Somatostatin receptors are biologically active before they are inserted into the plasma membrane

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 receptormediated 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⋅30 min, which did not differ from the control value. Cells incubated with digitonin demonstrated normal basal (160 ± 9 ng/10⁶ cells ⋅ 30 min) and GH-releasing factorstimulated GH release (564 ± 11 ng/10⁶ cells⋅30 min). However, the inhibitory action of SRIF in these cells was approximately 30% greater (98 ± 8 ng/10⁶ cells⋅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.