cytochalasin-b and Adenoma--Islet-Cell

Tumoral pancreatic islet cells of the RINm5F line are equipped with two classes of [3H]cytochalasin B binding sites with respective Kd of 0.4 and 7 microM. The binding of the fungal metabolite and its dissociation from the binding sites display rapid time courses. The binding is inhibited by D-glucose, more than by L-glucose, by phlorizin and by cytochalasin E. These findings are considered in the light of the dual action of cytochalasin B upon hexose transport and motile activity in islet cells.

Topics: Adenoma, Islet Cell; Binding Sites; Cytochalasin B; Cytochalasins; Glucose; Islets of Langerhans; Kinetics; Pancreatic Neoplasms; Phlorhizin; Tumor Cells, Cultured

The rat insulinoma-derived RINm5F cell line retains many differentiated functions of islet beta-cells. However, it fails to recognize glucose as an insulin secretagogue in the physiological concentration range. With this cell line, glucose-transport kinetics were investigated, by using a double-label technique with the non-metabolizable glucose analogue 3-O-methylglucose (OMG). RINm5F cells possess a passive glucose-transport system with high capacity and low affinity. Equilibration across the plasma membrane of extracellular OMG concentrations up to at least 20 mM is achieved within 2 min at 37 degrees C. The half-saturation of OMG uptake occurs at 32 mM. At lower temperatures OMG uptake is markedly retarded, with a temperature coefficient (Q10) of 2.9. As indicated by efflux measurements, transport is symmetrical. Cytochalasin B at micromolar concentrations and phlorrhizin in millimolar concentrations are potent inhibitors of OMG uptake. Neutralization of the secreted insulin with antibodies does not alter OMG uptake kinetics. The glucose metabolism of RINm5F cells is much exaggerated compared with that of islet beta-cells. Nonetheless, when measured in parallel to uptake, transport exceeds by far the rate of metabolism at glucose concentrations above 3 mM. Measurements of intracellular D-glucose reveal a lower intracellular glucose concentration relative to the extracellular in RINm5F cells. This seems to be due to abnormalities in the subsequent steps of glucose metabolism, rather than to abnormalities in hexose uptake. The loss of glucose-induced insulin release in RINm5F cells cannot be explained by alterations in hexose transport.

Topics: 3-O-Methylglucose; Adenoma, Islet Cell; Animals; Biological Transport; Cell Line; Cytochalasin B; Glucose; Insulin; Insulin Antibodies; Insulinoma; Methylglucosides; Pancreatic Neoplasms; Phlorhizin; Rats

Cytochalasin B (17-3 microM) virtually abolished 3-O-methyl-D-[U-14C]glucose uptake and D-[5-3H]glucose utilization in tumoral insulin-producing cells of the RINm5F line. This coincided with a marked decrease in D-[U-14C]glucose oxidation and suppression of the stimulant action of D-glucose upon insulin release. Cytochalasin B, however, augmented basal insulin release by the tumoral cells. The RINm5F cells appeared much more sensitive than normal islet cells to cytochalasin B, as judged by the relative magnitude of inhibition in either hexose uptake or utilization. In both cell types, the inhibitory action of cytochalasin B upon glucose metabolism seemed to be competitive, being more marked at low than high glucose concentration. These results are interpreted in support of the view that a decreased efficiency of hexose transport across the plasma membrane represents an essential deficiency of the RINm5F cells.

Topics: 3-O-Methylglucose; Adenoma, Islet Cell; Animals; Cell Line; Cytochalasin B; Glucose; Glycolysis; Insulin; Insulin Secretion; Insulinoma; Islets of Langerhans; Kinetics; Methylglucosides; Methylglycosides; Rats

We studied the release of insulin, glucagon, and somatostatin in response to glucose, glyceraldehyde (GA), and alpha-ketoisocaproate (KIC) from rat kidneys containing transplanted insulinomas. Kidneys were perfused about 11 wk after transplantation when the plasma glucose concentration of the fed animals had decreased from 180 +/- 7 to 95.1 +/- 9.9 mg/dl and plasma insulin concentrations had increased from 2.6 +/- 0.5 to 14.2 +/- 2.0 ng/ml. The insulin content of the tumor-containing kidney ranged from 40 to 679 micrograms; the glucagon and somatostatin concentrations ranged from undetectable levels to 3.7 micrograms and 248 ng, respectively. The average response to 30 mM glucose and 10 mM GA was a four- to fivefold increase in insulin secretion, whereas 30 mM KIC caused a 16- to 28-fold increase. In vitro stimulation of the insulinoma with 30 mM glucose primed the beta-cell response to a second stimulus following a short rest period. Cytochalasin B did not enhance this primed glucose response. Diazoxide inhibited glucose, GA, and KIC-stimulated insulin release. Glucose, GA, and KIC stimulated glucagon release in 2 of 17 insulinomas studied here. Somatostatin release was not seen in any of the experiments. These findings show that this islet cell tumor transplanted under the kidney capsule releases insulin in response to physiologic and model fuel substances. Thus, this particular transplantable tumor offers an opportunity to study the biochemistry and biophysics that underlie fuel-stimulated insulin release.

Topics: Adenoma, Islet Cell; Animals; Cytochalasin B; Diazoxide; Glucagon; Glucose; Glyceraldehyde; Humans; In Vitro Techniques; Insulin; Insulin Secretion; Insulinoma; Islets of Langerhans; Keto Acids; Kidney; Male; Neoplasm Transplantation; Pancreatic Neoplasms; Radioimmunoassay; Rats; Somatostatin