67 Effect of Different Dietary Protein Sources on Skeletal Muscle Growth Characteristics and Satellite Cell Mitotic Activity of Young Piglets

Abstract The neonatal period in pigs is distinguished by rapid growth as a function of extensive protein synthesis. Muscle satellite cells also referred to as stem cells (SC) have an essential role in postnatal hypertrophic muscle growth by increasing myofibrillar protein synthesis potential through fusion with existing myofibers to increase the DNA template available for transcription of these important muscle proteins. Thus, the impact of protein sources and differences in their amino acid availability on SC-mediated hypertrophic growth over time in neonatal pigs is of interest. To evaluate how different dietary protein sources fed to young piglets affect muscle fiber cross-sectional area (CSA), fiber density, and SC proliferative activity, a randomized complete block design experiment was conducted using piglets (n = 123) from 5 different farrowing groups. At birth, piglets were assigned to 1 of 4 treatments consisting of a control group and 3 liquid dietary treatments. The commercial milk replacer-based diets contained: whey only (MLK), MLK + spray dried plasma (SDP), and MLK + soy protein concentrate (SPC). After birth, all piglets were allowed to nurse at least 36-h on the sow to allow colostrum intake, assigned to 1 of 3 dietary treatments, transferred to Rescue Decks, and reared until tissue collection on d 6, 14, 19, and 25. The control group was left in the farrowing crate and nursed by the sow until sampling (SOW). On sampling day, pigs were injected with 5’-bromo-2’-deoxyuridine (BrdU) to label mitotically active cells. One-hour post-injection, pigs were euthanized and longissimus dorsi (LM) and biceps femoris (BF) muscle samples were collected for immunofluorescence microscopy analysis. Cell populations were classified as either mitotically inactive SC (MyoD+:BrdU-; Pax7+:BrdU-; MyoD+:Pax7+:BrdU-) or mitotically active SC (MyoD+:BrdU+; Pax7+:BrdU+; MyoD+:Pax7:BrdU+). Data were analyzed as a 2-way ANOVA using SAS V9.4 PROC GLIMMIX and means were separated using PDIFF at P ≤ 0.05. Tendencies were declared when 0.0501 < P ≤ 0.10. At d 14 and 19, MLK-fed piglets had the least BWG (P ≤ 0.0810). On d 14 and 19, MLK-fed piglet LM fibers had the smallest CSA and greatest density (P ≤ 0.0459) though they were similar in fiber density to SOW piglets on d 14. The MLK-fed piglets exhibited the least density of Pax7+:BrdU+ SC in BF (P = 0.0185) and the greatest density of MyoD+ SC in LM (P = 0.0831) on d 6 as well as the greatest density of MyoD+ SC in LM