Modulation of Somatostatin Binding To Rat Pituitary Membranes by Exogenously Administered Growth Hormone*

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Abstract
The effect of exogenously administered GH [growth hormone] on somatostatin (SRIF) receptor regulation was studied in rat anterior pituitary membranes. A single class, high affinity specific receptor for SRIF was identified by binding studies with [125I-Tyr11]SRIF [binding capacity (mean .+-. SD), 129.4 .+-. 23.3 fmol/mg protein; binding affinity, 2.8 .+-. 0.6 .times. 1010/M]. A single injection of rat GH (150 .mu.g) caused a significant reduction in capacity, but not in affinity, of SRIF receptors 2 and 6 h after injection (mean decrease, 23% and 24%, respectively) from that in controls. Mean SRIF binding capacity 24 h after a single injection of rat GH was increased 48% above control values, but affinity was unaffected. Measurement of membrane SRIF content indicated that these changes could not be explained by alteration in receptor occupancy. When rat GH was injected repeatedly for 3 days (150 .mu.g/rat .cntdot. day), the mean binding capacity, though not the affinity, of SRIF receptors was decreased 23% from that in controls 24 h after the last injection. The results can be explained by stimulation of SRIF release from the hypothalamus by GH and somatomedins, with subsequent internalization of the pituitary plasma membrane SRIF receptor. They suggest yet another level of neuroendocrine regulation of GH secretion.