Studies on the Chemical Composition of Muscle Tissue
Authors: Robertson James D. 
Published: April 2021

Comprehensive analyses have been made of the muscle and plasma for inorganic ions, organic phosphates, soluble carbohydrates, amino acids and nitrogenous bases. Compared with a total of 1108 mg. ions/kg. water in the plasma, the muscle had 1045, of which 515 were mg. ions (Na, K, Ca, Mg, Cl, SO4, lactate, HCO3, inorganic and arginine phosphates, adenosine triphosphate, hexose phosphates, etc.), and the remainder amino acids (405 mM.) and the nitrogenous compounds trimethylamine oxide (59 mM.) and betaine (66 mM.). Measurements of the apparent extracellular spaces in the muscle following injections of inulin, sucrose and sodium thiosulphate have led to the conclusion that the inulin space or permeation, 12·1 % ±0·69 (S.E.) of the total muscle water, is the best indication of total extracellular fluid in the muscle. The copper space, 4·11 ± 0·37, represents a maximum value for the blood space (that filled with haemocyanin), leaving 8·0% as the volume occupied by interstitial fluid. Intracellular concentrations of potassium and chloride do not conform to values expected on the basis of a Donnan equilibrium with plasma: [Ki]/[K0] = 21·9, [Cl0]/[ Cli] = 9·9. Mean values in mg. ions/kg. water for the whole muscle concentrations of phosphate ions are inorganic phosphate 18·3, arginine phosphate 71·0, adenosine triphosphate 11·6, remaining acid-soluble phosphate, chiefly hexose phosphates, 14·4, total 115·2. Total non-protein amino acids of muscle from measurements of a-amino N are 339–551 mM., mean 476. These are chiefly glycine, proline, arginine, taurine, alanine and glutamine. Most of the arginine is combined as arginine phosphate. Direct measurements of osmolality of the plasma using Krogh-Baldes thermocouples give a mean value of 100·4 ± 0·34 (N = 9), taking sea water as 100. Measurements of the juice expressed from muscle with a tissue-press give a mean value compared with plasma (100) of 102·0 + 0·70 if done within 1·5 hr. of removing the muscle, 101·7 ± 0·7 for seven estimations done within an hour; the latter result is not significantly different from the plasma values. Delay in measurement leads to higher values (plus 5–8% on plasma), owing to the breakdown of labile compounds. Compared with whole muscle on a water-content basis, the concentrations of ions in the muscle-juice are Na 76%, K.74%, Ca 10%, Mg 40%, Cl 100%, total acid-soluble P 78%. This is interpreted as showing binding by proteins of fractions of all these ions except chloride. If the juice is corrected for its extracellular component, approximate percentages of ions bound inside the muscle cells are Na 82%, K 26%, Ca 100%, Mg 64% and P 22%; all the cellular chloride is free. A final osmotic balance sheet for Nephrops muscle cells, in which consideration has been given to such factors as binding of ions, osmotic coefficients of ions and probable ideal behaviour of the nitrogenous compounds, shows that about 92% of the total osmotic concentration (in milliosmoles) has been accounted for. Increasing the concentration of potassium and calcium in sea water leads to increases in the values of these ions in both plasma and muscle.