Year

2000

Degree Name

Doctor of Philosophy

Department

Department of Biomedical Science

Abstract

This study investigates the mechanism by which CPZ lowers the cation content of rat and toad liver cells. Results indicate that, even in the presence of ouabain, high concentrations of CPZ are able to decrease the intracellular sodium content of isolated liver cells by activating a sodium efflux pathway. CPZ was able to decrease total intracellular sodium (flame photometry), as well as sodium activity (22Na+), indicting that the activation of sodium efflux was not due to an enhancement of sodium: sodium exchange activity. Although sodium efflux occurred in the presence of saturating concentrations of ouabain, or in the absence of potassium, activation of the sodium pump by CPZ was considered as a possible mechanism. However, CPZ was seen to decrease intracellular potassium, along with sodium, and inhibit ATPase activity in liver and kidney homogenates, therefore, it was considered unlikely that the sodium pump was involved in the activation of sodium efflux by CPZ.

The involvement of an energy requiring system, such as an active transporter was investigated according to four specific attributes: 1) sensitivity to metabolic inhibition, 2) dependence on cellular oxygen consumption, 3) ability to transport ions against a concentration gradient and 4) temperature sensitivity. A sensitivity of CPZ activated cation transport to treatments which inhibit active transport was determined. Sodium efflux seen in the presence of CPZ, occurs against the natural sodium concentration gradient that exists in living cells. In addition, CPZ activated sodium transport was seen to be completely inhibited by pre-incubation of liver cells in cyanide or by incubation of liver cells in the cold (0°C). In contrast, CPZ activated potassium efflux occurs down a naturally occurring concentration gradient. The decrease in intracellular potassium seen in the presence of CPZ, showed partial sensitivity to cyanide and the cold. However, the CPZ induced potassium efflux was not affected by increasing concentrations of extracellular potassium (5 to 150mM) suggesting, that potassium efflux in the presence of CPZ can occur against an inwardly directed potassium concentration gradient.

A decrease in cell volume following CPZ treatment is demonstrated. However, the decrease in cell volume is not sufficient to account for the decrease in intracellular cations measured following CPZ treatment, as indicated by the decrease in the sodium concentration in CPZ treated cells.

Considering the results of this investigation it is possible that a non-specific cation transport mechanism may exist in liver cells and may be activated by CPZ to lower intracellular sodium and potassium.

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