On the relation of glucose and insulin secretion in the fasting state

In the isolated perfused rat pancreas the relationship between glucose concentration and insulin release was altered by fasting for 1-3 days. The maximum effect decreased progressively, while there was no change in the glucose concentration, yielding half of maximum response. Prolonged perfusion of the pancreas from fasting rats with glucose (20 mM) failed to restore the reduced secretion. The insulin content of the pancreas gland decreased after the second day of starvation. Blood glucose, however, was unchanged for 4 days after an initial fall. It is concluded that the capacity of the secretory mechanism, rather than its sensitivity to glucose, is reduced by starvation.

Inhibition of insulin secretion from the perfused pancreas of the rat by pizotifen

1 In the perfused rat pancreas the effects of pizotifen on insulin release induced by 20 mM glucose were studied. 2 Pizotifen (10 and 100 muM) significantly reduced the insulin release during a 25 min perfusion period to 49% and 7% of the controls. 3 The same concentrations of the structurally related agents cyproheptadine, doxepin, and chlorpromazine produced a comparable inhibition.

Insulin release by tolbutamide and glibenclamide. A comparative study on the perfused rat pancreas

Glibenclamide, a "second generation" sulfonylurea, produced the same pattern of insulin release from the perfused rat pancreas as did tolbutamide. The stimulatory effect was closely dependent on the glucose concentration present. Both agents enhanced insulin secretion at 5--10 mM glucose, whereas no additional insulin was released when maximally stimulating levels of glucose (20 and 30 mM) were present. The concentrations of glibenclamide stimulating insulin release were 100--400 times lower than equieffective levels of tolbutamide. At glucose levels of 3 or 8 mM, however, glibenclamide did not liberate significantly more insulin from the pancreas than did tolbutamide. Thus the differences of tolbutamide and glibenclamide were quantitative rather than qualitative. Although the active concentrations differed the effects produced were comparable.

Effects of possible beta-cell membrane label, metahexamide-isothiocyanate, on insulin release

To synthetize a specifically and covalently reacting label for the sulfonylurea receptor site, the sulfonylurea metahexmide was converted to its isothiocyano-derivative (MITC), and the effects of MITC on insulin release from the perfused rat pancreas were studied. MITC (2 micrometer) stimulated a large insulin release that persisted after the end of the MITC-infusion. At a higher concentration (50 micrometer) MITC produced only a short lasting stimulation, and thereafter inhibited either the sulfonylurea or the glucose-induced insulin release. It is suggested that the irreversible stimulation of insulin release by MITC reflects the convalent linkage of the label to the sulfonylurea receptor site, while excess isothiocyanate inhibited insulin release by reacting on less specific binding sites involved in the secretion process.

Uptake of tolbutamide by islets of Langerhans and other tissues

3-H-tolbutamide was distributed in a volume exceeding the space occupied by 14-C-sucrose in islets as well as in liver, kidney, muscle, and fat. In contrast to previous reports, the findings suggest that tolbutamide is not restricted to the extracellular space of islets.

Subcellular B cell calcium and insulin secretion in vitro. Comparative ultracytochemical studies after glucose stimulation and cyproheptadine inhibition

Using the ultracytochemical pyroantimonate technique different patterns of calcium containing precipitates were found in the B cells of the isolated perfused rat pancreas under conditions of stimulated and inhibited insulin secretion. The calcium specificity of the ultracytochemical method was assessed by perfusion with a EGTA containing calcium-free medium, which markedly reduced the extent of precipitation. Perfusion with 20 mM D-glucose over a period of 30 min resulted in calcium distribution patterns which could be related to the biphasic insulin release. The calcium patterns differed significantly in their quality and quantitative morphometry from those after 5 mM D-glucose or cyproheptadine (CPH) perfusion (20 mM D-glucose plus 0.1 mM CPH). After 3--5 min of 20 mM glucose perfusion there was an increased calcium precipitation along the inner side of the B cell membranes. After 20--30 min an additional increase in precipitation was found in the cytoplasmic matrix and in the secretory granules. B cells in a CPH-inhibited state of secretion and also after perfusion with 5 mM glucose lacked these findings. The data suggest that an increase in the membrane associated calcium may induce the first phase of insulin secretion by triggering the exocytosis of peripheral granules, while the cytoplasmic calcium may be involved in long term regulation of insulin release.

Effects of polymer-linked sulfonylurea derivatives on insulin release

In order to elucidate whether polymerlinked sulfonylurea derivatives may serve as tools to locate the sulfonylurea receptor site, an easily detectable sulfonylurea was synthetized and coupled to cyanogen-bromide activated dextran. The conjugate proved to be unstable and to release sufficient amounts of free agent to explain its insulinotropic activity. Based on these findings previous results claiming that dextran-linked sulfonylureas retain biological activity may be questioned. A stable conjugate was obtained when the acrylyl derivative of the sulfonylurea was co-polymerized with acrylamide. Both a high (greater than 50000) and a low (1500) molecular weight fraction of this conjugate failed to stimulate insulin release from the perfused rat pancreas, questioning the ability of macromolecular conjugates to combine with the sulfonylurea receptor site.

The inhibition of insulin secretion from the perfused rat pancreas after thyroxine treatment

Thyroxine treatment did not significantly affect the immediate insulin secretory response of the perfused rat pancreas, but it inhibited the late phase of D-glucose-induced insulin secretion. Thyroxine treatment did not inhibit D-glyceraldehyde-, D-mannose-, and tolbutamide-induced insulin release from the perfused pancreas. An increase in the D-glucose concentration of the perfusion medium as well as feeding of the rats did not restore insulin secretion after thyroxine treatment. The inhibition of D-glucose-induced insulin release in response to thyroxine treatment was reversed after addition of either D-glyceraldehyde, dihydroxyacetone, DL-glyceric acid, pyruvate, or alpha-ketobutyrate to the perfusion medium. Tolbutamide, L-glucose, D-fructose, D-mannose, L-lactate, and propionic acid were not able to overcome the inhibition of D-glucose-induced insulin secretion. Except for alpha-ketobutyrate all substances which were effective in reversing the inhibition of D-glucose-induced insulin release were glycolytic intermediates. Comparing the glycolytic alpha-ketoacid pyruvate and the non-glycolytic ketoacid alpha-ketobutyrate, the only part common to both substances was the ketoacid moiety. It is concluded from these findings that the ketoacid moiety of the alpha-ketoacids plays an important role in reversing the effect of thyroxine on D-glucose-induced insulin release.

Inhibition of insulin and glucagon release from the perfused rat pancreas by cyproheptadine (Periactinol, Nuran)

The tricyclic compound cyproheptadine (Periactinol, Nuran) inhibited glucose-induced insulin release from the perfused rat pancreas. Tolbutamide-stimulated insulin release was significantly reduced in the presence and completely suppressed in the absence of a substimulatory glucose concentration (5 mM). Arginine produced a slow rise of insulin release, which was completely abolished by cyproheptadine. Furthermore the biphasic glucagon release due to the stimulus was inhibited. Oxidation of 14C-glucose in isolated islets was unaltered in the presence of cyproheptadine, and pyruvate added to the perfusion medium failed to reverse the inhibitory effect on glucose induced insulin release, indicating that impaired glucose metabolism is not responsible for the inhibition. In addition, the inhibition remained unchanged when phentolamine was present, suggesting that the effect is not mediated by inhibitory adrenergic alpha receptors. Theophylline, in contrast, partly overcame the inhibition. When the calcium concentration of the medium was enhanced, the inhibitory effect of cyproheptadine was still visible, although the relative inhibition had become smaller. The results suggest that cyproheptadine blocks insulin release by affecting a fundamental step of the stimulus-secretion coupling common to peptide hormones. A participation of a calcium-antagonizing effect in the inhibition is discussed.

Thyroid function and insulin secretion from the perfused pancreas in the rat

The influence of thyroid function on the kinetics of glucose-induced insulin secretion from the isolated perfused rat pancreas has been studied. L-Thyroxine (L-T4) administration did not modify the immediate insulin secretory response of the perfused pancreas to glucose. L-Triiodothyronine (L-T3) treatment as well as propylthiouracil (PTU) treatment decreased the immediate insulin secretory response of the pancreas slightly. Only thyroidectomy (Tx) reduced the immediate secretory response of the pancreas significantly. L-T4 and L-T3 treatment inhibited the late phase of glucose-induced insulin secretion from the isolated perfused rat pancreas, whereas TX and PTU treatment resulted in increased insulin secretion. D-Thyroxine (D-T4) did not affect glucose-induced insulin release from the pancreas. Concomitantly, several parameters indicative of thyroid function were determined in these animals. When changes in body weight, rectal temperature, plasma glucose, plasma cholesterol, and plasma butanol-extractable iodine (BEI) in these rats were compared with the insulin secretory responses, it was evident that experimental hyperthyroidism results in decreased insulin release, whereas experimental hypothyroidism induces increased insulin secretion from the pancreas. The transitions from hypothyroid to euthyroid to hyperthyroid states are accompanied by a steady decrease in glucose-induced insulin release from the rat pancreas. Inhibition of glucose-induced insulin secretion from the pancreas is therefore a specific effect of thyroid hormones.

Inhibition o'f insulin release by cyproheptadine: effects on 3',5'-cyclic-AMP-content and 45Ca-accumulation of incubated mouse islets

Cyproheptadine (1, 10 and 100 muM) significantly reduced insulin release from isolated mouse islets in response to glucose. In contrast, 1 mM cyproheptadine induced a large release of insulin into the incubation medium probably due to islet cell damage, since the islets had lost a considerable amount of their protein content. 3',5'-cyclic-AMP-levels of the islets were not significantly affected by 10 muM cyproheptadine in the presence as well as in the absence of theophylline (10 mM). As the inhibitory effect of cyproheptadine on insulin release was correlated with reduced accumulation of calcium-45, the agent may inhibit insulin release by interfering with the calcium handling of the beta-cell.