Effects of plasma total ammonia content and pH on urea excretion in Nile tilapia

Nile tilapia (Oreochromis niloticus) were infused with ammonium salts, acid , and base to investigate the effects of changes in arterial plasma total a mmonia content (T-amm) and pH (pHa) on plasma urea-nitrogen (urea-N) levels and urea-N excretory fluxes (J(urea-N)). The tilapia did not possess a fun ctional hepatic ornithine urea-cycle (no significant carbamyl phosphate syn thetase III activity). Infused substances were dissolved in a saline vehicl e and injected twice (5 mL kg(-1)), the first infusion to "prime" the anima l and promote a more marked response to the second infusion, given 2.5 h la ter. The results reported are those of the second infusion. Infusion of 200 mM NH4Cl increased T-amm, reduced pHa, and increased plasma urea-N and J(u rea-N). Two hundred mM NH4HCO3 increased T-amm and arterial plasma total CO 2 content (Ta-CO2), reduced pHa, and increased J(urea-N). Fifty mM HCl redu ced pHa but had no effects on urea dynamics. Fifty mM NaOH increased pHa, p lasma urea-N levels, and J(urea-N). Two hundred mM NaHCO3 increased pHa, Ta -CO2, plasma urea-N levels, and J(urea-N). Infusion of the saline vehicle w as without effect. The results indicate that ammonia loading and plasma alk alosis both stimulate urea excretion in uricolytic fish. The responses to h yperammonemia or alkalosis were not modified when combined with elevated pl asma bicarbonate levels.