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I Atwater

Publications and source records attributed to I Atwater.

At least 55 records · Page 3Linked to original sources
PubMed1988

The ATP-sensitive potassium channel in pancreatic B-cells is inhibited in physiological bicarbonate buffer.

The effects of bicarbonate buffer (HCO3-/CO2) on the activity of the two K+ channels proposed by some to control the pancreatic B-cell membrane response to glucose were studied. Single K+-channel records from membrane patches of cultured B-cells dissociated from adult rat islets exposed to a glucose- and bicarbonate-free medium (Na-Hepes in place of bicarbonate) exhibit the activity of both the ATP-sensitive as well as the [Ca2+]i-activated K+ channels. However, in the presence of bicarbonate-buffered Krebs solution, the activity of the ATP-sensitive K+ channel is inhibited leaving the activity of the K+ channel activated by intracellular [Ca2+]i unaffected. In the absence of bicarbonate (Hepes/NaOH in place of bicarbonate), lowering the external pH from 7.4 to 7.0 also has differential effects on the two K+ channels. While the K+ channel sensitive to ATP is inhibited, the K+ channel activated by a rise in [Ca2+]i is not affected. To determine whether the response of the B-cell in culture to bicarbonate is also present when the B-cell is functioning within the islet syncytium, the effects of bicarbonate removal on membrane potential of B-cells from intact mouse islets were compared. These studies showed that glucose-evoked electrical activity is also blocked in bicarbonate-free Krebs solution. Furthermore, in the absence of bicarbonate and presence of glucose (11 mM), electrical activity was recovered by lowering the pHo from 7.4 to 7.0. The ATP-sensitive K+-channel activity is greatly reduced by physiologically buffered solutions in pancreatic B-cells in culture. The most likely explanation for the bicarbonate effects is that they are mediated by cytosolic pH changes. Removal of bicarbonate (keeping the external pH at 7.4 with Hepes/NaOH as buffer) would increase the pHi. Since the activity of the [Ca2+]i-dependent K+ channels is not affected by the removal of the bicarbonate buffer, our patch-clamp data in cultured B-cells indicate an involvement of [Ca2+]i-activated K+ channels in the control of the membrane potential. For the B-cell in the islet, we propose that the burst pattern of electrical activity (Ca2+ entry) is controlled, at least in part, by the [Ca2+]i-activated K+ channel.

Adenosine Triphosphate↗
PubMed1988

Characterization and control of pulsatile secretion of insulin and glucagon.

Periodic oscillation of insulin and glucagon by isolated mice islets has been studied. Pulsatile secretion of insulin and glucagon was observed at all glucose concentrations tested. The frequency of oscillation per 20 min for glucagon was 5.0 +/- 0.26 and for insulin 4.0 +/- 0.26 (n = 6), approximating to periodicities of 4 and 5 min, respectively. These did not change by increasing the glucose concentration to 11.1 or 22.2 mM from 5.5 mM (basal). The maximal amplitude of glucagon secretion was not altered by raising the glucose concentration to 11.1 mM from basal. However, 22.2 mM glucose significantly suppressed the amount of glucagon released when compared with glucagon secretion in the presence of 5.5 mM glucose. In contrast, the maximal amplitude of insulin increased from 444.2 +/- 37.7 to 777.2 +/- 61.4 and from 271.8 +/- 35 to 701 +/- 26.5 pg/min (p less than 0.01, n = 6) by switching from basal to 11.1 and 22.2 mM glucose, respectively. We conclude from this study that the pacemaker controlling pulsatile secretion of insulin and glucagon is within the islet. Although the amplitude of secretion of these hormones is regulated by the ambient glucose concentration, the frequency of their pulsatile secretion is not.

Animals↗
PubMed1986

Effect of temperature upon potassium-stimulated insulin release and calcium entry in mouse and rat islets.

The effect of cooling to 27 degrees C was studied in islets of Langerhans exposed to 5 and 50 mM potassium in the absence of glucose. Membrane potential and insulin release were measured simultaneously from microdissected mouse islets while 45Ca outflow and insulin release were measured from collagenase-isolated rat islets. Cooling inhibited potassium-induced insulin release in both preparations. However, calcium entry estimated from electrical records and from 45Ca outflow experiments was only slightly affected by decreasing the temperature to 27 degrees C. It is concluded that the inhibition of insulin release caused by cooling to 27 degrees C can, within limits, be dissociated from calcium influx.

Animals↗
PubMed1986

Direct identification of electrophysiologically monitored cells within intact mouse islets of Langerhans.

Cells found to be electrically active within microdissected mouse islets of Langerhans perifused with high (greater than or equal to 11.1 mM) glucose concentrations were labeled by injecting Lucifer yellow through the recording electrode. After fixation, these cells were located by fluorescence microscopy on sections serially cut throughout the islets and were subsequently identified by immunofluororescence staining with specific anti-islet hormone sera. Electrophysiologic control confirmed that the electrode tip had remained within the same cell throughout the experiment and showed that Lucifer yellow labeling did not affect the electrical activity of the impaled cell. Upon individual impalements, Lucifer yellow labeled either the impaled cell alone or this cell and some of its neighbors to which it was dye coupled. Immunofluorescence staining of the Lucifer yellow-labeled cells revealed that glucose-induced electrical activity was recorded from individual B-cells or groups of dye-coupled B-cells as well as from A-cells coupled to B-cells.

Action Potentials↗
PubMed1985

Dissociation by methylamine of insulin release from glucose-induced electrical activity in isolated mouse islets of Langerhans.

The effect of methylamine on electrical activity and simultaneously measured insulin release was investigated in single perifused islets of normal mice. Methylamine, (2 mmol/L or 6 mmol/L) failed to affect beta-cell input resistance and only caused a modest and transient inhibition of electrical activity of islets exposed to 11.1 mmol/L glucose. Methylamine (2 mmol/L) inhibited insulin release evoked by a five-minute rise in glucose concentration from 5.6 to 22.2 mmol/L, even when the glucose-induced electrical activity remained unaltered. Methylamine, at 2 or 5 mmol/L, partially inhibited insulin release but failed to affect the continuous electrical activity in islets exposed throughout to 22.2 mmol/L glucose. At 10 mmol/L, methylamine reduced both insulin release and electrical activity. These data reinforce the idea that the glucose-induced changes in beta-cell membrane potential represent an early event in the process of stimulus-secretion coupling and can be dissociated from the subsequent process of insulin release.

Animals↗
PubMed1985

Chaotic and irregular bursting electrical activity in mouse pancreatic B-cells.

The glucose-induced B-cell electrical activity was recorded in islets of Langerhans isolated from Swiss Webster albino mice originating from different suppliers. 23 out of 25 islets obtained from mice bred at the Charles River Breeding Station (CR mice) exhibited irregular or chaotic burst patterns of electrical activity, while 36 out of 40 islets isolated from mice bred locally at the National Institutes of Health displayed the typical bursting activity. The CR mice tended to recover a regular pattern after 1 mo on the National Institutes of Health mouse diet. The irregular or chaotic bursting electrical activity is proposed to result from changes in B-cell membrane composition or cellular metabolism, possibly induced by differences in diet.

Animals↗
PubMed1985

Glucose-induced oscillatory changes in extracellular ionized potassium concentration in mouse islets of Langerhans.

Liquid membrane [K+]-sensitive microelectrodes (1-2 micron tip diameter) were used to measure the extracellular ionized potassium concentration in mouse pancreatic islets of Langerhans. With the tip of the microelectrode at the surface of the islet, the time course of the [K+]-sensitive electrode potential changes in response to the application of rapid changes in [K+]o (from 1.25 to 5 mM), could be reproduced by the equation for K+-diffusion through a 100-micron-thick unstirred layer around the islet (diffusion coefficient for K+ at 27 degrees C, DK,o, taken as 1.83 X 10(-5) cm2/s). The time to reach 63% of the steady-state electrode response with the tip in the chamber at the surface of the islet was from 5 to 6 s. When the tip of the [K+]-sensitive electrode was placed in the islet tissue, the time for the response to reach 63% of the steady-state level increased. The time course of the [K+]-sensitive electrode response could be reproduced using the same diffusion model assuming that K+ diffusion into the islet tissue takes place in a tortuous intercellular path with an apparent diffusion coefficient, DK,I, about half of DK,o, in series with the unstirred layer around the islet. In the absence of glucose the potassium concentration in the extracellular space, [K+]I, was found to be higher than the concentration in the external modified Krebs solution, [K+]o. The difference in concentration [K+]I - [K+]o was greater when [K+]o was smaller than 2 mM. In the presence of glucose (between 11 and 16 mM), under steady-state conditions, small oscillatory changes in the [K+], (1.48 +/- 0.94 mM) were detected. Simultaneous recording of membrane potential from one B-cell and [K+], in the same islet indicated that the potassium concentration increased during the active phase of the bursts of electrical activity. Maximum concentration in the intercellular was reached near the end of the active phase of the bursts. We propose that the space between islet cells constitutes a restricted diffusion system where potassium accumulates during the transient activation of potassium channels.

Animals↗
PubMed1985

Effects of the calcium channel agonist, BAY K 8644, on electrical activity in mouse pancreatic B-cells.

We studied the effects of the dihydropyridine derivative BAY K 8644 on the membrane potential of B-cells in mouse pancreatic islets. BAY K 8644, in a dose-dependent manner, decreased the spike frequency but increased the duration of the spikes elicited by glucose with or without quinine or tetraethylammonium (TEA). These effects were antagonized by cobalt and nifedipine but not by tetrodotoxin. The interval between spikes was proportionate to the duration of the spikes and the ratio of the interval to the spike duration was constant at all concentrations of BAY K 8644 tested. Peak inward current, estimated from the derivative of the action potential recorded in the presence of TEA, was increased by BAY K 8644 and decreased by nifedipine. BAY K 8644 elicited spike activity when the membrane was moderately depolarized by either 5.6 mM glucose or 15 mM K+, but did not change the membrane potential of the resting hyperpolarized B-cell. These results suggest that BAY K 8644 acts on the open Ca2+-channels. The threshold occurs at a membrane potential of -50 mV. Also, the modifications of the shape of the spikes appear to reflect specific changes in Ca2+ entry. We propose the existence of a Ca2+-induced Ca2+-channel inactivation process in the pancreatic B-cell.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1985

Membrane potential measurements in islets of Langerhans from ob/ob obese mice suggest an alteration in [Ca2+]i-activated K+ permeability.

High-resistance micro-electrodes were used to measure membrane potentials in beta-cells from islets of Langerhans of ob/ob obese mice (Norwich colony). In the presence of glucose the burst pattern of electrical activity recorded in ob/ob beta-cells, although similar to the burst pattern recorded from normal beta-cells, presents important differences. The membrane potential of the ob/ob beta-cells in the presence of 11 mM glucose in the modified Krebs solution oscillates between a silent-phase level at -48 mV and an active-phase level at -36 mV, similarly to normal mouse islet beta-cells. However, the average active-phase duration is 20 s in ob/ob beta-cells compared with 5 s in normal beta-cells. The average burst frequency is 1.8 bursts/min in ob/ob beta-cells compared with 3 bursts/min in normal beta-cells. While normal beta-cells show continuous spike activity above 16 mM glucose, ob/ob beta-cells often exhibit a burst pattern of electrical activity at glucose concentrations as high as 33 mM. Compared with normal beta-cells, the relationship between spike frequency and glucose concentration is shifted towards lower concentrations in ob/ob beta-cells. Thus, the concentration for half-maximal spike frequency is 6.9 mM for the ob/ob beta-cells and 10.2 mM for the normal beta-cells. In ob/ob beta-cells, the mitochondrial inhibitor carbonyl-cyanide m-chlorophenylhydrazone induces hyperpolarization of the membrane, consistent with its effect of stimulating K+ permeability in normal islets. However, quinine and the sulphonylurea glibenclamide did not block the silent phase between the bursts of electrical activity. Both drugs block the [Ca2+]i-activated K+ permeability thought to control the membrane potential at the silent phase in normal beta-cells. The modified pattern of response to glucose and decreased sensitivity to quinine and glibenclamide suggest that the beta-cell membrane of the ob/ob islet of Langerhans has a modified [Ca2+]i-activated K+ permeability.

Animals↗
PubMed1984

The response of pancreatic beta-cell membrane potential to potassium-induced calcium influx in the presence of glucose.

Membrane potential measurements were made in pancreatic beta-cells from microdissected islets from normal mice. In the presence of 11 mM glucose, depolarization of the membrane for 1 min with 50 mM potassium is followed by an inhibition of electrical activity before the normal burst pattern resumes. This inhibitory period, called the recovery time, is steady for each beta-cell after three consecutive pulses of 50 mM potassium. The mean recovery time is 109 s. During the recovery time, the membrane is hyperpolarized and the input resistance is decreased, indicating that potassium permeability is high over this period. The recovery time is dependent on the size of the depolarization: 1 min exposure to potassium concentrations below 50 mM reduces the recovery time with a half-maximal effect at 38.5 mM potassium, corresponding to -27 mV. Also, increasing the extracellular calcium concentration lengthens the recovery time. Increasing the glucose concentration, however, shortens the recovery time. It is postulated that the recovery time represents activation of the calcium-gated potassium permeability and is a reflexion of the time taken for the beta-cell to buffer the increased intracellular calcium resulting from the potassium depolarization.

Animals↗
PubMed1984

Electrophysiological evidence for the inhibition of potassium permeability in pancreatic beta-cells by glibenclamide.

The effects of glibenclamide on the electrical activity of the beta-cells of the islets of Langerhans of normal mice have been investigated in the absence and presence of glucose (11.1 mM). Glibenclamide depolarized the cell membrane and this has been interpreted in terms of an increase in the ratio of the Na+ and K+ permeabilities, PNa/PK. This ratio increased from 0.05 to 0.24 in the presence of 4 microM glibenclamide and zero glucose. The input resistance of the beta-cells also increased. These observations indicate a decrease in K+ permeability. The effect is only slowly reversed after removal of glibenclamide. Uncouplers of oxidative phosphorylation do not reverse the depolarization induced by glibenclamide. It is suggested that glibenclamide is acting directly to inhibit the [Ca2+]i-gated K+ permeability in the beta-cell membrane.

Animals↗
PubMed1984

Cooling dissociates glucose-induced insulin release from electrical activity and cation fluxes in rodent pancreatic islets.

Insulin release and beta-cell membrane potentials in response to glucose at 37 and 27 degrees C have been measured simultaneously in single, micro-dissected, perifused islets of Langerhans from normal mice. Insulin release and 45Ca outflow in response to glucose at 37 and 27 degrees C have been measured simultaneously from perfused islets isolated by collagenase digestion from normal rats. The effect of cooling on beta-cell membrane potassium permeability was assessed by changes in measured membrane potential and input resistance (in the mouse) and by changes in 86Rb outflow (in the rat). Resting and active beta-cell membrane parameters (i.e. membrane potential, spike frequency, input resistance, 45Ca outflow and 86Rb outflow), in both mouse and rat islets, were affected only slightly by cooling to 27 degrees C, with temperature coefficients of 2 or lower. At 27 degrees C glucose-stimulated insulin release was inhibited completely in mouse islets and almost completely in rat islets. The temperature coefficients in both preparations were greater than 5. It is concluded that beta-cell electrical activity and changes in membrane permeability induced by glucose are not consequences of insulin release.

Animals↗
PubMed1984

Effects of Zn2+ on glucose-induced electrical activity and insulin release from mouse pancreatic islets.

The effects of Zn2+ and CO2+ on glucose-induced beta-cell electrical activity and on insulin release from microdissected mouse pancreatic islets were studied. In 11 mM glucose the electrical activity is characterized by a burst pattern with a bimodal distribution of spike amplitudes along the plateau phase. Zn2+ at 0.05 mM induced a reduction in the number of spikes during the bursts and preferentially blocked the large action potentials. Zn2+ at 0.1 mM and CO2+ at 1.0 mM completely inhibited the electrical activity in response to glucose. Zn2+ inhibition of electrical activity was poorly reversible, whereas CO2+ inhibition was rapidly and completely reversible. Zn2+ and CO2+ inhibited the glucose-stimulated insulin release from microdissected perifused islets. Half-maximal inhibition occurred at about 0.3 mM for both metals. Zn2+ also inhibited K+-induced insulin release in the absence of glucose, indicating that Zn2+ inhibition does not involve glucose metabolism. It is proposed that Zn2+ blocks the voltage-gated Ca2+ channels in pancreatic beta-cells.

Action Potentials↗
PubMed1984

The topography of electrical synchrony among beta-cells in the mouse islet of Langerhans.

beta-Cells in microdissected islets of Langerhans produce rhythmical bursts of electrical activity. This was monitored with two micro-electrodes simultaneously and the frequency and phase (collectively referred to as synchrony) of the two signals was investigated. At any instant two impaled cells produced bursts of the same frequency even when separated by up to 400 micron. When the electrode tips were separated by less than about 20 micron and current injection showed the cells to be ionically coupled the two signals were in phase and had almost identical shape. The phase relations between cells further apart were variable, the leading cell usually being located deeper within the islet than the other impaled cell. Increasing the glucose concentration increased electrical activity, reduced any phase lags and made the shape of the bursts more similar. There was less lag between the responses from two cells when the glucose concentration was suddenly reduced, than when it was suddenly increased. Qualitatively similar observations were made in glibenclamide-treated mice, a treatment previously shown to increase dye coupling between islet cells. However, the response to increasing glucose concentrations showed less phase lag; likewise the phase lag between bursts was reduced. Furthermore the response to current injected into one cell could be detected at much larger distances (up to 80 micron) than in control islets. This suggests that electrical coupling of beta-cells was improved in sulphonylurea-treated mice. Electron microscopy of both control and glibenclamide-treated mouse islets fixed at the end of each electrophysiological experiment showed the region impaled by the electrodes to be well preserved and, whenever the electrodes penetrated at least 20 micron into the islet, to contain a large proportion of beta-cells. The data support the view that, within an islet, most but not necessarily all cells are electrically synchronized, and that the coupling can be modulated by natural and pharmacological secretagogues.

Action Potentials↗