Proteasome activities in the rectus abdominis muscle of young and older individuals.

Sarcopenia is one of the most striking effects of age, the causes and the pathogenic mechanisms being largely unknown. Unfortunately, there is limited information on the effect of aging on muscle protein breakdown in basal conditions. The present study aimed at investigating if skeletal muscle ubiquitn mRNA levels and proteasome activities vary with age in healthy individuals. Ub mRNA levels were measured by northern blot analysis whereas proteasome activities were determined by evaluating the cleavage of specific fluorogenic substrates in the rectus abdominis muscle of 14 healthy male individuals. Patients were divided in three groups according to the age: (1) 20-30 years (N = 3); (2) 31-64 years (N = 5); (3) > or = 65 years (N = 6). Quantitation of the ubiquitin mRNA levels (expressed in arbitrary units) (mean (SD) showed no differences among the three groups of age (20-30 years: 1352 +/- 441; 31-64 years: 1324 +/- 439; > or = 65 years: 884 +/- 400; P = 0.33). The correlation between age and muscle ubiquitin mRNA levels was not statistically significant (r = -0.4, P = 0.26). The three proteasome activities, chymotrypsin-like (CTL), trypsin-like (TL) and peptidyl-gutamyl-peptidase (PGP), expressed as nkatal x 10(-3)/mg protein, were similar in the three groups of patients stratified according to the age. There was no correlation between age with either CTL (r = 0.22, P = 0.4), PGP (r = 0.002, P = 0.9), and TL (r = 0.28, P = 0.33) activities. In conclusion, the present study shows that the skeletal muscle proteasome activities do not differ with age in healthy male individuals.

Palmitate acutely induces insulin resistance in isolated muscle from obese but not lean humans.

Exposure to high fatty acids (FAs) induces whole body and skeletal muscle insulin resistance. The globular form of the adipokine, adiponectin (gAd), stimulates FA oxidation and improves insulin sensitivity; however, its ability to prevent lipid-induced insulin resistance in humans has not been tested. The purpose of this study was to determine 1) whether acute (4 h) exposure to 2 mM palmitate would impair insulin signaling and glucose transport in isolated human skeletal muscle, 2) whether muscle from obese humans is more susceptible to the effects of palmitate, and 3) whether the presence of 2 mM palmitate + 2.5 mug/ml gAd (P+gAd) could prevent the effects of palmitate. Insulin-stimulated (10 mU/ml) glucose transport was not different, relative to control, following exposure to palmitate (-10%) or P+gAd (-3%) in lean muscle. In obese muscle, the absolute increase in glucose transport from basal to insulin-stimulated conditions was significantly decreased following palmitate (-55%) and P+gAd (-36%) exposure (control vs. palmitate; control vs. P+gAd, P < 0.05). There was no difference in the absolute increase in glucose transport between palmitate and P+gAd, indicating that in the presence of palmitate, gAd did not improve glucose transport. The palmitate-induced reduction in insulin-stimulated glucose transport in muscle from obese individuals may have been due to reduced Ser Akt (control vs. palmitate; P+gAd, P < 0.05) and Akt substrate 160 (AS160) phosphorylation (control vs. palmitate; P+gAd, P < 0.05). FA oxidation was significantly increased in muscle of lean and obese individuals in the presence of gAd (P < 0.05), suggesting that the stimulatory effects of gAd on FA oxidation may not be sufficient to entirely prevent palmitate-induced insulin resistance in obese muscle.

Myosin heavy chain isoform expression and Ca2 +-stimulated ATPase activity in single fibres of toad rectus abdominis muscle.

Segments of single fibres from the rectus abdominis (RA) muscles of adult and juvenile cane toads (Bufo marinus) were examined for myosin heavy chain (mHC) isoform expression and Ca2+-stimulated MgATPase activity. mHC isoform analyses were carried out using the recently developed alanine-SDS-PAGE method, which separates one tonic (BmHCT) and three twitch (BmHC1, BmHC2, BmHC3) mHC isoforms in toad skeletal muscle. Ca2+-stimulated MgATPase activity was measured by spectrophotometric determination of Pi, under conditions in which the ATPase associated with the sarcoplasmic reticulum (SR ATPase) was suppressed by feedback inhibition. The mHC-based fibre types identified in this study include three pure twitch fibre types (t1, t2 and t3), expressing BmHC1, BmHC2 or BmHC3 respectively, and seven hybrid fibre types co-expressing a combination of two or three twitch and tonic or twitch and twitch mHC isoforms. The fibre populations dissected from juvenile and adult toad muscles contained 49.4% (juvenile) and 73.7% (adult) mHC hybrids. The average values for Ca2+-stimulated MgATPase in pure twitch fibres and in fibres expressing predominantly (> or = 95%) the tonic mHC isoform (Tp fibres) differed significantly (P < 0.05) from each other and decreased in the order t1 > t2 > t3 > Tp. We conclude that (i) in RA muscles of both juvenile and adult cane toads there is a large proportion of mHC hybrids, some of which co-express twitch and tonic mHC isoforms and (ii) ATPase activities associated with the four mHC isoforms expressed in toad skeletal muscles decrease in the order BmHC1 > BmHC2 > BmHC3 > BmHCT.

Muscular contractile failure in septic patients: role of the inducible nitric oxide synthase pathway.

Skeletal muscle failure is a frequent manifestation of sepsis that affects prognosis and rehabilitation by impairing respiration and ambulation. Animal studies have shown that the inducible NO synthase (NOS2) is expressed in skeletal muscles during sepsis, likely affecting muscular function, by promoting the formation of the strong oxidant peroxynitrite. In contrast, whether human skeletal muscle expresses a functional NOS2 in similar conditions is unknown. We studied NOS2 expression (mRNA and protein) and activity and its role in contractile function in samples from rectus abdominis muscle obtained during surgical procedure in 16 septic patients and in 21 controls. Peroxynitrite formation was detected by immunohistochemical detection of nitrotyrosine residues. The main results of this study are as follows: (1) A significant increase in NOS2 mRNA, protein, and activity was found in muscles from septic patients, the expression of NOS2 protein positively correlating with sepsis severity. (2) Contractile force was significantly lower in septic than in control muscles. This phenomenon was not reverted by muscle incubation ex vivo with the NOS inhibitor L-NMMA, indicating that NO was not involved in force reduction at the time of biopsy. (3) NOS2 expression in skeletal myocytes was strongly co-localized with nitrotyrosine, revealing muscular peroxynitrite generation during the septic process, before the muscle was biopsied. Exposure of control muscles to an amount of peroxynitrite similar to that generated in septic muscles during the septic process resulted in a nonreversible reduction in force generation. These results suggest that NOS2 could be involved in the decreased muscular force of septic patients via the local generation of peroxynitrite.

The effects of aging on enzyme activities and metabolite concentrations in skeletal muscle from sedentary male and female subjects.

Aging affects the metabolic capacity of skeletal muscle, in particular the glycolytic and respiratory capacities. The purpose of this study was to quantify biochemical alterations due to aging in muscular metabolic capacity in human skeletal muscles in sedentary subjects. The activities of various marker enzymes and metabolites related to glycolysis, Krebs' cycle and the electron transfer chain and high energy phosphate compounds were measured in muscle biopsies from the rectus abdominis, vastus lateralis, and gluteus maximus muscles of 76 sedentary subjects (32 males and 44 females) between 15 and 91 yr. No significant differences between males and females were found, but changes related to age were: a decrease in hexokinase and lactate dehydrogenase activities in the rectus abdominis; a decrease in citrate synthase activity and citrate in the vastus lateralis; an increase in pyruvate kinase activity and a decrease in ATP and creatine phosphate concentrations in the gluteus maximus. These data suggest that distinct muscles may respond differently to aging regardless of sex in sedentary subjects.

Rectus abdominis rhabdomyolysis after sit ups: unexpected detection by bone scan.

Uptake of technetium-99m methylene diphosphonate by the rectus abdominis muscle was unexpectedly found in a 29 year old man who had started to perform 30 to 40 sit ups a day for five days before the bone scan. After a week of rest, serum creatine kinase activity was still abnormal but muscle uptake of technetium-99m methylene diphosphonate had ceased. This specific muscle injury after short term sit ups as well as the resolution of the phenomenon within a week are of interest.

Effects of chronic renal failure on enzymes of energy metabolism in individual human muscle fibers.

In order to improve knowledge about the mechanisms underlying the alterations of energy metabolism recently observed in the skeletal muscle of patients suffering from chronic renal failure, this study was designed to test (1) whether changes in the activity of key enzymes of energy metabolism occur in the muscle of these patients, and if so (2) whether the different muscle fiber types are equally altered in their metabolic machinery. For this, the maximum activities of 14 enzymes were measured in individual muscle fibers microdissected from biopsies of rectus abdominis muscle obtained from seven normal subjects and seven patients with end-stage renal failure before renal replacement therapy. A large decrease in the activities of beta-hydroxyacyl-coenzyme A dehydrogenase, a key enzyme of the beta-oxidation pathway, of citrate synthase, which initiates the tricarboxylic acid cycle, and of fructose-1,6-bisphosphatase, which contributes to the synthesis of glycogen from lactate, was observed in the three fiber types (slow-twitch oxidative, fast-twitch oxidative-glycolytic, and fast-twitch glycolytic). A smaller reduction of the activities of phosphofructokinase and/or pyruvate kinase, two key enzymes of glycolysis, was also observed in slow-twitch oxidative and/or fast-twitch oxidative-glycolytic fibers. These results demonstrate that the abnormalities of muscle energy metabolism observed in patients with chronic renal failure are due, at least in part, to intrinsic changes in the key enzymes of major energy-providing pathways; they also offer a satisfactory explanation for the defect of oxidative metabolism recently demonstrated in the muscle of these patients.