THE APPEARANCE, DISTRIBUTION, AND LONGEVITY OF RECEPTOR [I-125] T-3 COMPLEXES WITHIN THE NUCLEI OF ISOLATED RAT HEPATOCYTES

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.