Nitric oxide synthase (NOS) inhibitor L-NAME activates inducible NOS/NO system and drives multidimensional regulation of Na+/K+-ATPase in ionocyte epithelia of immersion-stressed air-breathing fish (Anabas testudineus Bloch)

Nitric oxide (NO) has been implicated in Na+ homeostatic control in water-breathing fishes. It is, however, uncertain whether air-breathing fish relies on NO to coordinate Na+/K+-ATPase (NKA)-driven Na+ transport during acute hypoxemia. We. thus, examined the action of nitric oxide synthase (NOS) inhibitor, L-NAME on NO availability, inducible NOS (iNOS) protein abundance and the regulatory dynamics of NKA in osmoregulatory epithelia of Anabas testudineus kept at induced hypoxemia. As expected in nonstressed fish, in vivo L-NAME (100 ng g(-1)) challenge for 30 min declined NO production in serum (40%) and osmoregulatory tissues (average 51.6%). Surprisingly, the magnitude of such reduction was less in hypoxemic fish after L-NAME challenge due to the net gain of NO (average 23.7%) in these tissues. Concurrently, higher iNOS protein abundance was found in branchial and intestinal epithelia of these hypoxemic fish. In nonstressed fish, L-NAME treatment inhibited the NKA activity in branchial and intestinal epithelia while stimulating its activity in renal epithelia. Interestingly in hypoxemic fish, L-NAME challenge restored the hypoxemia-inhibited NKA activity in branchial and renal epithelia. Similar recovery response was evident in the NKA alpha protein abundance in immunoblots and immunofluorescence images of branchial epithelia of these fish. Analysis of Nka alpha 1 isoform transcript abundance (Nka alpha 1a, alpha 1b, alpha 1c) also showed spatial and preferential regulation of Nka alpha 1 isoform switching. Collectively, the data indicate that L-NAME challenge activates iNOS/NO system in the branchial ionocyte epithelia of hypoxemia-stressed Anabas and demands multidimensional regulation of NKA to restore the Na+ transport rate probably to defend against acute hypoxemia.

Automated summary

In acute hypoxemia, L-NAME challenge activates the iNOS/NO system in branchial ionocytes of Anabas, leading to multidimensional regulation of NKA to restore the Na+ transport rate.