Regulation of H+ and K+ Gradients by In Vitro 3,5-Diiodothyronine in Hepatocyte Explants of Hypoxic Air-Breathing Fish Anabas testudineus Bloch

Thyroid hormone metabolite 3, 5-diiodothyronine (T2) has been shown to possess physiological actions in vertebrates including fishes. It is, however, not certain if T2 has a role in cation transport in fish hepatocytes, particularly in a stressed condition. We, therefore, tested the in vitro action of T2 on the activities of ion transporters such as Na+/K+ ATPase, H+/ K+ ATPase, Na+/NH4 + ATPase, vacuolar H+-ATPase, Plasma Membrane Ca2+ ATPase (PMCA), mitochondrial Ca2+ and mitochondrial H+-ATPase as these ATPases are known for their roles in maintaining systemic and cellular cation gradients including proton and potassium gradients. Hepatocyte explants of air-breathing fish (Anabas testudineus, Bloch), either in non-stressed or hypoxic condition, were incubated with varied doses of T2 (10-9, 10-8 and 10-7 M) for 15 min and the specific activities of these cation-dependent ATPases were analyzed. We found that T2 exposure evoked higher sensitivity to vacuolar and mitochondrial H+-ATPases and H+/K+ ATPase and not to PMCA or mitochondrial Ca2+ ATPase. The data also indicated that T2 has a similar sensitivity to vacuolar and mitochondrial H+-ATPases and H+/K+ ATPase in the hepatocytes of both non-stressed and hypoxia-stressed fish. The data thus provide evidence for a direct action of T2 on the regulation of proton and potassium gradients in the hepatocytes of both non-stressed and hypoxicair-breathing fish.