Somatostatin Receptors Are Biologically Active before They Are Inserted into the Plasma Membrane*

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Abstract
We have examined the biological activity of intracellular somatostatin (SRIF) receptors in cultured rat anterior pituitary cells. We used digitonin-permeabilized cells to introduce free SRIF intracellularly and chloroquine-treated cells to promote intracellular accumulation of SRIF via a receptor-mediated pathway. At a concentration of 0.001%, digitonin (3-min incubation at 37 C) allowed [125I]SRIF to enter the cells without affecting cell viability. Autoradiography of [125I]SRIF demonstrated its association with secretion vesicles (28%), nuclei (25), and other intracellular organelles. An acid wash technique that removes cell surface-bound ligand revealed that both digitonin-permeabilized cells and chloroquine-treated cells accumulated approximately twice as much intracellular SRIF as did control cells. The biological activities of intracellular SRIF accumulated via two different pathways, receptor mediated and through digitonin-produced pores in the plasma membrane, were different. In chloroquine-treated cells, the accumulation of intracellular SRIF did not result in its additional biological effect. SRIF inhibited GH-releasing factor-induced GH release from 578 .+-. 12 to 168 .+-. 9 ng/106 cells .cntdot. 30 min, which did not differ from the control value. Cells incubated with digitonin demonstrated normal basal (160 .+-. 9 ng/106 cells .cntdot. 30 min) and GH-releasing factor-stimulated GH release (564 .+-. 11 ng/106 cells .cntdot. 30 min). However, the inhibitory action of SRIF in these cells was approximately 30% greater (98 .+-. 8 ng/106 cells .cntdot. 30 min) than that in either control or chloroquine-treated cells, suggesting that SRIF freely admitted intracellularly produces additional biological activity. These observations confirm the presence of the intracellular receptors and suggest that these receptors exist in a biologically active form.