LUMINAL TRANSPORT STEP OF PARA-AMINOHIPPURATE (PAH) - TRANSPORT FROM PAH-LOADED PROXIMAL TUBULAR CELLS INTO THE TUBULAR LUMEN OF THE RAT-KIDNEY IN-VIVO

Proximal tubular cells were loaded for 10 s with [H-3]para-aminohippur ate ([H-3]PAH) by microperfusing the peritubular capillaries with Ring er solution containing 0.05 mmol/l PAH. Immediately thereafter [H-3]PA H influx from cells into a column of equilibrium solution injected int o the oil-filled tubular lumen was measured by re-aspirating the fluid after 1-10 s of contact time. The rise of luminal PAH concentration w ithin 2 s of contact time was almost linear, reaching a luminal ! capi llary concentation ratio of 1.6 after 2 s and of 3.2 after 5 s. The 2- s PAH concentration ratio was not changed when different manoeuvres we re applied to depolarize proximal tubular cells. Also, the 2-s PAH con centration ratio was not influenced by varying the luminal pH from 6.0 to 8.0 or the luminal Cl- concentration from zero to 134 mmol/l or wh en either 5 mmol/l urate or 25 mmol/l lactate was in the luminal perfu sate. A decrease in the 2-s PAH concentration ratio, i.e. trans-inhibi tion, was observed when 25 or 50 mmol/l HCO3- (-50%) was in the lumina l perfusate. Trans-inhibition was also seen with 5 mmol/l of the follo wing substituted benzoates: 2-hydroxy-benzoate (-58%), 2-methoxy-benzo ate (-46%), 2-hydroxy-benzoate-acetyl ester (-36%), 2-hydroxy-3,5-dini tro-benzoate (-48%), 3,5-dichlorobenzoate (-49%), and 2,3,5-trichloro- benzoate (-45%). No effect was seen with benzoate, 3-hydroxy-benzoate, 2-chloro-benzoate, 2-nitro-benzoate, 2,5-dinitro-benzoate, 3-sulfamoy l-benzoate and 4-sulfamoyl-benzoate. However, analogues of the latter two compounds possessing two additional side groups, such as furosemid e and piretanide, or a hydrophobic moiety, such as probenecid, were in hibitory (by -62, -41 and -49% respectively). Phenoxyacetate had no ef fect; however, it inhibited if in addition it had three chloro groups, as in 2,4,5-trichlorophenoxyacetate (-71%) or a hydrophobic carbamoyl side group, as in mersalylic acid (salyrgan, -75%). Benzene-sulfonate trans-inhibited (-33%), as did phenolsulfonphthalein (phenol red, -39 %) and sulfofluorescein (-55%). However, the trans-inhibitory effect o f the corresponding carboxy-compounds was absent (phenolphthalein) or weaker (fluorescein, -42%). The trans-inhibitory effect of the uricosu rics ethacrynic acid (-53%), tienilic acid (-55%) indacrinone (-72%) a nd benzbromarone (-42%) could be attributed to two chloro or bromo sid e groups on the benzene ring. Other trans-inhibiting uricosuric substa nces were indomethacin (-42%), sulfinpyrazone (-38%), losartan (-80%) its metabolite EXP 3174 (-55%), and AA 193 (-65%). These organic acids , with pKa values between 2.8 and 4.9, possess chloro and sulfin group s, as well as heterocyclic 5-ring and hydrophobic ring or chain areas. No significant effect was seen with 5 mmol/l PAH, a-oxo-glutarate, LI DS, cGMP, prostaglandin E-2, cortisol, benzylamiloride, pyrazinoic aci d and 25 mmol/l lactate. Our data indicate that in situ the secretory luminal PAH transport proceeds in a non-rheogenic fashion, per exclusi onem by anion exchange. The observed trans-inhibition of PAH secretion seems to correlate with the affinity for the luminal PAH transporter and, for uricosuric substances, with their uricosuric potency.