Journal of Applied Physiology, Vol 51, Issue 5 1157-1161, Copyright © 1981 by American Physiological Society
Proposed mechanism for increased thyroxine deiodination in cold-acclimated rats
J. G. Scammell, C. C. Barney and M. J. Fregly
Groups of male rats were administered isoproterenol (ISO), thyroxine (T4),
or both (ISO + T4) daily for 20 days in an attempt to mimic the effect of
cold acclimation on the rate of outer-ring deiodination of T4 to
triiodothyronine (T3) by 2,000-g supernatants of homogenates of liver and
kidney. The rates of hepatic and renal deiodination of T4 to T3 in an
additional group of rats exposed to 4 +/- 1 degree C for 20 days were 53
and 71% higher, respectively, than control. Administration of ISO (100
micrograms . kg-1 . day-1) did not affect the rate of deiodination of T4 to
T3 by either hepatic or renal tissue. Administration of T4 (50 micrograms .
kg-1 . day-1) resulted in rates of hepatic and renal deiodination of T4
that were 297 and 222% higher, respectively, than control. Administration
of ISO + T4 resulted in a rate of conversion of T4 to T3 not significantly
different from that observed when T4 was administered alone. Serum
concentration of T4 was elevated after administration of both T4 and ISO +
T4, whereas serum concentration of T3 was elevated significantly above that
of control only in the cold-acclimated group. These results suggest that
the increased rate of 5'-monodeiodination of T4 by hepatic and renal
homogenates from cold-acclimated rats is not a result of increased
beta-adrenergic activity but can be accounted for by the increase in
thyroid activity observed in these animals.
