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Biomedical subjects

I Atwater

Publications and source records attributed to I Atwater.

At least 73 records · Page 4Linked to original sources
PubMed1983

Resistance to apamin of the Ca2+-activated K+ permeability in pancreatic B-cells.

The bee venom neurotoxin apamin failed to affect 86Rb outflow and insulin release from rat pancreatic islets stimulated by D-glucose or the Ca2+-ionophore A23187. Apamin, in contrast to quinine or A23187, also failed to affect bioelectrical activity in mouse islet cells. These findings suggest that, like in erythrocytes, and at variance with the situation found in smooth muscle, liver or neuroblastoma cells, the Ca2+-activated K+ permeability in the pancreatic B-cell is resistant to apamin.

Animals↗
PubMed1983

Properties of the Ca-activated K+ channel in pancreatic beta-cells.

The existence of [Ca2+]i-activated K+-channels in the pancreatic beta-cell membrane is based in two observations: quinine inhibits K+-permeability and, increasing intracellular Ca2+ stimulates it. The changes in K+-permeability of the beta-cell have been monitored electrically by combining measurements of the dependence of the membrane potential on external K+ concentration and input resistance. The changes in the passive 42K and 86Rb efflux from the whole islet have been measured directly. Intracellular Ca2+ has been increased by various means, including increasing extracellular Ca2+, addition of the Ca2+-ionophore A23187 or noradrenaline and application of mitochondrial uncouplers and blockers. In addition to quinine, many other substances have been found to inhibit or modulate the [Ca2+]i-activated K+-channel. The most important of these is the natural stimulus for insulin secretion, glucose. Glucose may inhibit K+-permeability by lowering intracellular Ca2+. Glibenclamide, a hypoglycaemic sulphonylurea, is about 25 times more active than quinine in blocking the K+-channel in beta-cells. The methylxanthines, c-AMP, various calmodulin inhibitors and Ba2+ also inhibit K+-permeability. Genetically diabetic mice have been studied and show an alteration in the [Ca2+]i-activated K+-channel. It is concluded that the [Ca2+]i-activated K+-channel plays a major role in the normal function of the pancreatic beta-cell. The study of its properties should prove valuable for the understanding and treatment of diabetes.

Animals↗
PubMed1983

Beta cell membrane potential and insulin release; role of calcium and calcium:magnesium ratio.

Glucose-induced insulin release from perfused rat pancreas was compared with glucose-induced changes in membrane potential of beta cells from mouse islets. Extracellular concentrations of Ca and Mg were varied as steps, simultaneously or separately, from 10% to 200% of normal in the presence of 11.1 mM glucose. A change in Ca induced a transient change in electrical activity paralleled by a transient change in insulin release. If the Ca/Mg ratio was maintained, steady-state insulin release remained constant between 10% and 200% Ca, while electrical activity showed alterations. Analysis of burst parameters indicated that increased or decreased Ca entry was balanced by decreased or increased excitability. It is postulated that the beta cell contains a compensator mechanism for the regulation of Ca influx.

Animals↗
PubMed1983

Effects of adrenaline and noradrenaline on glucose-induced electrical activity of mouse pancreatic beta cell.

The effects of adrenaline and noradrenaline on membrane potential and glucose-induced electrical activity were studied in micro-dissected mouse Islets of Langerhans. Both catecholamines induced hyperpolarization and blocked electrical activity in the presence of 11.1 mM glucose. Phentolamine, but not propranalol, blocked these effects, indicating predominantly alpha receptor action. Quinine, but not tetraethylammonium ions, antagonized the inhibitory effects of the catecholamines. The data are consistent with the hypothesis that alpha receptor activation induces a transient increase in intracellular Ca2+ concentration which in turn leads to an increase in K+ permeability.

Drug Antagonism↗
PubMed1982

Potassium-induced insulin release and voltage noise measurements in single mouse islets of Langerhans.

Insulin release and membrane potential fluctuations in response to increased extracellular potassium [K+]o have been measured in single perifused islets of Langerhans from normal mice. An increase in [K+]o from 5 mM to 50 mM induced a transient insulin release with a peak at about 1 min. The peak value was [K+]o-dependent but the half-time t1/2 for the decline was constant at nearly 1 min. 2.5 mM cobalt completely inhibited the potassium-induced stimulation of insulin release. The insulin release elicited by 28 and 50 mM [K+]o was similar in terms of peak, total release and half-time from maximum release. Stepwise increase in [K+]o from 10 to 28 to 50 mM resulted in a normal response to 28 mM but no peak of release after the 28 to 50 mM increase. The results indicate good correlation between excess voltage noise, thought to reflect calcium channel activity, and insulin release evoked by changing extracellular potassium.

Animals↗
PubMed1981

A method for the simultaneous measurement of insulin release and B cell membrane potential in single mouse islets of Langerhans.

A method has been developed for the simultaneous measurement of insulin release and electrical activity in single micro-dissected mouse islets of Langerhans. The effects of D-glucose have been studied in individual islets. Each islet was exposed to 0, 5.6, 11.1, 16.7, 22.2, 27.8, and 33.3 mmol/l glucose in a stepwise fashion. The minimum glucose concentration required to elicit spike activity is lower than that required to stimulate insulin release above basal levels and the maximum spike frequency occurs at a lower glucose concentration than does maximum insulin release. Following a reduction in glucose from 27,8 (or 33.3) to 5.6 mmol/l, membrane potentials returned to resting values within 2 min whereas insulin returned to basal values after 20 min. Increasing glucose from 5.6 to 27.8 mmol/l induced spike activity within 10 s; the insulin response was detected within 40 s. Thus, it is possible to use the single mouse islet for simultaneous measurements of insulin release and electrical activity.

Animals↗
PubMed1981

Voltage noise measurements across the pancreatic beta-cell membrane: calcium channel characteristics.

1. Membrane potential fluctuations were measured in cells from mouse Islets of Langerhans identified as beta-cells by the characteristic pattern of electrical activity induced by 11 mM-D-glucose. 2. The membrane potential was controlled by adjusting the external potassium concentration, [K+]o, keeping the sum [Na+]o plus [K+]o constant. In the absence of glucose, when [K+]o is raised, the resulting depolarization is accompanied by a significant increase in voltage noise. 3 The amplitude and time course of the voltage noise were measured under various experimental conditions. The variance of the fluctuating voltage decreased monotonically along the depolarization induced by sudden increase in [K+]o, suggesting a monotonic reduction in the number of elementary events. 4. The frequency characteristics of the excess noise could be analysed as the sum of 1/f and 1/f2 components. While the 1/f component remained unaffected by the external application of 20mM-tetraethylammonium (TEA) and either 2 mM-Mn2+ or 2 mM-Co2+, the 1/f2 component was suppressed by both Mn2+ and Co2+. 5. The corner frequency, fc, of the 1/f2 component depended on membrane potential, which was adjusted by adjusting the [K+]o jump. These results support the idea that fc in these experiments is a measure of the channel relaxation. 6. Measurements of the input resistance in the frequency range from 0 to 25 Hz were used to obtain a rough estimate of the size of the channel conductance as 5 x 10(-12) omega (-1).

Animals↗
PubMed1981

Calcium affects insulin release and membrane potential in islet beta-cells.

Insulin release from perfused rat pancreas was compared with membrane potentials of single beta-cells from perifused mouse islets during glucose stimulation (11.1 mM) in the presence of varying Ca and Mg concentrations. Depolarization was associated with insulin release and hyperpolarization with its suppression, irrespective of Ca concentration. After sudden reduction of Ca and Mg (to 0.05 and 0.01 mM, respectively), glucose-stimulated insulin release was maintained while the cell membrane depolarized, leading to a reversed pattern of burst activity. Readdition of Ca and Mg caused suppression of insulin release that paralleled hyperpolarization of the cell membrane. This suppression was transient, lasting < 5 min, and was due mainly to readdition of Ca. Patterns of insulin release during reduction of Ca and Mg in the presence of valinomycin (1 microM), diphenylhydantoin (25 microgram/ml), and ethyleneglycol-bis(beta-aminoethylether)-N,N'-tetraacetic acid (0.037 and 1.0 mM) were also studied. In conclusion, the relative concentrations of Ca and Mg and the membrane potential per se are important in the release of insulin. Also, under certain experimental conditions, Ca can block glucose-stimulated insulin release, possibly by increasing K+ permeability.

Animals↗
PubMed1980

Similarities in the stimulus-secretion coupling mechanisms of glucose- and 2-keto acid-induced insulin release.

The stimulus-secretion coupling of 2-keto acid-induced insulin release was investigated using 2-ketoisocaproate (4-methyl-2-oxopentanoate) as the principal model secretagogue. 2-Ketoisocaproate and 2-ketocaproate (2-oxo-, hexanoate) provoked changes in B cell electrical behavior characterized by an initial depolarization of the membrane potential, followed by rapid spike activity, which appeared either in a bursting pattern or as continuous activity. The onset of spike activity induced by 2-ketoisocaproate (5 mM) was biphasic in nature. The dynamic pattern of 2-ketoisocaproate-induced insulin release was also biphasic. 2-[U-14C]Ketoisocaproate (10 mM) was oxidized in islet tissue at a rate equivalent to that of [U-14C]glucose (17 mM) and a t a higher rate than 2-ketoisovalerate (3-methyl-2-oxobutyrate) and 2-keto-3-methyl-valerate, which were poor secretagogues. Like glucose, 2-ketoisocaproate provoked characteristic changes in 86Rb and 45Ca efflux from prelabeled islets and stimulated 45Ca net uptake. Proinsulin synthesis was stimulated by 2-ketoisocaproate through both a general effect on protein synthesis and a specific effect on hormonal biosynthesis. 2-Ketoisocaproate and 2-ketocaproate reproduced the effect of glucose on the islet content of ATP, ADP, AMP, NAD+, NADH, NADP+, and NADPH. These findings together with a series of observations on the effects upon the above parameters of site-specific inhibitors, e.g. respiratory inhibitors, suloctidil, theophylline, and epinephrine, suggested that the stimulus-secretion-coupling mechanisms for 2-ketoisocaproate- and glucose-induced release are similar. It is postulated that glucose- and 2-keto acid-induced insulin release may be initiated by a common signal.

Adenine Nucleotides↗
PubMed1980

The nature of the oscillatory behaviour in electrical activity from pancreatic beta-cell.

The typical burst pattern of electrical activity recorded from beta-cells of mouse islets in the presence of 11.1 mM glucose has been compared from 225 cells in terms of two parameters: relative duration of the active phase and periodicity of the oscillations between active and silent phases. The two parameters show different distributions, Gaussian and bimodal respectively. Burst periodicity was found to increase dramatically at external potassium concentrations below 2mM, while the relative duration of the active phase was little affected at concentrations as low as 0.5 mM. Increasing external calcium concentration significantly increased the amplitude of the bursts but greatly decreased the relative duration of the active phase. Quinine, in the absence of glucose, induced depolarization and electrical activity and stimulated insulin release from perifused mouse islets; spike frequency and insulin release followed a monophasic pattern. A model has been proposed for the beta-cell explaining the experimental results in terms of a feedback mechanism between calcium permeability activated by depolarization of the cell membrane and potassium permeability activated by an increase in the concentration of intracellular ionized calcium [Ca2+]i. The results are in agreement with the hypothesis that a [Ca2+]i-dependent potassium permeability, specifically blocked by quinine, controls membrane potential and insulin release.

Action Potentials↗
PubMed1980

Fuel and signal function of 2-keto acids in insulin secretion.

The stimulation of insulin release from pancreatic islet tissue by 2-ketoisocaproate was accompanied by a characteristic electrophysiological response and by increases in islet net uptake of 45Ca and respiratory activity. These parameters were closely correlated to changes in islet keto acid content, islet NADPH/NADP+ ratio and to the rates of 2-ketoisocaproate catabolism and amination. The production of 14CO2 from [U-14C]2-ketoisocaproate was reduced by the addition of valine, isovalerate, glutamine or pyruvate to the incubation media. Such an effect, however, did not correspond to the response of insulin secretion of 45Ca uptake observed in the presence of these substrates. These findings suggest that the metabolic sequence between the initial conversion of 2-ketoisocaproate to isovaleryl CoA and its subsequent transformation to acetoacetate and acetyl CoA was not the site of generation of a signal which initiated insulin release. The total rate of oxidation deduced from the sum of the rates of 14CO2 production from [U-14C]2-ketoisocaproate and the [U-14C]-labelled second substrate, however, did follow the secretory response of the tissue. It is proposed that reactions associated with the tricarboxylic acid cycle are important to the stimulus-secretion coupling mechanism of 2-ketoisocaproate-induced insulin release. The question of whether such reactions participate in the generation of a signal which initiates insulin secretion or merely reflect the energetic demands of the secretory process is discussed.

Animals↗
PubMed1980

The interplay between metabolic and cationic events in islet cells: coupling factors and feedback mechanisms.

In the mechanism of glucose-stimulated insulin release, the coupling between glucose metabolism and the remodelling of cationic fluxes in the B-cell apparently represents a multifactorial process involving changes in the generation rate of H+, reducing equivalents and ATP. This process is susceptible to feedback regulatory mechanisms through which primary changes in cationic movements affect glucose metabolism. The interplay between metabolic and ionic events may participate in the rhythmogenesis of bioelectrical and secretory phenomena.

Adenosine Triphosphate↗
PubMed1979

Mouse pancreatic beta-cells: tetraethylammonium blockage of the potassium permeability increase induced by depolarization.

1. Membrane potentials and input resistance were measured in beta-cells from mouse pancreatic islets of Langerhans in the presence or absence of D-glucose. 2. Tetraethylammonium (TEA) (a specific blocker of the K permeability increase induced by shifts in membrane potential from negative to positive values) was externally applied and its effects on potentials and input resistance evaluated. 3. In the absence of glucose, addition of TEA up to 20 mM to the perifusion medium did not affect the resting potential and the input resistance, the selectivity ratio PK/PNa (calculated from the constant field equation) remaining unchanged at about 30. 4. The characteristic response of the beta-cell membrane potential, in the presence of glucose, is a fluctuation between a silent phase at about -50 mV and an active phase at about -40 mV giving rise to a train of spikes. TEA abolishes this pattern and very much reduces the graded response of spike frequency normally seen with different concentrations of glucose. 5. Addition of glucose in the presence of up to 20 mM-TEA induces an increase in membrane resistance of about 4.10(7) omega. 6. TEA lowers the glucose level required to trigger the electrical activity from about 5.6 to about 4.6 mM. 7. TEA blocks the repolarization phase of action potentials induced by the addition of glucose or by depolarizing intracellular current injection. 8. In the presence of 11.1 mM-glucose and 20 mM-TEA the action potentials frequently crossed the zero line, the membrane potential reaching up to 25 mV during the peak of the spikes.

Action Potentials↗
PubMed1979

Potassium permeability activated by intracellular calcium ion concentration in the pancreatic beta-cell.

1. Membrane potentials and input resistance were measured in beta-cells from mouse pancreatic islets of Langerhans in a study designed to assess the role of a K permeability specifically blocked by quinine or quinidine and activated by intracellular calcium ion concentration ([Ca2+])i-activated PK). 2. Addition of 100 microM-quinine to the perifusion medium resulted in a 10--30 mV depolarization of the membrane and an increase in the input resistance of ca. 4.10(7) omega. 3. In the absence of glucose, 100 microM-quinine induced electrical activity. 4. In the presence of glucose, 100 microM-quinine abolished the burst pattern of electrical activity and very much reduced the graded response of spike frequency normally seen with different concentrations of glucose. 5. Addition of mitochondrial inhibitors, KCN, NaN3, DNP, CCCP, FCCP, to the perifusion medium containing glucose rapidly hyperpolarized the beta-cell membrane, inducing a concomitant decrease in input resistance. 6. In the presence of glucose, these mitochondrial inhibitors reversibly blocked electrical activity; upon removal of the inhibitor, recovery of electrical activity followed a biphasic pattern. 7. The effects of mitochondrial inhibitors were partially reversed by 100 microM-quinine. 8. It is proposed that the membrane potential of the beta-cell in the absence of glucose is predominantly controlled by the [Ca2+]i-activated PK. It is further suggested that this permeability to K controls the level for glucose stimulation and leads to the generation of the burst pattern.

Action Potentials↗
PubMed1978

Cyclic changes in potential and resistance of the beta-cell membrane induced by glucose in islets of Langerhans from mouse.

1. The effects of KCl on the membrane potential were studied in cells from mouse islets of Langerhans identified as beta-cells by the characteristic pattern of electrical activity induced by 11.1 mM glucose. 2. In the absence of glucose, when the beta-cell membrane does not exhibit electrical activity, the dependence of the membrane potential upon external potassium [K+]o, could be described by the constant field equation using a PK/PNa ratio between 30 and 75. 3. In 11.1 mM glucose, when the beta-cell membrane potential fluctuates between a silent phase at about -50 mV and an active phase at about -40 mV giving rise to a train of spikes, the dependence of the membrane potential upon [K+]o could also be described with the constant field equation using a smaller PK/PNa of about 15, during the silent phase, and of about 8, during the active phase (foot of the spikes during the burst). 4. A bridge amplifier for measuring the changes in membrane potential during the application of pulses of current through the same micro-electrode was used to estimate the input resistance of a beta-cell. In 11.1 mM glucose, rough estimates of the membrane resistance during the silent phase averaged 1.2 X 10(8) omega. 5. The time course of the changes in input resistance of the cell when switching from 0 to 11.1 mM glucose showed a transient decrease from 0.9 X 10(8) to 0.7 X 10(8) omega followed by an increase to 1.2 X 10(8) omega. 6. The burst pattern was shown to result from the superposition of two potential changes: (a) 5--10 mV depolarization (lasting about 10 sec in 11.1 mM glucose), and (b) 10--50 mM spikes (lasting about 0.1 sec). Only the latter could be suppressed by hyperpolarizing current injection. 7. Application of pulses of current during the various phases of the electrical activity in 11.1 mM glucose enable us to compare the resistance during the silent and active phases. This was found to be oscillating between a high resistance value at about 1.2 X 10(8) omega before each burst and a low resistance value at 0.9 X 10(8) omega during the active phase at the foot of the spikes. In some cells the resistance during the silent phase remained fairly constant. In other cells it increased gradually from 1.1 X 10(8) to 1.3 X 10(8) omega measured just before each burst of spikes. 8. The observed increase in resistance induced by glucose together with the measured dependency of the membrane potential on [K+]o with and without glucose can be explained by postulating that in the presence of glucose the K+ permeability of the beta-cell membrane is reduced. 9. The oscillations between a high and a low resistance state in the presence of 11.1 mM glucose could be due to a sudden decrease in K+ permeability followed by a much larger increase in permeability to other ions, presumably Na+ and Ca2+.

Action Potentials↗