Extension of cell viability in rested post-mortem teleost white muscle

Abstract

The objective of this thesis was to investigate how cell viability can be maintained in rested ischemic teleost white muscle (WM) during post-mortem (PM) storage. For cells to remain viable they require energy in the form of ATP for cellular processes. Exercise prior to anaesthesia severely depleted the ATP generating potential of the WM (estimated by WM pH) from yellow-eye mullet (Aldrichetta forsteri) prior to PM storage, reducing the pre-rigor period. Fish anaesthetised in a rested state had a high ATP generating potential (intact ATP, PCr and glycogen WM stores) with ATP being able to be supplied to cells during PM storage, extending the pre-rigor period. To further extend cell viability during PM storage either the rate of ATP utilisation needed to be curbed or more ATP needed to be generated in the WM. As aerobic generation of ATP is more efficient than anaerobic, an attempt was made to deliver oxygen to the ischemic WM during PM storage. WM from rested mullet was stored under hyperbaric, hyperoxic conditions (620 ± 10 kPa with humidified oxygen flow of 50 mL/min ± 1.25%) and the rate of acidification was slowed by two-thirds compared with storage under normobaric, hyperoxic conditions (humidified oxygen flow of 50 mL/min ± 1.25%). Onset of ATP depletion and lactate accumulation was delayed for ~27 h suggesting a period of aerobic ATP generation. In chinook salmon (Oncorhynchus tshawytscha) the delay was ~ 12 h and in snapper (Pagrus auratus) ~ 37 h. Physical condition and functional differences between species played a significant role in determining the benefit from the storage treatment. As aerobic A TP generation did not continue indefinitely in the hyperbaric, hyperoxic WM, the investigation turned to the site of oxidative phosphorylation (the mitochondria) in order to determine what inhibits their function during PM storage. Varying the incubation medium pH and CO2 concentration (over a physiological range) did not inhibit mitochondria respiration in vitro. Only very high levels of CO2 in combination with low pH may inhibit mitochondria. Extracting mitochondria from WM during PM storage showed that mitochondria from hyperbaric, hyperoxic WM may become damaged due to the high levels of oxygen, consequently leading to an inhibition of oxidative phosphorylation. Cold acclimation of fish often results in compensatory biochemical modifications to maintain aerobic flux. It was hypothesised that fish acclimated to cold temperatures could generate ATP aerobically for longer when stored under hyperbaric, hyperoxic conditions. However, the PM period of aerobic ATP generation was greatest in yelloweye mullet acclimated to summer temperatures. Winter mullet may rely solely on carbohydrate to be used for both aerobic and anaerobic ATP generation, whereas in summer both lipid and carbohydrate, may be available, i.e. summer mullet have more available substrate.