Characterization of Longissimus dorsi skeletal muscle fiber characteristics in finishing Mangalica pigs supplemented with ractopamine hydrochloride

Abstract

Heritage pig breeds such as Mangalica are gaining popularity in US niche markets and Mangalica pork is currently sold as specialty meat due to higher meat quality characteristics. Widespread commercial production of this breed is limited by their slow growth rate, poor lean yield, and high adiposity. However, practices like inclusion of ractopamine hydrochloride (RAC) may improve these poor performance characteristics. The skeletal muscle of Mangalica pigs has not been well described nor have the effect of feeding RAC on muscle fiber myosin heavy chain (MHC) type composition. Therefore, the current study aimed to characterize skeletal muscle fiber type and cross-sectional area (CSA) of Mangalica pigs and to evaluate the effect of feeding RAC on these parameters during the finisher phase. Pigs were randomly assigned to 1 of 2 finisher diets fed for 21 d consisting of either 20 mg per kg RAC (n = 6) or RAC free (CTL; n = 8). Longissimus dorsi (LD) muscle samples were collected during carcass fabrication after a withdrawal period of 23 d. Immunofluorescence staining of 5-µm-thick cryosections with MHC antibodies was performed. Type 2B fibers were determined by subtraction. Fibers (≥ 150 per image) were assessed for fiber type and their CSA was measured at 200X magnification. Data were analyzed as a 1-way ANOVA using the GLIMMIX procedure of SAS V9.4. Means were separated with the PDIFF option and declared different when P ≤ 0.05. Tendencies were declared when 0.0501 ≤ P ≤ 0.1000. Feeding RAC decreased the density of type 2A (P = 0.0136) and type 2B (P = 0.0186) fibers in the longissimus muscle of Mangalica pigs. A tendency for RAC to reduce the density of type 1 (6.61 vs. 16.14 fiber per mm2; P = 0.0505) fibers and increase the density of fibers co-expressing type 1 and 2A MHC isoforms (10.23 vs. 2.88 fiber per mm2; P = 0.0524) simultaneously was also observed. When analyzed as a proportion of total fibers, RAC reduced the proportion of fibers expressing only type 2A (11.8 vs. 6.70%; P = 0.0468) and type 2B (0.19 vs. 1.32%; P = 0.0101) MHC isoforms but increased the percentage of fibers expressing both Type 1 and Type 2A MHC isoforms (4.66 vs. 1.13%; P = 0.0280) simultaneously. Feeding RAC did not impact the density or proportion of fibers expressing type 2X MHC (P ≥ 0.386). Inclusion of RAC decreased density of oxidative fibers (type 1 + type 1:2A + type 2A; P = 0.0337) while density of intermediate (type 2A:X) and glycolytic (type 2X + type 2B) fibers was not impacted (P = 0.3584). No differences were observed in the proportion of oxidative, intermediate, or glycolytic muscle fibers (P = 0.1617) with the inclusion of RAC. When fibers were analyzed according to their speed of contraction, feeding RAC did not impact (P = 0.134) density and proportion of slow-twitch (type 1 + type 1:2A) or fast-twitch (type 1:2A + type 2A + type 2A:X + type 2X + type 2B) fibers. Similarly, inclusion of RAC did not change density and proportion of red (type 1 + type 1:2A + type 2A + type 2A:X) or white fibers (type 2A:X + type 2X + type 2B) when fibers were classified according to their visual appearance. Mean LD muscle fiber CSA was not impacted by feeding Mangalica pigs RAC for 21 d (P = 0.4815). Neither the CSA of the smallest and largest fibers nor fiber distribution were influenced by RAC inclusion in the diet of Mangalica pigs (P ≥ 0.484). In conclusion, the MHC fiber type profile of Mangalica pigs was characterized for the first time. Feeding RAC to Mangalica pigs during the finisher phase and following a 23 d withdrawal period impacted fiber MHC isoform expression in the longissimus muscle without altering fiber CSA. Future studies will explore the impact of RAC on the CSA of the various fiber types as well as the impact on other muscles in the Mangalica pig.