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The in vitro Generation of Avian Urate‐Containing Spherules

Uricotelic animals such as birds excrete nitrogen in the form of urate‐containing spherules to prevent uric acid crystallization in the nephron. This could be a concern for uricotelic animals due to the low aqueous solubility of uric acid and its salts. 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 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 a 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. Three permutations and a control were run 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. The results suggest that calcium is necessary for sphere formation and that cholesterol is not. It appears that divalent cations are necessary for the formation of the urate and protein‐containing spheres.