Calcium and potassium currents recorded from pancreatic beta-cells under voltage clamp control

Physiology of pancreatic β-cells: Ion channels and molecular mechanisms implicated in stimulus-secretion coupling

The human and mouse islet of Langerhans is an endocrine organ composed of five different cells types; insulin-secreting β-cells, glucagon-producing α-cells, somatostatin-producing δ-cells, pancreatic polypeptide-secreting PP cells and ɛ-cells that secretes ghrelin. The most important cells are the pancreatic β-cells that comprise around 45-50% of human islets and 75-80% in the mouse. Pancreatic β-cells secrete insulin at high glucose concentration, thereby finely regulating glycaemia by the hypoglycaemic effects of this hormone. Different ion channels are implicated in the stimulus-secretion coupling of insulin. An increase in the intracellular ATP concentration leads to closure K_ATP channels, depolarizing the cell and opening voltage-gated calcium channels. The increase of intracellular calcium concentration induced by calcium entry through voltage-gated calcium channels promotes insulin secretion. Here, we briefly describe the diversity of ion channels present in pancreatic β-cells and the different mechanisms that are responsible to induce insulin secretion in human and mouse cells. Moreover, we described the pathophysiology due to alterations in the physiology of the main ion channels present in pancreatic β-cell and its implication to predispose metabolic disorders as type 2 diabetes mellitus.

Pancreatic β-Cell Electrical Activity and Insulin Secretion: Of Mice and Men

The pancreatic β-cell plays a key role in glucose homeostasis by secreting insulin, the only hormone capable of lowering the blood glucose concentration. Impaired insulin secretion results in the chronic hyperglycemia that characterizes type 2 diabetes (T2DM), which currently afflicts >450 million people worldwide. The healthy β-cell acts as a glucose sensor matching its output to the circulating glucose concentration. It does so via metabolically induced changes in electrical activity, which culminate in an increase in the cytoplasmic Ca2+ concentration and initiation of Ca2+-dependent exocytosis of insulin-containing secretory granules. Here, we review recent advances in our understanding of the β-cell transcriptome, electrical activity, and insulin exocytosis. We highlight salient differences between mouse and human β-cells, provide models of how the different ion channels contribute to their electrical activity and insulin secretion, and conclude by discussing how these processes become perturbed in T2DM.

Glucagon regulates orexin A secretion in humans and rodents

AIMS/HYPOTHESIS

Orexin A (OXA) modulates food intake, energy expenditure, and lipid and glucose metabolism. OXA regulates the secretion of insulin and glucagon, while glucose regulates OXA release. Here, we evaluate the role of glucagon in regulating OXA release both in vivo and in vitro.

METHODS

In a double-blind crossover study, healthy volunteers and type 1 diabetic patients received either intramuscular glucagon or placebo. Patients newly diagnosed with type 2 diabetes underwent hyperinsulinaemic-euglycaemic clamp experiments, and insulin-hypoglycaemia tests were performed on healthy volunteers. The primary endpoint was a change in OXA levels after intramuscular glucagon or placebo administration in healthy participants and patients with type 1 diabetes. Secondary endpoints included changes in OXA in healthy participants during insulin tolerance tests and in patients with type 2 diabetes under hyperinsulinaemic-euglycaemic conditions. Participants and staff conducting examinations and taking measurements were blinded to group assignment. OXA secretion in response to glucagon treatment was assessed in healthy and obese mice, the streptozotocin-induced mouse model of type 1 diabetes, and isolated rat pancreatic islets.

RESULTS

Plasma OXA levels declined in lean volunteers and in type 1 diabetic patients injected with glucagon. OXA levels increased during hyperinsulinaemic hypoglycaemia testing in healthy volunteers and during hyperinsulinaemic euglycaemic conditions in type 2 diabetic patients. Plasma OXA concentrations in healthy lean and obese mice and in a mouse model of type 1 diabetes were lower after glucagon treatment, compared with vehicle control. Glucagon decreased OXA secretion from isolated rat pancreatic islets at both low and high glucose levels. OXA secretion declined in pancreatic islets exposed to diazoxide at high and low glucose levels, and after exposure to an anti-insulin antibody. Glucagon further reduced OXA secretion in islets pretreated with diazoxide or an anti-insulin antibody.

CONCLUSIONS/INTERPRETATION

Glucagon inhibits OXA secretion in humans and animals, irrespective of changes in glucose or insulin levels. Through modifying OXA secretion, glucagon may influence energy expenditure, body weight, food intake and glucose metabolism.

The physiology of rodent beta-cells in pancreas slices

Beta-cells in pancreatic islets form complex syncytia. Sufficient cell-to-cell electrical coupling seems to ensure coordinated depolarization pattern and insulin release that can be further modulated by rich innervation. The complex structure and coordinated action develop after birth during fast proliferation of the endocrine tissue. These emergent properties can be lost due to various reasons later in life and can lead to glucose intolerance and diabetes mellitus. Pancreas slice is a novel method of choice to study the physiology of beta-cells still embedded in their normal cellulo-social context. I present major advantages, list drawbacks and provide an overview on recent advances in our understanding of the physiology of beta-cells using the pancreas slice approach.

A novel approach to in situ characterization of pancreatic beta-cells

The tissue-slice technique has enabled major insights into neural and neuroendocrine physiology. Our aim was to adapt this technique to study the function of the endocrine pancreas. The preparation combines an in situ approach, as in gland perfusion, with a resolution characteristic of electrophysiological studies on single cells. The membrane potential in beta-cells in the slices recorded using the whole-cell patch-clamp was close to the calculated reversal potential for K+. With sufficient ATP in the recording pipette the beta-cells depolarized rapidly on exposure to an increased glucose concentration or stimulation with tolbutamide. The cells preserved bursting and spiking capacity for tens of minutes despite the whole-cell dialysis. In addition, the voltage clamp was used to monitor the changes in the membrane capacitance and to allow correlation of the electrical activity and the cytosolic calcium changes. The pancreatic tissue slice preparation is a novel method for studying the function of the beta- and other pancreatic endocrine and exocrine cells under near-physiological conditions.

Inactivation of Delayed Potassium Current in Cultured Bovine Chromaffin Cells

Potassium currents of cultured bovine chromaffin cells in the presence of CdCl2 (0.5 mM) or CoCl2 (2 - 4 mM) may be described as a component that follows an n4 kinetics over the first 100 ms. However, during prolonged voltage clamp steps (5 - 20 s) this current reaches a peak and then declines to a non-zero steady-state level. In the absence of potassium accumulation, slow decay has been attributed to slow inactivation of the K+ conductance. Slow inactivation of calcium-independent potassium currents responds to the following: (i) the inactivation has a time course that may be fitted by a sum of two exponentials; (ii) inactivation is use-dependent (increasing frequency or duration of depolarizing pulses increases the extent of inactivation). Use-dependent inactivation does not appear when short (100 ms) hyperpolarizing prepulses are applied; (iii) at a membrane potential of - 60 mV, approximately 20% of the channels are inactivated; (iv) recovery from inactivation is very slow. At a holding potential of - 70 mV it takes about 20 s to reach the original value. The rate of inactivation recovery is voltage-dependent. The more hyperpolarized the membrane potential, the faster the recovery. The presence of use-dependent inactivation with slow recovery could be relevant for the stimulus - secretion coupling modulation in chromaffin cells.

Expression of a rapid, low-voltage threshold K current in insulin-secreting cells is dependent on intracellular calcium buffering

Depolarization-activated outward currents ranging in amplitude from 100-1000 pA were studied in cultured, insulin-secreting HIT cells and mouse B-cells using the whole-cell patch clamp. Outward current was identified as a K current since it was blocked by K channel blockers and its tail current reversed near EK. The K currents of HIT cells dialyzed with internal solutions containing 0.1-10 mM EGTA with no added calcium (Ca), or 10 mM EGTA with 2 mM added Ca, activated rapidly with depolarization. However, the stronger Ca buffer BAPTA (5 mM; no added Ca) blocked the rapidly activating current to reveal an underlying more slowly activating K current. With intracellular EGTA, application of the Ca channel blocker cadmium mimicked the effect of intracellular BAPTA. These data suggest that the rapid K current was mediated by low-voltage threshold, Ca-activated K channels while the slower K current was mediated by high threshold delayed rectifier K channels. Mouse B-cells also had both K current components. Dialyzing these cells with either BAPTA (5 mM, no added Ca) or high EGTA (10 mM with 2 mM Ca) blocked the rapid Ca-activated K current observed when cells were filled with 0.1 to 1 mM EGTA. It is concluded that the extent of Ca-activated K current activation in either HIT or adult mouse B-cells depends on the degree of intracellular Ca buffering.