Interactions of urea transport and synthesis in hepatocytes of the gulf toadfish, Opsanus beta

Although urea transport is receiving increased attention in mammalian systems, little is known about urea transport in fish tissues. Recently, we identified a phloretin-sensitive urea efflux pathway in hepatocytes of gulf toadfish (Opsanus beta), a ureogenic teleost. The present study sought to further examine this transport system and its potential interrelation with metabolic urea production. The transport inhibitors phloretin and 1-(3,4-dichlorophenyl)-2-thiourea (DCPTU) both had substantial inhibitory effects on hepatocyte urea production rates. These effects could be at least partially explained by inhibition of glutamate dehydrogenase (by phloretin) and argininosuccinate synthetase (by DCPTU) activities in vitro. Whereas phloretin substantially inhibited the efflux transport of urea, DCPTU did not. The metabolic effects of phloretin could be nearly eliminated by preparing hepatocytes in the absence of hyaluronidase, enabling its transport effects to be studied in isolation. In so doing we found that transporter inhibition did not lead to a substantial short-term buildup of intracellular urea concentration in hepatocytes. These results are discussed in the context of in vitro vs in vivo rates and pathways of urea transport or excretion.