Renal excretion of chlorphenol red and related organic acids in the intact flounder Pseudopleuronectes americanus

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Abstract
Renal clearance experiments were performed on unanesthetized winter flounder from which bladder urine was collected continuously and caudal vein blood was sampled periodically; renal tissue was also obtained terminally for comparison of test organic acid content in vivo and after incubation in vitro. Urine flow rates and inulin U/P (urine to plasma concentration) ratios were relatively constant for a given fish and averaged 1.0 ml/hr × kg fish and 2.6, respectively. In contrast, U/P ratios for all three test acids cycled from minima of near 100 to maxima of over 1000 roughly every 24 hr when plasma concentrations of unbound acid were below 1 μM; correction of plasma protein binding was required in the case of chlorphenol red, but not PAH or Diodrast. Both in vivo and in vitro the organic acid content of renal tissue was intermediate between plasma and urine concentrations. These results demonstrate that kidneys of intact flounder exhibit the remarkable concentrative capacity for exogenous organic acids previously observed with isolated tubules and suggest that the tubular urine concentration is established in two steps by cell transport first at the peritubular and second at the luminal membrane. The anterior kidney position and the magnitude of maximal PAH and Diodrast clearances, about 1000 ml/hr × kg flounder, are consistent with most of the cardiac output returning to the heart through the renal portal circulation; a regulatory shunt bypassing the peritubular capillaries is proposed to explain cycling of organic acid clearances to minimal values.