Index Of Muscle Protein Breakdown Research Articles

Effects of leucine and its metabolite β‐hydroxy‐β‐methylbutyrate on human skeletal muscle protein metabolism

Maintenance of skeletal muscle mass is contingent upon the dynamic equilibrium (fasted losses–fed gains) in protein turnover. Of all nutrients, the single amino acid leucine (Leu) possesses the most marked anabolic characteristics in acting as a trigger element for the initiation of protein synthesis. While the mechanisms by which Leu is ‘sensed’ have been the subject of great scrutiny, as a branched-chain amino acid, Leu can be catabolized within muscle, thus posing the possibility that metabolites of Leu could be involved in mediating the anabolic effect(s) of Leu. Our objective was to measure muscle protein anabolism in response to Leu and its metabolite HMB. Using [1,2-13C2]Leu and [2H5]phenylalanine tracers, and GC-MS/GC-C-IRMS we studied the effect of HMB or Leu alone on MPS (by tracer incorporation into myofibrils), and for HMB we also measured muscle proteolysis (by arteriovenous (A–V) dilution). Orally consumed 3.42 g free-acid (FA-HMB) HMB (providing 2.42 g of pure HMB) exhibited rapid bioavailability in plasma and muscle and, similarly to 3.42 g Leu, stimulated muscle protein synthesis (MPS; HMB +70%vs. Leu +110%). While HMB and Leu both increased anabolic signalling (mechanistic target of rapamycin; mTOR), this was more pronounced with Leu (i.e. p70S6K1 signalling ≤90 min vs. ≤30 min for HMB). HMB consumption also attenuated muscle protein breakdown (MPB; −57%) in an insulin-independent manner. We conclude that exogenous HMB induces acute muscle anabolism (increased MPS and reduced MPB) albeit perhaps via distinct, and/or additional mechanism(s) to Leu.

3-Methylhistidine Excretion as an Index of Muscle Protein Breakdown in Birds in Different States of Malnutrition

Abstract Using 3-methylhistidine (3-MeHis) excretion as an index, we evaluated the abilities of molting and non-molting sparrows to adjust muscle protein degradation rates in response to three types of nutritional limitations: total food shortage, protein deficiency, or sulfur amino acid (SAA) deficiency. Regardless of nutritional status, molting birds excreted daily at least two-fold as much 3-MeHis as their non-molting counterparts. Both molting and non-molting birds significantly reduced daily excretion of 3-MeHis in response to a 1-day fast. Likewise, both molting and non-molting birds promptly reduced daily excretion of 3-MeHis in response to protein deficiency (within 1 day) and significantly so by day 5 and through day 9 of dietary treatment. Reductions in excretion of 3-MeHis by fasted or protein-limited birds could not be explained solely on the basis of reduced body protein mass and provided evidence of adjustments in muscle protein degradation rates. Responses of birds to SAA deficiency differed between molting and non-molting birds and from responses to the other nutritional limitations. In these birds, 3-MeHis excretion increased significantly on day 1 of dietary treatment. By day 5 and through day 9 of SAA deficiency, non-molting birds excreted 3-MeHis at rates similar to those of their well-nourished counterparts. In contrast, excretion of 3-MeHis by molting birds experiencing SAA deficiency remained significantly higher than by their well-nourished counterparts. Neither molting nor non-molting birds readily reduced muscle protein degradation rates in response to SAA deficiency and it is suggested that their failure to do so may provide evidence of a direct and (or) indirect role for glutathione in the regulation of muscle protein degradation.

3-Methylhistidine excretion by molting and non-molting sparrows

Excretion of 3-methylhistidine (3-mehis) provides an index of muscle protein breakdown when 3-mehis is contained mainly in the skeletal muscle of an animal and is excreted quantitatively. In sparrows (Zonotrichia leucophrys gambelii), at least 84% of total 3-mehis in the body was found to be contained in skeletal muscle. When injected with 14C-3-mehis, these birds excreted, on average, 93% of the injected dose, as 3-mehis, within 24 hr. We measured the rate of 3-mehis excretion in well-nourished molting and non-molting sparrows to determine if degradation rates of muscle (myofibrillar) protein increased during molt. Molting sparrows excreted significantly more 3-mehis per day than non-molting sparrows. We estimated that the increased rate of muscle protein breakdown during molt nearly equalled the increased rate of muscle protein synthesis reported earlier for molting sparrows (Murphy and Taruscio, 1995). The results from this investigation provide independent evidence that sparrows accelerate whole-body protein turnover during molt.

The retention and metabolism of N tau-methylhistidine by cockerels: implications for the measurement of muscle protein breakdown determined from the excretion of N tau-methylhistidine in excreta.

Excreta were collected for four consecutive days from 4- to 18-week-old cockerels following subcutaneous injection of N tau-[14CH3]methylhistidine. The recoveries of radioactivity in excreta were incomplete and progressively decreased with increasing age. Most of the radioactivity not recovered in excreta after 4 d was found in skeletal muscle where greater than 55% of the radioactivity present was in the N tau-methylhistidine-containing dipeptide, balenine. This peptide appeared to be relatively stable so that most of the labelled N tau-methylhistidine incorporated was not released during the period of the recovery measurements. The total pool of non-protein bound N tau-methylhistidine (free (free N tau-methylhistidine + balenine) in pectoral and mixed thigh muscles increased with age and relative to the daily excretion of N tau-methylhistidine. At 18 weeks the pool was 3.3 times the daily excretion of N tau-methylhistidine. These observations account for the decreasing recoveries of radioactivity in excreta described previously, due to progressive dilution of labelled N tau-methylhistidine in an expanding pool of non-protein-bound N tau-methylhistidine, part of which was relatively stable. It is concluded that excretion of N tau-methylhistidine by 4- to 18-week-old cockerels cannot be used as a reliable index of muscle protein breakdown in vivo.

Synergism of triiodothyronine and corticosterone on muscle protein breakdown

The concerted effect of triiodothyronine (T 3) and corticosterone on muscle protein synthesis and breakdown was studied. Thyroidectomized young male rats were treated with T 3 (1.5 μg/100 g body weight per day), corticosterone (10 mg/100 g body weight per day) and both T 3 and corticosteronee for 4 days. On the 3rd day of the experiment urine was collected to measure N τ -methylhistidine excretion as an index of muscle protein breakdown. On the last day of the experiment, the rates of protein synthesis in skeletal muscles were measured by the large-dose [ 3H]phenylalanine method. N τ -Methylhistidine excretion was slightly increased by T 3 treatment and it was increased about 3-times by corticosterone treatment. When both T 3 and corticosterone were administered, it was increased about 6-fold. The rate of muscle protein breakdown calculated from the difference between the rate of protein synthesis and the growth rate was consistent with these findings. The rate of muscle protein synthesis was increased by T 3, and it was decreased by corticosterone. The rate was the same as that of the thyroidectomized control group when the animals were given T 3 and corticosterone, showing that T 3 restrained the inhbiting effect of corticosterone on muscle protein synthesis. The results indicate that a physiological level of T 3 enhances the catabolic action of pharmacological doses of glucocorticoids on muscle protein breakdown.

Genetic studies on the muscle protein turnover rate of coturnix quail.

The validation of the urinary excretion of N tau-methylhistidine (N tau-MH) by quail as an index of the muscle protein turnover rate was tested using the criterion of the rate of recovery of radioactivity in urine following an intraperitoneal dose of L-[3-14C] methylhistidine. A genetic study on muscle protein turnover in quail was conducted using three genetically diverse lines (LL, large body size; SS, small body size; RR, random-bred control line) selected for body size. When L-[3-14C] methylhistidine was administered to 20-week-old male and female coturnix quail by direct intraperitoneal injection, approximately 90% of the L-[3-14C] methylhistidine was recovered by 96 hr postinjection. Recoveries were low in the egg and muscle. These results show that N tau-MH released from myofibrillar protein is not reutilized and the excretion of N tau-MH is a satisfactory index of muscle protein breakdown. In all lines, the amount of urinary N tau-MH excretion and fractional synthesis (Ks) and degradation (Kd) rates at the high growing period were higher than those at the low growing period. The Ks and Kd are significantly different among selected lines at both 3 and 6 weeks of age. At 3 weeks of age, the fractional rate of synthesis of the LL line (13.2%/day) was higher than that of the RR line (11.5%/day), whereas the SS (8.1%/day) was lower than that of the RR line (11.5%/day). The fractional rates of degradation of both the LL line (4.1%/day) and the SS line (5.6%/day) were lower than that of the RR line (7.0%/day) at 3 weeks of age. From these results, it was recognized that selection for body size gave rise to the changes in the muscle protein turnover rate.

N tau-methyl histidine excretion by poultry: not all species excrete N tau-methyl histidine quantitatively.

The rate of elimination of administered N tau-[14CH3]methyl histidine was used to assess the validity of N tau-methyl histidine excretion as an index of muscle protein breakdown in poultry. Broiler chicks (2-3 and 4-5 weeks old), laying hens, adult quail (Coturnix coturnix japonica), adult cockerels and turkey poults (2-4 weeks old) were tested. All except the turkey poults showed quantitative recoveries of N tau-[14CH3]methyl histidine within 1 week. Turkeys showed a different pattern of N tau-[14CH3]methyl histidine output; the mean total recovery after 14 d was less than 50% of the injected dose. The majority of the label remaining after this time was found in breast muscle. All birds tested excreted N tau-methyl histidine unchanged, although a small amount sometimes appeared as another metabolite. No significant oxidation of N tau-[14CH3]methyl histidine by broiler chicks turkey poults or adult quail was found. The results show that excretion of N tau-methyl histidine is a useful measure of muscle protein breakdown in the domestic fowl and quail but not in turkeys.

Metabbolism of perfused ruminant muscle

SUMMARYSubstrate and nitrogen metabolism of ruminant muscle was studiedin vitrousing a perfused sheep hemidiaphragm preparation. Glucose, acetate, propionate and butyrate were taken up by the perfused muscle at a constant rate over 2 h. Rate of acetoacetate uptake declined over the perfusion period. Net production of 3-hydroxybutyrate over the perfusion period was observed.Release of alanine and glycine by the perfused muscle could not be accounted for by protein breakdown, implicating muscle as a site of synthesis of these amino acids. The perfused muscle metabolized aspartate and arginine to a considerable extent, but the branched-chain amino acids only to a small extent. Only a small proportion, approximately 5%, of the alanine produced by the perfused muscle was derived from glucose.In the presence of amino acids, at twice the concentration found in plasma, and insulin, some amino acids showed a net uptake by the diaphragm but when cysteine was omitted from the perfusate, all amino acids with the exception of arginine showed a net output. Evidence was obtained that protein synthesis was reduced in the absence of cysteine. The output of Nτ methylhistidine by the preparation was much higher than could be accounted for by estimates of protein breakdown using isotope dilution techniques. These data are, however, consistent with the view that Nτ methylhistidine is not a good index of muscle protein breakdown in sheep.

Urinary excretion of 3-methylhistidine: An assessment of muscle protein catabolism in adult normal subjects and during malnutrition, sepsis, and skeletal trauma

The urinary excretion of 3-methylhistidine (3 MEH) has been shown to be a reliable index of muscle protein breakdown. It is decreased in protein-calorie malnutrition and increased during the hypercatabolic phase of sepsis and thermal trauma. Losses of 3 MEH after moderate to severe skeletal trauma in man and animals are reported as increased or unchanged. To clarify this response, 24 male and 6 female skeletal trauma patients were evaluated for 24 hr urinary losses of 3 MEH, nitrogen and creatinine. Eight of the 24 males also received a catabolic steroid for treatment of a head injury. In addition, 3 male and 1 female septic patients were similarly evaluated. Controls consisted of 10 volunteers on a meat free diet for 4 days and of 8 volunteers who were given only intravenous 5% dextrose in water for 3 days. The 3 MEH excretion for all control males was 3.6 μmole/Kg/day and for females was 2.8. Skeletal trauma produced a 280% increase for the males and a 225% increase for the females. Trauma with steroids caused a 325% increase. Sepsis induced a 227% increase in 3 MEH losses for males and 292% for females during the febrile episode. Creatinine excretion also increased significantly in response to trauma and sepsis but the magnitude of the increase was less than for 3 MEH. This was reflected in the 3 MEH to creatinine molar ratio increase from 0.018 for controls to 0.030–0.040 in sepsis and trauma. Patients with extensive body weight loss showed decreases in 3 MEH and creatinine excretion and a molar ratio similar to controls. The calculated contribution of muscle protein to whole body protein breakdown in the trauma and septic groups showed a twofold increase compared to the control group. The data indicate that the increased muscle protein catabolic response following stress of skeletal trauma and sepsis provides an insight on the origin of the large urinary nitrogen losses following such insults.

The urinary excretion of N tau-methyl histidine by cattle: validation as an index of muscle protein breakdown.

1. The recoveries of radioactivity in cattle urine following the intravenous administration of N tau-[14CH3]methyl histidine were essentially quantitative in 5--7 d in non-lactating cows, bulls and steers and did not change with age. 2. The N tau-methyl histidine was excreted unchanged in urine. 3. N tau-methyl histidine occurred in muscle extracts both in the free form and as a perchloric acid-soluble, acid-labile form which accounted for approximately 85% of the total non-bound N tau-methyl histidine in muscle and appeared identical to a similar component identified in muscle extracts of sheep and pigs. 4. There was probably an age-related decrease in the concentration of the acid-labile component in muscle but which did not produce a measurable change in recovery of radioactivity in urine. 5. The daily excretion of N tau-methyl histidine (E, mumol) by male cattle was highly correlated with live weight (W, kg) by the equation: E = 50 . 4 + 3 . 536 (+/- 0 . 044)W (r 0. 997). The excretions progressively decreased from 4 . 04 mumol/d per kg at 100 kg weight to 3 . 62 mumol/d per kg at 600 kg. 6. By the criterion of the rate of clearance of labelled N tau-methyl histidine from the body, the excretion of N tau-methyl histidine in urine appears to be a valid index of muscle protein breakdown in cattle.

The inadequacy of urinary N tau-methyl histidine excretion in the pig as a measure of muscle protein breakdown.

1. The validity of the urinary excretion of N tau-methyl histidine (N tau-MH) by pigs as an index of muscle protein breakdown in vivo was tested using the criterion of the rate of recovery of radioactivity in urine following an intravenous dose of N tau-[14CH3]methyl histidine. 2. Urinary recoveries of radioactivity from five animals were less than 21% of dose in 7 d after which the daily recovery was less than 0 . 3% per day. 3. The incomplete recoveries of radioactivity were associated with the presence in muscle of a large pool of non-protein-bound N tau-MH, the concentration of which increased with age. 4. The N tau-MH in this pool was present as free N tau-MH and in a dipeptide which constituted more than 90% of the total non-protein-bound N tau-MH. The contribution of the peptide increased with age, reaching 99. 8% in older animals. 5. The pool of non-protein-bound N tau-MH was maintained and increased in both established and newly accreted tissue by retention of some of the N tau-MH released by muscle protein breakdown, only a proportion of which was therefore available for excretion. Hence, the urinary excretion of N tau-MH is not a valid index of muscle protein breakdown in pigs.

The urinary excretion of N tau-methyl histidine in sheep: an invalid index of muscle protein breakdown.

1. The validity of the urinary excretion of N tau-methyl histidine (N tau-MH) in sheep as a measure of the breakdown of muscle protein in vivo was assessed from the urinary recovery of radioactivity following the intravenous administration of N tau-[14CH3]methyl histidine. 2. Recoveries of radioactivity in urine from animals of 4 weeks to 7 years of age were incomplete in 7 d but progressively increased with the age of the animal, becoming almost quantitative (90%) in older animals after recovery for 3 weeks. 3. The incomplete urinary recoveries were not due to partial excretion of N tau-MH in faeces or its oxidation and elimination in expired gases but were related to the presence in muscle of a pool of non-protein-bound N tau-MH which was several times larger than the expected daily urinary excretion. 4. This pool in newly accreted muscle tissue was maintained by retention of some of the N tau-MH released by breakdown of muscle protein. Hence, only a proportion of the N tau-MH released from protein breakdown was available for excretion. This proportion increased with the age of the animal and was probably the main determinant of the improved recoveries of radioactivity obtained in urine from older animals. 5. The non-protein-bound N tau-MH in muscle consisted of free N tau-MH and a dipeptide containing N tau-MH, the latter comprising on average approximately 82% of the total non-protein-bound N tau-MH in muscle. This proportion did not change appreciably with the age of the animal. 6. The dipeptide appeared to be synthesized in muscle from free N tau-MH and was not a terminal product of protein breakdown. 7. The results show that urinary excretion of N tau-MH is not a reliable index of muscle protein breakdown in sheep.