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The difference in interposition ratio of pink muscle fiber among various fish species was studied to learn its influence in dorsal ordinary muscle on the post-mortem change of K-value. A remarkable mosaic pattern was exhibited in the interpositioned white and pink muscle fibers in many of the species. The relative number and area percentages of these fibers to all muscle fiber types differed greatly among fish species, and distributed from 12.9% to 56.53% in a number, and from 2.22% to 53.68% in an area. The relationship between the area and numerical percentages had significantly high positive correlation. Size of the pink muscle fiber was somewhat smaller than that of white muscle fiber.

To clarify the cause of differences in the temporal change of K-value among fish species living in the same habitat water temperature, the influence of the interposition of pink muscle fibers into dorsal ordinary muscle on 5′-inosine monophosphate (5′-IMP) degrading activity was examined. Fourteen fish species from the Sakishima Islands (habitat water temperature 28°C) and from Nagasaki (habitat water temperature 17°C) were used for the sample fishes. Each of 5′-IMP and ρ-nitrophenol phosphate (ρ-NPP) degrading activities showed a peak at near pH 8.0 and near pH 5.0, respectively. These activities were somewhat higher in fish from Nagasaki than in fish from the Sakishima Islands. The interposition percentage of pink muscle fibers into dorsal ordinary muscle correlated significantly (P < 0.05) with the 5′-IMP degrading activity at pH 7.0. Furthermore, the activity at pH 7.0 correlated significantly (P < 0.001) with the increasing rate of K-value at 32°C (ΔK32). These results suggest that the difference in ΔK32 among fish species in the same habitat water temperature might be caused by differences in 5′-IMP degrading activity because of the differences in the interposition rates of pink muscle fibers into dorsal ordinary muscle.

Twenty-seven fish species from the Sakishima Islands and Nagasaki living in water temperatures of 28°C (12 fish species) and 17°C (15 fish species), respectively, were used as sample fishes. Postmortem temporal changes of K-values were measured at 1-h intervals in the dorsal ordinary muscle of each fish, of which the remaining body was then kept in water at a controlled temperature of 32°C. The increasing rate of K-value at 32°C (ΔK32) was calculated from the regression line formed from the relationship between the kept time and the K-value. Muscle fiber types were differentiated by the stability of actomyosin ATPase preincubated in either acid or alkaline conditions. ΔK32 varied widely according to fish species, ranging from 0.98%/h to 14.00%/h. Although the mean value of each body of water was somewhat higher for fishes from Nagasaki at 17°C than in fishes from Sakishima Islands at 28°C, the difference was not significant. The interposition percentage of pink muscle fibers into the dorsal ordinary muscle was also distributed widely, ranging from 0% to 56.53% in number and from 0% to 53.68% in area. A significant correlation was recognized in the relationship between the number or area percentage of pink muscle fibers and deviation percentage of ΔK32 for all fish species sampled; r = 0.411 (P < 0.05) for number percentage and r = 0.518 (P < 0.01) for area percentage. These results suggest that the interposition of pink muscle fibers into the dorsal ordinary muscle might accelerate a temporal change of K-value in fish.

The present study compared and examined the characteristics of actomyosin among white (W), pink (P), and red (R) muscle fiber types in carp (cultured). Both the superprecipitation reaction and the Mg2+-ATPase activity of actomyosin became higher with increased Ca2+ concentration (pCa 7.0–pCa 5.0) and with decreased adenosine triphosphate (ATP) concentration (3.0–0.5 mM) in all three muscle fiber types. A comparison of the three fiber types shows that the superprecipitation reaction of actomysoin was lower in the order of W P > R. A significantly positive correlation between both values was found for each of the three muscle fiber types, but these correlations were clearly different among the three muscle fiber types, and the superprecipitation reaction of actomyosin was lower in the order of W < P < R when Mg2+-ATPase activity was at the same level. In conclusion, the characteristics of actomyosin were remarkably different among white, pink, and red muscle fiber types.

SUMMARY: In order to clarify the influence of the interposition of pink muscle fiber into the dorsal ordinary muscle on the post-mortem temporal change of K-value, using carp, the dorsal muscle was divided into three muscle parts toward depth with the naked eye, as follows: dark muscle part, intermediate muscle part, and deepest ordinary muscle part. The muscle fiber types in these parts were discriminated by the inactivation of actomyosin ATPase activity for both acid and alkaline pre-incubations, and the temporal changes of ATP related compounds in these parts were then measured at a kept temperature of 32°C. Three muscle parts were organized from the muscle fiber types as follows: dark muscle part was organized from only red muscle fiber type, intermediate part from only pink muscle fiber type in a thin layer and from two muscle fiber types of not only pink muscle fiber but also white muscle fiber of the IIa or IIb subtype in a region of mosaic pattern, and deepest ordinary muscle part from two muscle fiber types of white muscle fiber IIa or IIb. The temporal change of K-value among muscle fiber types was considerably different and remarkably faster in the order of red muscle fiber, pink muscle fiber, and white muscle fiber. From these results, it was considered that the interposition of pink muscle fiber into the dorsal ordinary muscle might accelerate the temporal change of K-value.