BPDZ 154 Activates Adenosine 5′-Triphosphate-Sensitive Potassium Channels:In VitroStudies Using Rodent Insulin-Secreting Cells and Islets Isolated from Patients with Hyperinsulinism (original) (raw)
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Potassium Channels, Imidazolines, and Insulin-Secreting Cells
Annals of the New York Academy of Sciences, 1995
Leicester LE2 7LX. UK It has been recognized for many years that certain imidazoline-derived a-adrenergic receptor antagonists will promote insulin secretion from the p-cells of the pancreatic islets of Langerhans. These effects occur both in vivo and in vitro and are mediated through interaction with the P-cell membrane at a site dissimilar to the a-adrenoceptor. Through the combined approaches of radiolabeled tracer studies and electrophysiology, there is good evidence that many of these compounds will inhibit potassium channels. Most electrophysiological studies have focused on a particular type of K' channel, the adenosine-triphosphate (ATP)-regulated K+ (K+ATp) channel. These channels are inhibited by imidazolines, and because they also play a major role in governing changes in P-cell membrane potential, channel block accounts for the fact that such compounds will depolarize the membrane, promote the generation of Ca2+ action potentials, elevate intracellular Ca", and elicit insulin secretion. In this short review we provide some background on the effects of imidazolines on insulinsecreting cells, and we present recent data from our own laboratory which show that imidazolines block other types of K ' channels in the p-cell (including human tissue), that they elevate intracellular Ca2+, and finally that the site of interaction with the K+ 'Human pancreatic P-cell work in our laboratories is funded by the British Diabetic Association (M.D., R.F.L.J., N.J.M.L.) and the Wellcome Trust (M.D.).
The Journal of Physiology, 2010
BK channels are large unitary conductance K + channels cooperatively activated by intracellular calcium and membrane depolarisation. We show that BK channels regulate electrical activity in β-cells of mouse pancreatic islets exposed to elevated glucose. In 11.1 mm glucose, the non-peptidyl BK channel blocker paxilline increased the height of β-cell action potentials (APs) by 21 mV without affecting burst-or silent-period durations. In isolated β-cells, paxilline increased AP height by 16 mV without affecting resting membrane potential. In voltage clamp, paxilline blocked a transient component of outward current activated by a short depolarisation, which accounted for at least 90% of the initial outward K + current. This BK current (I BK) was blocked by the Ca 2+ channel blockers Cd 2+ (200 μm) or nimodipine (1 μm), and potentiated by FPL-64176 (1 μm). I BK was also 56% blocked by the BK channel blocker iberiotoxin (100 nm). I BK activated more than 10-fold faster than the delayed rectifier I Kv over the physiological voltage range, and partially inactivated. An AP-like command revealed that I BK activated and deactivated faster than I Kv and accounted for 86% of peak I K , explaining why I BK block increased AP height. A higher amplitude AP-like command, patterned on an AP recorded in 11.1 mm glucose plus paxilline, activated 4-fold more I Kv and significantly increased Ca 2+ entry. Paxilline increased insulin secretion in islets exposed to 11.1 mm glucose by 67%, but did not affect basal secretion in 2.8 mm glucose. These data suggest a modified model of β-cell AP generation where I BK and I Kv coordinate the AP repolarisation.
Biochemical and Biophysical Research Communications, 2001
Effects of the imidazoline compound RX871024 on cytosolic free Ca(2+) concentration ([Ca(2+)]i) and insulin secretion in pancreatic beta-cells from SUR1 deficient mice have been studied. In beta-cells from wild-type mice RX871024 increased [Ca(2+)]i by blocking ATP-dependent K(+)-current (K(ATP)) and inducing membrane depolarization. In beta-cells lacking a component of the K(ATP)-channel, SUR1 subunit, RX871024 failed to increase [Ca(2+)]i. However, insulin secretion in these cells was strongly stimulated by the imidazoline. Thus, a major component of the insulinotropic activity of RX871024 is stimulation of insulin exocytosis independently from changes in K(ATP)-current and [Ca(2+)]i. This means that effects of RX871024 on insulin exocytosis are partly mediated by interaction with proteins distinct from those composing the K(ATP)-channel.
Molecular Pharmacology, 2004
Evidence is accumulating that, in addition to regulating peripheral energy metabolism, insulin is an important modulator of neuronal function. Indeed, high levels of insulin and insulin receptors are expressed in several brain regions including the hippocampus. We have shown previously that insulin inhibits aberrant synaptic activity in hippocampal neurons via activation of large conductance Ca 2ϩ -activated K ϩ (BK) channels. In this study, we have examined further the effects of insulin on native hippocampal and recombinant (hSlo) BK channels expressed in human embryonic kidney (HEK) 293 cells. Pipette or bath application of insulin evoked a rapid increase in hippocampal BK channel activity, an action caused by activation of insulin receptors because insulin-like growth factor 1 (IGF-1) failed to mimic insulin action. In parallel studies, insulin, applied via the pipette or bath, also activated hSlo channels expressed in HEK293 cells. Although phosphoinositide 3-kinase is a key component of insulin and IGF-1 receptor signaling pathways, This work was supported by The Wellcome Trust (055291), Tenovus Scotland and The Royal Society.
Diabetes, 2017
Loss-of-function mutations of β-cell KATP channels cause the most severe form of congenital hyperinsulinism (KATPHI). KATPHI is characterized by fasting and protein-induced hypoglycemia that is unresponsive to medical therapy. For a better understanding of the pathophysiology of KATPHI, we examined cytosolic calcium, insulin secretion, oxygen consumption, and [U-(13)C]glucose metabolism in islets isolated from the pancreases of children with KATPHI who required pancreatectomy. Basal cytosolic calcium ([Ca(2+)] i ) and insulin secretion were higher in KATPHI islets compared to controls. Unlike controls, insulin secretion in KATPHI islets increased in response to amino acids but not to glucose. KATPHI islets have increased basal rate of oxygen consumption and mitochondrial mass. [U-(13)C]glucose metabolism showed a 2-fold increase in alanine levels and 6-fold increase in (13)C enrichment of alanine in KATPHI islets, suggesting increased rates of glycolysis. KATPHI islets also exhibite...
Journal of Pharmacology and Experimental Therapeutics, 2007
Sulfonylureas have been the leading oral antihyperglycemic agents and presently continue to be the most popular antidiabetic drugs prescribed for treatment of type 2 diabetes. However, concern has arisen over the side effects of sulfonylureas on the cardiovascular system. Here we tested the hypothesis that iptakalim, a novel vascular ATP-sensitive potassium (K ATP) channel opener, closes rat pancreatic β-cell K ATP channels and increases insulin release. Rat pancreatic βcell K ATP channels and heterologously expressed K ATP channels in both HEK 293 cells and Xenopus oocytes were used to test the pharmacological effects of iptakalim. Patch-clamp recordings, Ca 2+ imaging and measurements of insulin release were applied. Patch-clamp wholecell recordings revealed that iptakalim depolarized β-cells, induced action potential firing and reduced K ATP channel-mediated currents. Single-channel recordings revealed that iptakalim reduced the open probability of K ATP channels without changing channel sensitivity to ATP. By closing β-cell K ATP channels, iptakalim elevated intracellular Ca 2+ concentrations and increased insulin release. In addition, iptakalim inhibited the open probability of recombinant Kir6.2FL4A (a trafficking mutant of the Kir6.2) K ATP channels heterologously expressed in HEK 293 cells, suggesting that iptakalim suppressed the function of β-cell K ATP channels by directly inhibiting the Kir6.2 subunit. Finally, iptakalim inhibited Kir6.2/SUR1, but activated Kir6.1/SUR2B (vasculartype), K ATP channels heterologously expressed in Xenopus oocytes. Iptakalim bi-directionally regulated pancreatic-type and vascular-type K ATP channels, and this unique pharmacological property suggests the potential use of iptakalim as a new therapeutic strategy for treating type 2 diabetes with the additional benefit of alleviating vascular disorders.
Effects of IKs channel inhibitors in insulin-secreting INS-1 cells
Pflügers Archiv - European Journal of Physiology, 2005
Potassium channels regulate insulin secretion. The closure of K ATP channels leads to membrane depolarisation, which triggers Ca 2+ influx and stimulates insulin secretion. The subsequent activation of K + channels terminates secretion. We examined whether KCNQ1 channels are expressed in pancreatic b-cells and analysed their functional role. Using RT/ PCR cellular mRNA of KCNQ1 but not of KCNE1 channels was detected in INS-1 cells. Effects of two sulfonamide analogues, 293B and HMR1556, inhibitors of KCNQ1 channels, were examined on voltage-activated outwardly rectifying K + currents using the patch-clamp method. It was found that 293B inhibited 60% of whole-cell outward currents induced by voltage pulses from À70 to +50 mV with a concentration for half-maximal inhibition (IC 50 ) of 37 lM. The other sulfonamide analogue HMR1556 inhibited 48% of the outward current with an IC 50 of 7 lM. The chromanol 293B had no effect on tolbutamide-sensitive K ATP channels. Action potentials induced by current injections were broadened and after-repolarisation was attenuated by 293B. Insulin secretion in the presence but not in the absence of tolbutamide was significantly increased by 293B. These results suggest that 293B-and HMR1556-sensitive channels, probably in concert with other voltage-activated K + channels, influence action potential duration and frequency and thus insulin secretion.
The Journal of Physiology, 2010
Glucose-induced β-cell action potential (AP) repolarization is regulated by potassium efflux through voltage gated (Kv) and calcium activated (K Ca) potassium channels. Thus, ablation of the primary Kv channel of the β-cell, Kv2.1, causes increased AP duration. However, Kv2.1 −/− islet electrical activity still remains sensitive to the potassium channel inhibitor tetraethylammonium. Therefore, we utilized Kv2.1 −/− islets to characterize Kv and K Ca channels and their respective roles in modulating the β-cell AP. The remaining Kv current present in Kv2.1 −/− β-cells is inhibited with 5 μm CP 339818. Inhibition of the remaining Kv current in Kv2.1 −/− mouse β-cells increased AP firing frequency by 39.6% but did not significantly enhance glucose stimulated insulin secretion (GSIS). The modest regulation of islet AP frequency by CP 339818 implicates other K + channels, possibly K Ca channels, in regulating AP repolarization. Blockade of the K Ca channel BK with slotoxin increased β-cell AP amplitude by 28.2%, whereas activation of BK channels with isopimaric acid decreased β-cell AP amplitude by 30.6%. Interestingly, the K Ca channel SK significantly contributes to Kv2.1 −/− mouse islet AP repolarization. Inhibition of SK channels decreased AP firing frequency by 66% and increased AP duration by 67% only when Kv2.1 is ablated or inhibited and enhanced GSIS by 2.7-fold. Human islets also express SK3 channels and their β-cell AP frequency is significantly accelerated by 4.8-fold with apamin. These results uncover important repolarizing roles for both Kv and K Ca channels and identify distinct roles for SK channel activity in regulating calciumversus sodium-dependent AP firing.
Modulation of Ionic Channels and Insulin Secretion by Drugs and Hormones in Pancreatic Beta Cells
Molecular pharmacology, 2016
Pancreatic beta cells, unique cells that secrete insulin in response to an increase in glucose levels, play a significant role in glucose homeostasis. Glucose-stimulated insulin secretion (GSIS) in pancreatic beta cells has been extensively explored. In this mechanism, glucose enters the cells and subsequently the metabolic cycle. During this process, the ATP/ADP ratio increases, leading to ATP-sensitive potassium (KATP) channel closure, which initiates depolarization that is also dependent on the activity of TRP nonselective ion channels. Depolarization leads to the opening of voltage-gated Na(+) channels (Nav) and subsequently voltage-dependent Ca(2+) channels (Cav). The increase in intracellular Ca(2+) triggers the exocytosis of insulin-containing vesicles. Thus, electrical activity of pancreatic beta cells plays a central role in GSIS. Moreover, many growth factors, incretins, neurotransmitters, and hormones can modulate GSIS, and the channels that participate in GSIS are highly...