Modification of nucleic acid levels per mitochondrion induced by thyroidectomy or triiodothyronine administration.

The authors have determined the liver mitochondrial population (number of mitochondria/nucleus) in young rats, which has been thyroidectomized (T) or thyroidectomized and subsequently treated with triiodothyronine (T3). They have observed that thyroidectomy decreased such a population to 72.3% with respect to the normal one, while the T3 administration (at the dose of 10 mug/100 g body weight every second day, from day 50 to day 60 of age) restored the mitochondria number to 81.8% of normal ones. The average levels of proteins per mitochondrion were 8.90 X 10(-13) g in the liver of normal 60-day-old rats. This content was doubled in T rats of the same age while the levels of nucleic acids or the nucleic acid polymerase activities per mitochondrion were enhanced, notwithstanding that the specific values (referred to mg mitochondrial protein) decreased. The T3 administration severely lowered the content of protein per mitochondrion, and this may indicate that thyroid hormones control the normal assemblage of mitochondrial protein.