Post-natal changes in some histochemical and enzymatic characteristics of three pig muscles.

Post-natal changes in histoenzymatic and enzymatic characteristics were studied in porcine longissimus, psoas major and tibialis cranialis muscles. The experiment was carried out on thirty-five female large White pigs: fifteen were slaughtered between birth and 120 kg body weight (BW) and twenty at the market liveweight of 100 kg. The relative growth of each of the three muscles in relation to BW appeared to be monophasic, the allometric growth rates differed significantly between muscles: 1·21, 1·08 and 0·97, respectively. Myofibers were classified according to their contractile and metabolic properties as βR, αR and αW fibers. In the three muscles, the percentage of β fibers increased from birth up to 2 months of age and little change occurred thereafter. Glycolytic fibers were detectable only from about one month onwards. Their percentage then increased rapidly up to 30-50 kg BW to reach 60%, 30% and 12% in longissimus, psoas major and tibialis cranialis muscles, respectively. Thereafter, these values only changed slightly. This histoenzymatic differentiation led to the typical type grouping found in pigs. Throughout the period studied, the cross-sectional area of the myofibers increased with some differences according to fiber type and muscle. Lactate dehydrogenase (LDH) specific activity was similar in the three muscles at birth and then increased rapidly up to 4-5kg BW (15-20 days of age), although to a greater extent in the future glycolytic muscle (longissimus) This increase then abruptly slowed down. The period between 15 and 20 days of age corresponds to a critical stage already known in the rearing of pigs, i.e. growth rate, body composition and food intake. Changes in isocitrate dehydrogenase (ICDH) specific activity were much slighter and differed according to the muscle. There were close correlations between percentages of αW fibers and enzymatic kinetic measurements from one month onwards.