Protein metabolism in skeletal muscle

Two different approaches towards the study of protein turnover in skeletal muscle were made. The first approach involved the injection of 75Se Selenomethionine into rats and the subsequent measurement of the whole body decay rate of the 75Se activity in a 4 pi liquid scintillation counter. By this means it was hoped that the whole body decay curve could be analysed into exponentials representative of 'fast' (visceral protein) and 'slow' (muscle protein) pools. This proved not to be feasible. The special difficulties resulting from the use of 75Se-labelled amino acids are discussed. As a second approach a search was made for a technique for labelling muscle proteins so that radioactivity decay rates could be used directly to calculate rates of synthesis and catabolism without the usual errors arising from isotope reutilisation. 75Se selenomethionine, 14C-6 arginine, 14C-Na2 CO3 and 14C-1 glutamate were investigated. 14C-Na2 CO3 proved to be suitable especially if the decay rates of separate and glutamate isolated from muscle proteins labelled with 14C Na2 CO3 are measured. The lack of reutilisation of label is discussed in terms of the metabolic activity of the carboxyl groups of these dicarboxylic amino acids. The effects of acute deprivation of calories and protein on synthesis and catabolism of muscle and liver protein was measured in rats, using the 14C-Na2 CO3 labelling method. The synthesis rates for muscle proteins, 0.25 and 0.097 days-1 for sarcoplasmic and myofibrillar proteins respectively are the fastest reported in the literature. The total protein synthesised and catabolised in muscle sarcoplasmic and myofibrillar fractions was calculated and compared with liver. The protein free diet caused a reduction in synthesis rates in liver and muscle protein with no change in the distribution pattern between the tissues. The starved rats showed a shift in the distribution pattern of synthesis towards liver and a concomitant shift towards muscle in catabolism. The results are discussed in terms of the mobility and therefore importance of muscle protein metabolism in the economy of the whole animal (AU)

Protein turnover in skeletal muscle. I. The measurement of rates of synthesis and catabolism of skeletal muscle protein using [14C]Na2CO3 to label proteins

The turnover of rat skeletal muscle protein was studied using [75Se]selenomethionine, [6-14C]arginine and [14C]Na2CO3 to label protein. In rats labelled with both [75Se]selonomethionine and [14C]Na2CO3 the 14C activity of mixed skeletal muscle protein fell rapidly with a half-life of 6.0 days for the specific activity and 10.5 days for the total activity. There was no loss of 75Se activity from muscle protein during the 12 days of the experiment. Following the injection of [6-14C]arginine both sarcoplasmic and myofibrillar proteins continued to incorporate label for 6 days after which time the label was lost fairly rapidly. Following injection of [14C]Na2CO3 muscle protein was maximally labelled by 6h, at which time specific activity of the free amino acids had fallen to a very low level. Aspartate and glutamate in particular had lost over 99 percent of their maximum activity by this time in comparison to arginine which was still highly labelled after 24h. 14C activity was lost more rapidly from aspartate and glutamate isolated from sarcoplasmic and myofibrillar protein than from other labelled amino acids. The half-lives of the two protein fractions were 3.9 and 7.2 days from the specific activity curves and 6.0 and 19.0 days from the total activity curves. The differences between the half lives of muscle proteins labelled with amino acids are discussed in terms of the effects of reutilization of the labelled amino acids used. It is postulated that aspartate and glutamate labelled by the injection [14C]CO3= are only reutilized to a very small extent and therefore afford the means by which the rates of protein synthesis and catabolism in skeletal muscle can be measured with reasonable accuracy (Summary)

Total body potassium, muscle electrolytes, and glycogen in malnourished children

Total body potassium, muscle potassium, magnesium, and glycogen have been estimated in infants while they were malnourished, during recovery, and in several after they were fully recovered. Muscle potassium was curvilinearly related to the total body potassium. Muscle magnesium was reduced, and the potassium/magnesium ratio was depressed in children with low muscle potassium values, implying differential loss of muscle potassium. Muscle potassium was linearly related to muscle glycogen. Twenty-four-hour urinary excretion of creatinine was measured; by assuming that 1 mg. of creatinine was derived from 20 Gm. of muscle, calculations of muscle mass were made. In children with a total body potassium over 40 mEq. per kilogram of body weight, muscle potassium contributed approximately one half of the total body potassium; this ratio decreased significantly when body potassium fell to very low values. (AU)

Protein turnover in skeletal muscle. I. The effect of starvation and a protein-free diet on the synthesis and catabolism of skeletal muscle proteins in comparison to liver

Rates of synthesis and catabolism of liver and muscle sarcoplasmic and myofibrillar protein have been measured in young control, starved and protein (deprived) rats using [14C]Na2CO3 to label protein. Half-lives for synthesis of 1.35, 2.8 and 7.2 days for liver, sarcoplasmic and myofibrillar proteins, respectively were obtained, whilst half-lives for catabolism were 1.55, 3.6 and 15.6 days in each case in the control animals. The protein free diet for 3 days caused a small decrease in the rate of synthesis of liver and muscle proteins. The catabolic rate of liver protein was increased by 20 percent whilst there was a smaller increase in the catabolic rate of myofibrillar proteins. Starvation for 3 days caused a 20 percent reduction in the rate of liver protein synthesis whilst there were greater reductions in muscle protein synthesis. The catabolic rate of liver protein was only slightly increased whereas there was a 75 percent increase in the rate of myofibrillar protein breakdown. The total amount of protein synthesis and catabolism in liver and the 2 muscle protein fractions over the first 3 days of the 3 regimes were calculated. Muscle protein turnover, in particular myofibrillar, was shown to be very sensitive to dietary protein and/or calorie deficiency. These results are discussed in terms of the mobility and therefore importance of muscle protein metabolism in the economy of the whole animal. (AU)

The turnover of [75 Se] selenomethionine in infants and rats measured in a whole body counter

The gamma-emitting amino acid [75Se]selenomethionine was given to rats and to human infants, and the rate and route of excertion of 75Se was followed for several weeks by daily measurements in a 4 pi whole body counter. These data were used to calculate the turnover rate of total body protein, and the results were checked against other, technically more difficult, methods.(AU)