A B S T R A C T Somatostatin (SRIF) is localized in the hypothalamus, extrahypothalamic brain, and throughout the gastrointestinal tract. Release of gastrointestinal SRIF-like immunoreactivity (SRIF-LI) is under nutrient regulation but the effect of nutrients on neural SRIF-LI is unknown. The present studies examined the effects of glucose uptake and metabolism and hormones influencing glucose disposition on SRIF-LI release from medial basal hypothalamus (MBH) and cerebral cortex (Cx) incubated in Krebs-Ringer bicarbonate containing bacitracin. After a preincubation to achieve stable secretion, tissues were incubated for 20 min in 14 mM glucose (basal) and then, for 20 min in fresh medium with test materials. MBH SRIF-LI release was inversely related to medium glucose concentration with release in the absence of glucose (235±42 pg/MBH per 20 min) more than five times that in the presence of 25 mM glucose (46±4 pg/20 min). In the presence of 14 mM glucose MBH SRIF-LI release was stimulated above basal by agents interfering with glucose uptake including 3-0-methyl-D-glucose (42 mM Cx SRIF-LI release was slightly inhibited by decreases in medium glucose and unaffected by inhibition of glucose uptake or metabolism. These results provide evidence for nutrient regulation of MBH but not Cx SRIF-LI release and may explain inhibition of growth hormone seen in the rat in response to hypoglycemia.Insulin (10 nM-1 gM) stimulated MBH but not Cx SRIF-LI release while glucagon was without effect. Our previous demonstration that MBH SRIF-LI release was stimulated by somatomedin-C, but not insulin at physiologic concentrations, is consistent with an action of insulin through the somatomedin-C receptor at the doses studied. Our studies indicate a regional specificity for the control of SRIF secretion within the brain and suggests the possibility of a role for hypothalamic SRIF in metabolic regulation.

To study the incorporation of sulfate into thyroglobulin, human thyroid tissue was incubated with [35S]sulfate. Labeled thyroglobulin was purified by ammonium sulfate precipitation, gel exclusion chromatography, and equilibrium density gradient centrifugation, the last of these specifically to remove proteoglycans. [35S]Sulfate was found in thyroglobulin with low density, indicating that sulfate was incorporated into newly synthesized molecules before their iodination. Chondroitin ABC lyase and chondroitin AC lyase released equal amounts of [35S]sulfate, demonstrating the presence of chondroitin units, and TLC showed this to be predominantly chondroitin 6-sulfate. Additional [35S]sulfate was released by endoglycosidase-F (Endo-F), but not by Endo-H. The Endo-F-sensitive sulfate-labeled complex carbohydrate units were heterogeneous, one fraction passing through a Concanavalin-A-Sepharose column without delay and another fraction showing weak affinity for the lectin. More than 90% of the incorporated [35S]sulfate was accounted for by the chondroitin ABC lyase and Endo-F-sensitive fractions. Thus, human thyroglobulin contains sulfate in at least three types of carbohydrate units, 1) chondroitin 6-sulfate units, 2) complex units with no affinity for Concanavalin-A (tri- or tetraantennary forms), and 3) complex units with weak affinity for Concanavalin-A (biantennary forms).

Human thyroglobulin (TG) is one of a growing number of glycoproteins that are known to contain sulfate. Among these TG is unusual because it contains sulfate on both its asparagine-linked complex carbohydrate units and its single chondroitin 6-SO4 unit. We incubated tissue fragments prepared from normal and neoplastic thyroid tissue with [35S]sulfate to study the incorporation of sulfate into these two types of carbohydrate acceptor sites. Incubation conditions (0.1 mM sulfate for 16 h) were selected that maintained linear incorporation of [35S]sulfate and retention of more than 90% of iodinated TG in the tissue. Enzyme susceptibility was used to determine incorporation into the complex carbohydrate units (endoglycosidase-F) and the chondroitin 6-SO4 unit (chondroitin ABC lyase). In a representative experiment, 29.8% of the incorporated sulfate was found in the chondroitin 6-SO4 unit during the first hour of incubation. This increased progressively to 72.5% in the chondroitin unit during the incubation period from 8-16 h. A reciprocal decrease occurred in the proportion of sulfate incorporated into the complex carbohydrate units. TG released into the medium and retained in the tissue had the same ratio of sulfate in the two types of carbohydrate units. Neoplastic thyroid tissue incorporated more [35S]sulfate into TG than normal thyroid tissue from the same patient. Neoplastic and normal tissue differed further in the ratios of sulfate incorporated into the two types of carbohydrate units. We conclude that the incorporation of sulfate into the two types of sulfate-containing carbohydrate units of TG does not occur in a fixed ratio and that this differential incorporation of sulfate does not appear to be related to its release from the tissue.

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