title: Somatostatin receptors are biologically active before they are inserted into the plasma membrane.
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/10(6) cells X 30 min, which did not differ from the control value. Cells incubated with digitonin demonstrated normal basal (160 +/- 9 ng/10(6) cells X 30 min) and GH-releasing factor-stimulated GH release (564 +/- 11 ng/10(6) cells X 30 min). However, the inhibitory action of SRIF in these cells was approximately 30% greater (98 +/- 8 ng/10(6) cells X 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.
authors.forename: C
authors.initials: C
authors.lastname: Steiner
authors.forename: R
authors.initials: R
authors.lastname: Dahl
authors.forename: N
authors.initials: N
authors.lastname: Sherman
authors.forename: M
authors.initials: M
authors.lastname: Trowbridge
authors.forename: A
authors.initials: A
authors.lastname: Vatter
authors.forename: R
authors.initials: R
authors.lastname: Robbins
authors.forename: B
authors.initials: B
authors.lastname: Draznin
mesh_terms: D000818:Animals; D002462:Cell Membrane / Q000378:metabolism*; D002463:Cell Membrane Permeability / Q000187:drug effects; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D002738:Chloroquine / Q000494:pharmacology; D004072:Digitonin / Q000494:pharmacology; D013006:Growth Hormone / Q000378:metabolism; D013007:Growth Hormone-Releasing Hormone / Q000494:pharmacology; D008854:Microscopy, Electron; D009940:Organoids / Q000378:metabolism; D010903:Pituitary Gland, Anterior / Q000378:metabolism* / Q000648:ultrastructure; D051381:Rats; D011956:Receptors, Cell Surface / Q000378:metabolism*; D017481:Receptors, Somatostatin; D013004:Somatostatin / Q000378:metabolism / Q000494:pharmacology
chemical_list: D011956:Receptors, Cell Surface; D017481:Receptors, Somatostatin; D013004:Somatostatin; D002738:Chloroquine; D013006:Growth Hormone; D013007:Growth Hormone-Releasing Hormone; D004072:Digitonin