Muscle unloading induces slow to fast transitions in myofibrillar but not mitochondrial properties. Relevance to skeletal muscle abnormalities in heart failure

Muscle deconditioning is a common observation in patients with congestive heart failure (CHF), chronic obstructive pulmonary disease, neuromuscular diseases or prolonged bed rest. To gain further insight into metabolic and mechanical properties of deconditioned slow-twitch (soleus) or fast-twitch (EDL) skeletal muscles, we induced experimental muscle deconditioning by hindlimb suspension (HS) in rats for 3 weeks. Cardiac muscle was also studied. Besides profound muscle atrophy, increased proportion of fast type II fibers as well as fast myosin isoenzymes, we found decreased calcium sensitivity of Triton X-100 skinned fiber bundles of soleus muscle directed towards the fast muscle phenotype. Glycolytic enzymes such as hexokinase and pyruvate kinase were increased, and the LDH isoenzyme pattern was clearly shifted from an oxidative to an anaerobic profile. Creatine kinase (CK) and myokinase activities were increased in HS soleus towards EDL values. Moreover, the M-CK mRNA level was greatly increased in soleus, with no change in EDL. However, oxygen consumption rate assessed in situ in saponin skinned fibers (12.5 +/- 0.8 in C and 15.1 +/- 0.9 micromol O2/min/g dw in HS soleus compared to 7.3 +/- 1.3 micromol O2/min/g dw in control EDL), as well as mitochondrial CK (mi-CK) and citrate synthase activities, were preserved in HS soleus. Following deconditioning no change in Km for ADP of mitochondrial respiration, either in the absence (511 +/- 92 in C and 511 +/- 111 microM in HS soleus compared to 9 +/- 4 microM in control EDL) or presence of creatine (88 +/- 10 in C and 95 +/- 16 microM in HS soleus compared to 32 +/- 9 microM in control EDL), was found. The results show that muscle deconditioning induces a biochemical and functional slow to fast phenotype transition in myofibrillar and cytosolic compartments of postural muscle, but not in the mitochondrial compartment, suggesting that these compartments are differently regulated under conditions of decreased activity.     

Apoptosis of skeletal muscle myofibers and interstitial cells in experimental heart failure

Congestive heart failure (CHF) is characterized by a limb skeletal muscle myopathy with shift from the slow aerobic, fatigue resistant fibers, to the fast, anaerobic ones, and muscle bulk loss. Apoptosis (A) has been recently demonstrated to play a role in several cardiovascular diseases. AIM OF THE STUDY: we have investigated the role of A in the skeletal muscle of the hindlimbs in an experimental model of CHF. ANIMALS AND METHODS: CHF was induced in 7 males 80-100 g Sprague-Dawley rats with 30 mg/kg monocrotaline. Five age and diet matched controls were also studied. The time course of A was also studied in additional animals at day 0, 17, 24 and 30 days. RESULTS: At day 27 the electrophoretic analysis of myosin heavy chains (MHCs) demonstrated in the CHF rats the occurrence of a myopathy, with disappearance of slow MHC1 in the Tibialis Anterior (TA), and a significant shift from the slow to the fast isoforms in the soleus and EDL. With in situ DNA nick-end labelling (TUNEL) we found in the TA of CHF animals a significantly higher number of TUNEL positive nuclei (0.43 +/- 0.24 v 0.08 +/- 0.02, P<0.02 and TUNEL positive myonuclei (0.031 +/- 0.012 v 0.0025 +/- 0.005, P<0.02). The time course of A showed a progressive rise in interstitial and myocyte A, accompanied by a drop in fibers cross-sectional area and muscle weight/body weight, that came out to be significant at 30 days. Western blot showed a lower expression of Bcl-2 at 27 days and a further drop at 30 days in the CHF rats. Double staining for TUNEL and antibody against anti-MHC2a and anti MHC2b + 2x showed that A occurs non-selectively in all the myofiber types. BetaANP and Right Ventricle Mass/Volume (RVM/V) correlated significantly with total apoptotic nuclei. CONCLUSIONS: In CHF myofibers A can lead to muscle atrophy. Endothelial cells A may produce an imbalance in myofibres nutrition with relative ischemia that triggers the preferential synthesis of fast anaerobic myosin as an adaptive mechanism or alternatively induce myofibres death.     

Altered expression of myosin heavy chain in human skeletal muscle in chronic heart failure

To explore further alterations in skeletal muscle in chronic heart failure (CHF), we examined myosin heavy chain (MHC) isoforms from biopsies of the vastus lateralis in nine male patients with class II-III (CHF) (left ventricular ejection fraction (LVEF) 26 +/- 11%, peak oxygen consumption (peak VO2) 12.6 +/- 2 mL.kg-1.min-1) and nine age-matched sedentary normal males (NL). The relative content of MHC isoforms I, IIa, and IIx was determined by gel electrophoresis as follows: The normal sedentary group (NL) had a higher percent of MHC type I when compared with the patients (NL 48.4 +/- 7% vs CHF patients 24 +/- 21.6%, P < 0.05, no difference between MCH IIa (NL 45.1 +/- 10.5% vs CHF 56.0 +/- 12.5%), and CHF patients had a higher relative content of MHC type IIx than did the normal group (NL 6.5 +/- 9.6% vs CHF 20.0 +/- 12.9%, P < 0.05. Three of nine patients had no detectable MHC type I. In patients relative expression of MHC type I (%) was related to peak VO2 (r = 0.70, P < 0.05). Our results indicate that major alterations in MHC isoform expression are present in skeletal muscle in CHF. These alterations parallel previously reported changes in fiber typing that may affect contractile function i skeletal muscle and possibly exercise performance. The absence of MHC type I in some CHF patients suggests that skeletal muscle changes in this disorder are not solely a result of deconditioning, buy may reflect a specific skeletal muscle myopathy in this disorder.     

Alterations in skeletal muscle gene expression in the rat with chronic congestive heart failure

Congestive heart failure is often associated with skeletal muscle abnormalities that contribute to early fatigue and acidosis. Up to the present time, however, the mechanisms responsible for these changes are unclear. Myocardial infarctions were produced by coronary ligation in adult Sprague-Dawley rats. At 20 weeks, 10 control rats, and 15 animals with heart failure [defined by elevated LVEDP (26.1 +/- 3.1 v 2.5 +/- 0.5 mmHg) and RV hypertrophy (300 +/- 21 g v 158 +/- 9 mg)] underwent in vivo measurements of total body, and soleus total protein and myosin heavy chain (MHC) synthesis by [3H]leucine constant infusion. Soleus muscle was also analysed for protein content, and MHC isoenzyme content by SDS-PAGE. Northern blotting also was used to determine levels of the mRNA's encoding type I, IIa, IIb, and IIx MHC, alpha-skeletal actin, COX III, SDH and GAPDH. Soleus muscles in heart failure rats were smaller than controls (112 +/- 6 v 126 +/- 5 mg) and the degree of atrophy was significant when corrected for body mass (0.38 +/- 0.02 v 0.46 +/- 0.02 mg/g. P = 0.007). Although there was no significant difference in plasma leucine flux (an index of whole-body protein synthesis), soleus muscle total and MHC synthesis was reduced in heart failure animals. Whereas the Type I MHC isoenzyme (beta MHC) was the only MHC detected in the soleus of control animals, type II MHC isoenzyme comprised 11.8 +/- 3.1% of the MHC in the heart failure group. Furthermore, steady-state mRNA levels encoding beta MHC were significantly depressed in the heart failure rats, where those encoding Types IIb and IIx MHC were increased. Steady-state mRNA levels of alpha-skeletal actin, cytochrome C oxidase (COX III) and succinate dehydrogenase (SDH) were also significantly depressed. This animal model of chronic heart failure is associated with quantitative and qualitative alterations in skeletal muscle gene expression that are similar to those reported in skeletal muscle of patients with chronic heart failure. The altered phenotype and impaired metabolic capacity may contribute to exercise intolerance in CHF.     

Effects of ovariectomy and hindlimb unloading on skeletal muscle

Female rats (7-8 mo old, n = 40) were randomly placed into the intact control (Int) and ovariectomized control (Ovx) groups. Two weeks after ovariectomy, animals were further divided into intact 2-wk hindlimb unloaded (Int-HU) and ovariectomized hindlimb unloaded (Ovx-HU). We hypothesized that there would be greater hindlimb unloading-related atrophy in Ovx than in Int rats. In situ contractile tests were performed on soleus (Sol), plantaris (Plan), peroneus longus (Per), and extensor digitorum longus (EDL) muscles. Body weight and Sol mass were approximately 22% larger in Ovx than in Int group and approximately 18% smaller in both HU groups than in Int rats (Ovx x HU interaction, P < 0.05), and there was a similar trend in Plan muscle (P < 0.07). There were main effects (P < 0.05) for both ovariectomy (growth) and hindlimb unloading (atrophy) on gastrocnemius mass. Mass of the Per and EDL muscles was unaffected by either ovariectomy or hindlimb unloading. Time to peak twitch tension for EDL and one-half relaxation times for Sol, Plan, Per, and EDL muscles were faster (P < 0.05) in Ovx than in Int animals. The results suggest that 1) ovariectomy led to similar increases of approximately 20% in body weight and plantar flexor mass; 2) hindlimb unloading may have prevented ovariectomy-related muscle growth; 3) greater atrophy may have occurred in Sol and Plan of Ovx animals compared with controls; and 4) removal of ovarian hormonal influence decreased skeletal muscle contraction times.     

Functional and bioenergetic consequences of postinfarction left ventricular remodeling in a new porcine model. MRI and 31 P-MRS study

BACKGROUND: The underlying mechanisms by which left ventricular remodeling (LVR) leads to congestive heart failure (CHF) are unclear. This study examined the functional and bioenergetic abnormalities associated with postinfarction ventricular remodeling in a new, large animal model. METHODS AND RESULTS: Remodeling was induced by circumflex coronary artery ligation in young pigs. LV mass, volume, ejection fraction (EF), the ratio of scar surface area to LV surface area, and LV wall stresses were calculated from magnetic resonance imaging anatomic data and simultaneously measured LV pressure. Hemodynamics, transmural blood flow, and high-energy phosphates (spatially localized 31P-nuclear magnetic resonance) were measured under basal conditions, during hyperperfusion induced by pharmacological vasodilation with adenosine, and during pyruvate infusion (11 mg/kg per minute IV). Six of 18 animals with coronary ligation developed clinical CHF while the remaining 12 animals had LV dilation (LVR) without CHF. The results were compared with 16 normal animals. EF decreased from 55.9 +/- 5.6% in normals to 34.6 +/- 2.3% in the LVR group (P < .05) and 24.2 +/- 2.8% in the CHF group (P < .05 versus LVR). The infarct scar was larger in CHF hearts than in LVR hearts (P < .05). In normals, LV myocardial creatine phosphate (CP)/ATP ratios were 2.10 +/- 0.10, 2.06 +/- 0.16, and 1.92 +/- 0.12 in subepicardium (EPI), mid myocardium (MID), and subendocardium (ENDO), respectively. In LVR hearts, the corresponding ratios were decreased to 1.99 +/- 0.13, 1.80 +/- 0.14, and 1.57 +/- 0.15 (ENDO P < .05 versus normal). In CHF hearts, CP/ATP ratios were 1.41 +/- 0.14, 1.33 +/- 0.15, and 1.25 +/- 0.15; (P < .05 versus LVR in EPI and MID). The calculated myocardial free ADP levels were significantly increased only in CHF hearts. CONCLUSIONS: Bioenergetic abnormalities in remodeled myocardium are related to the severity of LV dysfunction, which, in turn, is dependent on the severity of the initiating myocardial infarction.     

Amylin-mediated reduction in insulin sensitivity corresponds to reduced insulin receptor kinase activity in the rat in vivo

Studies were undertaken to elucidate further the mechanism whereby the pancreatic peptide amylin induces insulin resistance. Sixteen male Sprague-Dawley rats underwent hyperinsulinemic (14 pmol/kg/min, 0 to 120 minutes) euglycemic clamps in the presence or absence of amylin (500 pmol/kg/min, 60 to 120 minutes). Amylin induced insulin resistance at both the hepatic level (mean +/- SE: hepatic glucose output [HGO] with amylin 1.4 +/- 0.2 v without amylin -1.9 +/- 0.3 mmol/kg/h, P < .001) and peripheral level (glucose disposal [Rd] with amylin 5.0 +/- 0.2 v without amylin 8.5 +/- 0.6 mmol/kg/h, P < .001). Serum insulin levels were similar in the presence or absence of amylin alone (661 +/- 89 v 636 +/- 50 pmol/L, respectively, P = NS), but were significantly less when somatostatin (SRIF) was simultaneously infused (408 +/- 15 pmol/L, P < .02 v the other two groups). This suggests that endogenous insulin production was not suppressed by amylin under these study conditions. Similar findings were obtained in 18 animals in the absence of exogenous insulin infusion. In vitro kinase activity toward histone of skeletal muscle insulin receptors (IRs) activated by insulin in vivo was reduced in the presence of amylin to 6.0 +/- 0.8 versus 9.1 +/- 1.2 fmol phosphate into histone (insulin-infused) and 3.9 +/- 0.7 versus 6.9 +/- 1.4 (non-insulin-infused; P < .03 by ANOVA). Serum calcium was significantly decreased in amylin-treated animals (1.93 +/- 0.04 v 2.30 +/- 0.05 mmol/L, P < .001).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of high-intensity intermittent swimming on glucose transport in rat epitrochlearis muscle

Recently (K. Kawanaka, I. Tabata, and M. Higuchi. J. Appl. Physiol. 83: 429-433, 1997), we demonstrated that glucose transport activity after repeated 10-s-long in vitro tetani in rat epitrochlearis (Epi) muscle was negatively correlated with the postcontraction muscle glycogen concentration. Therefore, we examined whether high-intensity intermittent swimming, which depletes muscle glycogen to a lower level than that observed after ten 10-s-long in vitro tetani, elicits higher glucose transport than that observed after ten 10-s-long in vitro tetani, which has been regarded as the exercise-induced maximal stimulus for glucose transport. In male rats, 2-deoxy-D-glucose transport rate in Epi muscle after eight bouts of high-intensity intermittent swimming with a weight equal to 18% of body mass (exercise duration: 20 s, rest duration between exercise bouts: 40 s) was higher than that observed after the ten 10-s-long tetani (2.25 +/- 0.08 vs. 1.02 +/- 0.16 micromol . ml intracellular water-1 . 20 min-1). Muscle glycogen concentration in Epi after eight bouts of high-intensity intermittent swimming was significantly lower than that observed after ten 10-s-long in vitro tetani (7.6 +/- 0.5 vs. 14.8 +/- 1.4 micromol glucose/g muscle). These observations show that the high-intensity intermittent swimming increases glucose transport in rat Epi to a much higher level than that induced by ten 10-s-long in vitro tetani, which has been regarded as the exercise-related maximal stimulus for glucose transport. Furthermore, this finding suggests that the lower muscle glycogen level after high-intensity intermittent swimming than after in vitro tetani may play a role, because there was a significant negative correlation between glucose transport and muscle glycogen concentration in Epi after high-intensity swimming and in vitro tetani.     

Effects of Mg2+ on Ca2+ handling by the sarcoplasmic reticulum in skinned skeletal and cardiac muscle fibres

The influence of myoplasmic Mg2+ (0.05-10 mM) on Ca2+ accumulation (net Ca2+ flux) and Ca2+ uptake (pump-driven Ca2+ influx) by the intact sarcoplasmic reticulum (SR) was studied in skinned fibres from the toad iliofibularis muscle (twitch portion), rat extensor digitorum longus (EDL) muscle (fast twitch), rat soleus muscle (slow twitch) and rat cardiac trabeculae. Ca2+ accumulation was optimal between 1 and 3 mM Mg2+ in toad fibres and reached a plateau between 1 and 10 mM Mg2+ in the rat EDL fibres and between 3 and 10 mM Mg2+ in the rat cardiac fibres. In soleus fibres, optimal Ca2+ accumulation occurred at 10 mM Mg2+. The same trend was obtained with all preparations at 0.3 and 1 microM Ca2+. Experiments with 2,5-di-(tert-butyl)-1,4-benzohydroquinone, a specific inhibitor of the Ca2+ pump, revealed a marked Ca2+ efflux from the SR of toad iliofibularis fibres in the presence of 0.2 microM Ca2+ and 1 mM Mg2+. Further experiments indicated that the SR Ca2+ leak could be blocked by 10 microM ruthenium red without affecting the SR Ca2+ pump and this allowed separation between SR Ca2+ uptake and SR Ca2+ accumulation. At 0.3 microM Ca2+, Ca2+ uptake was optimal with 1 mM Mg2+ in the toad iliofibularis and rat EDL fibres and between 1 and 10 mM Mg2+ in the rat soleus and trabeculae preparations. At higher [Ca2+] (1 microM), Ca2+ uptake was optimal with 1 mM Mg2+ in the iliofibularis fibres and between 1 and 3 mM Mg2+ in the EDL fibres.(ABSTRACT TRUNCATED AT 250 WORDS)     

Aorta and skeletal muscle NO synthase expression in experimental heart failure

Nitric oxide (NO), the free radical that accounts for the biological activity of endothelium-derived relaxing factor, is synthesized from L-arginine by NO synthase (NOS). There is evidence that NO availability is reduced in the peripheral vasculature of patients with congestive heart failure (CHF). The aim of this study was to investigate the expression of NOS in the descending aorta and in the skeletal muscles of rats subjected to heart failure. The alkaloid, monocrotaline, was used to induce pulmonary hypertension and cardiac failure in rats. The expression of both the constitutive (ecNOS) and the inducible (iNOS) isoforms of the enzyme was assessed by Western blot analysis. In CHF animals, the ecNOS location in the aorta is altered: the endothelial protein expression is substantially reduced (from 0.083 +/- 0.012 to 0.003 +/- 0.004 OD/microgram total proteins, P < 0.001) whereas the expression of ecNOS in the smooth muscle is increased (from 0.024 +/- 0.004 to 0.059 +/- 0.009 OD/ microgram total proteins, P < 0.01). The total aortic ecNOS is diminished in CHF respect to control animals (0.062 +/- 0.009 v 0.107 +/- 0.013 OD/microgram total proteins, P < 0.01). On the contrary, no difference in ecNOS protein expression was observed in the extensor digitorum longus and soleus muscles. Furthermore, iNOS was not detected in any of the tissues considered. In conclusion, experimental CHF causes a re-setting of the ecNOS protein expression in the descending aorta but not in skeletal muscles. The reduced abundance of ecNOS in the aortic endothelium is consistent with the impairment of the vasodilating function reported in patients with CHF.     

Effects of felodipine on left ventricular systolic and diastolic performance in congestive heart failure

We studied the effects of a dihydropyridine calcium blocker, felodipine, on left ventricular (LV) contractile performance and diastolic filling dynamics in conscious dogs with pacing-induced congestive heart failure (CHF) before and after autonomic blockade. Eleven conscious dogs were instrumented to measure micromanometer LV and left atrial (LA) pressure (P) and to determine LV volume (V) from three dimensions. CHF was induced by 4 to 5 weeks of right ventricular pacing. After CHF, the mean LV end-diastolic (ED) P increased (9.7 +/- 2.9 vs. 27.9 +/- 6.8 mm Hg, P < .05), LVEDV and end-systolic (ES) V increased and stroke volume (SV) decreased (15.3 +/- 2.4 vs. 9.6 +/- 3.0 ml, P < .05). The time constant of LV relaxation (T) (25.9 +/- 2.9 vs. 37.9 +/- 5.1 msec, P < .05) and LVES wall stress (WS) (63.4 +/- 21.0 vs. 74.6 +/- 23.7 g/cm2, P < .05) also increased. After CHF, felodipine (25 nmol/kg i.v., plasma concentrations 17.4 +/- 3.2 nmol/L) produced significant decreases in LVESP (119 +/- 12 vs. 96 +/- 11 mm Hg, P < .05), arterial elastance, total systolic resistance (TSR) (0.11 +/- 0.04 vs. 0.07 +/- 0.03 mm Hg/ml/min, P < .05) and LVESWS (74.6 +/- 23.7 vs. 60.2 +/- 17.3 g/cm2, P < .05). Felodipine increased the slopes of the ESP-V relation (5.6 +/- 1.5 vs. 7.8 +/- 0.7 mm Hg/ml, P < .05), the dP/dtmax-EDV relation (51.4 +/- 6.1 vs. 85.3 +/- 10.1 mm Hg/ml sec, P < .05) and the stroke work-EDV relation (69.8 +/- 7.1 vs. 78.9 +/- 7.1 mm Hg, P < .05) and shifted all three relations to the left, indicating enhanced contractile performance.(ABSTRACT TRUNCATED AT 250 WORDS)     

Passive Ca2+ binding in ventricular myocardium of neonatal and adult rats

In this study, passive Ca2+ binding was determined in ventricular homogenates (VH) from neonatal (4-6 days) and adult rats, as well as in digitonin-permeabilized adult ventricular myocytes. Ca2+ binding sites, both endogenous and exogenous (Indo-1 and BAPTA) were titrated. Sarcoplasmic reticulum and mitochondrial Ca2+ uptake were blocked by thapsigargin and Ru360, respectively. Free [Ca2+] ([Ca2+]F) was measured with Indo-1 and bound Ca2+ ([Ca2+]B) was the difference between [Ca2+]F and total Ca2+. Apparent Ca2+ dissociation constants (Kd) for BAPTA and Indo-1 were increased by 10-20 mg VH protein/ml (from 0.35 to 0.92 microM for Indo-1 and from 0.20 to 0.76 microM for BAPTA) and also by ruthenium red in the case of Indo-1. Titration with successive CaCl2 additions (2.5-10 nmoles) yielded delta[Ca2+]B/delta[Ca2+]F for the sum of [Ca2+]B at all three classes of binding sites. From this function, the apparent number of endogenous sites (Ben) and their Kd (Ken) were determined. Similar Ken values were obtained in neonatal and adult VH, as well as in adult myocytes (0.68 +/- 0.14 microM, 0.69 +/- 0.13 microM and 0.53 +/- 0.10 microM, respectively). However, Ben was significantly higher in adult myocytes than in adult VH (1.73 +/- 0.35 versus 0.70 +/- 0.12 nmol/mg protein, P < 0.01), which correspond to approximately 300 and 213 mumol/l cytosol. This indicates that binding sites are more concentrated in myocytes than in other ventricular components and that Ben determined in VH underestimates cellular Ben by 29%. Although Ben values in nmol/mg protein were similar in adult and neonatal VH (0.69 +/- 0.12), protein content was much higher in adult ventricle (125 +/- 7 versus 80 +/- 1 mg protein/g wet weight, P < 0.01). Expressing Ben per unit cell volume (accounting for fractional mitochondrial volume, and 29% dilution in homogenate), the passive Ca2+ binding capacity at high-affinity sites is approximately 300 and 176 mmol/l cytosol in adult and neonatal rat ventricular myocytes, respectively. Additional estimates suggest that passive Ca2+ buffering capacity in rat ventricle increases markedly during the first two weeks of life and that adult levels are attained by the end of the first month.     

Effects of CO2-induced acidification on the fatigue resistance of single mouse muscle fibers at 28 degrees C

The role of reduced muscle pH in the development of skeletal muscle fatigue is unclear. This study investigated the effects of lowering skeletal muscle intracellular pH by exposure to 30% CO2 on the number of isometric tetani needed to induce significant fatigue. Isolated single mouse muscle fibers were stimulated repetitively at intervals of 4-2.5 s by using 80-Hz, 400-ms tetani at 28 degrees C in Tyrode solution bubbled with either 5 or 30% CO2. Stimulation continued until tetanic force had fallen to 40% of the initial value. Exposure to 30% CO2 caused a significant fall in intracellular pH of approximately 0.3 pH unit but did not cause any significant changes in initial peak tetanic force. During the course of repetitive stimulation, intracellular pH fell by approximately 0.3 pH unit in both normal and acidified fibers. The number of tetani needed to reduce force to 40% of the initial value was not significantly different in 5 and 30% CO2 Tyrode. The sole effect of acidosis was to reduce the rate of relaxation of force, especially in fatigued fibers. It is concluded that, at 28 degrees C, acidosis per se does not accelerate the development of fatigue during repeated tetanic stimulation of isolated mouse skeletal muscle fibers.     

Effect of zidovudine (AZT) on the structure and function of rat skeletal muscle

The role of the anti-HIV agent zidovudine (ZDV = AZT) in the generation of mitochondrial myopathies and subsequent skeletal muscle contractile deficiencies was evaluated in male rats given ZDV in drinking water (1 mg/mL). After 6 weeks, there was no difference in treadmill run time between experimental (n = 6) and control (n = 6) rats. ZDV did not affect tension output by the gastrocnemius-plantaris-soleus muscle group when stimulated in situ at frequencies of 15, 30, 45, and 75 tetani/min, nor did the drug affect the cytochrome oxidase activity of fast glycolytic, fast oxidative glycolytic, or slow oxidative fiber types after 6 or 15 weeks of treatment. A group of female rats, similarly evaluated after 6 weeks of ZDV at 1 (n = 4) or 2 (n = 4) mg/mL, also did not display any discernable deficiencies. However, when the data from all 10 control rats were compared with those of the 19 ZDV rats, the cytochrome oxidase activity of fast oxidative glycolytic muscle of the ZDV rats was significantly higher (35.0 +/- 1.36 versus 40.7 +/- 1.14 mumol.min-1.g-1; p < 0.05). No ultrastructural abnormalities were observed in 15-week ZDV-treated cardiac muscle or in any of the three skeletal muscle fiber types. These results suggest that ZDV-related myopathies observed in AIDS patients may be due to interactions between the drug and complications associated with HIV infection.     

More tetanic contractions are required for activating glucose transport maximally in trained muscle

Exercise training increases contraction-stimulated maximal glucose transport and muscle glycogen level in skeletal muscle. However, there is a possibility that more muscle contractions are required to maximally activate glucose transport in trained than in untrained muscle, because increased glycogen level after training may inhibit glucose transport. Therefore, the purpose of this study was to investigate the relationship between the increase in glucose transport and the number of tetanic contractions in trained and untrained muscle. Male rats swam 2 h/day for 15 days. In untrained epitrochlearis muscle, resting glycogen was 26.6 micromol glucose/g muscle. Ten, 10-s-long tetani at a rate of 1 contraction/min decreased glycogen level to 15.4 micromol glucose/g muscle and maximally increased 2-deoxy-D-glucose (2-DG) transport. Training increased contraction-stimulated maximal 2-DG transport (+71%; P < 0.01), GLUT-4 protein content (+78%; P < 0.01), and resting glycogen level (to 39.3 micromol glucose/g muscle; P < 0.01) on the next day after the training ended, although this training effect might be due, at least in part, to last bout of exercise. In trained muscle, 20 tetani were necessary to maximally activate glucose transport. Twenty tetani decreased muscle glycogen to a lower level than 10 tetani (18.9 vs. 24.0 micromol glucose/g muscle; P < 0.01). Contraction-stimulated 2-DG transport was negatively correlated with postcontraction muscle glycogen level in trained (r = -0.60; P < 0.01) and untrained muscle (r = -0.57; P < 0.01).     

Physical training and baroreceptor control of sympathetic nerve activity in humans

In nine sedentary subjects (16.5 +/- 0.4 years, mean +/- SEM) we measured blood pressure (Finapres device), heart rate (electrocardiogram), and postganglionic muscle sympathetic nerve activity (microneurography from the peroneal nerve) at rest and during intravenous infusion of phenylephrine and nitroprusside. These measurements were performed before and after 10 weeks of endurance training (2 h/d, 5 d/wk) that increased maximum oxygen consumption from 34.8 +/- 2.1 to 40.4 +/- 1.8 mL/kg per minute (P < .02). Basal mean blood pressure and muscle sympathetic nerve activity were lower after than before endurance training (86.5 +/- 2.6 versus 97.5 +/- 1.8 mm Hg, P < .05, and 14.0 +/- 1.8 versus 21.2 +/- 2.3 bursts per minute, P < .02), and the changes in these variables were closely related (r = .95, P < .01). Similar mean blood pressure increases induced by phenylephrine caused greater reductions in heart rate and muscle sympathetic nerve activity after than before endurance training (-8.6 +/- 0.8 versus -6.1 +/- 1.1 beats per minute, P = NS, and -78.0 +/- 4.6% versus -53.6 +/- 4.8%, P < .05). Likewise, similar mean blood pressure reductions induced by nitroprusside caused greater increases in heart rate and muscle sympathetic nerve activity after than before endurance training (18.6 +/- 3.0 versus 12.4 +/- 2.4 beats per minute, P < .05, and 128.1 +/- 26% versus 63.2 +/- 11%, P < .02). No alteration in hemodynamics, oxygen consumption, muscle sympathetic nerve activity, and baroreceptor reflex sensitivity occurred in four other age-matched sedentary subjects studied before and after a 10-week observation period without endurance training.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lipoic acid reduces glycemia and increases muscle GLUT4 content in streptozotocin-diabetic rats

Alpha lipoic acid (lipoate [LA]), a cofactor of alpha-ketodehydrogenase, exhibits unique antioxidant properties. Recent studies suggest a direct effect of LA on glucose metabolism in both human and experimental diabetes. This study examines the possibility that LA positively affects glucose homeostasis in streptozotocin (STZ)-induced diabetic rats by altering skeletal muscle glucose utilization. Blood glucose concentration in STZ-diabetic rats following 10 days of intraperitoneal (i.p.) injection of LA 30 mg/kg was reduced compared with that in vehicle-treated diabetic rats (495 +/- 131 v 641 +/- 125 mg/dL in fed state, P = .003, and 189 +/- 48 v 341 +/- 36 mg/dL after 12-hour fast, P = .001). No effect of LA on plasma insulin was observed. Gastrocnemius muscle crude membrane GLUT4 protein was elevated both in control and in diabetic rats treated with LA by 1.5- and 2.8-fold, respectively, without significant changes in GLUT4 mRNA levels. Gastrocnemius lactic acid was increased in diabetic rats (19.9 +/- 5.5 v 10.4 +/- 2.8 mumol/g muscle, P < .05 v nondiabetic rats), and was normal in LA-treated diabetic rats (9.1 +/- 5.0 mumol/g muscle). Insulin-stimulated 2-deoxyglucose (2 DG) uptake into isolated soleus muscle was reduced in diabetic rats compared with the control group (474 +/- 15 v 568 +/- 52 pmol/mg muscle 30 min, respectively, P = .05). LA treatment prevented this reduction, resulting in insulin-stimulated glucose uptake comparable to that of nondiabetic animals. These results suggest that daily LA treatment may reduce blood glucose concentrations in STZ-diabetic rats by enhancing muscle GLUT4 protein content and by increasing muscle glucose utilization.     

Peripheral and respiratory muscles in chronic heart failure

It is well-established that in patients with congestive heart failure (CHF), exercise is limited by fatigue and shortness of breath. The poor correlation between the fatigue and indices of central haemodynamic function might indicate that peripheral muscle alterations contribute to impaired exercise capacity. Intrinsic abnormalities of the skeletal muscles have been suggested as a possible explanation. Since the shortness of breath correlates poorly with changes in lung function, changes in the respiratory muscles have been investigated. Studies have demonstrated diaphragmatic myopathy and atrophy similar, in part, to the changes in peripheral skeletal muscles. In CHF, type I (slow twitch) fibre atrophy is seen in respiratory as well as in peripheral muscles. The mechanism of these alterations remains to be elucidated. Studies into the mechanism of muscle dysfunction in congestive heart failure are relevant to the prospect of treatment of the changes in peripheral and respiratory muscles.     

Pharmacokinetics and distribution over the blood-brain barrier of zalcitabine (2',3'-dideoxycytidine) and BEA005 (2', 3'-dideoxy-3'-hydroxymethylcytidine) in rats, studied by microdialysis

Microdialysis was applied to sample the unbound drug concentration in the extracellular fluid in brain and muscle of rats given zalcitabine (2',3'-dideoxycytidine; n = 4) or BEA005 (2', 3'-dideoxy-3'-hydroxymethylcytidine; n = 4) (50 mg/kg of body weight given subcutaneously). Zalcitabine and BEA005 were analyzed by high-pressure liquid chromatography with UV detection. The maximum concentration of zalcitabine in the dialysate (Cmax) was 31.4 +/- 5. 1 microM (mean +/- standard error of the mean) for the brain and 238. 3 +/- 48.1 microM for muscle. The time to Cmax was found to be from 30 to 45 min for the brain and from 15 to 30 min for muscle. Zalcitabine was eliminated from the brain and muscle with half-lives 1.28 +/- 0.64 and 0.85 +/- 0.13 h, respectively. The ratio of the area under the concentration-time curve (AUC) (from 0 to 180 min) for the brain and the AUC for muscle (AUC ratio) was 0.191 +/- 0.037. The concentrations of BEA005 attained in the brain and muscle were lower than those of zalcitabine, with Cmaxs of 5.7 +/- 1.4 microM in the brain and 61.3 +/- 12.0 microM in the muscle. The peak concentration in the brain was attained 50 to 70 min after injection, and that in muscle was achieved 30 to 50 min after injection. The half-lives of BEA005 in the brain and muscle were 5.51 +/- 1.45 and 0.64 +/- 0.06 h, respectively. The AUC ratio (from 0 to 180 min) between brain and muscle was 0.162 +/- 0.026. The log octanol/water partition coefficients were found to be -1.19 +/- 0.04 and -1.47 +/- 0.01 for zalcitabine and BEA005, respectively. The degrees of plasma protein binding of zalcitabine (11% +/- 4%) and BEA005 (18% +/- 2%) were measured by microdialysis in vitro. The differences between zalcitabine and BEA005 with respect to the AUC ratio (P = 0.481), half-life in muscle (P = 0.279), and level of protein binding (P = 0.174) were not statistically significant. The differences were statistically significant in the case of the half-life in the brain (P = 0.032), clearance (P = 0.046), volume of distribution (P = 0.027) in muscle, and octanol/water partition coefficient (P = 0.019).     

Absence of an effect of fatigue on muscle efficiency during high-intensity exercise in rat skeletal muscle

The effect of fatigue was studied on rat skeletal muscle efficiency during maximal dynamic exercise of 10s duration. After the initial 4s of exercise, power output decreased rapidly to 46.2 +/- 6.7% (mean +/- SD; n = 6) after 6s of stimulation and further to 17.5 +/- 5.8% in the last contraction. Both the rates of total work output and high-energy phosphate consumption decreased with increasing exercise duration. As a result muscle efficiency was not affected by exercise time in the present experiments. This result indicates that fatigue in severe maximal exercise is induced by a feed-back mechanism, which in the case of high ATP utilisation rates will reduce ATP splitting probably by reducing Ca(2+)-release from the sarcoplasmic reticulum.