Ionic composition of urate-containing spheres in the urine of domestic fowl

Birds excrete urate in their urine in the form of small spherical concretions. In addition to urate, these concretions contain protein and inorganic ions. Energy dispersive x-ray microanalysis was used to determine the inorganic ion composition of the urate-containing spheres to gain a better understanding of how the spheres are formed. Ureteral urine was collected from six female white leghorn domestic fowl, Gallus gallus. The urine was filtered to separate the spheres into size categories to determine if the ionic composition varied with the size of the spheres. The spheres were placed on scanning electron microscope stubs, dried and coated with carbon. All samples were examined at an accelerated voltage (kV) of 20. The results showed that in all birds, the spheres contained the ions calcium and potassium (approximately 70% and 30%, respectively, of the total inorganic ions present). However, in one bird, the percent of calcium and potassium was reversed (i.e., calcium 30% and potassium 70%). In all birds, chloride and magnesium were also detected, but in comparatively small amounts (less than 1%). There were no significant differences in the ion content of spheres of different sizes. These data suggest that calcium and potassium may play an important role in the formation of the spheres and are not random inclusions as the spheres are formed. We suggest that the spheres form to facilitate the excretion of urate without blocking or damaging the renal tubules. (C) 1997 Elsevier Science Inc.