Salinity adaptation in fish: effect of thyroxine on mitochondrial status.

Upon transfer of the fresh-water fish, Sarotherodon mossambicus, to 50% sea water, extensive changes take place in the functions of the gill mitochondria. The changes are (i) loss of ADP/O and RCI; (ii) loss of the ability to contract upon addition of ATP-Mg2+; (iii) lowered energy-dependent 45Ca uptake; (iv) increased amino acid incorporation capacity; (v) increased adenine nucleotide content; and (vi) a higher endogenous Ca2+ content. Administration of thyroxine to the fish reversed these changes, and the effect of thyroxine was also not transient. It is suggested that thyroxine promotes mitochondriogenesis, thereby effecting a restoration of the stress-affected mitochondrial functions.