Effects of Lowering External Na+ Concentration on Cytoplasmic pH and Ca2+ Concentration in Mouse Pancreatic, .BETA.-Cells: Mechanism of Periodicity of Spike-Bursts.

Abstract

The periodic spike-burst response of pancreatic beta-cells varies in duration with an increase of external glucose within the range 5-20 mM. To elucidate the mechanism determining the length of spike-burst, we studied the low-Na+ induced change in electrical response to glucose which is similar to the change induced by high glucose. Cytoplasmic pH (pHi) and Ca2+ concentration ([Ca2+]i) were measured by the microfluorometric method under normal and low-Na+ conditions in mouse pancreatic islets. Lowering external Na+ concentration from 135 to 25 mM by replacing Na+ with Tris+ induced progressive alkalinization in islet cells in the presence of 11.1 mM glucose. In contrast, reduction of external Na+ by replacement with Li+ caused intracellular acidification. Both manipulations described above caused a marked increase in [Ca2+]i, suggesting the presence of Na+/Ca(2+)-antiport activity. Although the change in pHi induced by decreasing external Na+ varied in direction depending on the species of cations used for replacing Na+, the pattern of electrical activity consistently changed from the spike-burst type to the continuous spike-generation type without regard for the difference in species of cations replacing Na+. These findings lead to the following hypothesis: A decrease in Na+ influx could cause a decrease in ATP-consumption by Na+/K(+)-pumps that prevents the fall of intracellular ATP concentration. The resultant continuation of high concentrations of intracellular ATP may be responsible for the abolishment of the silent phase.

Action potentials and insulin secretion: new insights into the role of Kv channels

“…The amount of K slow current closely follows the Ca 2+ levels and its inactivation follows a time course similar to the termination of the Ca 2+ wave at the end of the slow wave [35]. Inhibition of Ca 2+ influx eliminates the K slow current and removal of extracellular Ca 2+ induces a continued slow wave of depolarization [17,30,31,35,45].…”

“…Extracellular Na + is also required for normal insulin secretion from both human and rodent islets [25–30]. Rodent β‐cell APs fire in the absence of extracellular Ca 2+ during glucose‐induced depolarization; however, when Na + is also removed with Ca 2+ the APs fail to fire, implicating an important role for Na + influx during the AP [30,31]. Human and canine β‐cells on the other hand require external Na + regardless of extracellular Ca 2+ for a normal pattern of glucose‐induced APs [29].…”

Involvement of Calmodulin in Glucagon‐Like Peptide 1(7‐36) Amide‐Induced Inhibition of the ATP‐Sensitive K+ Channel in Mouse Pancreatic β‐Cells

“…The pancreases were quickly removed and the pancreatic islets were isolated by collagenase (2 mg ml _1 ; Wako Pure Chemical Industries, Osaka, Japan) digestion and separated into single cells using Dispase (Godo Shusei Co. Ltd., Tokyo, Japan) as previously described (Hattori et al 1994). The cell suspension was plated on poly-Dlysine-coated coverslips and maintained for up to 5 days in RPMI 1640 tissue-culture medium (Gibco BRL, UK), supplemented with 10 % (v/v) heat-inactivated fetal calf serum, 100 i.u.…”

Bursting and calcium oscillations in pancreatic β-cells: specific pacemakers for specific mechanisms

“…However, the small remaining K ATP conductance in elevated glucose is still comparable with the conductances of other channels, exchangers, and pumps (133). For this reason, cyclical changes in the K ATP channel conductance have been proposed as a possible mechanism underlying oscillatory behavior of ␤-cells (3,68,110,126,133 (34,51). We illustrated this mechanism using our mathematical model (51) that includes a simulation of ATP consumption due to the work of Ca 2ϩ pumps (PM and ER) as well as the Ca 2ϩ -activated ATP consumption in some cytoplasmic processes.…”