pH modulation of glucose-induced electrical activity in B-cells: Involvement of Na/H and HCO3/Cl antiporters

Abstract

Regulation of intracellular pH is an essential function and may be especially significant in the B-cell in which the influence of glucose on electrical activity is modulated by alterations in pH. Two possible regulatory processes have been examined: Na/H and HCO3/Cl exchange, by using inhibitors, an ionophore, and changes of ionic concentrations. In the presence of 11.1 mM glucose we found that DIDS, an inhibitor of anion exchange, elicited a dose-response increase in the relative duration of the active phase with an ED50 of 99 microM. Probenecid (0.5 mM), an inhibitor of anion fluxes, also augmented the electrical activity (EA) due to glucose. Withdrawal of HCO-3 elicited constant spike activity followed by a resumption of burst activity with a greater duration of the active phase compared to control. These data are consistent with predicted cellular acidification. However, reduction of Cl-o by isethionate substitution produced no marked effect on EA. In contrast, SO-4- substitution for Cl- resulted in variable effects characterized by constant spike activity or a decrease in the duration of the active and silent phases along with silent hyperpolarization. Tributyltin, a Cl/OH, ionophore enhanced EA at 0.25 microM with 120 mM Cl-o, but reduced EA with 10 mM Cl- as would be predicted with either cellular acidification or alkalinization, respectively. Amiloride at 100 microM elicited constant spike activity perhaps due to inhibition of Na/H exchange. Reduction of Na+o from 142.8 to 40.8 mM had a similar effect and enhanced the influence of amiloride. It appears therefore that interference with putative pH regulatory mechanisms in the B-cell are consistent with the hypothesis that cell pH is involved in regulation of EA.

The Na+-driven Cl−/HCO3−Exchanger

“…3b) . These properties are similar to those of the Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger described in native cells (14,16,(27)(28)(29), indicating that the cloned NCBE is a Na ϩ -driven Cl Ϫ / HCO 3 Ϫ exchanger. The functional properties of NCBE are different from those of the recently identified Drosophila NDAE1, which does not require HCO 3 Ϫ for transport activity (30).…”

“…It has been shown that glucose-induced insulin secretion is accompanied by a rise in pH i in pancreatic islet ␤-cells (17)(18)(19)(20)(21). Although several pH i regulators have been suggested to be present in pancreatic ␤-cells (16,21), the molecular basis of these regulators is not known. NCBE is the first pH i -regulating exchanger of which the primary structure and functional properties have been determined.…”

“…Three AEs (3) and three NBCs (6 -10) have been cloned and functionally characterized, but the molecular structure of the K ϩ -HCO 3 Ϫ cotransporter and the Na ϩ -driven Cl Ϫ / HCO 3 Ϫ exchanger has remained unknown. The Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger was first discovered in invertebrate neurons (11) and was found later in vertebrate neurons and non-neuronal cells, including the brain (12), vascular endothelial cells (13), sperm (14), kidney (15), and pancreatic ␤-cells (16). The Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger is an intracellular pH regulator that transports extracellular Na ϩ and HCO 3 Ϫ into cells in exchange for intracellular Cl Ϫ and H ϩ playing an important role in cellular alkalinization (5,11).…”

“…It has been shown that glucose metabolism induces an increase in pH i in pancreatic ␤-cells (17)(18)(19)(20)(21) and that this glucose-induced rise in pH i is evoked primarily by the action of the Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger (16). To determine the structure and functional roles of the Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger, we attempted to clone the Na ϩ -driven Cl Ϫ /HCO 3 Ϫ exchanger from the mouse insulinsecreting cell line, MIN6.…”

Chloride Channels Regulate HIT Cell Volume but Cannot Fully Account for Swelling-Induced Insulin Secretion

“…This is true even though it has been known for some time that islets have glucose-sensitive anion fluxes (2) and that anion substitution significantly modifies insulin secretion (2,3). In addition, anion channel blockers have been shown to strongly modulate glucose-induced islet electrical activity (4). Most of these earlier studies of the effects of anions on islet physiology presumed that anion transporters or pumps, rather than anion channels, were the predominant anion transport mechanisms of islet cells.…”