A 13C-n.m.r. investigation of the metabolism of amino acids in renal proximal convoluted tubules of normal and streptozotocin-treated rats and rabbits.

13C-n.m.r. spectroscopy was used to determine the metabolic fate of alanine and aspartate in rat and rabbit kidney proximal tubules. The main purpose of the present study was to investigate the effect of streptozotocin-induced diabetes on the influx of 13C label from [3-13C]alanine into the tricarboxylic acid cycle and through the fructose-1,6-bisphosphatase pathway. This influx was calculated from the relative enrichment of 13C in the various glutamate and glutamine carbon atoms. The relative proportion of 13C label which entered the tricarboxylic acid cycle via pyruvate carboxylase relative to the proportion that entered via pyruvate dehydrogenase was 1.92 +/- 0.02 in fed control rats and 2.27 +/- 0.04 in streptozotocin-treated rats. However, streptozotocin-induced diabetes did not significantly affect this ratio in rabbit proximal convoluted tubular cells. Only in rat proximal convoluted tubular cells did we observe an increase in flux through the fructose-1,6-bisphosphatase pathway by streptozotocin treatment compared with fed controls. The data suggest that streptozotocin-induced diabetes in rats causes the same metabolic changes as does chronic acidosis.