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 K_ATP channel modulators affect α-cell [Ca²⁺]_c.

Research Design and Methods: GYY mice (expressing EYFP in α-cells) and NMRI mice were used. [Ca²⁺]_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, [Ca²⁺]_c oscillated in some α-cells and was basal in the others. Increasing glucose to 15 mmol/l decreased [Ca²⁺]_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 [Ca²⁺]_c whereas diazoxide and azide abolished [Ca²⁺]_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 Ca²⁺ channels are the main conduits for Ca²⁺ influx in α-cells. GABA and zinc did not decrease α-cell [Ca²⁺]_c, and insulin, although lowering [Ca²⁺]_c very modestly, did not affect glucagon secretion.

Conclusions: α-cells display similarities with β-cells: K_ATP channels control Ca²⁺ influx mainly through L-type Ca²⁺ 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 decreased [Ca²⁺]_c. Hence, glucose and K_ATP channel modulators exert distinct effects on α-cell [Ca²⁺]_c. The direct small glucose-induced drop in α-cell [Ca²⁺]_c contributes likely only partly to the strong glucose-induced inhibition of glucagon secretion in islets.