Glycolate and glyoxylate, documented lithogenic precursors of oxalate, were administered acutely and chronically to Wistar-strain rats in order to study their effects on oxalate excretion and subsequent stone formation. Urinary oxalate increased significantly within 4 hours, with a maximum being reached between 4-8 hours after a single administration of glycolic acid (200mg) or glyoxylic acid (200mg). The 24-hour increase in urinary oxalate was about 3% of each amount given. Hyperoxaluria developed immediately and persisted throughout the experimental period in all the rats, which were fed on a diet containing glycolic acid or glyoxylic acid at a 3% level. Microscopically amorphous substances accumulated in the renal tubules at one week. Significant crystal formation appeared in the tubules after two weeks in both experimental groups and consistently increased both in number and in volume until the 4th week.
Therefore, the oral administration of either glycolate or glyoxylate increased urinary oxalate comparably much within a few hours, but a few weeks of hyperoxaluria may be necessary to develop crystals in the convoluted tubules.