising and lowering a weight (15% maximal voluntary contraction) followed by 2 s rest while the other leg raised and lowered the weight.

RESULTS

In the ET group the time to 63% (T63%) of the approximately exponential increase in MBV following 10 days of training (8.6 +/- 1.2 s, mean +/- s.e.) was significantly faster than the Day 0 response (14.2 +/- 2.1 s, P < 0.05). The T63% of femoral artery vascular conductance (VCfa) was also faster following 10 days of ET (9.4 +/- 0.9 s) versus Day 0 (16.0 +/- 2.5 s) (0.05). There was no change in the T63% of both MBV and VCfa for the C group. The kinetics of CO were not significantly affected by ET, but the amplitude of CO in the adaptive phase, and at steady state, were significantly greater (P < 0.05) at Day 10 compared to Day 0 for the ET group with no change in the C group.

CONCLUSIONS

These data supported the hypothesis that endurance training resulted in faster adaptation of blood flow to exercising muscle, and further showed that this response occurred early in the training program.

Progressive effect of endurance training on metabolic adaptations in working skeletal muscle

We investigated the hypothesis that a program of prolonged endurance training, previously shown to decrease metabolic perturbations to acute exercise within 5 days of training, would result in greater metabolic adaptations after a longer training duration. Seven healthy male volunteers [O2 consumption = 3.52 +/- 0.20 (SE) l/min] engaged in a training program consisting of 2 h of cycle exercise at 59% of pretraining peak O2 consumption (VO2peak) 5-6 times/wk. Responses to a 90-min submaximal exercise challenge were assessed pretraining (PRE) and after 5 and 31 days of training. On the basis of biopsies obtained from the vastus lateralis muscle, it was found that, after 5 days of training, muscle lactate concentration, phosphocreatine (PCr) hydrolysis, and glycogen depletion were reduced vs. PRE (all P < 0.01). Further training (26 days) showed that, at 31 days, the reduction in PCr and the accumulation of muscle lactate was even less than at 5 days (P < 0.01). Muscle oxidative potential, estimated from the maximal activity of succinate dehydrogenase, was increased only after 31 days of training (+41%; P < 0.01). In addition, VO2peak was only increased (10%) by 31 days (P < 0.05). The results show that a period of short-term training results in many characteristic training adaptations but that these adaptations occurred before increases in mitochondrial potential. However, a further period of training resulted in further adaptations in muscle metabolism and muscle phosphorylation potential, which were linked to the increase in muscle mitochondrial capacity.

Failure of prolonged exercise training to increase red cell mass in humans

The purpose of this study was to investigate the time-dependent effects of long-term prolonged exercise training on vascular volumes and hematological status. Training using seven untrained males [age 21.1 +/- 1.4 (SE) yr] initially consisted of cycling at 68% of peak aerobic power (VO2peak) for 2 h/day, 4-5 days/wk, for 11 wk. Absolute training intensity was increased every 3 wk. Red cell mass (RCM), obtained using 51Cr, was unchanged (P > 0.05) with training (2,142 +/- 95, 2,168 +/- 86, 2,003 +/- 112, and 2,080 +/- 116 ml at 0, 3, 6, and 11 wk, respectively) as were serum erythropoietin levels (17.1 +/- 4.3, 13.9 +/- 3.5, and 17.0 +/- 2.0 U/l at 0, 6, and 11 wk, respectively). Plasma volume measured with 125I-labeled albumin and total blood volume (TBV) were also not significantly altered. The increase in mean cell volume that occurred with training (89.7 +/- 0.95 vs. 91.0 +/- 1.0 fl, 0 vs. 6 wk, P < 0.05) was not accompanied by changes in either mean cell hemoglobin or mean cell hemoglobin concentration. Serum ferritin was reduced 73% with training (67.4 +/- 13 to 17.9 +/- 1 microgram/l, 0 vs. 11 wk, P < 0.05). Total hemoglobin (HbTot) calculated as the product of hemoglobin concentration and TBV was unaltered (P > 0.05) at both 6 and 11 wk of training. The 15% increase in VO2peak (3.39 +/- 0.16 to 3.87 +/- 0.14 l/min, 0 vs. 11 wk, P < 0.05) with training occurred despite a failure of training to change TBV, RCM, or HbTot.

The interaction between short-term exercise training and a diuretic-induced hypovolemic stimulus

Nine healthy untrained males [mean (SEM) age, 20.2 (1) years; peak oxygen uptake (VO2max, 48.2 (2) ml.kg-1.min-1] took part in a study to examine whether short-term exercise training (cycle exercise 2 h.day-1 for 3 days at 60% VO2max), which normally results in an expansion of plasma volume (PV), can counteract a diuretic-induced hypovolemic stimulus (100 mg triamterene + 50 mg hydrochlorothiazide.day-1 for 5 days concurrent with exercise training) and restore PV to control levels. Resting and exercise responses (90 min, 60% VO2max) in the diuretic plus exercise training condition (D+E) were compared to a control (C) and a diuretic (D) condition in which no exercise was performed. Following the short-term training, PV was still decreased (P < 0.05) below C by -8.3 (3)% in D+E and was similar (P > 0.05) to the reduction in D [-12.4 (2)%]. The reduced PV in response to the diuretic was associated with similar (P > 0.05) elevations in resting aldosterone (ALDO) and norepinephrine (NOREPI) levels (ng.100 ml-1) in D [101 (12), 61 (4)] and D+E [85 (16), 60 (10)] above (P < 0.05) C [22 (5), 37 (4)]. During exercise, ALDO levels were increased (P < 0.05) by 66 (5) and 70 (10) ng.100 ml-1 in D and D+E, respectively, and the increase was greater (P < 0.05) than C [44 (8) ng.100 ml-1]. The rise in NOREPI during exercise was lower (P < 0.05) in D+E [164 (44) ng.100 ml-1] than in D [244 (24) ng.100 ml-1] with levels similar to C [176 (25) ng.100 ml-1]. Thus, the ALDO response to the diuretic was heightened at rest and during exercise but was not additionally affected by the short-term training session. Results suggest that 3 days of exercise training are unable to counteract the hypovolemic effects of a diuretic and restore PV to control levels despite chronic elevations in NOREPI and ALDO.

Progressive effect of endurance training on VO2 kinetics at the onset of submaximal exercise

The rates of increase in O2 uptake (VO2) after step changes in work rate from 25 W to 60% of pretraining peak VO2 (VO2 peak) were measured at various times during an endurance training program (2 h/day at 60% pretraining VO2 peak). Seven untrained males [23 +/- 1 (SE) yr] performed a series of repeated step changes in work rate before training (PRE) and after 4 days (4D), 9 days (9D), and 30 days (30D) of training. VO2 kinetic responses were determined from breath-by-breath data averaged across four repetitions and analyzed using a two-component exponential model. Mean response time (time taken to reach 63% of steady-state VO2) was faster (P < 0.01) than PRE (38.1 +/- 2.6 s) at both 4D (34.9 +/- 2.4 s) and 9D (32.5 +/- 1.8 s) and was faster (P < 0.01) at 30D than at all other times (28.3 +/- 1.0 s). Blood lactate concentrations (after 6 min of cycling) were also lower at 4D and 9D than PRE (P < 0.01) and were lower at 30D than at all other times (P < 0.01). VO2 peak was unchanged from PRE (3.52 +/- 0.20 l/min) at 8D (3.55 +/- 0.20 l/min) but was increased (P < 0.01) at 30D (3.89 +/- 0.18 l/min). Muscle oxidative capacity (maximal citrate synthase activity) was not significantly increased until 30D (P < 0.01). It is concluded that at least part of the acceleration of whole body VO2 kinetics with endurance training is a rapid phenomenon, occurring before changes in VO2 peak and/or muscle oxidative potential.

Increased clearance of lactate after short-term training in men

A short-term training model previously shown to result in a tighter metabolic control in working muscle in the absence of an increase in mitochondrial potential was used to examine changes in lactate turnover. Lactate flux was studied before and after 10 days of cycle training [2 h/day at 59% maximal oxygen consumption (VO2max)] in untrained men [VO2max = 45.5 +/- 2.4 (SE) ml.kg-1.min-1). A primed constant infusion of L-[1-13C]lactate was used to examine lactate kinetics during a prolonged exercise protocol (90 min at 59% VO2max). Rate of appearance of lactate increased with exercise (P < 0.01), both pretraining (rest = 30.3 +/- 4.9 ml.kg-1.min-1, exercise = 115 +/- 14 ml.kg-1.min-1) and posttraining (rest = 28.4 +/- 4.7 ml.kg-1.min-1, exercise = 112 +/- 13 ml.kg-1.min-1). Despite a lower blood lactate concentration (P < 0.05) during exercise after training, there was no difference in the rate of appearance of lactate. Training increased (P < 0.05) the metabolic clearance rate of lactate during exercise from 36.8 +/- 4.8 to 51.4 +/- 6.8 ml.kg-1.min-1. These findings indicate that at least part of the lower exercising blood lactate observed after training is due to an increase in metabolic clearance rate. In addition, the lower intramuscular lactate levels suggest a decreased recruitment of glycolysis particularly early in exercise.

Gas exchange, blood lactate, and plasma catecholamines during incremental exercise in hypoxia and normoxia

The interrelationships among blood lactate (La-) and plasma norepinephrine (NE) and epinephrine (Epi) were studied simultaneously with measures of ventilation (VE) and gas exchange during incremental exercise to exhaustion in nine healthy young men. We wanted to observe whether the tight coupling that exists during normoxic exercise between the concentrations of La- ([La-]) and of both NE and Epi would also be found in hypoxia (inspired O2 fraction = 0.14). In addition, we used recently advocated methods of V slope [CO2 output vs. O2 uptake (VO2)] to select the ventilatory threshold (VT) and log-log transformation of [La-] and VO2 to select the lactate threshold (LT). Peak VO2 was reduced from 4,164 +/- 184 ml/min in normoxia to 3,635 +/- 144 ml/min in hypoxia (P < 0.05). The increase in [La-] was linearly related to the increases in both NE and Epi concentrations in the normoxic and hypoxic tests (r = 0.92-0.96). Estimates of VO2 at VT were significantly greater than those at LT in both normoxia and hypoxia, but these estimates were poorly correlated (r = -0.11-0.46). VT and LT were reduced by hypoxia. Visual interpretation of the VT by examination of VE vs. VO2 and VE/VO2 vs. VO2 did not differ from the LT, but they were less than the VTs by the V-slope method (P < 0.05); yet, all were poorly correlated. The tight coupling between the increase in [La-] and the increase in plasma catecholamines might indicate a common mechanism for the increase or a causative link. VT and LT provided estimates of the general trend in the data, but the poor correlation between them questions the utility of attempting to predict one from the other.

Short-term training, muscle glycogen, and cycle endurance

The objective of this study was to test the hypothesis that the increased glycogen concentration found in the working muscles following short-term training would result in an increase in endurance performance. Endurance performance was examined in 8 active but untrained males who cycled until fatigue at 65% VO2max prior to and following 3 consecutive days of training. Training consisted of cycling for 2 hrs a day at the same power output used in the prolonged fatigue trials. A 39% increase in cycle time, from 103 +/- 7.7 to 143 +/- 14 min (p < 0.05), was observed following training. At fatigue prior to training, glycogen concentration in the vastus lateralis muscle was depleted by 75% (317 +/- 17 to 78.8 +/- 32 mmol.glucosyl units .kg-1 d.w). Following training, glycogen concentration at a comparable work time was 2.3 times higher. The elevated glycogen level following training was due both to higher glycogen at rest and during exercise. The energy cost of the activity as measured by the VO2 at selected intervals was unchanged with training. No change (p > 0.05) in exercise time was observed in a control group who were subjected to similar exercise protocols approximately 1 to 2 weeks apart. It is concluded that short-term training at least in untrained individuals (VO2max averaging 43.6 +/- 2.9 ml.kg-1.min-1) substantially elevates submaximal exercise tolerance and that the increase in resistance to fatigue is related to the elevated availability of glycogen.

Decreased glucose turnover after short-term training is unaccompanied by changes in muscle oxidative potential

This study investigated the hypothesis that training-induced reductions in exercise blood glucose utilization can occur independently of increases in muscle mitochondrial potential. To induce a training adaptation, eight active participants (23 +/- 1 yr, 80.6 +/- 3.7 kg, mean +/- SE) with a maximal oxygen consumption (VO2max) of 45.5 +/- 2.4 ml.kg-1.min-1, cycled at 59% VO2max for 2 h per day for 10 consecutive days. Measurements of blood glucose appearance (Ra) and disappearance (Rd), using a primed continuous infusion of [6,6-2H2]glucose, were made during 90 min of cycle exercise (59% VO2max) performance before and after training. Training resulted in a 25% decrease (P < 0.01) in mean glucose Ra during exercise (43.0 +/- 3.7 to 34.4 +/- 2.8 mumol.kg-1.min-1). Since blood glucose concentration was not different between training conditions, glucose metabolic clearance rate was also depressed (P < 0.05). Exercise-induced glycogen depletion in vastus lateralis muscle was reduced (P < 0.05) with training. Calculation of carbohydrate and fat oxidation based on the respiratory exchange ratio supported a shift toward greater preference for fat. Because training did not elicit changes in the maximal activities of citrate synthase and malate dehydrogenase, two enzymes of the citric acid cycle, it would appear that increases in mitochondrial potential are not necessary for the adaptations that occurred.

Effects of prolonged low frequency stimulation on skeletal muscle sarcoplasmic reticulum

The role of prolonged electrical stimulation on sarcoplasmic reticulum (SR) Ca2+ sequestration measured in vitro and muscle energy status in fast white and red skeletal muscle was investigated. Fatigue was induced by 90 min intermittent 10-Hz stimulation of rat gastrocnemius muscle, which led to reductions (p < 0.05) in ATP, creatine phosphate, and glycogen of 16, 55, and 49%, respectively, compared with non-stimulated muscle. Stimulation also resulted in increases (p < 0.05) in muscle lactate, creatine, Pi, total ADP, total AMP, IMP, and inosine. Calculated free ADP (ADPf) and free AMP (AMPf) were elevated 3- and 15-fold, respectively. No differences were found in the metabolic response between tissues obtained from the white (WG) and red (RG) regions of the gastrocnemius. No significant reductions is SR Ca2+ ATPase activity were observed in homogenate (HOM) or a crude SR fraction (CM) from WG or RG muscle following exercise. Maximum Ca2+ uptake in HOM and CM preparations was similar in control (C) and stimulated (St) muscles. However, Ca2+ uptake at 400 nM free Ca2+ was significantly reduced in CM from RG (0.108 +/- 0.04 to 0.076 +/- 0.02 mumol.mg-1 protein.min-1 in RG - C and RG - St, respectively). Collectively, these data suggest that reductions in muscle energy status are dissociated from changes in SR Ca2+ ATPase activity in vitro but are related to Ca2+ uptake at physiological free [Ca2+ bd in fractionated SR from highly oxidative muscle. Dissociation of SR Ca2+ ATPase activity from Ca2+ uptake may reflect differences in the mechanisms evaluated by these techniques.

Metabolic adaptations to short-term training are expressed early in submaximal exercise

In previous studies we have been able to demonstrate tighter metabolic control of muscle metabolism during prolonged steady-state exercise 5 to 6 days after the initiation of training and well before changes in oxidative potential. To examine whether the metabolic adaptations are manifested during the non-steady-state adjustment to submaximal exercise, 11 male subjects (Vo2 peak, 45 +/- 2.4 mL.kg(-1). min(-1), X +/- SE) performed 98 min of cycle exercise at 67% of Vo2 peak prior to and following 3 to 4 days of training for 2 h per day. Analysis of lactate concentration (mmol/kg dry weight) in samples rapidly extracted from vastus lateralis indicated reductions (p < 0.05) of 44% at 3 min ( 42.1 +/- 7.1 vs. 23.6 +/- 7.7), 29% at 15 min (35.4 +/- 6.4 vs. 25.0 +/- 6.0), and 32% at 98 min (22.9 +/- 6.9 vs. 15.6 +/- 3.2) with training. Training also resulted in higher phosphocreatine and lower creatine and P(i) values that were not specific to any exercise time point. In addition, Vo2 was not altered either during the non-steady state or during the steady-state phases of exercise. These results suggest that at least part of the tightening of the metabolic control and the apparent reduction in glycogenolysis and glycolysis in response to short-term training occurs during the adjustment phase to steady-state exercise.