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Desensitization to growth hormone-releasing factor (GRF) is associated with down-regulation of GRF-binding sites
Abstract:
The time course, concentration dependence, and mechanism of rat anterior pituitary desensitization to GRF were studied. Chronic stimulation of cultures of rat anterior pituitary cells with rat GRF (rGRF) resulted in desensitization to a subsequent challenge with the peptide. Despite a slight enhancement of GH synthesis, prolonged exposure to GRF caused substantial depletion of cellular GH pools. As a result, acute secretory responses were markedly blunted. Depletion was accompanied by a time-dependent decrease in sensitivity to rGRF; GRF EC50 values for GH release of 0.5 nM rGRF-pretreated cells were 24.8 +/- 6 (+/- SEM) pM after 2 h, 46.2 +/- 2.4 pM after 4 h, and 154.7 +/- 31 pM after 8 h compared to 9.2 +/- 0.6 pM for control cells. The process of desensitization was complete within 8 h, as cells pretreated for 24 h exhibited sensitivity to rGRF comparable to that of cells pretreated for 8 h. Desensitization was associated with a time-dependent decrease in rat anterior pituitary cell GRF-binding capacity; a 48% loss of binding sites was evident after a 2-h pretreatment with 0.5 nM rGRF, with a maximum loss occurring after 8 h. The dose of rGRF required to produce an attenuation of responsiveness did not completely correlate with the dose requirement for down-regulation of binding sites. The decrease in GRF-binding sites was not associated with any alteration of apparent Kd values, which were 0.36 (0.18-0.72) nM in control and 0.1 (0.01-0.82) nM after 8 h of exposure to 0.5 nM rGRF. Both the reduction in GRF-binding capacity and decreased sensitivity to GRF were reversible after 24 h, although cellular GH pools were not restored to control levels. These results suggest that rat anterior pituitary cells become desensitized to rGRF after chronic stimulation with a maximal concentration of the peptide. One mechanism for this decrease in apparent sensitivity to rGRF may be the pronounced reduction or down-regulation of GRF-binding sites.