Somatostatin discriminates between the intracellular pathways of secretory and membrane proteins.

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Abstract
Somatostatin is a 14-amino acid peptide hormone that inhibits the secretion of a variety of other polypeptide hormones, including growth hormone. An experimental system used to determine whether somatostatin can discriminate in its inhibition between secretory and plasma membrane proteins is described. Growth hormone-secreting cells (GH3) [rat pituitary tumor] were infected with vesicular stomatitis virus and pulse-chased with [35S]methionine to follow the simultaneous intracellular transit of growth hormone and the viral membrane glycoprotein, G protein. Secretion of growth hormone was monitored by immunoprecipitation of chase media, while appearance of G protein on the plasma membrane was detected by cell surface labeling and virus purification. In the presence of somatostatin (10 .mu.g/ml), the secretion of growth hormone was inhibited by 80%. In contrast, G protein appeared on the plasma membrane with slightly enhanced kinetics. When cells were treated with the ionophore monensin (0.2 .mu.M), there was a dramatic inhibition of both the secretion of growth hormone and the incorporation of G protein into plasma membranes. Results on the differential effect of somatostatin provide evidence for sorting of secretory and membrane proteins into distinct compartments in the secretory pathway. This sorting event apparently occurs late in the Golgi complex or after proteins exit from that organelle.