Saturable triiodothyronine-binding sites in the pituitary nuclei of salmonid teleost fish

High-affinity, limited-capacity, 3,5,3′-triiodo- L-thyronine (T 3)-binding sites were established by in vitro saturation analysis in cell nuclei of the pituitary gland of arctic charr. The sites were extracted from the purified nuclei using 0.4 M NaCl and incubated with [ 125I]T 3 in the presence of 0.2 M NaCl. T 3 saturable binding attained equilibrium after 18–24 hr of incubation at 4°. The association constant ranged from 6.7 to 20.1 liters mo −1 × 10 9, indicating a T 3 affinity greater than that for T 3-binding sites in rainbow trout liver. The maximal binding capacity ranged from 0.93 to 2.05 10 −13 mol · mg DNA −1, representing a mean site abundance corresponding to 60% of that for nuclei from trout liver. Thyroxine (T 4) completely displaced [ 125I]T 3 in the pituitary nuclei of arctic charr and T 3 completely displaced [ 125I]T 4 in the pituitary nuclei of rainbow trout, suggesting that in salmonids both T 4 and T 3 bind to the same single class of sites. However, the site affinity for T 4 was approximately 20–50x less than that for T 3. The possible roles of these sites in pituitary function as well as their relationship to other nuclear T 3 binding sites in salmonid fish are discussed.