Factors influencing the affinity and capacity of T3-binding sites in hepatic nuclei of rainbow trout,Oncorhynchus mykiss

Abstract

The effects of several variables were tested on the in vitro binding affinity (Ka) and maximum binding capacity (MBC) of saturable 3,5,3′-triiodo-L-thyronine (T3)-binding sites (putative thyroid hormone receptors) extracted from hepatic nuclei of immature rainbow trout held on a 12L:12D photocycle at 12°C. Neither Ka nor MBC varied during the daily photocycle, nor were they influenced by the growth rate, food ration, or a 13-fold chronic elevation in plasma T3. However, MBC was inversely correlated with fish size and was depressed significantly by two injections of 17β-estradiol (E2) or starvation for 3 or 12 days. The Ka was uninfluenced in either case. Since both E2 and starvation also depress T3 levels in the blood, the decrease in MBC could be secondary to this change. The drug ipodate, which depresses plasma T3, also lowered the MBC. However, MBC remained low in ipodate-treated trout given exogenous T3, thus suggesting that the effect of ipodate was not due to T3 deprivation. It remains to be shown, therefore, that plasma T3 depression itself can modify MBC. It is concluded that increasing body size and catabolic states, both of which are associated with a decrease in specific growth rate, tend to decrease the abundance of hepatic T3 receptors.