The Journal of General Physiology

The whole activity cycle of muscle can be reproduced in glycerol-extracted muscle fibers by suspending them in solutions containing adenosinetriphos-phate (ATP) and certain ions, particularly Mg and Ca ions (1). Quantitatively, however, the agreement between the responses of normal and extracted muscle fibers is disappointing. Relaxation induced by ATP in extracted muscle fibers is too slow and is associated with a high rate of P liberation. Also, in the contracted state enzymatic activity is much lower in extracted than in normal muscle (2). In the search for a better reconstruction of muscular activity the action of phosphocreatine (PC) was investigated. Indeed, this substance was found to intensify the actions of ATP to a surprising degree, to increase the tension of contracted fibers, and to accelerate relaxation. The interpretation of these phenomena which will be presented here is based on the accepted view that PC does not directly furnish energy for contraction, but is important for the rapid phosphorylation of adenosinemonophosphate (AMP) and adenosine-diphosphate (ADP). Beyond that, the study of the effects of PC has led to the conclusion that contraction requires only a small amount of adenine nucleotide firmly bound to the contractile elements. Technique The rabbit's psoas muscle preserved in 50 per cent glycerol at about-1 5 ° was used. The preparations, which had a total cross-sectional area of about 0.2 ram3, were suspended in a small chamber as described previously (1). They were divided into at least 12 strands to facilitate diffusion (2). The mechanical responses were recorded on a smoked drum by an isometric lever. Before an experiment the fibers were transferred into a solution which contained 0.16 M KC1 and 2 m • MgCI~ and which will be referred to as saline. All other solutions used contained nearly the same concentrations of KC1 and MgC12. Solutions of PC were prepared from commercial Na salt (Sigma Chemical Co., St. Louis), those of ATP were prepared from the Na or Ba salt (Nutrition Biochemical Corporation, Cleveland, and Sigma Chemical Company, respectively). These solutions were usually buffered by 0.02 M glycylglycine of pH 6.9, but exactly the same results were obtained without buffer. PC was always used at a concentration of 20 m~t per liter, approximately that in normal muscle. Most experiments were carried out at room temperature (21-24°).