Lack of effect of low [Ca²⁺], La³⁺, and pyrazinoate on urate transport by isolated, perfused snake renal tubules

Effects of low medium calcium concentration, of lanthanum, and of pyrazinoate on urate transport by isolated, perfused snake (Thamnophis spp.) distal-proximal renal tubules were studied. Removal of calcium from perfusate with 0.18 mmol/l calcium in bathing medium had no effect on net urate secretion (J _urate ^net ) or on net fluid absorption (J _v). In the presence of calcium (1.8 mmol/l), lanthanum (2.0 mmol/l) in perfusate alone, in bathing medium alone, or in both perfusate and bathing medium had no effect on J _urate ^net . These findings suggest that urate transport, in contrast to para-aminohippurate (PAH) transport, is not sensitive to calcium entry into the cells and support the concept that urate and PAH are transported by separate mechanisms in these renal tubules. Pyrazinoate (1.0 mmol/l) in the bathing medium had no effect on J _urate ^net or J _v. These findings do not support the idea of a primary urate secretory process uniquely sensitive to pyrazinoate among the vertebrates.