The kidney is the target for a wide variety of chemical agents, including heavy metals, haloalkenes, analgesics and antibiotics. The functional and metabolic characteristics of the proximal tubule (PT) predispose it as the primary site for xenobiotic damage. The aim of this study was to isolate and characterise human and rat PT cells in suspension and primary culture for use as defined models to investigate drug-induced PT cell damage in vitro . A second aim was to compare the response of human and rat systems to known nephrotoxins. Human and rat PT cells (90&'37 viable) were isolated from kidney cortex by collagenase digestion followed by isopycnic Percoll density centrifugation. This resulted in the formation of two distinct bands of cell at densities 1.040g/ml (A) and 1.060g/ml (B) for both preparations. Characterisation of human cells in terms of morphology, marker enzymes, retention of active transport systems and responsiveness to parathyroid hormone indicated that &'62 95&'37 of the cells in band B were proximal tubular. Each transport system demonstrated Michaelis-Menten kinetics; kinetic parameters suggested that a higher proportion of PT cells from the S1-S2 segment of the nephron were present in human isolates. Human isolated cells also contained levels of glutathione and cytochrome P450, in particular ethoxyresorufin-O-deethylase activity, a marker for the P4501A family, similar to the intact kidney. Both human and rat cells were successfully cultured in serum-supplemented medium (10&'37 v/v) with human cells reaching confluence by 3-4 days and rat by 5-6. Maximal attachment was seen when cells from both preparations were inoculated onto collagen coated plates with an additional layer of fibronectin. Only human cells were able to reach confluence on porous membranes and demonstated an enhanced morphology when compared to normal cultured cells. Cultured cells from both preparations retained an epithelial morphology and showed minimal secondary cell contamination as shown by light microscopy and in the case of human cultures additionally through immunohistochemical staining. Immunohistochemical staining also demonstrated that human cells in culture were depositing components of the extracellular matrix. The maintenance of PT cell function, throughout the time in culture, was shown following maintenance of active transport systems, in particular the glucose carrier system and on porous membranes the organic anion system. Only rat cells maintained the organic cation system in primary culture. In addition human cells maintained the preferential response to parathyroid hormone. Except for the transport of organic cations, the other carrier systems and responsiveness to hormones were evident at both sub-confluent and confluent stages of cell culture.

Freshly isolated hepatocytes from fed and starved rats were used as a model in the investigation of the mechanisms by which cadmium chloride exerts its toxic effects at the cellular level. Exposure to cadmium chloride resulted in a slight decrease in viability, more pronounced in hepatocytes from starved rats. Morphological changes preceded the increase in membrane permeability. Hepatocytes exhibited a rapid initial uptake of cadmium chloride, followed by a second slower phase. The accumulation of more metal in hepatocytes from starved rats may contribute to their enhanced susceptibility to cadmium chloride. Adverse metabolic effects of cadmium chloride included an increase in the lactate:pyruvate ratio in hepatocytes from fed rats, with a concomitant decrease in the 3-hydroxybutyrate:acetoacetate ratio in hepatocytes from fed and starved rats. Incubation with cadmium chloride resulted in increased glycogenolysis and glycolysis. Decreased rates of gluconeogenesis from lactate and pyruvate reflected the decreased uptake of gluconeogenic precursors. Studies of intracellular lactate concentrations could not resolve whether the decrease in gluconeogenesis was due to an inhibition of lactate transport into the hepatocyte or due to a decrease in its metabolism. Cadmium chloride caused a slight decrease in the basal and pyruvate-stimulated rates of cellular respiration, a marked dose-related decrease with lactate, and no significant effects with succinate. Carbonyl- cyanide-m-chlorophenylhydrazone was less effective in stimulating respiration in hepatocytes incubated with cadmium chloride, this effect being more pronounced with lactate and pyruvate than with succinate. Cadmium chloride had little effect on the uncoupled rates of FADH2 oxidation with succinate suggesting that electron transport from succinate dehydrogenase to cytochrome a/a3 was not impaired. The results from these studies suggest a primary effect of cadmium chloride on mitochondrial function and cellular energy production, resulting in secondary metabolic changes in an attempt to overcome the declining levels of ATP within the cell.

The biological activities of some homeosterically related analogs of dihydrofolic acid have been examined involving pyrimido[4,5-b][l,4]oxazine (8-oxapteridine) derivatives. It is anticipated that these compounds might interfere with the essential intermediary metabolic functions of the vitamin and thus serve as potential chemotherapeutic agents. Preliminary toxicity studies in microbial assay systems were disappointing; however, inhibitory effects were demonstrated in cell free enzyme systems. A comparison of the structure/activity relationships was determined using two folic acid coenzyme systems, dihydrofolate reductase and thymidylate synthetase. The 2-amino-4-hydroxy-6-(substituted)-8-oxapteridines were generally more effective inhibitors than the corresponding 2,4-diamino analogs. The relative biological activity of a series of 2-amino-4-hydroxy-6-ω-phenylalkyl derivatives were examined, and the most active derivative was the 6-phenylethyl analog which appears to function as a mixed-type inhibitor involving partially competitive and partially non-competitive inhibition.