The Dynamics of Somatogenic and Lactogenic Growth Hormone Binding: Internalization to Golgi Fractions in the Male Rat

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Abstract
Rapid turnover of the GH receptor has previously been shown, and its turnover (t½) has been estimated to be 30–85 min. In a companion study, we found that lactogenic and somatogenic GH binding sites on rat liver membranes were down-regulated immediately after an episode of GH secretion and reappeared at the plasma membrane in time for the next secretory surge of the hormone. In the present in vivo study, we followed the fate of the down-regulated membrane GH binding sites in the Golgi membranes. Male rats were killed at 1000, 1100, and 1200 h, and their livers were removed for preparation of Golgi membranes. Lactogenic and somatogenic [125I]human GH binding to Golgi membranes was measured. The results of the present study show that Golgi receptors are related to the endogenous pulsation of serum GH. After the GH surge, an increase in the capacity of the lactogenic and somatogenic receptors in the Golgi membranes takes place. Most of these receptors were occupied by ligand and represent, therefore, internalized receptors. Two hours after the GH secretory peak, the occupied receptors had disappeared from the Golgi membranes and appeared in the plasma membrane, suggesting that receptors were recycled to the plasma membrane, awaiting the next GH surge. The following model is proposed: A surge of GH secretion causes immediate down-regulation of the plasma membrane receptors to GH. The receptors that disappear from the plasma membrane are internalized to the Golgi complex. A fraction of the Golgi receptors is recycled to the membrane, along with newly synthesized receptors, awaiting the next GH pulse.