The In Vitro Generation of Avian Urate-Containing Spherules

Abstract

Uricotelic animals such as birds excrete approximately 70% of their waste nitrogen in the form of uric acid. This could be a concern due to the low aqueous solubility of uric acid and its salts, yet crystals and stones rarely form in the avian kidney. Our objective was to generate in vitro the urate-containing spherules found in avian urine. A dialysis membrane with a molecular weight cut off of 100 Da was used in chambers to separate two solutions. On one side of the membrane was a solution mimicking avian glomerular filtrate and on the other was solution mimicking avian plasma. The artificial plasma had an osmotic potential higher than that of the filtrate to draw fluid from the filtrate side and thus concentrate the uric acid on the filtrate side. Three permutations and a control were conducted to examine the effect of calcium and cholesterol on sphere formation. The increase in uric acid concentration lead to the formation of spheres about 1 micron in diameter in the permutations that included calcium. It is not certain whether cholesterol contributes to the spheres. However, it appears that divalent cations such as calcium are necessary for the formation of the urate and protein-containing spheres.