Removal of rabbit psoas strips immediately after death and incubation in a saline solution containing 1 mM Ca 2 + and 5 MM Mg2+ for 9 hr at 37°C and pH 7 .1 causes complete Zline removal but has no ultrastructurally detectable effect on other parts of the myofibril . Z lines remain ultrastructurally intact if I mm 1, 2-bis-(2-dicarboxymethylaminoethoxy)-ethane (EGTA) is substituted for 1 mm Ca 2+ and the other conditions remain unchanged . Z lines are broadened and amorphous but are still present after incubation for 9 hr at 37°C if 1 mm ethylenediaminetetraacetate (EDTA) is substituted for 1 mm Ca 2+ and 5 MM Mg2+ in the saline solution . A protein fraction that causes Z-line removal from myofibrils in the presence of Ca2+ at pH 7 .0 can be isolated by extraction of ground muscle with 4 mat EDTA at pH 7 .0-7 .6 followed by isoelectric precipitation and fractionation between 0 and 40% ammonium sulfate saturation . Z-line removal by this protein fraction requires Ca2 + levels higher than 0

SUMMARY– Investigations were conducted on the effect of three storage temperatures, 2°, 16°, and 37°, on the changes and relationships of certain chemical and physical properties of post‐mortem bovine semitendinosus and psoas muscle. Post‐mortem muscle shortening was measured with the isometer. Isometric tension development was maximal at 2°, minimal at 16°, and at 37° tension was approximately one‐half that developed at 2°. The large tension development at 2° very likely originates from the same events as those in “cold shortening.” Differences in isometric tension parameters were noted between muscles in that psoas muscle developed tension and lost the ability to maintain tension more quickly than did the semitendinosus. Loss of ability to maintain tension was observed only at 2°, and this could correspond to a “resolution” of rigor mortis. Adenosine triphosphate (ATP) degradation was measured by two methods, ammonia production and bioluminescent enzymic method; the bioluminescent method proved to be the more satisfactory. A common relationship was observed between pH and ATP for both muscles and the three temperatures studied. No direct relationship was found between ATP degradation and shear resistance with the possible exception of muscle stored at 37°. Isometric tension parameters and shear resistance were related somewhat at 2° in semitendinosus muscle, but no relationship existed at 16° and 37°. Although considerable tension developed in psoas and semitendinosus muscle at 37°, shear resistance values decreased continuously, indicating that factors other than shortening are more important at high temperature and that these factors are temperature‐dependent. The role of ATP degradation in tension development was difficult to interpret, since at 2°, only a small change occurred in ATP level during large tension development, and the level of ATP at 2° did not differ from ATP level in muscle stored at 16° which developed little tension. Differences in post‐mortem muscle shortening at 2 and 37 are discussed.

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