Glucose-dependent regulation of rhythmic action potential firing in pancreatic beta-cells by K(ATP)-channel modulation

Abstract

The regulation of a K(+) current activating during oscillatory electrical activity (I(K,slow)) in an insulin-releasing beta-cell was studied by applying the perforated patch whole-cell technique to intact mouse pancreatic islets. The resting whole-cell conductance in the presence of 10 mM glucose amounted to 1.3 nS, which rose by 50 % during a series of 26 simulated action potentials. Application of the K(ATP)-channel blocker tolbutamide produced uninterrupted action potential firing and reduced I(K,slow) by approximately 50 %. Increasing glucose from 15 to 30 mM, which likewise converted oscillatory electrical activity into continuous action potential firing, reduced I(K,slow) by approximately 30 % whilst not affecting the resting conductance. Action potential firing may culminate in opening of K(ATP) channels by activation of ATP-dependent Ca(2+) pumping as suggested by the observation that the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) inhibitor thapsigargin (4 microM) inhibited I(K,slow) by 25 % and abolished bursting electrical activity. We conclude that oscillatory glucose-induced electrical activity in the beta-cell involves the opening of K(ATP)-channel activity and that these channels, in addition to constituting the glucose-regulated K(+) conductance, also play a role in the graded response to supra-threshold glucose concentrations.

Figure 1. Activation of I_K,slow influences the β-cell membrane potential

A, electrical activity recorded from a β-cell in an intact islet. During the period indicated by * and bar, the amplifier was switched into the voltage-clamp mode and the cell subjected to the train of simulated action potentials as indicated. The dashed horizontal line corresponds to the most repolarised potential during two successive bursts before going into the voltage-clamp mode. B, I_K,slow (top) and stimulation protocol (lower) displayed on an expanded time base. The dotted lines indicate the pre-stimulatory current level.

Figure 2. Tolbutamide-sensitive component of activity-dependent K⁺ current in bursting β-cells

A, electrical activity recorded in the presence of 10 mm glucose alone (glc; left), in the presence of 0.1 mm tolbutamide (middle) and after withdrawal of the drug (right). At the times indicted by (a), (b) and (c), the membrane potential recording was interrupted to record I_K,slow voltage-clamp currents. B, electrical activity in the presence of tolbutamide on an expanded time base. The expanded segment has been indicated in A by the horizontal line. C, lower, voltage-clamp stimulation protocol. C, upper, I_K,slow recorded in the presence of 10 mm glucose alone (a), in the presence of 10 mm glucose and 0.1 mm tolbutamide (b) and following wash-out of tolbutamide and the resumption of oscillatory electrical activity (c). The horizontal dotted lines indicate (from top to bottom) the peak current amplitude at 10 mm glucose, the pre-stimulatory current level recorded at - 40 mV and the holding current at - 70 mV.

Figure 3. Glucose modulation of I_K,slow and β-cell electrical activity

A, membrane potential recording. The glucose concentration (glc) was varied between 15 and 30 mm as indicated schematically above. At times indicated (a and b), the amplifier was switched from the current- to voltage-clamp mode to monitor I_K,slow. B, I_K,slow recorded at 15 mm (a) and 30 mm (b) glucose. The horizontal dotted lines indicate (from top to bottom) the peak current amplitude at 15 mm glucose, the pre-stimulatory current level recorded at −40 mV and the holding current at −70 mV.

Figure 4. Modulation of I_K,slow and β-cell electrical activity by thapsigargin

A, electrical activity recorded at 10 mm glucose before and after treatment of the islet for 2 min with 4 μm thapsigargin. B, I_K,slow recorded under control conditions (left) and following treatment with thapsigargin (right). The horizontal lines indicate (from top to bottom) the peak amplitude observed under control conditions, the steady-state currents recorded at −40 mV pre- stimulation and the holding current at −70 mV.