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Feedback Control of the ATP-Sensitive K⁺ Current by Cytosolic Ca²⁺ Contributes to Oscillations of the Membrane Potential in Pancreatic ß-Cells

From the Unité d’Endocrinologie et Métabolisme, University of Louvain, Brussels, Belgium

During glucose stimulation, pancreatic ß-cells display membrane potential oscillations that correspond to intermittent Ca²⁺ influx, leading to oscillations of the cytosolic free calcium concentration ([Ca²⁺]_c) and insulin secretion. The role of ATP-sensitive K⁺ (K⁺-ATP) channels in the control of these oscillations was investigated by measuring the K⁺-ATP current (I_KATP) with the perforated mode of the patch-clamp technique. No oscillations of I_KATP were observed when glucose-stimulated ß-cells were kept hyperpolarized, thus with low and stable [Ca²⁺]_c. However, increasing [Ca²⁺]_c by Ca²⁺ influx (depolarizing pulses) or Ca²⁺ mobilization (acetylcholine) transiently augmented I_KATP. This effect was abolished by tolbutamide, attenuated by increasing the glucose concentration in the medium, and prevented by abrogation of the [Ca²⁺]_c rise, which demonstrates that the current is really I_KATP and that its increase is Ca²⁺-dependent. Injection of a current of a similar amplitude to that of the Ca²⁺-induced increase in I_KATP was sufficient to repolarize glucose-stimulated ß-cells. These results suggest that, in the absence of [Ca²⁺]_c oscillations, no metabolic oscillations affect I_KATP in pancreatic ß-cells. In contrast, [Ca²⁺]_c oscillations evoke I_KATP oscillations. This mechanism may constitute the feedback loop controlling the glucose-induced oscillating electrical activity in ß-cells.

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