Effects of L-Thyroxine and L-Triiodothyronine on Protein and Nucleic Acid Contents of Liver of 6N-2-Propylthiouracil Treated Hypothyroid Singi Fish,Heteropneustes fossilisBloch

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Abstract
Three consecutive injections of 12.5 × 10-10 and 25 × 10-10 moles/g of L-thyroxine (T4) or a single injection of L-triiodothyronine (T3) at 7.5 × 10-10 moles/g to Singi fish caused an increase in liver protein and RNA contents, whereas similar injections of 50 × 10-10 moles/g of T4 or 75 × 10-10 moles/g of T3 caused a fall in these cellular constituents in liver. Treatments of Singi fish with thiourea (1 mg/ml) for 30 days caused a fall in the protein and RNA contents in liver which were restored to the euthyroid control level by a single injection of 7.5 × 10-10 moles/g of T3 or three consecutive injections of T4 at 12.5 × 10-10 moles/g dose. Administration of T4 (12.5 × 10-10 moles/g, three consecutive injections) along with 6N-2-propylthiouracil (PTU) at 20 μg/g of b. w. in six consecutive injections to the thiourea treated (hypothyroid) fish failed to cause any change in hepatic protein and RNA contents in comparison to only PTU-treated hypothyroid fish, but a single injection of 7.5 × 10-10 moles/g of T3 to the PTU-treated hypothyroid fish increased these cellular constituents of liver. A dose-dependent biphasic nature of thyroid hormone action, a higher potency of T3 than T4 and the probable ‘prohormone’ nature of T4 have been documented in case of Singi fish in the present experiments.

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