Consumer demand for lean, inexpensive meat products has resulted in unprecedented growth of the poultry industry. Turkey breeders responded with intensive genetic selection to increase body weight, breast muscle yield, and feed conversion efficiency. However, this success has come at the cost of increased susceptibility to heat stress in the fast-growing birds, which led to increased frequency of pale, soft, exudative (PSE) meat. Previous studies have led to a general understanding of the etiology of PSE meat; however, the precise molecular mechanisms, which could inform development of a mitigation strategy for this defect, remain unclear. This dissertation comprises two studies. The first study was based on an observation that pyruvate dehydrogenase kinase 4 (PDK4) is significantly less abundant in PSE samples compared to normal turkey meat. We hypothesized that reduced PDK4 expression would result in reduced phosphorylation of pyruvate dehydrogenase which would alter the course of postmortem metabolism. Turkeys from two lines were exposed to a pre-market thermal challenge. Following slaughter and exsanguination (5 minutes postmortem), pectoralis major samples were removed, and the birds were then processed according to standard commercial practice. Meat quality data were collected at 24 hours postmortem and used to categorize meat samples as PSE or normal. The phosphorylation state of pyruvate dehydrogenase (PDH) in normal and PSE turkey meat and metabolite abundances were analyzed in 5-minute postmortem muscles using the western blot technique. Interestingly, results of this study showed that PSE samples had a significantly lower mean total PDH. However, when normalized to the PDH level in each sample, there was no significant difference in the phosphorylation state of PDH between normal and PSE samples. Analysis of these samples for various metabolites associated with aerobic and anaerobic metabolism showed no significant differences in the levels of glycogen, lactate, glycolytic potential, or ATP when compared to control samples. Previous studies showed that exposure of broilers to thermal challenge during embryonic development improved their ability to withstand pre-market heat stress. Thus, the second study aimed to elucidate the effects of embryonic thermal manipulation on post-hatch thermotolerance and its consequent effects on meat quality of turkeys. Eggs from two turkey lines were exposed to pre-hatch thermal challenge; birds were hatched, raised, then exposed to three days of pre-market thermal challenge at 16 weeks. Following slaughter, breast muscle samples were collected at 15 minutes postmortem, and birds were processed according to standard industry practice. Histological analysis was conducted for perimysial space and fiber diameter from the 15 min postmortem samples. Metabolite levels were quantified at 15 min and 24 h postmortem, and samples were assessed for meat quality parameters including pH at 15 min, color, drip loss, cook loss, and marinade uptake. The results showed that the pre-hatch treatment did not improve the thermotolerance of a premarket heat stress, and there were no significant differences in the fiber diameter, perimysial space, and meat quality characteristics. Moreover, embryonic thermal manipulation did not affect the levels of metabolites from samples of 24 h postmortem, nor of the levels of glucose, G6P, lactate, glycolytic potential, and ADP at 15 min postmortem. However, the glycogen and ATP levels at 15 min postmortem were significantly higher while the levels of IMP and AMP were significantly lower in the samples exposed to embryonic thermal manipulation compared to control when both were exposed to the pre-market thermal challenge. The results of this research do not support a mechanistic role for reduced PDK4 abundance in altering postmortem muscle metabolism which would possibly lead to reduced meat quality. In addition, the embryonic thermal manipulation was not successful in improving thermotolerance of turkeys. Future studies are needed to define the molecular mechanisms associated with development of PSE meat and to develop a successful mitigation strategy.
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