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.