The effect of glucose on insulin release and ion movements in isolated pancreatic islets of rats in old age.

1. The effect of glucose on 86Rb+ efflux, 45Ca2+ net uptake and insulin secretion of pancreatic islets from 3- and 24-month-old rats was studied. 2. Raising the glucose concentration from 3 to 5.6 and 16.7 mM had no effect on 86Rb+ efflux from islets of 24-month-old male rats whereas that from 24-month-old female rats was decreased. 3. At 16.7 mM-glucose, net uptake of 45Ca2+ was significantly diminished in islets of 24-month-old rats compared to islets of 3-month-old rats. 4. In the presence of 16.7 mM-glucose, islets of 24-month-old rats exhibited only 60-70% of the insulin release obtained with islets from 3-month-old rats. 5. Neither net uptake of 45Ca2+ nor insulin secretion appear to differ between the sexes. 6. These data suggest that the decreased insulin secretory response to glucose during old age is due, at least in part, to inadequate inhibition of K+ efflux and diminished net uptake of Ca2+.

Effects of propylthiouracil and methylthiouracil on cyclic AMP and ion movements in rat pancreatic islets.

Propylthiouracil and methylthiouracil have been shown to potentiate glucose-induced insulin secretion from rat pancreatic islets: the effect of methylthiouracil being less pronounced than that of propylthiouracil. In this study the effects of these substances on cAMP levels, 86Rb+ efflux, 45Ca2+ net uptake, and 45Ca2+ efflux were tested in isolated rat islets in order to obtain information on their possible mechanism of action. Propylthiouracil and to a lesser extent methylthiouracil increased islet cyclic AMP in a concentration-related manner. Maximum increases at the highest concentrations tested were 261% and 190% respectively. In the presence of 3 mM glucose propylthiouracil and methylthiouracil led to a decrease in the 86Rb efflux rate. With 5.6 mM glucose, both thiourea derivatives produced an increase in the 86Rb+ efflux rate which was independent of the presence or absence of calcium in the medium. Propylthiouracil and methylthiouracil augmented the 45Ca2+ efflux rate in the presence as well as in the absence of external calcium at various glucose concentrations. Propylthiouracil did not change, and methylthiouracil only slightly augmented, 45Ca2+ net uptake into the isolated islets. It is suggested that the synergistic effect of propylthiouracil and methylthiouracil on glucose-induced insulin release is at least in part due to an increase in islet cAMP levels. Whether the two substances have additional direct effects on ionic fluxes which contribute to their insulinotropic action or whether the observed changes in ion movements are secondary to the elevation of cAMP levels remains to be unclear and needs further investigation.

Potentiation of the locomotor activity of memantine by aminophylline.

1-Amino-3,5-dimethyladamantane hydrochloride (memantine, D-145, Akatinol) and aminophylline were shown to increase the locomotor activity of mice in a dose-dependent manner; yet the effect of aminophylline took place and decreased faster than that of memantine. The ED25 for memantine was found to be 4 mg/kg; the moderate increase of locomotor activity caused by this dose of memantine was potentiated by aminophylline. The combination of both led to an over-additive immediate onset of effect and was longer lasting than that produced by memantine alone. An optimal potentiation of memantine effect was reached when it was combined with aminophylline in molar ratio of 1:2. The possible mechanism of this synergistic effect is discussed.

Effect of thioloxidant diazene dicarboxylic acid bis-(N'-methylpiperazide) (DIP) on 45Ca2+ net uptake into rat pancreatic islets.

The effect of DIP (an oxidant of glutathione) on 45Ca2+ net uptake induced by a variety of stimulators of insulin secretion was studied in rat pancreatic islets. In addition the effect of exogenous glutathione (GSH) on 45Ca2+ net uptake in response to glucose was tested. DIP (0.1 mM) inhibited the increase of 45Ca2+ net uptake in the presence of glucose (16.7 mM) and glyceraldehyde (10 mM). A similar inhibitory effect could be demonstrated, when 45Ca2+ net uptake was enhanced by tolbutamide (100 micrograms/ml), glibenclamide (0.5 micrograms/ml), b-BCH (20 mM), 2-ketoisocaproate (20 mM), arginine (20 mM) in the presence of 3 mM glucose or by high extracellular potassium (20 mM). The increase of 45Ca2+ net uptake stimulated by leucine (20 mM) plus glucose (3 mM) was further augmented by DIP. Exogenous GSH did not affect 45Ca2+ net uptake in the presence of (5.6-16.7 mM) glucose. It is suggested that 45Ca2+ net uptake of pancreatic islets depends on the redox state of islet thiols regardless of whether uptake is promoted via inhibition of potassium efflux (nutrients, sulfonylureas) or by high potassium and arginine. The voltage sensitive calcium-channel is the site of action of critical thiols. It is possible that these thiols are localized at the inner side of the plasma membrane.

A possible role of intracellular and membrane thiols of rat pancreatic islets in calcium uptake and insulin release.

To distinguish between the significance of membrane thiols and intracellular thiols of rat pancreatic islets in calcium uptake and insulin secretion, lanthanum-nondisplaceable uptake of 45Ca and secretion of insulin were studied in the presence of the membrane-penetrating thiol oxidant diazene dicarboxylic acid bis-(N'-methylpiperazide) (DIP), on the one hand, and the corresponding nonpenetrating thiol oxidant salt of DIP (bis-N'-methyl iodide, i.e. DIP + 2), on the other hand. In contrast to DIP + 2, DIP inhibited glucose-induced (20 mM) uptake of 45Ca and insulin secretion. Furthermore, DIP + 2 stimulated 45Ca uptake and insulin secretion in the presence of low glucose (3 mM). DIP inhibited these effects of DIP + 2. 45Ca uptake and insulin release evoked by the ionophore A-23187 were not inhibited by DIP, however. The present data collected from isolated pancreatic islets have several implications. Firstly, the reduced state of the intracellular glutathione to GSSG ratio, which causes the superficial thiol to remain in the reduced state, constitutes a prerequisite for glucose-stimulated 45Ca uptake and insulin release. Secondly, direct oxidation of superficial thiols leads to 45Ca uptake and subsequent secretion of insulin. Finally, the redox state of the intracellular glutathione system does not appear to be of importance in the stimulus-secretion coupling steps following the uptake of calcium.