Year

2000

Degree Name

Doctor of Philosophy

Department

Department of Biological Sciences

Abstract

Intracellular sodium and potassium homeostasis was examined in tissue slices made from liver and brain from a variety of species. All tissue slices were allowed to recover from the process of slicing. The degree of damage to tissue slices was assessed by measurement of the activity of the cytoplasmic enzyme, lactate dehydrogenase. Intracellular Na+/K+ contents were measured following the washout of extracellular space with a Na+-free and K+-free solution. The activity of the Na+/K+-pump was measured as the active uptake of 86Rb+, and the passive Na+-leak was measured using 22Na+, whilst the Na+/K+- ATPase enzyme activity was also measured. Total water space and extracellular space of tissue slices were respectively measured with 3H20 and 14C-dextran, and the intracellular water volume was calculated as the difference between them.

Ion homeostasis at 37°C was compared in liver slices from five species of vertebrate. They were three species of endothermic vertebrates (ie. the rat, Rattus norvegicus; the pigeon, Columba livia and the tammar wallaby, Macropus eugenii) and two species of ectothermic vertebrates (ie. the cane toad, Bufo marinus and the bearded dragon lizard, Pogona vitticeps). Liver slices from all the five species of vertebrates maintained cellular Na+/K+ homeostasis at approximately the same intracellular concentrations of K+ and Na+. However in maintaining tissue ion homeostasis, the Na+,K+-pump activity was approximately 2-3 times greater in the endotherms than in the ectotherms. The liver slices of the rat also had a greater "leak" to both Na+ and K+ than the liver slices from the cane toad.

The effect of temperature on ion homeostasis was compared in liver and brain slices from the rat and the cane toad. The influence of both hypothyroidism or dietary fat composition on the relationship between temperature and ion homeostasis of liver and brain slices from the laboratory rat was also examined. Both liver and brain slices from the rat were unable to maintain tissue ion homeostasis at temperatures below 19°C, whereas tissue slices from the toad were able to maintain ion homeostasis at 13°C and higher temperatures. The temperature at which the rat tissues were unable to maintain ion homeostasis is similar to the reported lower lethal body temperature of rat for 24 h hypothermia (ie. 18~19°C). These findings at the tissue level are consistant with the fact that the ectothermic toad can tolerate lower body temperatures than the endothermic rat. The inability to maintain tissue ion homeostasis was due to the finding that the Na+-leak was less temperature-sensitive than the activity of the Na+,K+-pump.

Hypothyroidism induced by removal of thyroid gland had little effect on tissue ion homeostasis at different temperatures. This was because hypothyroidism decreased both Na+-leak and Na+,K+-pump activity of tissue slices to a similar degree.

Dietary fat composition was found to have little effect on tissue ion homeostasis at different temperatures. Although the effect of dietary fat profile on leak and/or pump activity of tissue slices was different, each dietary fat composition had similar influence on Na+- leak and on Na+,K+-pump activity of each kind of tissue slices. These resulted in the similar maintenance of tissue ion homeostasis between dietary groups.

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