Intracellular Transport of 3,5,3′-Triiodo-L-Thyronine in Rat Skeletal Myoblasts*

Publisher Website
Google Scholar
Abstract
Since in skeletal muscle circulating L-T3 is the only source for the hormone bound to nuclei, we investigated the intracellular and intranuclear transport of L-T3 in L6E9 rat skeletal muscle cells. The characteristics of this process were assessed by analyzing the nuclear bound L-T3 as a marker of the internalized hormone and by determining the initial rat of L-T3 uptake. [125I]L-T3 cellular uptake at 37.degree. C reached a plateau at 2 h when the nuclear uptake, after an initial lag phase, was still increasing and represented 4.7% of the cellular uptake. Incubation at 4.degree. C caused [125I]L-T3 cellular uptake to decrease by 77% and nuclear uptake to be abolished. A similar effect on [125]L-T3 nuclear uptake was obtained after myoblasts were incubated at 37.degree. C with a 1000-fold excess of unlabeled L-T3. The addition of various inhibitors of ATP production, cytoskeleton integrity, endocytosis, and Na+, K+-ATPase that did not interfere with [125I]L-T3 binding to the cell surface or to isolated nuclei caused a dose-dependent reduction of both extranuclear and nuclear uptake, ranging from 34-85%. Scatchard analysis revealed the presence on the myoblast surface of L-T3 high affinity (Kd = 1.6 .times. 109 M-1) and low affinity (Ka = 5.4 .times. 106 M-1) binding sites; other iodothyronines exhibited lower affinity for both sites. Kinetic analysis of L-T3 transport after 60-sec incubation at 23.degree. C defined a process with a Km of 17 .+-. 5.6 nM and a maximum velocity of 83 .+-. 35 pmol/mg DNA. These results indicate the existence in rat myoblasts of a temperature-dependent, energy-requiring, saturable, and stereospecific L-T3 uptake mechanism, probably mediated through an endocytotic pathway, located on the myoblast plasma membrane, that may regulate L-T3 action in skeletal muscle.

This publication has 41 references indexed in Scilit: