Micropuncture study of urate transport by superficial nephrons in avian (Sturnus vulgaris) kidney.

Tubule fluid samples from proximal tubules of superficial, reptilian-type nephrons of the European starling (Sturnus vulgaris) were obtained by micropuncture and, along with plasma samples, were analyzed for urate by high performance liquid chromatography with electrochemical detection, as well as for the elements sodium, chloride, calcium, magnesium, phosphorus and potassium by electron microprobe analysis. All proximal tubules sampled showed net secretion of urate, but the magnitude of transport was highly variable. Mean fractional delivery of urate to the point of micropuncture was 4.04 +/- 0.37 (mean +/- SE, n = 40 tubules). Fractional delivery of urate could not be correlated with fractional delivery of any of the measured elements or with relative fluid absorption. Samples collected from surface segments of cortical collecting ducts suggest that, although much net secretion of urate occurs early within the proximal tubule of superficial nephrons, some additional secretion may occur in more distal segments. Fractional excretion of urate at the level of the cloaca averaged 4.72 +/- 0.97 for nine birds, confirming the result of overall net secretion of urate previously reported for other avian species. These results clearly demonstrate the presence of significant secretory transport of urate in the proximal tubule of superficial reptilian-type nephrons, but do not rule out the possibility of additional secretory or reabsorptive fluxes at other sites.