Glucose and Pharmacological Modulators of ATP-Sensitive K+ Channels Control [Ca2+]c by Different Mechanisms in Isolated Mouse α-Cells

Abstract

OBJECTIVE— We studied how glucose and ATP-sensitive K + (K ATP ) channel modulators affect α-cell [Ca 2+ ] c . RESEARCH DESIGN AND METHODS— GYY mice (expressing enhanced yellow fluorescent protein in α-cells) and NMRI mice were used. [Ca 2+ ] c , the K ATP current (I KATP , perforated mode) and cell metabolism [NAD(P)H fluorescence] were monitored in single α-cells and, for comparison, in single β-cells. RESULTS— In 0.5 mmol/l glucose, [Ca 2+ ] c oscillated in some α-cells and was basal in the others. Increasing glucose to 15 mmol/l decreased [Ca 2+ ] c by ∼30% in oscillating cells and was ineffective in the others. α-Cell I KATP was inhibited by tolbutamide and activated by diazoxide or the mitochondrial poison azide, as in β-cells. Tolbutamide increased α-cell [Ca 2+ ] c , whereas diazoxide and azide abolished [Ca 2+ ] c oscillations. Increasing glucose from 0.5 to 15 mmol/l did not change I KATP and NAD(P)H fluorescence in α-cells in contrast to β-cells. The use of nimodipine showed that L-type Ca 2+ channels are the main conduits for Ca 2+ influx in α-cells. γ-Aminobutyric acid and zinc did not decrease α-cell [Ca 2+ ] c , and insulin, although lowering [Ca 2+ ] c very modestly, did not affect glucagon secretion. CONCLUSIONS— α-Cells display similarities with β-cells: K ATP channels control Ca 2+ influx mainly through L-type Ca 2+ channels. However, α-cells have distinct features from β-cells: Most K ATP channels are already closed at low glucose, glucose does not affect cell metabolism and I KATP , and it slightly decreases [Ca 2+ ] c . Hence, glucose and K ATP channel modulators exert distinct effects on α-cell [Ca 2+ ] c . The direct small glucose-induced drop in α-cell [Ca 2+ ] c contributes likely only partly to the strong glucose-induced inhibition of glucagon secretion in islets.

The dicarboxylate carrier plays a role in mitochondrial malate transport and in the regulation of glucose-stimulated insulin secretion from rat pancreatic beta cells

“…Similar observations have been made in beta cells, which lack functional K ATP channels [4,5]. Our group, as well as others, has shown an apparent disconnect between nutrient-stimulated increase in the ATP/ADP ratio and insulin release [3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21]. Although ATP is a critical factor in the control of GSIS, an important question arises from these studies: what are the other important metabolic signalling molecule(s) for insulin secretion?…”

The α-Cell Conundrum: ATP-Sensitive K+ Channels and Glucose Sensing

“…The authors confirm that K ATP channel currents are mostly closed in low glucose (0.5 mmol/l) and can be activated by diazoxide or metabolic poisons. To address whether this may be due to increased metabolism in ␣-cells under low glucose conditions, the authors determined changes in the metabolic product NAD(P)H. Levels of NAD(P)H in ␤-cells were found to respond to glucose, as expected; however, ␣-cell NAD(P)H levels did not vary substantially, thereby corroborating previous ␣-cell studies in which equivalent ATP-to-ADP ratios and NAD(P)H or FAD fluorescence under low-and high-glucose conditions were observed (16,17). Thus, this work leads to several important conclusions concerning ␣-cell glucose-sensing mechanisms.…”

“…In summary, the work of Quoix et al provides evidence that the K ATP channel is not the primary ␣-cell target responsible for hypoglycemia-induced glucagon secretion (16). Interestingly, the work also shows that, at least in isolated cells, Zn 2ϩ , insulin, or GABA minimally regulates ␣-cell glucose-induced Ca 2ϩ flux.…”

“…␣-Cells may require interaction with an intact islet or islet paracrine factors for normal glucose sensing. Quoix et al (16) found that on single ␣-cells the islet paracrine factors Zn 2ϩ , insulin, or GABA have modest effects on ␣-cell Ca 2ϩ levels under low glucose conditions (2-6). However, the ␣-cell Ca 2ϩ levels are reduced by only 28% when switching from low to high glucose, thus implicating another unknown modulator that may work with glucose to further reduce ␣-cell electrical signaling and Ca 2ϩ influx.…”

Voltage-dependent K+ channels are positive regulators of alpha cell action potential generation and glucagon secretion in mice and humans

“…Various reports have suggested that stimulation of alpha cells with glucose causes either membrane hyperpolarisation [21] or depolarisation [3,18]. Our recent work demonstrates that, although glucose-dependent inhibition of K ATP channels may not represent a major or sole mechanism contributing to the suppression of glucagon secretion [23], titrating K ATP activity with diazoxide can re-activate alpha cell Ca 2+ signalling and glucagon secretion in the presence of high glucose. While this provides indirect evidence of the involvement of membrane depolarisation in suppression of glucagon secretion [18] and the Ca 2+ and Na + channels involved in action potential depolarisation are clearly subject to voltage-dependent inactivation [3,18], the relationship between membrane depolarisation and termination of alpha cell action potential firing is unclear.…”