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Activation of Ca(2+)-dependent K(+) channels contributes to rhythmic firing of action potentials in mouse pancreatic beta cells.

Göpel SO, Kanno T, Barg S, Eliasson L, Galvanovskis J, Renström E, Rorsman P - J. Gen. Physiol. (1999)

Bottom Line: The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium.This is similar to the interval between two successive bursts of action potentials.We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiological Sciences, Division of Molecular and Cellular Physiology, Lund University, SE-223 62 Lund, Sweden.

ABSTRACT
We have applied the perforated patch whole-cell technique to beta cells within intact pancreatic islets to identify the current underlying the glucose-induced rhythmic firing of action potentials. Trains of depolarizations (to simulate glucose-induced electrical activity) resulted in the gradual (time constant: 2.3 s) development of a small (<0.8 nS) K(+) conductance. The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium. Upon cessation of electrical stimulation, the current deactivated exponentially with a time constant of 6.5 s. This is similar to the interval between two successive bursts of action potentials. We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

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Parallel recordings of glucose-induced changes of the membrane potential and membrane conductance in a β cell within an intact islet. (A) Membrane potential recorded in the absence of glucose, after elevation of glucose to 15 mM, and in the simultaneous presence of 15 mM glucose and 100 μM tolbutamide. At the times indicated (1–3), the amplifier was switched from the current-clamp into the voltage-clamp mode and the membrane conductance was monitored by application of ±10-mV voltage pulses (duration: 500 ms; frequency: 0.5 Hz). (B) Membrane currents measured during the voltage steps to −80 mV. The current responses shown in A–C were taken as indicated in A. (C) Net change of whole-cell KATP produced by 0.1 mM tolbutamide added in the presence of 10 or 15 mM glucose and the amplitude of the Kslow current elicited by a train of depolarizations.
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Figure 6: Parallel recordings of glucose-induced changes of the membrane potential and membrane conductance in a β cell within an intact islet. (A) Membrane potential recorded in the absence of glucose, after elevation of glucose to 15 mM, and in the simultaneous presence of 15 mM glucose and 100 μM tolbutamide. At the times indicated (1–3), the amplifier was switched from the current-clamp into the voltage-clamp mode and the membrane conductance was monitored by application of ±10-mV voltage pulses (duration: 500 ms; frequency: 0.5 Hz). (B) Membrane currents measured during the voltage steps to −80 mV. The current responses shown in A–C were taken as indicated in A. (C) Net change of whole-cell KATP produced by 0.1 mM tolbutamide added in the presence of 10 or 15 mM glucose and the amplitude of the Kslow current elicited by a train of depolarizations.

Mentions: We finally compared the amplitude of the Kslow current with the changes of the KATP conductance associated with the transition from oscillatory into uninterrupted electrical activity. Fig. 6 shows parallel recordings of membrane potential and KATP conductance in a β cell within an islet. In the absence of glucose, the membrane potential was approximately −80 mV. The membrane conductance under these conditions (determined by switching the amplifier into the voltage-clamp mode, holding at −70 mV, and applying ±10-mV voltage pulses) exceeded 5 nS (Fig. 6 B, 1). Addition of 15 mM glucose resulted in membrane depolarization and the induction of oscillatory electrical activity. This was associated with a >75% reduction in resting membrane conductance, which fell to ≈1.2 nS (Fig. 6 B, 2). Subsequent addition of 100 μM tolbutamide, to inhibit remaining KATP channel activity, evoked continuous spiking and a further reduction of the membrane conductance to ≤1 nS (Fig. 6, Fig. 3). In a series of six experiments, the membrane conductance measured in the absence of glucose averaged 3.9 ± 1.0 nS. In the presence of 10 (five cells) or 15 (four cells) mM glucose (i.e., when the β cells generated oscillatory electrical activity), the membrane conductance dropped to 1.4 ± 0.2 nS (n = 9; P < 0.001 vs. that observed in the glucose-free solution). The corresponding value in the simultaneous presence of 10 or 15 mM glucose and 100 μM tolbutamide was 1.0 ± 0.1 nS (n = 7). The decrease in membrane conductance obtained by addition of tolbutamide (100 μM) to islets already exposed to 10 or 15 mM glucose thus amounted to 0.4 ± 0.1 nS (n = 7; P < 0.025). This value is smaller than the 0.8 ± 0.1 (n = 30) that can be derived for the Kslow current from its amplitude and reversal potential (28 ± 2 pA at −40 and −73 mV, respectively).


Activation of Ca(2+)-dependent K(+) channels contributes to rhythmic firing of action potentials in mouse pancreatic beta cells.

Göpel SO, Kanno T, Barg S, Eliasson L, Galvanovskis J, Renström E, Rorsman P - J. Gen. Physiol. (1999)

Parallel recordings of glucose-induced changes of the membrane potential and membrane conductance in a β cell within an intact islet. (A) Membrane potential recorded in the absence of glucose, after elevation of glucose to 15 mM, and in the simultaneous presence of 15 mM glucose and 100 μM tolbutamide. At the times indicated (1–3), the amplifier was switched from the current-clamp into the voltage-clamp mode and the membrane conductance was monitored by application of ±10-mV voltage pulses (duration: 500 ms; frequency: 0.5 Hz). (B) Membrane currents measured during the voltage steps to −80 mV. The current responses shown in A–C were taken as indicated in A. (C) Net change of whole-cell KATP produced by 0.1 mM tolbutamide added in the presence of 10 or 15 mM glucose and the amplitude of the Kslow current elicited by a train of depolarizations.
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Related In: Results  -  Collection

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Figure 6: Parallel recordings of glucose-induced changes of the membrane potential and membrane conductance in a β cell within an intact islet. (A) Membrane potential recorded in the absence of glucose, after elevation of glucose to 15 mM, and in the simultaneous presence of 15 mM glucose and 100 μM tolbutamide. At the times indicated (1–3), the amplifier was switched from the current-clamp into the voltage-clamp mode and the membrane conductance was monitored by application of ±10-mV voltage pulses (duration: 500 ms; frequency: 0.5 Hz). (B) Membrane currents measured during the voltage steps to −80 mV. The current responses shown in A–C were taken as indicated in A. (C) Net change of whole-cell KATP produced by 0.1 mM tolbutamide added in the presence of 10 or 15 mM glucose and the amplitude of the Kslow current elicited by a train of depolarizations.
Mentions: We finally compared the amplitude of the Kslow current with the changes of the KATP conductance associated with the transition from oscillatory into uninterrupted electrical activity. Fig. 6 shows parallel recordings of membrane potential and KATP conductance in a β cell within an islet. In the absence of glucose, the membrane potential was approximately −80 mV. The membrane conductance under these conditions (determined by switching the amplifier into the voltage-clamp mode, holding at −70 mV, and applying ±10-mV voltage pulses) exceeded 5 nS (Fig. 6 B, 1). Addition of 15 mM glucose resulted in membrane depolarization and the induction of oscillatory electrical activity. This was associated with a >75% reduction in resting membrane conductance, which fell to ≈1.2 nS (Fig. 6 B, 2). Subsequent addition of 100 μM tolbutamide, to inhibit remaining KATP channel activity, evoked continuous spiking and a further reduction of the membrane conductance to ≤1 nS (Fig. 6, Fig. 3). In a series of six experiments, the membrane conductance measured in the absence of glucose averaged 3.9 ± 1.0 nS. In the presence of 10 (five cells) or 15 (four cells) mM glucose (i.e., when the β cells generated oscillatory electrical activity), the membrane conductance dropped to 1.4 ± 0.2 nS (n = 9; P < 0.001 vs. that observed in the glucose-free solution). The corresponding value in the simultaneous presence of 10 or 15 mM glucose and 100 μM tolbutamide was 1.0 ± 0.1 nS (n = 7). The decrease in membrane conductance obtained by addition of tolbutamide (100 μM) to islets already exposed to 10 or 15 mM glucose thus amounted to 0.4 ± 0.1 nS (n = 7; P < 0.025). This value is smaller than the 0.8 ± 0.1 (n = 30) that can be derived for the Kslow current from its amplitude and reversal potential (28 ± 2 pA at −40 and −73 mV, respectively).

Bottom Line: The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium.This is similar to the interval between two successive bursts of action potentials.We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiological Sciences, Division of Molecular and Cellular Physiology, Lund University, SE-223 62 Lund, Sweden.

ABSTRACT
We have applied the perforated patch whole-cell technique to beta cells within intact pancreatic islets to identify the current underlying the glucose-induced rhythmic firing of action potentials. Trains of depolarizations (to simulate glucose-induced electrical activity) resulted in the gradual (time constant: 2.3 s) development of a small (<0.8 nS) K(+) conductance. The current was dependent on Ca(2+) influx but unaffected by apamin and charybdotoxin, two blockers of Ca(2+)-activated K(+) channels, and was insensitive to tolbutamide (a blocker of ATP-regulated K(+) channels) but partially (>60%) blocked by high (10-20 mM) concentrations of tetraethylammonium. Upon cessation of electrical stimulation, the current deactivated exponentially with a time constant of 6.5 s. This is similar to the interval between two successive bursts of action potentials. We propose that this Ca(2+)-activated K(+) current plays an important role in the generation of oscillatory electrical activity in the beta cell.

Show MeSH
Related in: MedlinePlus