Matrix extracellular phosphoglycoprotein (MEPE): a putative phosphatonin

The introduction to the thesis discusses the control of phosphate balance and considers recent developments in the field. The concept of novel circulating substances which influence, and perhaps regulate, plasma phosphate (‘phosphatonins’), and which are important in tumour-induced hypophosphataemia, hereditary hypophosphataemias and, potentially, in normal phosphate physiology, is introduced. Finally, the isolation of the putative phosphatonin MEPE, and the current state of knowledge about this molecule, is described. The initial practical studies deal with the development and validation of a novel micromethod for measurement of phosphate and, simultaneously, other anions in biological fluids, using capillary electrophoresis. The accuracy and reproducibility of the method are demonstrated, and the method is compared with an established micromethod for measurement of individual ions. Finally, capillary electrophoresis is applied to the measurement of phosphate and other anions in urine, plasma and tubular fluid samples. In the next section, renal clearance experiments are performed in anaesthetized rats, in order to assess the effects of intravenously administered MEPE and to compare them with those of parathyroid hormone. MEPE is shown to be markedly phosphaturic. It does not alter glomerular filtration rate, blood pressure, plasma phosphate concentration or filtered phosphate load, but does cause large, dose-dependent increases in absolute and fractional phosphate excretion. The final series of experiments uses micropuncture methods in anaesthetized rats, in which fluid is collected from individual proximal convoluted tubules. The phosphaturic effect of MEPE is confirmed, and is shown to result, at least in part, from a reduction in fractional phosphate reabsorption in the proximal convoluted tubule, without a change in filtered phosphate load. The results obtained in these studies are discussed in relation to previous knowledge