Membrane potential and intercellular calcium during glucose challenge in mouse islet of Langerhans

The present work was undertaken to study the effects of the addition and removal of glucose on the calcium concentration of the intercellular space [Ca2+]o of islets of Langerhans. The removal of glucose hyperpolarize the beta-cell membrane to near- 60 mV and induced a biphasic response in [Ca2+]o. On the other hand, the addition of glucose (11 mM) depolarized beta-cell, inducing a biphasic electrical activity and concomitant changes in the [Ca2+]o. During the continuous membrane electrical activity, which characterized the first phase of glucose evoked biphasic response, the [Ca2+]o decreased. In the second phase, the [Ca2+]o displayed an oscillatory behavior concomitantly with the membrane potential.

The diabetogenic agent alloxan increases K+ permeability by a mechanism involving activation of ATP-sensitive K(+)-channels in mouse pancreatic beta-cells

The effects of the diabetogenic agent, alloxan, on membrane potential, input resistance and electrical activity of normal mouse pancreatic beta-cells were studied. Tetraethylammonium (TEA), quinine and Glyburide were used to block K(+)-channels and to elucidate the mechanisms underlying alloxan's effects on beta-cell membrane potential. Exposure of the islet to alloxan (75-100 microM) in the presence of glucose (11 mM), produced a rapid (15 sec), transient inhibition of electrical activity, often accompanied by hyperpolarization of the membrane, and this was followed by recovery of the burst pattern. This early effect of alloxan was followed after approximately 15 min by a complete inhibition of electrical activity and hyperpolarization. The inhibition accompanied by hyperpolarization was associated with a decrease in input resistance, indicating increased K(+)-conductance. Both the transient and delayed effects of alloxan were blocked by glucose (33 mM), quinine and glyburide but not by other conditions which induce continuous electrical activity such as elevated external [K+] (10 mM), ouabain, K+ removal, or TEA (20 mM). The transient inhibition induced by alloxan may be due to a direct competition with glucose transport/metabolism since it did not occur when alpha-keto isocaproic acid (KIC) was used to induce electrical activity. The delayed inhibition may reflect indirect effects of accumulation of this agent or its metabolites within the cell. Since both effects of alloxan are blocked by glyburide they appear to involve activation of the ATP-sensitive K(+)-channel (K-ATP).

Ascorbic acid and insulin secretion in pancreatic islets

The effect of ascorbic acid on glucose-induced insulin release from single pancreatic islets was measured using a new, ultra-sensitive enzyme-linked immunosorbent insulin assay. Within 20 s ascorbic acid inhibited insulin secretion; inhibition was dose dependent and completely reversible. There was a 50% inhibition of the secretory response with 200 microM ascorbic acid and 90% inhibition with 400 microM ascorbic acid. The decrease in insulin secretion was recorded as a reduction of the amplitudes of the fast insulin transients, which give rise to the oscillatory nature of insulin secretion. The inhibition of glucose-induced insulin release by ascorbic acid was associated with hyperpolarization of the pancreatic beta-cell. Suppression of glucose-induced membrane depolarization was evident after 20 s, was dose dependent, and was completely reversible. The data here may provide the first explanation of why plasma ascorbate concentrations are tightly controlled.

Blockade of the 32P phosphate flush of pancreatic beta cells from adult rats who received a low-protein diet during early lactation

In order to study the effect of nutrition on the onset of disturbances in Wistar rat pancreatic beta cells, we compared the effects of a low protein diet (8% protein) and a normal protein diet (25% protein) supplied to the dams (6 in each group) during the first 12 days of lactation. The parameter evaluated was the beta cells phosphate flush in response to stimulatory concentration of glucose (16.7 mM) of isolated islets of Langerhans from 60-day old pups. Using a collagenase digestion technique, islets were isolated from the pups and the 32P fractional outflow rate (FOR) of the beta cells was used as a metabolic index in both experimental groups (N = 36). We observed that although the weights of the pups of the two groups were not significantly different at 60 days of age (control = 186 +/- 18 g; undernourished during lactation = 179 +/- 19 g), the typical phosphate flush response (FOR = 2.4 +/- 0.4%/min) to a stimulatory glucose concentration (16.7 mM) was abolished in the rats from undernourished mothers. Our data are consistent with the hypothesis that undernutrition may be an important cause of diabetes mellitus type II.