Cell specificity of the cytoplasmic Ca2+ response to tolbutamide is impaired in β-cells from hyperglycemic mice

Abstract

We recently reported that the timing and magnitude of the nutrient-induced Ca 2C response are specific and reproducible for each isolated b-cell. We have now used tolbutamide and arginine to test if the cell specificity exists also for the response to non-nutrient stimulation of b-cells and if so, whether it is disturbed in b-cells from hyperglycemic ob/ob and db/db mice. Zn 2C outflow measurements were used to study the correlation between Ca 2C response and insulin secretion in individual b-cells. Tolbutamide and arginine induced cellspecific Ca 2C responses in lean mouse b-cells both with regard to lag times for [Ca 2C ] i rise and peak [Ca 2C ] i heights. b-Cells within intact islets also showed cell-specific timing of their Ca 2C responses to tolbutamide. However, in tolbutamide-and arginine-stimulated single b-cells from ob/ob and db/db mice only the magnitude of Ca 2C response was cell-specific, not the timing. The lag time of tolbutamideinduced insulin secretion was cell-specific in lean mouse b-cells but not in ob/ob mouse cells. Therefore, cell specificity seems to be a robust mechanism, and probably important for an adequate b-cell function. The loss of temporal cell specificity for the response to tolbutamide in single b-cells from hyperglycemic mice may be a sign of K ATP -or voltagedependent calcium channel dysfunction.

Impaired Ca2+ Signaling in β-Cells Lacking Leptin Receptors by Cre-loxP Recombination

“…When Lepr flox/flox RIP-Cre islets were stimulated with tolbutamide a significant reduction in the Ca 2+ influx was found compared to control Lepr flox/flox islets. Similar observations have been reported from leptin receptor deficient db/db β-cells, which show a diminished response to tolbutamide [39]. In addition, defective electrical activity characterized by altered [Ca 2+ ] i -activated K + permeability or decreased L-type Ca 2+ currents has been described in islets from different diabetic models [40], [41].…”

The Physiology of Obese-Hyperglycemic Mice [ob/obMice]

“…OM ß-cells have an increased Na/K-ATPase activity [50] and may be more sensitive to voltage-dependent events [51], but an impaired function of voltage-dependent Ca 2+ channels [52] and a disturbed pattern of cytoplasmic calcium changes after glucose stimulation has also been reported [53]. They also do not show the same type of cell-specific Ca 2+ responses that are found in lean mouse islets [54]. There is an excessive firing of cytoplasmic Ca 2+ transients when OM ß-cells are stimulated with glucagon [55].…”

GPR55‐dependent stimulation of insulin secretion from isolated mouse and human islets of Langerhans

“…To identify the roles of the putative GPR55 ligands O‐1602, LPI and CBD in β‐cells, and to examine the requirement of GPR55 for their functional effects, [Ca 2+ ] i was measured by single cell microfluorimetry in islets isolated from WT and GPR55 −/− mice. Previous studies have identified β‐cell responses in dispersed islet populations through elevated [Ca 2+ ] i levels in response to stimulatory concentrations of glucose and to the K ATP channel blocker tolbutamide, so in our experiments dispersed islet cells were classified as β‐cells if they responded to 20 mM glucose and to 100 μM tolbutamide. Figure A shows that 10 μM O‐1602 caused a rapid and reversible increase in [Ca 2+ ] i in WT islet β‐cells at 2 mM glucose, and these cells then responded to 20 mM glucose with a similar amplitude of calcium increase (peak response, 340 : 380 ratio: 0.821 vs. 0.815).…”