An estimation of the true inorganic phosphate content of frog sartorius muscle.

W7hen a resting skeletal muscle is treated with cold perchloric or other acid extracting agent, and the solution is analyzed with one of the standard methods for inorganic phosphate, values are obtained that differ for various kinds of muscle, but have been reported as about 5 pmoles per g for the sartorius muscle of the frog (I; see also Tables IV and V). For several reasons, we have felt the need to investigate whether such analyses truly represent the inorganic phosphate level in the muscle cell. Potentially, phosphate could act as a rate-limiting factor for phosphorylative reactions (2, 3), but this would probably require lower concentrations of phosphate than indicated above. Secondly, reported failure to demonstrate a decomposition of either adenosine triphosphate or phosphocreatine in brief contractions (4-7) warrants the search for other participating high energy substances, which might be phosphate compounds so labile as to be decomposed, and appear as inorganic phosphate after the customary extraction and assay procedures. Finally, Stella (8) has estimated from the phosphate distribution between a muscle and its medium that the internal phosphate activity might be well below the analytical concentration as it was known at that time. In the present study, we have developed new methods for extracting the muscle at rest or in activity, and made use of more selective procedures for the determination of the true inorganic phosphate. It was found that phosphate is present in much smaller amounts than has previously appeared.