Gl. Pullen et al., THE APPEARANCE, DISTRIBUTION, AND LONGEVITY OF RECEPTOR [I-125] T-3 COMPLEXES WITHIN THE NUCLEI OF ISOLATED RAT HEPATOCYTES, Thyroid, 4(3), 1994, pp. 305-312
The nuclei of isolated rat hepatocytes were separable into three recep
tor compartments based upon their differential salt extractabilities:
nucleoplasmic receptors (NP) extractable with 0.15 M KCI, high-salt ex
tractable receptors (HSE) extractable with 0.4 M KCI, and salt-resista
nt receptors (SR) extractable with 0.4 M KCl/5 mM dithiothreitol. The
receptor distribution among the three compartments was approximately N
P, 45%; HSE, 30%; SR, 25%. The mean percent occupancy with endogenous
T-3 of the SR receptors (86%) was higher than the occupancies of the N
P receptors (68%) and the HSE receptors (63%). When hepatocytes were p
ulsed with 3 nM [I-125]T-3 at 37 degrees C for brief intervals, recept
or-[I-125]T-3 complexes were detectable in all three nuclear compartme
nts within 15 sec. With increasing pulse intervals up to 120 sec, the
receptor content of each nuclear compartment increased progressively a
nd without evidence of preferential accumulation in any of the three c
ompartments. To determine the life span and intercompartmental ''migra
tion'' pattern of nuclear receptors, hepatocytes were pulsed with 3 nM
[I-125]T-3 at 37 degrees C for 2.5 min or 5 min, followed by a chase
with a 500-fold excess of nonlabeled T-3. The population of receptor-[
I-125]T-3 complexes generated during the pulse was serially recovered
at increasing intervals after the chase. The complexes of each compart
ment dissociated with a half-life of approximately 3 min and manifeste
d no predilection to accumulate in any of the compartments, Exposure o
f isolated hepatocytes to 3 nM T-3 for 5 min or 10 min at 37 degrees C
induced no change in the gross intercompartmental distribution of rec
eptors compared to control hepatocytes incubated without T-3. These st
udies support the concept that extracellular T-3 is rapidly taken up b
y the hepatocyte and enters the nucleus well within 1 min. Within the
nucleus, the T-3 has immediate access to the unoccupied receptors of e
ach compartment. The receptor-T-3 complexes formed therefrom exist for
only a few minutes (t(1/2) similar or equal to 3 min), during which t
ime there is no net shift of the complexes from one compartment to ano
ther. Exposure to T-3 does not appear to drive or influence the gross
intercompartmental receptor distribution.