Ultrastructural changcs in bovine, porcine, and rabbit muscle have been studied during the first 24 hours post-mortem. Samples were taken for phase and electron microscopy immediately after death, after 4, 8, and 24 hours of post-mortem storage at 2' and 37"C, and after 24 hours post-mortem at 16" and 25°C. The results show that two kinds of structural changes occur in muscle during the first 24 hours post-mortem: ( a ) a variable amount of shortening, this shortening occurring via a sliding of filaments in all species and at all post-mortem storage temperatures examined, and ( b ) degradation of the 2 line, and at higher storage temperaturcs, of the M line also. Shortening of unrestrained muscle occurs soonest post-mortem at 37°C in all three species and is completed within four hours post-mortem in porcine and rabbit muscle and within eight hours post-mortem in bovine muscle. Post-mortem shorlening of unrestrained rabbit and porcine muscle is greatest at 37°C (sarcomere lengths of 1.5 b ) : shortening of rabbit muscle is minimal at 2'C (sarcomere length of 1.7 b j , but shortening of porcine muscle is minimal at 25°C (sarcomere length of 1.8 E L ) and is slightly greater at 2°C (sarcomere length of 1.6 p ) than at 16'C. Post-mortem shortening of bovine muscle is greatest at 2°C (sarcomere length of 1.3 p ) , is minimal at 16-25°C (sarcomerc lcngth of 1.8 p ) , and increases between 25-37°C (sarcomere lcngth of 1.5 p at 37°C). Sarconiere length measurements show that some variation occurs i n the extent 01 post-mortem shortening within the same muscle.Z line degradation occurs sooner post-mortem and to a greater extent at storage temperatures of 25°C or above than at temperatures of 16°C or below. Also, bovine muscle Z lines are clearly more resistant to post-mortem degradation than porcine or rabbit muscle 2 lines. Loss of fibrillar structure in porcine or rabhit muscle Z lines occurs during the first four hours post-mortem at 37'C, but cight hours of post-mortcni storage at 373C are required to caiise loss of fibrillar structure of bovine muscle 2 lines. After 24 hours at 25 or 37"C, Z lines of rabbit and porcine muscle are usually campletely absent; M lines are also frequently absent in this muscle.Changes that occur i n muscle during the onset of rigor mortis are of interest from several points of view. First, since postmortem muscle always experiences a loss of oxygen, ATP * and creatine phosphate as well as a variable decline in pH (BateSmith and Bendall, '49; Bendall, ' 6 0 ) , a study o f post-mortem muscle may provide insight into the effects of various nonphysiological conditions on the contractile apparatus. Secondly, it has been shown that muscle strips will shorten or develop isometric tension (Busch et al., '67) as they pass into rigor. This shortening or tension development resembles in. vizo contraction in several ways: (a) i t is initiated by an efflux of Ca++ from the sarcoplasrnic reticulum (Nauss and Davies, '66); (b) it uses ATP as an energy source (Bendall, '51, '60; Nauss and Da...
