Determinants of the renal handling of 2, 4-dichlorophenoxyacetic acid by winter flounder

The factors determining the renal handling of 2,4-dichlorophenoxyacetic acid (2,4-D) were examined in winter flounder, Pseudopleuronectes americanus, using isolated tubules and clearance techniques. In vitro, extensive energy-dependent uptake was seen with tissue/medium ratios of 30-fold at 1 micronM 2,4-D. The velocity of uptake was concentration-dependent with apparent Km and Vmax v,lues of 70 micronM and 3.6 micronmol/g of tubules per hr, respectively. Uptake was inhibited by other organic acids and 2,4-D competitively inhibited p-aminohippurate uptake. 2,4-D did not inhibit organic cation transport by the tubules. In vivo, 2,4-D was actively secreted with clearances of nearly 500 times the glomerular filtration rate at 1 micronM 2,4-D in plasma. At higher plasma concentrations (10-60 micronM) a transport maximum of 0.85 micronmol/g of kidney per hr was observed. Secretion was inhibited by other organic acids. 2,4-D also inhibited p-aminohippurate secretion in vivo. Little metabolism was noted; approximately 10% was excreted as the taurine conjugate. Plasma binding was 70%. Examination of the effects of added proteins on in vitro uptake showed that protein binding could limit 2,4-D transport but that flounder plasma (low in albumin) was far less effective than bovine serum albumin in binding and inhibition of transport. The roles of plasma binding, intracellular binding and metabolism in determining the rate of 2,4-D elimination by the kidney are discussed.