Oscillatory patterns of electrical activity in mouse pancreatic islets of Langerhans recorded in vivo

Binding of sulphonylureas to plasma proteins - A KATP channel perspective

Sulphonylurea drugs stimulate insulin secretion from pancreatic β-cells primarily by inhibiting ATP sensitive potassium (K_ATP) channels in the β-cell membrane. The effective sulphonylurea concentration at its site of action is significantly attenuated by binding to serum albumin, which makes it difficult to compare in vitro and in vivo data. We therefore measured the ability of gliclazide and glibenclamide to inhibit K_ATP channels and stimulate insulin secretion in the presence of serum albumin. We used this data, together with estimates of free drug concentrations from binding studies, to predict the extent of sulphonylurea inhibition of K_ATP channels at therapeutic concentrations in vivo. K_ATP currents from mouse pancreatic β-cells and Xenopus oocytes were measured using the patch-clamp technique. Gliclazide and glibenclamide binding to human plasma were determined in spiked plasma samples using an ultrafiltration-mass spectrometry approach. Bovine serum albumin (60g/l) produced a mild, non-significant reduction of gliclazide block of K_ATP currents in pancreatic β-cells and Xenopus oocytes. In contrast, glibenclamide inhibition of recombinant K_ATP channels was dramatically suppressed by albumin (predicted free drug concentration <0.1%). Insulin secretion was also reduced. Free concentrations of gliclazide and glibenclamide in the presence of human plasma measured in binding experiments were 15% and 0.05%, respectively. Our data suggest the free concentration of glibenclamide in plasma is too low to account for the drug’s therapeutic effect. In contrast, the free gliclazide concentration in plasma is high enough to close K_ATP channels and stimulate insulin secretion.

Oscillatory membrane potential response to glucose in islet beta-cells: a comparison of islet-cell electrical activity in mouse and rat

In contrast to mouse, rat islet beta-cell membrane potential is reported not to oscillate in response to elevated glucose despite demonstrated oscillations in calcium and insulin secretion. We aim to clarify the electrical activity of rat islet beta-cells and characterize and compare the electrical activity of both alpha- and beta-cells in rat and mouse islets. We recorded electrical activity from alpha- and beta-cells within intact islets from both mouse and rat using the perforated whole-cell patch clamp technique. Fifty-six percent of both mouse and rat beta-cells exhibited an oscillatory response to 11.1 mm glucose. Responses to both 11.1 mm and 2.8 mm glucose were identical in the two species. Rat beta-cells exhibited incremental depolarization in a glucose concentration-dependent manner. We also demonstrated electrical activity in human islets recorded under the same conditions. In both mouse and rat alpha-cells 11 mm glucose caused hyperpolarization of the membrane potential, whereas 2.8 mm glucose produced action potential firing. No species differences were observed in the response of alpha-cells to glucose. This paper is the first to demonstrate and characterize oscillatory membrane potential fluctuations in the presence of elevated glucose in rat islet beta-cells in comparison with mouse. The findings promote the use of rat islets in future electrophysiological studies, enabling consistency between electrophysiological and insulin secretion studies. An inverse response of alpha-cell membrane potential to glucose furthers our understanding of the mechanisms underlying glucose sensitive glucagon secretion.

Individual mice can be distinguished by the period of their islet calcium oscillations: is there an intrinsic islet period that is imprinted in vivo?

Pulsatile insulin secretion in vivo is believed to be derived, in part, from the intrinsic glucose-dependent intracellular calcium concentration ([Ca2+]i) pulsatility of individual islets. In isolation, islets display fast, slow, or mixtures of fast and slow [Ca2+]i oscillations. We show that the period of islet [Ca2+]i oscillations is unique to each mouse, with the islets from an individual mouse demonstrating similar rhythms to one another. Based on their rhythmic period, mice were broadly classified as being either fast (0.65 +/- 0.1 min; n = 6 mice) or slow (4.7 +/- 0.2 min; n = 15 mice). To ensure this phenomenon was not an artifact of islet-to-islet communication, we confirmed that islets cultured in isolation (period: 2.9 +/- 0.1 min) were not statistically different from islets cultured together from the same mouse (3.1 +/- 0.1 min, P > 0.52, n = 5 mice). We also compared pulsatile insulin patterns measured in vivo with islet [Ca2+]i patterns measured in vitro from six mice. Mice with faster insulin pulse periods corresponded to faster islet [Ca2+]i patterns, whereas slower insulin patterns corresponded to slower [Ca2+]i patterns, suggesting that the insulin rhythm of each mouse is preserved to some degree by its islets in vitro. We propose that individual mice have characteristic oscillatory [Ca2+]i patterns, which are imprinted in vivo through an unknown mechanism.

Glucose metabolism and oscillatory behavior of pancreatic islets

A variety of oscillations are observed in pancreatic islets. We establish a model incorporating two oscillatory systems of different time scales: One is the well-known bursting model in pancreatic cells and the other is the glucose-insulin feedback model which considers direct and indirect feedback of secreted insulin. These two are coupled to interact with each other in the combined model, and two basic assumptions are made on the basis of biological observations: The conductance gK(ATP) for the ATP-dependent potassium current is a decreasing function of the glucose concentration whereas the insulin secretion rate is given by a function of the intracellular calcium concentration. Obtained via extensive numerical simulations are complex oscillations including clusters of bursts, slow and fast calcium oscillations, and so on. We also consider how the intracellular glucose concentration depends upon the extracellular glucose concentration, and examine the inhibitory effects of insulin.

FRET-based voltage probes for confocal imaging: membrane potential oscillations throughout pancreatic islets

Insulin secretion is dependent on coordinated pancreatic islet physiology. In the present study, we found a way to overcome the limitations of cellular electrophysiology to optically determine cell membrane potential ( Vm) throughout an islet by using a fast voltage optical dye pair. Using laser scanning confocal microscopy (LSCM), we observed fluorescence (Förster) resonance energy transfer (FRET) with the fluorescent donor N-(6-chloro-7-hydroxycoumarin-3-carbonyl)-dimyristoylphosphatidyl-ethanolamine and the acceptor bis-(1,3-diethylthiobarbiturate) trimethine oxonol in the plasma membrane of essentially every cell within an islet. The FRET signal was approximately linear from Vm−70 to +50 mV with a 2.5-fold change in amplitude. We evaluated the responses of islet cells to glucose and tetraethylammonium. Essentially, every responding cell in a mouse islet displayed similar time-dependent changes in Vm. When Vmwas measured simultaneously with intracellular Ca2+, all active cells showed tight coupling of Vmto islet cell Ca2+changes. Our findings indicate that FRET-based, voltage-sensitive dyes used in conjunction with LSCM imaging could be extremely useful in studies of excitation-secretion coupling in intact islets of Langerhans.

Biophysical evidence that connexin-36 forms functional gap junction channels between pancreatic mouse beta-cells

Connexin-36 (Cx36) is the only gap junction protein that has been unambiguously identified in rodent pancreatic beta-cells. However, properties of gap junction channel unitary currents between beta-cells remain unrevealed. To address whether Cx36 forms functional channels in beta-cells, we characterized biophysical properties of macro- and microscopic junctional currents recorded from dual whole cell voltage clamp isolated pairs of dispersed mouse beta-cells. Electrical coupling was recorded in 80% of cell pairs with a junctional conductance (g(j)) of 355 +/- 45 pS (n = 20). Transjunctional voltage dependence was identified in three of seven cell pairs with high-input membrane resistances. Normalized steady-state g(j) (Gj) and transjunctional-voltage relation were well described by a two-state Boltzmann equation [maximal conductance (Gmax) = 1.0, voltage-insensitive conductance (Gmin) = 0.3 and 0.28, voltage gating sensitivity (A) = 0.21 and 0.23, and voltage at which one-half of the initial voltage-dependent conductance was reached (Vo) = -85 and 87 mV for negative and positive potentials, respectively]. Halothane reversibly uncoupled beta-cell pairs, and, during recovery, unitary conductances of 5-10 pS were recorded while using patch pipettes containing mainly CsCl. Although these properties are similar to those previously described for Cx36 channels in mammalian cell systems, we found that beta-cell junctional currents were insensitive to quinine. Cx36 transcript and protein expression in islets and freshly dispersed cell preparations was confirmed by RT-PCR and immunofluorescence. In conclusion, biophysical properties of junctional channels between beta-cells are similar but not identical to those previously described for homomeric Cx36 channels. Cell type-specific mechanisms that may account for these differences are discussed.

Metabolic setpoint control mechanisms in different physiological systems at rest and during exercise

Using a number of different homeostatic control mechanisms in the brain and peripheral physiological systems, metabolic activity is continuously regulated at rest and during exercise to prevent catastrophic system failure. Essential for the function of these regulatory processes are baseline "setpoint" levels of metabolic function, which can be used to calculate the level of response required for the maintenance of system homeostasis after system perturbation, and to which the perturbed metabolic activity levels are returned to at the end of the regulatory process. How these setpoint levels of all the different metabolic variables in the different peripheral physiological systems are created and maintained, and why they are similar in different individuals, has not been well explained. In this article, putative system regulators of metabolic setpoint levels are described. These include that: (i) innate setpoint values are stored in a certain region of the central nervous system, such as the hypothalamus; (ii) setpoint values are created and maintained as a response to continuous external perturbations, such as gravity or "zeitgebers", (iii) setpoint values are created and maintained by complex system dynamical activity in the different peripheral systems, where setpoint levels are regulated by the ongoing feedback control activity between different peripheral variables; (iv) human anatomical and biomechanical constraints contribute to the creation and maintenance of metabolic setpoints values; or (v) a combination of all these four different mechanisms occurs. Exercise training and disease processes can affect these metabolic setpoint values, but the setpoint values are returned to pre-training or pre-disease levels if the training stimulus is removed or if the disease process is cured. Further work is required to determine what the ultimate system regulator of metabolic setpoint values is, why some setpoint values are more stringently protected by homeostatic regulatory mechanisms than others, and the role of conscious decision making processes in determining the regulation of metabolic setpoint values.

Contribution of the endoplasmic reticulum to the glucose-induced [Ca(2+)](c) response in mouse pancreatic islets

Thapsigargin (TG), a blocker of Ca(2+) uptake by the endoplasmic reticulum (ER), was used to evaluate the contribution of the organelle to the oscillations of cytosolic Ca(2+) concentration ([Ca(2+)](c)) induced by repetitive Ca(2+) influx in mouse pancreatic beta-cells. Because TG depolarized the plasma membrane in the presence of glucose alone, extracellular K(+) was alternated between 10 and 30 mM in the presence of diazoxide to impose membrane potential (MP) oscillations. In control islets, pulses of K(+), mimicking regular MP oscillations elicited by 10 mM glucose, induced [Ca(2+)](c) oscillations whose nadir remained higher than basal [Ca(2+)](c). Increasing the depolarization phase of the pulses while keeping their frequency constant (to mimic the effects of a further rise of the glucose concentration on MP) caused an upward shift of the nadir of [Ca(2+)](c) oscillations that was reproduced by raising extracellular Ca(2+) (to increase Ca(2+) influx) without changing the pulse protocol. In TG-pretreated islets, the imposed [Ca(2+)](c) oscillations were of much larger amplitude than in control islets and occurred on basal levels. During intermittent trains of depolarizations, control islets displayed mixed [Ca(2+)](c) oscillations characterized by a summation of fast oscillations on top of slow ones, whereas no progressive summation of the fast oscillations was observed in TG-pretreated islets. In conclusion, the buffering capacity of the ER in pancreatic beta-cells limits the amplitude of [Ca(2+)](c) oscillations and may explain how the nadir between oscillations remains above baseline during regular oscillations or gradually increases during mixed [Ca(2+)](c) oscillations, two types of response observed during glucose stimulation.

Primary in vivo oscillations of metabolism in the pancreas

The role of metabolism in the generation of plasma insulin oscillations was investigated by simultaneous in vivo recordings of oxygen tension (pO(2)) in the endocrine and exocrine pancreas and portal blood insulin concentrations in the anesthetized rat. At the start of the experiment, the blood glucose concentration of seven rats was 6.2 +/- 0.1 mmol/l and the arterial blood pressure was 116 +/- 5 mmHg. These values did not differ from those obtained at the end of the experiment. Islet pO(2) was measured by impaling superficially located islets with a miniaturized Clark electrode. The pO(2) measurements revealed slow (0.21 +/- 0.03 min(-1)) with superimposed rapid (3.1 +/- 0.3 min(-1)) oscillations. The average pO(2) was 39 +/- 5 mmHg. Simultaneous recordings of pO(2) in the exocrine pancreas were significantly lower (16 +/- 6 mmHg), but showed a slow and a rapid oscillatory activity with similar frequencies as seen in the endocrine pancreas. Corresponding measurements of portal insulin concentrations revealed insulin oscillations at a frequency of 0.22 +/- 0.02 min(-1). The results are the first in vivo recordings of an oscillatory islet parameter with a frequency corresponding to that of plasma insulin oscillations; they support a primary role of metabolic oscillations in the induction of plasma insulin oscillations.

Feedback control of the ATP-sensitive K(+) current by cytosolic Ca(2+) contributes to oscillations of the membrane potential in pancreatic beta-cells

During glucose stimulation, pancreatic beta-cells display membrane potential oscillations that correspond to intermittent Ca(2+) influx, leading to oscillations of the cytosolic free calcium concentration ([Ca(2+)](c)) and insulin secretion. The role of ATP-sensitive K(+) (K(+)-ATP) channels in the control of these oscillations was investigated by measuring the K(+)-ATP current (I(KATP)) with the perforated mode of the patch-clamp technique. No oscillations of I(KATP) were observed when glucose-stimulated beta-cells were kept hyperpolarized, thus with low and stable [Ca(2+)](c). However, increasing [Ca(2+)](c) by Ca(2+) influx (depolarizing pulses) or Ca(2+) mobilization (acetylcholine) transiently augmented I(KATP). This effect was abolished by tolbutamide, attenuated by increasing the glucose concentration in the medium, and prevented by abrogation of the [Ca(2+)](c) rise, which demonstrates that the current is really I(KATP) and that its increase is Ca(2+)-dependent. Injection of a current of a similar amplitude to that of the Ca(2+)-induced increase in I(KATP) was sufficient to repolarize glucose-stimulated beta-cells. These results suggest that, in the absence of [Ca(2+)](c) oscillations, no metabolic oscillations affect I(KATP) in pancreatic beta-cells. In contrast, [Ca(2+)](c) oscillations evoke I(KATP) oscillations. This mechanism may constitute the feedback loop controlling the glucose-induced oscillating electrical activity in beta-cells.

Role of oscillations in membrane potential, cytoplasmic Ca2+, and metabolism for plasma insulin oscillations

A model for the relationship between ionic and metabolic oscillations and plasma insulin oscillations is presented. It is argued that the pancreatic beta-cell in vivo displays two intrinsic frequencies that are important for the regulation of plasma insulin oscillations. The rapid oscillatory activity (2--7 oscillations [osc] per minute), which is evident in both ionic and metabolic events, causes the required elevation in cytoplasmic Ca(2+) concentration ([Ca(2+)](i)) for the exocytosis of insulin granules. This activity is important for regulation of the amplitude of plasma insulin oscillations. The frequency of the rapid oscillatory ionic activities is regulated by glucose and allows the beta-cell to respond in an analogous way, with gradual changes in [Ca(2+)](i) and insulin release in response to the alterations in glucose concentration. The slower oscillatory activity (0.2--0.4 osc/min), which is evident in the metabolism of the beta-cell, has a frequency corresponding to the frequency observed in plasma insulin oscillations. The frequency is not affected by changes in the glucose concentration. This activity is suggested to generate energy in a pulsatile fashion, which sets the frequency of the plasma insulin oscillations. It is proposed that the slow oscillations in [Ca(2+)](i) observed in vitro are a manifestation of the metabolic oscillations and do not represent an in vivo phenomenon.

Regulation by tolbutamide and diazoxide of the electrical activity in mouse pancreatic beta-cells recorded in vivo

1. The glucose-dependence of beta-cell electrical activity and the effects of tolbutamide and diazoxide were studied in anaesthetized mice. 2. In untreated animals there was a direct relationship between glycaemia and the burst pattern of electrical activity. Animals with high glucose concentration showed continuous electrical activity. The application of insulin led to a steady decrease in blood glucose concentration and a transition from continuous to oscillatory activity at 7.7+/-0.1 mM glucose (mean+/-s.d.) and a subsequent transition from oscillatory to silent at 4.7+/-0.6 mM glucose. 3. At physiological blood glucose concentrations the electrical activity was oscillatory. The injection of tolbutamide (1800 mg kg[-1]) transformed this oscillatory pattern into one of continuous electrical activity. The increased electrical activity was associated with a decrease in blood glucose concentration from 7.1+/-0.9 (control) to 5.5+/-1.0 mM (10 min after tolbutamide injection). The effects of tolbutamide are consistent with a direct blocking effect on the K(ATP) channel that leads to membrane depolarization. 4. The injection of diazoxide (6000 mg kg[-1]) hyperpolarized the cells and transformed the oscillatory pattern into a silent one. This is consistent with a direct stimulant effect by diazoxide on the K(ATP) channel. The use of tolbutamide or diazoxide correspondingly led to the lengthening or shortening of the active phase of electrical activity, respectively. This indicates that in vivo, such activity can be modulated by the relative degree of activation or inhibition of the K(ATP) channel. 5. These results indicate that under physiological conditions, tolbutamide and diazoxide have direct and opposite effects on the electrical activity of pancreatic beta-cells, most likely through their action on K(ATP) channels. This is consistent with previous work carried out on in vitro models and explains the drugs hypo- and hyperglycaemic effects.