ALTERATIONS OF ION-DEPENDENT ATPASE ACTIVITIES IN THE BRAIN OF SINGI FISH, HETEROPNEUSTES-FOSSILIS (BLOCH), BY TRIIODOTHYRONINE

The activities of Na+K+- and Mg2+-ATPases in mitochondrial, microsomal, and cytosolic fractions of Singi fish (Heteropneustes fossilis Bloch) brain were investigated after injections of various doses (0.012, 0.025, 0.05 and 0.10 μg/g) of triiodothyronine (T3) for 3 consecutive days. Both ATPases were found in the mitochondrial and microsomal fractions. The cytosolic fraction showed only Mg2+-ATPase activity. Mitochondrial Na+K+-ATPase activity increased to almost the same level in fish treated with 0.025, 0.05, or 0.10 μg of T3/g, while the T3 dose of 0.012 μg/g was ineffective in this respect. Microsomal Na+K+-ATPase activity increased to about the same level with all of the doses of T3 used. No detectable amount of Na+K+-ATPase was found in the brain cytosolic fraction. Mitochondrial Mg2+-ATPase activity was enhanced with 0.025, 0.05, and 0.10 μg of T3/g. The last dose, however, produced a higher increase in activity than the other two doses. Surprisingly, microsomal and cytosolic Mg2+-ATPase activity was not increased by T3 treatment. Although T3 concentrations rose sharply after each T3 injection, the serum T3 level in T3-injected fish was not different from that in the control as observed on the fourth day. The T3-induced rise of Na+K+- and Mg2+-ATPase activities was inhibited by cycloheximide treatment. Immersion of Singi fishes in thiourea significantly reduced brain Na+K+-ATPase activity in microsomal and mitochondrial fractions but decreased Mg2+-ATPase activity only in the mitochondrial fraction. Three consecutive daily injections of T3 (0.10 μg/g) into the thioureatreated fishes increased their ATPase activities even beyond the control level. Our results thus indicate that T3 has a controlling influence on the ATPase system in fish brain. © 1992.