Pancreatic Beta-cell KATP Channel Research Articles

α2A‐Adrenoceptor antagonism increases insulin secretion and synergistically augments the insulinotropic effect of glibenclamide in mice

The imidazoline-type alpha2-adrenoceptor antagonists (+/-)-efaroxan and phentolamine increase insulin secretion and reduce blood glucose levels. It is not known whether they act by antagonizing pancreatic beta-cell alpha2-adrenoceptors or by alpha2-adrenoceptor-independent mechanisms. Many imidazolines inhibit the pancreatic beta-cell KATP channel, which is the molecular target of sulphonylurea drugs used in the treatment of type II diabetes. To investigate the mechanisms of action of (+/-)-efaroxan and phentolamine, alpha2A-adrenoceptor knockout (alpha2A-KO) mice were used. Effects of (+/-)-efaroxan, 5 mg kg(-1), and phentolamine, 1 mg kg(-1), on blood glucose and insulin levels were compared with those of the non-imidazoline alpha2-adrenoceptor antagonist [8aR,12aS,13aS]-5,8,8a,9,10,11,12,12a,13,13a-decahydro-3-methoxy-12-(ethylsulphonyl)-6H-isoquino[2,1-g][1,6]naphthyridine (RS79948-197), 1 mg kg(-1), and the sulphonylurea glibenclamide, in alpha2A-KO and control (wild type (WT)) mice. In fed WT mice, (+/-)-efaroxan, phentolamine and RS79948-197 reduced blood glucose and increased insulin levels. Fasting abolished these effects. In fed alpha2A-KO mice, (+/-)-efaroxan, phentolamine and RS79948-197 did not alter blood glucose or insulin levels, and in fasted alpha2A-KO mice, blood glucose levels were increased. Glibenclamide, at a dose only moderately efficacious in WT mice (5 mg kg(-1)), caused severe hyperinsulinaemia and hypoglycaemia in alpha2A-KO mice. This was mimicked in WT mice by co-administration of RS79948-197 with glibenclamide. These results suggest that (+/-)-efaroxan and phentolamine increase insulin secretion by inhibition of beta-cell alpha2A-adrenoceptors, and demonstrate a critical role for alpha2A-adrenoceptors in limiting sulphonylurea-induced hyperinsulinaemia and hypoglycaemia.

Ketotic hypoglycaemia in children with diazoxide responsive hyperinsulinism of infancy

Hyperinsulinism of infancy (HI) is a cause of persistent and recurrent hypoglycaemia in infancy and childhood, which if untreated can cause significant brain damage and mental retardation. The biochemical hallmark of hyperinsulinism is hypofattyacidaemic, hypoketotic hyperinsulinaemic hypoglycaemia. Diazoxide is the first line medical treatment for persistent HI. Diazoxide is an agonist of the pancreatic beta-cell KATP channel and inhibits insulin secretion. Children who develop recurrent hypoglycaemia while on therapy with diazoxide are thought to be unresponsive to this medication or non compliant with medical therapy. We report a novel observation of "ketotic" hypoglycaemia in two children on diazoxide therapy for persistent HI. Detailed assessment of the intermediary metabolites and hormones at the time of the hypoglycaemia showed appropriate insulin suppression with appropriate increases in the serum levels of non-esterified fatty acids and ketone bodies as well as an intact counter-regulatory hormone response. The precise mechanism of the hypoglycaemia is unclear. These cases illustrate that recurrent hypoglycaemia while on diazoxide therapy may be due to other mechanisms and does not imply diazoxide unresponsiveness or non-compliance.

Pyridine nucleotide regulation of the KATP channel Kir6.2/SUR1 expressed in Xenopus oocytes.

The pancreatic beta-cell type of ATP-sensitive potassium (KATP) channel (Kir6.2/SUR1) is inhibited by intracellular ATP and ADP, which bind to the Kir6.2 subunit, and is activated by Mg-nucleotide interaction with the regulatory sulphonylurea receptor subunits (SUR1). The nicotinamide adenine dinucleotides NAD and NADP consist of an ADP molecule with a ribose group and a nicotinamide moiety attached to the terminal phosphate. Both these molecules block native KATP channels in pancreatic beta-cells at concentrations above 500 microM, and activate them at lower concentrations. We therefore investigated whether NAD and NADP interact with both Kir6.2 and SUR1 subunits of the KATP channel by comparing the potency of these agents on recombinant Kir6.2DeltaC and Kir6.2/SUR1 channels expressed in Xenopus oocytes. Our results show that, at physiological concentrations, NAD and NADP interact with the nucleotide inhibitory site of Kir6.2 to inhibit Kir6.2/SUR1 currents. They may therefore contribute to the resting level of channel inhibition in the intact cell. Importantly, our data also reveal that this interaction is dependent on the presence of SUR1, which may act by increasing the width of the nucleotide-binding pocket of Kir6.2.

Molecular and electrophysiological investigation of ATP-sensitive K+ channels in lower urinary tract function: the aims for clinical treatment of unstable detrusor

It is a common sequelae of bladder outlet obstruction caused by benign prostatic hyperplasia in adult males and gives rise to significant bladder dysfunction such as frequency and urgency of micturition. The unstable detrusor contractions may lead to urge incontinence. Since it has been reported that experimentally-induced bladder instability can be abolished by ATP-sensitive K+ channel (KATP channel) openers, various types of detrusor-selective KATP channels have been newly synthesized, targeting KATP channels in urinary bladder. Thus, the significant differences in molecular and pharmacological properties of KATP channels between urinary bladder and urethra hold out some hope for the development of tissue-selective KATP channel openers for urge urinary incontinence, and detrusor-selective KATP channel openers should be screened against urethral as well as vascular smooth muscle. In functional expression experiments, pharmacological and electrophysiological studies have reported that SUR1/Kir6.2 represents the pancreatic beta-cell KATP channel and that SUR2A/Kir6.2 is thought to represent the cardiac KATP channel, whereas SUR2B/Kir6.1 represents the smooth muscle-type KATP channel. In general, the smooth muscle type-KATP channel is (i) of a relatively small conductance (about 20 pS under quasi-physiological conditions, approximately 40 pS in symmetrical 140 mM K+ conditions), (ii) intracellular Ca(2+)-insensitive, (iii) inhibited by intracellular ATP, (iv) abolished by glibenclamide at a submicromolar concentration, and (v) reactivated by intracellular nucleoside diphosphates (NDPs). There has been no report concerning the properties of KATP channels in human detrusor by use of single-channel recordings. We would like to introduce our recent evidence of novel synthesized detrusor-selective KATP channel openers and properties of KATP channels in the lower urinary tract.

The structure and function of the ATP-sensitive K+ channel in insulin-secreting pancreatic beta-cells.

ATP-sensitive K+ channels (KATP channels) play important roles in many cellular functions by coupling cell metabolism to electrical activity. The KATP channels in pancreatic beta-cells are thought to be critical in the regulation of glucose-induced and sulfonylurea-induced insulin secretion. Until recently, however, the molecular structure of the KATP channel was not known. Cloning members of the novel inwardly rectifying K+ channel subfamily Kir6.0 (Kir6.1 and Kir6.2) and the sulfonylurea receptors (SUR1 and SUR2) has clarified the molecular structure of KATP channels. The pancreatic beta-cell KATP channel comprises two subunits: a Kir6.2 subunit and an SUR1 subunit. Molecular biological and molecular genetic studies have provided insights into the physiological and pathophysiological roles of the pancreatic beta-cell KATP channel in insulin secretion.

A view of sur/KIR6.X, KATP channels.

ATP-sensitive potassium channels, termed KATP channels, link the electrical activity of cell membranes to cellular metabolism. These channels are heteromultimers of sulfonylurea receptor (SUR) and KIR6.X subunits associated with a 1:1 stoichiometry as a tetramer (SUR/KIR6.X forms the pores, whereas SUR regulates their activity. Changes in [ATP]i and [ADP]i gate the channel. The diversity of KATP channels results from the assembly of SUR and KIR6.X subtypes KIR6.1-based channels differ from KIR6.2 channels mainly by their smaller unitary conductance. SUR1- and SUR2-based channels are distinguished by their differential sensitivity to sulfonylureas, whereas SUR2A-based channels are distinguished from SUR2B channels by their differential sensitivity to diazoxide. Mutations that result in the loss of KATP channels in pancreatic beta-cells have been identified in SUR1 and KIR6.2. These mutations lead to familial hyperinsulinism. Understanding the mutations in SUR and KIR6.X is allowing insight into how these channels respond to nucleotides, sulfonylureas, and potassium channel openers, KCOs.

Loss of functional KATP channels in pancreatic beta-cells causes persistent hyperinsulinemic hypoglycemia of infancy.

Persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is a disorder of childhood associated with inappropriate hypersecretion of insulin by the pancreas. The pathogenesis of the condition has hitherto remained controversial. We show here that insulin-secreting cells from a homogeneous group of five infants with PHHI lack ATP-sensitive K+ channel (KATP) activity. As a consequence, PHHI beta-cells are spontaneously electrically active with high basal cytosolic Ca2+ concentrations due to Ca2+ influx. Our findings define the pathogenesis of this disease as a novel K+ channel disorder.