Voltage-activated currents in guinea pig pancreatic alpha 2 cells. Evidence for Ca2+-dependent action potentials

Evidence that glucose can control insulin release independently from its action on ATP-sensitive K+ channels in mouse B cells

Glucose stimulation of insulin release involves closure of ATP-sensitive K+ channels, depolarization, and Ca2+ influx in B cells. Mouse islets were used to investigate whether glucose can still regulate insulin release when it cannot control ATP-sensitive K+ channels. Opening of these channels by diazoxide (100-250 mumol/liter) blocked the effects of glucose on B cell membrane potential (intracellular microelectrodes), free cytosolic Ca2+ (fura-2 method), and insulin release, but it did not prevent those of high K (30 mmol/liter). K-induced insulin release in the presence of diazoxide was, however, dose dependently increased by glucose, which was already effective at concentrations (2-6 mmol/liter) that are subthreshold under normal conditions (low K and no diazoxide). This effect was not accompanied by detectable changes in B cell membrane potential. Measurements of 45Ca fluxes and cytosolic Ca2+ indicated that glucose slightly increased Ca2+ influx during the first minutes of depolarization by K, but not in the steady state when its effect on insulin release was the largest. In conclusion, there exists a mechanism by which glucose can control insulin release independently from changes in K(+)-ATP channel activity, in membrane potential, and in cytosolic Ca2+. This mechanism may serve to amplify the secretory response to the triggering signal (closure of K(+)-ATP channels--depolarization--Ca2+ influx) induced by glucose.

Role of voltage-dependent ionic currents in coupling glucose stimulation to insulin secretion in canine pancreatic islet B-cells

Glucose-induced electrical activity in canine pancreatic islet B cells is distinct from that in rodent islets, though both display Ca(2+)-dependent insulin secretion. Rodent islet B cells undergo regular bursts of Ca(2+)-dependent action potentials, while canine islet B cells generate isolated Na(+)-dependent action potentials which often give way to a plateau depolarization. Here we present evidence to reconcile the species difference in electrical activity with the similarity of Ca2+ dependence of secretion. (i) In canine B cells increasing glucose concentrations produce membrane depolarization and increasing frequency of Nao-dependent action potentials until a background membrane potential (approximately -40 mV) is reached where Na+ currents are inactivated. (ii) Voltage-dependent Ca2+ currents are present which are activated over the voltage excursion of the action potential (-50 to +20 mV) and inactivate slowly, (over seconds) in the range of the plateau depolarization (-40 to -25 mV). Hence, they are available to contribute to both phases of depolarization. (iii) Tetrodotoxin (TTX) reduces by half an early transient phase of glucose-stimulated insulin secretion but not a subsequent prolonged plateau phase. The transient phase of secretion often corresponds well in time to the period of initial high frequency action potential activity. These latter results suggest that in canine B cells voltage-dependent Na+ and Ca2+ currents mediate biphasic glucose-induced insulin secretion. The early train of Na(+)-dependent action potentials, by transiently activating Ca2+ channels and allowing pulsatile Ca2+ entry, may promote an early transient phase of insulin secretion. The subsequent sustained plateau depolarization, by allowing sustained Ca2+ entry, may permit steady insulin release.

Voltage-activated calcium and potassium currents in human pancreatic beta-cells

1. The whole-cell configuration of the patch clamp technique was used to study inward and delayed outward currents in beta-cells isolated from human pancreatic islets. 2. The delayed outward current activated at about -20 mV and increased linearly with further depolarization. The instantaneous current-voltage (I-V) relation, measured by tail current analysis, reversed at -70 mV. This is close to the K+ equilibrium potential and suggests the outward current is carried primarily by potassium ions. In support of this idea, outward currents were abolished when internal K+ was replaced by the impermeant cation N-methyl-D-glucamine (NMG). 3. The voltage dependence of K+ current activation could be fitted by a sigmoidal function with a mid-point at +1 mV. K+ currents showed voltage-dependent inactivation which was half-maximal at -25 mV. 4. Inward currents were studied after outward currents were suppressed by replacing internal potassium with NMG. In 5 mM [Ca2+]o, the inward current activated between -50 and -40 mV, had a peak amplitude at -10 mV and reversed at potentials positive to +60 mV. The voltage dependence of inward current activation was sigmoidal with half-maximal activation at -10 mV in 5 mM [Ca2+]o and at -22 mV in 5 mM [Ba2+]o. 5. Inward currents were unaffected by tetrodotoxin (TTX), but could be blocked by cadmium ions. Barium was also capable of carrying inward current. This pharmacology is consistent with inward currents flowing through Ca2+ channels. 6. The inactivation of the inward current was dependent on calcium entry. In two-pulse experiments, the voltage dependence of inactivation was U-shaped, and resembled that of the calcium current. Barium currents showed little inactivation. 7. In two-pulse experiments the degree of inward current inactivation during the test pulse was related to the amount of calcium entry during the first pulse. Calcium entering at more positive potentials was less effective at producing inactivation. 8. Calcium and barium currents also showed a slow, voltage-dependent inactivation when the holding potential was changed between -100 and -40 mV. This inactivation developed with a time course of seconds. 9. The Ca2+ and K+ currents described here are similar to those reported for rodent beta-cells and indicate the rodent beta-cell provides a good model for that of man.

Effects of glimepiride and glibenclamide on insulin and glucagon secretion by the perfused rat pancreas

Glimepiride and glibenclamide act apparently in a closely comparable manner upon both insulin and glucagon release. Except for the decreased efficiency of D-glucose in suppressing glucagon release after a prior exposure of the pancreas to the hypoglycemic sulfonylureas, no evidence was obtained to suggest that a positive glucagonotropic action of the latter drugs would counteract their hypoglycemic action, as mainly attributable to stimulation of insulin release.

Biophysical properties of gap junctions between freshly dispersed pairs of mouse pancreatic beta cells

Coupling between beta cells through gap junctions has been postulated as a principal mechanism of electrical synchronization of glucose-induced activity throughout the islet of Langerhans. We characterized junctional conductance between isolated pairs of mouse pancreatic beta cells by whole-cell recording with two independent patch-clamp circuits. Most pairs were coupled (67%, n = 155), although the mean junctional conductance (gj) (215 +/- 110 pS) was lower than reported in other tissues. Coupling could be recorded for long periods, up to 40 min. Voltage imposed across the junctional or nonjunctional membranes had no effect on gj. Up to several hours of treatment to increase intracellular cAMP levels did not affect gj. Electrically coupled pairs did not show transfer of the dye Lucifer yellow. Octanol (2 mM) reversibly decreased gj. Lower concentrations of octanol (0.5 mM) and heptanol (0.5 mM) than required to uncouple beta cells decreased voltage-dependent K+ and Ca2+ currents in nonjunctional membranes. Although gj recorded in these experiments would be expected to be provided by current flowing through only a few channels of the unitary conductance previously reported for other gap junctions, no unitary junctional currents were observed even during reversible suppression of gj by octanol. This result suggests either that the single channel conductance of gap junction channels between beta cells is smaller than in other tissues (less than 20 pS) or that the small mean conductance is due to transitions between open and closed states that are too rapid or too slow to be resolved.

Sodium channels contribute to action potential generation in canine and human pancreatic islet B cells

Pancreatic islet B cells depolarize and display trains of action potentials in response to stimulatory concentrations of glucose. Based on data from rodent islets these action potentials are considered to be predominantly Ca2+ dependent. Here we describe Na(+)-dependent action potentials and Na+ currents recorded from canine and human pancreatic islet B cells. Current-clamp recording using the nystatin "perforated-patch" technique demonstrates that B cells from both species display tetrodotoxin-sensitive Na+ action potentials in response to modest glucose-induced depolarization. In companion "whole-cell" voltage-clamp experiments on canine B cells, the underlying Na+ current displays steep voltage-dependent activation and inactivation over the range of -50 to -40 mV. The Na+ current is sensitive to tetrodotoxin block with a KI = 3.2 nM and has a reversal potential which changes with [Na+]o as predicted by the Nernst equation. These results suggest that a voltage-dependent Na+ current may contribute significantly to action potential generation in some species outside the rodent family.

Glucose-inhibition of glucagon secretion involves activation of GABAA-receptor chloride channels

The endocrine part of the pancreas plays a central role in blood-glucose regulation. It is well established that an elevation of glucose concentration reduces secretion of the hyperglycaemia-associated hormone glucagon from pancreatic alpha 2 cells. The mechanisms involved, however, remain unknown. Electrophysiological studies have demonstrated that alpha 2 cells generate Ca2+-dependent action potentials. The frequency of these action potentials, which increases under conditions that stimulate glucagon release, is not affected by glucose or insulin. The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is present in the endocrine part of the pancreas at concentrations comparable to those encountered in the central nervous system, and co-localizes with insulin in pancreatic beta cells. We now describe a mechanism whereby GABA, co-secreted with insulin from beta cells, may mediate part of the inhibitory action of glucose on glucagon secretion by activating GABAA-receptor Cl- channels in alpha 2 cells. These observations provide a model for feedback regulation of glucagon release, which may be of significance for the understanding of the hypersecretion of glucagon frequently associated with diabetes.

'Perforated patch recording' allows long-term monitoring of metabolite-induced electrical activity and voltage-dependent Ca2+ currents in pancreatic islet B cells

We describe the application of 'perforated patch recording' using the pore-forming antibiotic nystatin, to monitor the electrical activity and underlying ionic currents of rat and human pancreatic islet B cells. We demonstrate that glucose-induced electrical activity is seen even in single B cells during current-clamp recordings lasting hours 'L-type' Ca2+-channel currents can also be monitored over this period of time. This technique may prove useful in examining hormone and neurotransmitter modulation of electrical activity in B cells, while minimizing the effects of cytoplasmic 'wash-out'.

Cyclic AMP raises cytoplasmic calcium in pancreatic alpha 2-cells by mobilizing calcium incorporated in response to glucose

The cytoplasmic Ca2+ concentration ([Ca2+]i) was monitored in individual guinea-pig pancreatic alpha 2-cells exposed to modulators of glucagon release. Addition of the stimulatory amino acid arginine resulted in a sustained increase in [Ca2+]i, whereas the inhibitor glucose had the opposite effect. Epinephrine, the beta-adrenergic agonist isoproterenol, the adenylate cyclase activator forskolin and 8-bromo-cAMP transiently raised [Ca2+]i provided that the cells had been pretreated with glucose. However, simultaneous presence of glucose was not required and the effect occurred even in the absence of extracellular Ca2+. Carbachol, the alpha 2-adrenergic agonist clonidine and the sulfonylurea tolbutamide lacked effects on [Ca2+]i. In addition to providing support for the concept that glucagon release is positively modulated by [Ca2+]i, the results demonstrate that cAMP raises [Ca2+]i in the alpha 2-cells by mobilizing calcium incorporated in response to glucose.

Control of insulin secretion by sulfonylureas, meglitinide and diazoxide in relation to their binding to the sulfonylurea receptor in pancreatic islets

Sulfonylureas inhibit an ATP-dependent K+ channel in the B-cell plasma membrane and thereby initiate insulin release. Diazoxide opens this channel and inhibits insulin release. In mouse pancreatic islets, we have explored whether other targets for these drugs must be postulated to explain their hypo- or hyperglycaemic properties. At non-saturating drug concentrations the rates of increase in insulin secretion declined in the order tolbutamide = meglitinide greater than glipizide greater than glibenclamide. The same rank order was observed when comparing the rates of disappearance of insulin-releasing and K+ channel-blocking effects. The different kinetics of response depend on the lipid solubility of the drugs, which controls their penetration into the intracellular space. Allowing for the different kinetics, the same maximum secretory rates were caused by saturating concentrations of tolbutamide, meglitinide, glipizide and glibenclamide. A close correlation between insulin-releasing and K+ channel-blocking potencies of these drugs was observed. The relative potencies of tolbutamide, meglitinide, glipizide and glibenclamide corresponded well to their relative affinities for binding to islet-cell membranes, suggesting that the binding site represents the sulfonylurea receptor. The biphasic time-course of dissociation of glibenclamide binding indicates a complex receptor-drug interaction. For diazoxide there was no correlation between affinity of binding to the sulfonylurea receptor and potency of inhibition of insulin secretion. Thus, opening or closing of the ATP-dependent K+ channel by diazoxide or sulfonylureas, respectively, appears to be due to interaction with different binding sites in the B-cell plasma membrane. The free concentrations of tolbutamide, glipizide, glibenclamide and diazoxide which are effective on B-cells are in the range of therapeutic plasma concentrations of the free drugs. It is concluded that the hypo- and hyperglycaemic effects of these drugs result from changing the permeability of the ATP-dependent K+ channel in the B-cell plasma membrane.

Two types of Ca2+ currents with different sensitivities to organic Ca2+ channel antagonists in guinea pig pancreatic alpha 2 cells

The possibility that guinea pig pancreatic alpha 2 cells are equipped with more than one type of Ca2+ channel was explored using the patch-electrode voltage-clamp technique. At a holding potential of -100 mV, a slowly developing (tau m approximately 5 ms at -40 mV assuming m2 kinetics) Ca2+ current appeared. This conductance first became detectable at potentials of about -60 mV and reached a maximum amplitude of 50-100 pA between -30 and -20 mV. During long depolarizations, it inactivated completely (tau h approximately 100 ms at -40 mV). Half-maximal steady state inactivation was observed at about -60 mV. A second, more rapidly developing (tau m approximately 2 ms at 0 mV) Ca2+ current was observed during pulses to -40 mV and above. It had a peak amplitude of 150-200 pA between 0 and 10 mV, was less dependent on the holding potential, and inactivated very little, even during long pulses. Both conductances were blocked by Co2+ but were unaffected by tetrodotoxin. The rapidly developing current differed from the slowly developing one in being sensitive to the antagonists D-600 and nifedipine, conducting Ba2+ better than Ca2+, increasing upon exposure to forskolin, and showing time-dependent decay (rundown). These findings indicate that the alpha 2 cells are equipped with two kinds of Ca2+ channels.