Adaptation of Liver Membrane Somatogenic and Lactogenic Growth Hormone (GH) Binding to the Spontaneous Pulsation of GH Secretion in the Male Rat

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Abstract
The secretory pattern of GH is pulsatile in both man and experimental animals. The present study was undertaken to investigate the effect of the endogenous pulsatility of serum GH on the dynamics of plasma membrane GH binding sites. In order to characterize somatogenic and total specific binding, male rats 45 days old were decapitated at 15-min intervals, and rat liver membranes were prepared. Binding studies were done in parallel with and without excess ovine PRL, 1 μg/tube. Removal of endogenous ligand was performed by exposing the membranes to 3 M MgCl2. A composite picture of serum GH showed relatively good synchronization of the secretory episodes among the animals. Peak levels of the spontaneous secretion pulse of rat GH were followed by an immediate decrease in free somatogenic binding. Two hours later the binding activity returned to presecretion levels, essentially resetting the cell for another cycle. The kinetics of desaturated somatogenic binding were similar. The occupancy of liver GH somatogenic binding sites was maximal at the time of peak serum GH. High levels of the endogenous hormone caused an immediate sharp down-regulation of lactogenic desaturated binding. Up-regulation of the binding sites occurred about 90 min later. Scatchard analysis showed that binding activity of the membranes was a result of the altered number of binding sites, while the binding affinity remained fairly constant. Thus, the centrally regulated GH secretion in the male rat is complemented by appropriate dynamics of the GH receptor turnover, which in turn recognizes individual pulses and allows individual pulse-related responses to occur.