Pre-acclimation to low ammonia improves ammonia handling in common carp (Cyprinus carpio) when exposed subsequently to high environmental ammonia

We tested whether exposing fish to low ammonia concentrations induced acclimation processes and helped fish to tolerate subsequent (sub)lethal ammonia exposure by activating ammonia excretory pathways. Common carp (Cyprinus carpio) were pre-exposed to 0.27 mM ammonia (∼10% 96 h LC50) for 3, 7 and 14 days. Thereafter, each of these pre-exposed and parallel naïve groups were exposed to 1.35 mM high environmental ammonia (HEA, ∼50% 96 h LC50) for 12 h and 48 h to assess the occurrence of ammonia acclimation based on sub-lethal end-points, and to lethal ammonia concentrations (2.7 mM, 96 h LC50) in order to assess improved survival time. Results show that fish pre-exposed to ammonia for 3 and 7 days had a longer survival time than the ammonia naïve fish. However, this effect disappeared after prolonged (14 days) pre-exposure. Ammonia excretion rate (Jamm) was strongly inhibited (or even reversed) in the unacclimated groups during HEA. On the contrary, after 3 days the pre-exposure fish maintained Jamm while after 7 days these pre-acclimated fish were able to increase Jamm efficiently. Again, this effect disappeared after 14 days of pre-acclimation. The efficient ammonia efflux in pre-acclimated fish was associated with the up-regulation of branchial mRNA expression of ammonia transporters and exchangers. Pre-exposure with ammonia for 3-7 days stimulated an increment in the transcript level of gill Rhcg-a and Rhcg-b mRNA relative to the naïve control group and the up-regulation of these two Rhcg homologs was reinforced during subsequent HEA exposure. No effect of pre-exposure was noted for Rhbg. Relative to unacclimated fish, the transcript level of Na+/H+ exchangers (NHE-3) was raised in 3-7 days pre-acclimated fish and remained higher during the subsequent HEA exposure while gill H+-ATPase activities and mRNA levels were not affected by pre-acclimation episodes. Likewise, ammonia pre-acclimated fish with or without HEA exposure displayed pronounced up-regulation in Na+/K+-ATPase activity and mRNA expression relative to the corresponding ammonia naïve groups. Overall, these data suggest that ammonia acclimation was evident for both lethal and the sub-lethal endpoints through priming mechanisms in ammonia excretory transcriptional processes, but these acclimation effects were transient and disappeared after prolonged pre-exposure.