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Pharmacological properties and functional role of Kslow current in mouse pancreatic beta-cells: SK channels contribute to Kslow tail current and modulate insulin secretion.

Zhang M, Houamed K, Kupershmidt S, Roden D, Satin LS - J. Gen. Physiol. (2005)

Bottom Line: AZ inhibition of K(slow) also supported mediation by SK, rather than cardiac-like slow delayed rectifier channels since bath application of AZ to HEK 293 cells expressing SK3 cDNA reduced SK current.These results strongly support a functional role for SK channel-mediated K(slow) current in beta-cells, and suggest that drugs that target SK channels may represent a new approach for increasing glucose-dependent insulin secretion.The apamin insensitivity of beta-cell SK current suggests that beta-cells express a unique SK splice variant or a novel heteromultimer consisting of different SK subunits.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacology and Toxicology, School of Medicine, Virginia Commonwealth University Medical Center, Richmond, VA 23264, USA.

ABSTRACT
The pharmacological properties of slow Ca(2+)-activated K(+) current (K(slow)) were investigated in mouse pancreatic beta-cells and islets to understand how K(slow) contributes to the control of islet bursting, [Ca(2+)](i) oscillations, and insulin secretion. K(slow) was insensitive to apamin or the K(ATP) channel inhibitor tolbutamide, but UCL 1684, a potent and selective nonpeptide SK channel blocker reduced the amplitude of K(slow) tail current in voltage-clamped mouse beta-cells. K(slow) was also selectively and reversibly inhibited by the class III antiarrythmic agent azimilide (AZ). In isolated beta-cells or islets, pharmacologic inhibition of K(slow) by UCL 1684 or AZ depolarized beta-cell silent phase potential, increased action potential firing, raised [Ca(2+)](i), and enhanced glucose-dependent insulin secretion. AZ inhibition of K(slow) also supported mediation by SK, rather than cardiac-like slow delayed rectifier channels since bath application of AZ to HEK 293 cells expressing SK3 cDNA reduced SK current. Further, AZ-sensitive K(slow) current was extant in beta-cells from KCNQ1 or KCNE1 mice lacking cardiac slow delayed rectifier currents. These results strongly support a functional role for SK channel-mediated K(slow) current in beta-cells, and suggest that drugs that target SK channels may represent a new approach for increasing glucose-dependent insulin secretion. The apamin insensitivity of beta-cell SK current suggests that beta-cells express a unique SK splice variant or a novel heteromultimer consisting of different SK subunits.

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(A) β-cells from KCNE1  mice lacking cardiac type IKs delayed rectifier K+ channels also express AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNE1 (−/−) mouse β-cells, both β-cells from KCNE1 (+/+) and KCNE1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNE1 (+/+) and KCNE1 (−/−) mice (n = 38–45, P > 0.05), AZ significantly inhibited Kslow currents in both group mice (**, P < 0.01, n = 6). (B) β-cells from KCNQ1  mice lacking cardiac type IKs delayed rectifier K+ channels still have AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNQ1 (−/−) mouse β-cells. Both β-cells from KCNQ1 (+/+) and KCNQ1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNQ1 (+/+) and KCNQ1 (−/−) (P > 0.05, n = 12), AZ significantly inhibited Kslow currents in both groups of mice (*, P < 0.05, n = 5).
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fig3: (A) β-cells from KCNE1 mice lacking cardiac type IKs delayed rectifier K+ channels also express AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNE1 (−/−) mouse β-cells, both β-cells from KCNE1 (+/+) and KCNE1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNE1 (+/+) and KCNE1 (−/−) mice (n = 38–45, P > 0.05), AZ significantly inhibited Kslow currents in both group mice (**, P < 0.01, n = 6). (B) β-cells from KCNQ1 mice lacking cardiac type IKs delayed rectifier K+ channels still have AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNQ1 (−/−) mouse β-cells. Both β-cells from KCNQ1 (+/+) and KCNQ1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNQ1 (+/+) and KCNQ1 (−/−) (P > 0.05, n = 12), AZ significantly inhibited Kslow currents in both groups of mice (*, P < 0.05, n = 5).

Mentions: As shown in Fig. 3 (A and B), islet β-cells isolated from global KCNE1 or KCNQ1 knockout mice that lack cardiac IKs (Kupershmidt et al., 1999) still exhibited slowly activating and deactivating, and AZ-sensitive Kslow currents. The mean t1/2 of Kslow deactivation in wild-type β-cells was 1.9 ± 0.2 s, n = 16, and was not significantly different in β-cells from either KCNQ1 −/− or KCNE1 −/− islet β-cells (n = 16 or n = 24). The bar graphs shown in Fig. 3 (A and B) confirm that Kslow current density was not significantly reduced in β-cells from either the KCNQ1 −/− or KCNE1 −/− mice. Furthermore, it is clearly apparent that 3 μM AZ retained its ability to inhibit Kslow current despite the lack of IKs. This strongly suggests that AZ does not inhibit Kslow by targeting channels composed of KCNQ1/KCNE1, and that, by extension, IKs is unlikely to mediate Kslow current in pancreatic β-cells.


Pharmacological properties and functional role of Kslow current in mouse pancreatic beta-cells: SK channels contribute to Kslow tail current and modulate insulin secretion.

Zhang M, Houamed K, Kupershmidt S, Roden D, Satin LS - J. Gen. Physiol. (2005)

(A) β-cells from KCNE1  mice lacking cardiac type IKs delayed rectifier K+ channels also express AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNE1 (−/−) mouse β-cells, both β-cells from KCNE1 (+/+) and KCNE1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNE1 (+/+) and KCNE1 (−/−) mice (n = 38–45, P > 0.05), AZ significantly inhibited Kslow currents in both group mice (**, P < 0.01, n = 6). (B) β-cells from KCNQ1  mice lacking cardiac type IKs delayed rectifier K+ channels still have AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNQ1 (−/−) mouse β-cells. Both β-cells from KCNQ1 (+/+) and KCNQ1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNQ1 (+/+) and KCNQ1 (−/−) (P > 0.05, n = 12), AZ significantly inhibited Kslow currents in both groups of mice (*, P < 0.05, n = 5).
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fig3: (A) β-cells from KCNE1 mice lacking cardiac type IKs delayed rectifier K+ channels also express AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNE1 (−/−) mouse β-cells, both β-cells from KCNE1 (+/+) and KCNE1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNE1 (+/+) and KCNE1 (−/−) mice (n = 38–45, P > 0.05), AZ significantly inhibited Kslow currents in both group mice (**, P < 0.01, n = 6). (B) β-cells from KCNQ1 mice lacking cardiac type IKs delayed rectifier K+ channels still have AZ-sensitive Kslow currents. Top traces show representative Kslow currents from KCNQ1 (−/−) mouse β-cells. Both β-cells from KCNQ1 (+/+) and KCNQ1 (−/−) mice are sensitive to AZ. Bottom bar graph shows no difference of mean Kslow currents in KCNQ1 (+/+) and KCNQ1 (−/−) (P > 0.05, n = 12), AZ significantly inhibited Kslow currents in both groups of mice (*, P < 0.05, n = 5).
Mentions: As shown in Fig. 3 (A and B), islet β-cells isolated from global KCNE1 or KCNQ1 knockout mice that lack cardiac IKs (Kupershmidt et al., 1999) still exhibited slowly activating and deactivating, and AZ-sensitive Kslow currents. The mean t1/2 of Kslow deactivation in wild-type β-cells was 1.9 ± 0.2 s, n = 16, and was not significantly different in β-cells from either KCNQ1 −/− or KCNE1 −/− islet β-cells (n = 16 or n = 24). The bar graphs shown in Fig. 3 (A and B) confirm that Kslow current density was not significantly reduced in β-cells from either the KCNQ1 −/− or KCNE1 −/− mice. Furthermore, it is clearly apparent that 3 μM AZ retained its ability to inhibit Kslow current despite the lack of IKs. This strongly suggests that AZ does not inhibit Kslow by targeting channels composed of KCNQ1/KCNE1, and that, by extension, IKs is unlikely to mediate Kslow current in pancreatic β-cells.

Bottom Line: AZ inhibition of K(slow) also supported mediation by SK, rather than cardiac-like slow delayed rectifier channels since bath application of AZ to HEK 293 cells expressing SK3 cDNA reduced SK current.These results strongly support a functional role for SK channel-mediated K(slow) current in beta-cells, and suggest that drugs that target SK channels may represent a new approach for increasing glucose-dependent insulin secretion.The apamin insensitivity of beta-cell SK current suggests that beta-cells express a unique SK splice variant or a novel heteromultimer consisting of different SK subunits.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacology and Toxicology, School of Medicine, Virginia Commonwealth University Medical Center, Richmond, VA 23264, USA.

ABSTRACT
The pharmacological properties of slow Ca(2+)-activated K(+) current (K(slow)) were investigated in mouse pancreatic beta-cells and islets to understand how K(slow) contributes to the control of islet bursting, [Ca(2+)](i) oscillations, and insulin secretion. K(slow) was insensitive to apamin or the K(ATP) channel inhibitor tolbutamide, but UCL 1684, a potent and selective nonpeptide SK channel blocker reduced the amplitude of K(slow) tail current in voltage-clamped mouse beta-cells. K(slow) was also selectively and reversibly inhibited by the class III antiarrythmic agent azimilide (AZ). In isolated beta-cells or islets, pharmacologic inhibition of K(slow) by UCL 1684 or AZ depolarized beta-cell silent phase potential, increased action potential firing, raised [Ca(2+)](i), and enhanced glucose-dependent insulin secretion. AZ inhibition of K(slow) also supported mediation by SK, rather than cardiac-like slow delayed rectifier channels since bath application of AZ to HEK 293 cells expressing SK3 cDNA reduced SK current. Further, AZ-sensitive K(slow) current was extant in beta-cells from KCNQ1 or KCNE1 mice lacking cardiac slow delayed rectifier currents. These results strongly support a functional role for SK channel-mediated K(slow) current in beta-cells, and suggest that drugs that target SK channels may represent a new approach for increasing glucose-dependent insulin secretion. The apamin insensitivity of beta-cell SK current suggests that beta-cells express a unique SK splice variant or a novel heteromultimer consisting of different SK subunits.

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