Limits...
Membrane potential-dependent inactivation of voltage-gated ion channels in alpha-cells inhibits glucagon secretion from human islets.

Ramracheya R, Ward C, Shigeto M, Walker JN, Amisten S, Zhang Q, Johnson PR, Rorsman P, Braun M - Diabetes (2010)

Bottom Line: Inhibition of K(ATP)-channels with tolbutamide depolarized alpha-cells by 10 mV and reduced the action potential amplitude.Exocytosis was negligible at voltages below -20 mV and peaked at 0 mV.We propose that voltage-dependent inactivation of these channels underlies the inhibition of glucagon secretion by tolbutamide and glucose.

View Article: PubMed Central - PubMed

Affiliation: Oxford Centre for Diabetes Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, UK. matthias.braun@drl.ox.ac.uk

ABSTRACT

Objective: To document the properties of the voltage-gated ion channels in human pancreatic alpha-cells and their role in glucagon release.

Research design and methods: Glucagon release was measured from intact islets. [Ca(2+)](i) was recorded in cells showing spontaneous activity at 1 mmol/l glucose. Membrane currents and potential were measured by whole-cell patch-clamping in isolated alpha-cells identified by immunocytochemistry.

Result: Glucose inhibited glucagon secretion from human islets; maximal inhibition was observed at 6 mmol/l glucose. Glucagon secretion at 1 mmol/l glucose was inhibited by insulin but not by ZnCl(2). Glucose remained inhibitory in the presence of ZnCl(2) and after blockade of type-2 somatostatin receptors. Human alpha-cells are electrically active at 1 mmol/l glucose. Inhibition of K(ATP)-channels with tolbutamide depolarized alpha-cells by 10 mV and reduced the action potential amplitude. Human alpha-cells contain heteropodatoxin-sensitive A-type K(+)-channels, stromatoxin-sensitive delayed rectifying K(+)-channels, tetrodotoxin-sensitive Na(+)-currents, and low-threshold T-type, isradipine-sensitive L-type, and omega-agatoxin-sensitive P/Q-type Ca(2+)-channels. Glucagon secretion at 1 mmol/l glucose was inhibited by 40-70% by tetrodotoxin, heteropodatoxin-2, stromatoxin, omega-agatoxin, and isradipine. The [Ca(2+)](i) oscillations depend principally on Ca(2+)-influx via L-type Ca(2+)-channels. Capacitance measurements revealed a rapid (<50 ms) component of exocytosis. Exocytosis was negligible at voltages below -20 mV and peaked at 0 mV. Blocking P/Q-type Ca(2+)-currents abolished depolarization-evoked exocytosis.

Conclusions: Human alpha-cells are electrically excitable, and blockade of any ion channel involved in action potential depolarization or repolarization results in inhibition of glucagon secretion. We propose that voltage-dependent inactivation of these channels underlies the inhibition of glucagon secretion by tolbutamide and glucose.

Show MeSH

Related in: MedlinePlus

Effects of Ca2+-channel antagonists on glucagon secretion. A: Glucagon secretion measured in the absence (open bars) and presence (filled bars) of 10 μmol/l isradipine. *P < 0.01 versus 1 mmol/l glucose alone, †P < 0.05 versus 1 mmol/l glucose and 10 μmol/l isradipine. 100% = 10.5 ± 0.6 pg/islet/h (n = 9; 3 donors). B: Same as in A but effects of 200 nmol/l ω-agatoxin were tested. *P < 0.01 versus 1 mmol/l glucose alone, 100% = 21.1 ± 3.7 pg/islet/h (n = 9; 3 donors).
© Copyright Policy - creative-commons
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC2927942&req=5

Figure 6: Effects of Ca2+-channel antagonists on glucagon secretion. A: Glucagon secretion measured in the absence (open bars) and presence (filled bars) of 10 μmol/l isradipine. *P < 0.01 versus 1 mmol/l glucose alone, †P < 0.05 versus 1 mmol/l glucose and 10 μmol/l isradipine. 100% = 10.5 ± 0.6 pg/islet/h (n = 9; 3 donors). B: Same as in A but effects of 200 nmol/l ω-agatoxin were tested. *P < 0.01 versus 1 mmol/l glucose alone, 100% = 21.1 ± 3.7 pg/islet/h (n = 9; 3 donors).

Mentions: The L-type Ca2+-channel blocker isradipine inhibited glucagon secretion from human islets at 1 mmol/l glucose by 25% (Fig. 6A). Glucose retained an inhibitory action in the presence of isradipine (P < 0.05). Blocking the P/Q-type Ca2+-channels exerted a stronger suppressor effect and was as inhibitory as 20 mmol/l glucose (Fig. 6B). Glucose did not diminish glucagon secretion when applied in the presence of ω-agatoxin. The T-type Ca2+-channel blocker 55-0396 paradoxically increased secretion at 1 mmol/l glucose by 190 ± 30% (n = 3; P < 0.05) but did not affect the inhibitory action of 20 mmol/l glucose (65 ± 5%; n = 3; P < 0.05; not shown). The stimulation of glucagon secretion by the T-type Ca2+-channel blocker cannot be attributed to inhibition of somatostatin or insulin secretion (19,20) and may therefore reflect an unspecific action of the compound.


Membrane potential-dependent inactivation of voltage-gated ion channels in alpha-cells inhibits glucagon secretion from human islets.

Ramracheya R, Ward C, Shigeto M, Walker JN, Amisten S, Zhang Q, Johnson PR, Rorsman P, Braun M - Diabetes (2010)

Effects of Ca2+-channel antagonists on glucagon secretion. A: Glucagon secretion measured in the absence (open bars) and presence (filled bars) of 10 μmol/l isradipine. *P < 0.01 versus 1 mmol/l glucose alone, †P < 0.05 versus 1 mmol/l glucose and 10 μmol/l isradipine. 100% = 10.5 ± 0.6 pg/islet/h (n = 9; 3 donors). B: Same as in A but effects of 200 nmol/l ω-agatoxin were tested. *P < 0.01 versus 1 mmol/l glucose alone, 100% = 21.1 ± 3.7 pg/islet/h (n = 9; 3 donors).
© Copyright Policy - creative-commons
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2927942&req=5

Figure 6: Effects of Ca2+-channel antagonists on glucagon secretion. A: Glucagon secretion measured in the absence (open bars) and presence (filled bars) of 10 μmol/l isradipine. *P < 0.01 versus 1 mmol/l glucose alone, †P < 0.05 versus 1 mmol/l glucose and 10 μmol/l isradipine. 100% = 10.5 ± 0.6 pg/islet/h (n = 9; 3 donors). B: Same as in A but effects of 200 nmol/l ω-agatoxin were tested. *P < 0.01 versus 1 mmol/l glucose alone, 100% = 21.1 ± 3.7 pg/islet/h (n = 9; 3 donors).
Mentions: The L-type Ca2+-channel blocker isradipine inhibited glucagon secretion from human islets at 1 mmol/l glucose by 25% (Fig. 6A). Glucose retained an inhibitory action in the presence of isradipine (P < 0.05). Blocking the P/Q-type Ca2+-channels exerted a stronger suppressor effect and was as inhibitory as 20 mmol/l glucose (Fig. 6B). Glucose did not diminish glucagon secretion when applied in the presence of ω-agatoxin. The T-type Ca2+-channel blocker 55-0396 paradoxically increased secretion at 1 mmol/l glucose by 190 ± 30% (n = 3; P < 0.05) but did not affect the inhibitory action of 20 mmol/l glucose (65 ± 5%; n = 3; P < 0.05; not shown). The stimulation of glucagon secretion by the T-type Ca2+-channel blocker cannot be attributed to inhibition of somatostatin or insulin secretion (19,20) and may therefore reflect an unspecific action of the compound.

Bottom Line: Inhibition of K(ATP)-channels with tolbutamide depolarized alpha-cells by 10 mV and reduced the action potential amplitude.Exocytosis was negligible at voltages below -20 mV and peaked at 0 mV.We propose that voltage-dependent inactivation of these channels underlies the inhibition of glucagon secretion by tolbutamide and glucose.

View Article: PubMed Central - PubMed

Affiliation: Oxford Centre for Diabetes Endocrinology and Metabolism, University of Oxford, Churchill Hospital, Oxford, UK. matthias.braun@drl.ox.ac.uk

ABSTRACT

Objective: To document the properties of the voltage-gated ion channels in human pancreatic alpha-cells and their role in glucagon release.

Research design and methods: Glucagon release was measured from intact islets. [Ca(2+)](i) was recorded in cells showing spontaneous activity at 1 mmol/l glucose. Membrane currents and potential were measured by whole-cell patch-clamping in isolated alpha-cells identified by immunocytochemistry.

Result: Glucose inhibited glucagon secretion from human islets; maximal inhibition was observed at 6 mmol/l glucose. Glucagon secretion at 1 mmol/l glucose was inhibited by insulin but not by ZnCl(2). Glucose remained inhibitory in the presence of ZnCl(2) and after blockade of type-2 somatostatin receptors. Human alpha-cells are electrically active at 1 mmol/l glucose. Inhibition of K(ATP)-channels with tolbutamide depolarized alpha-cells by 10 mV and reduced the action potential amplitude. Human alpha-cells contain heteropodatoxin-sensitive A-type K(+)-channels, stromatoxin-sensitive delayed rectifying K(+)-channels, tetrodotoxin-sensitive Na(+)-currents, and low-threshold T-type, isradipine-sensitive L-type, and omega-agatoxin-sensitive P/Q-type Ca(2+)-channels. Glucagon secretion at 1 mmol/l glucose was inhibited by 40-70% by tetrodotoxin, heteropodatoxin-2, stromatoxin, omega-agatoxin, and isradipine. The [Ca(2+)](i) oscillations depend principally on Ca(2+)-influx via L-type Ca(2+)-channels. Capacitance measurements revealed a rapid (<50 ms) component of exocytosis. Exocytosis was negligible at voltages below -20 mV and peaked at 0 mV. Blocking P/Q-type Ca(2+)-currents abolished depolarization-evoked exocytosis.

Conclusions: Human alpha-cells are electrically excitable, and blockade of any ion channel involved in action potential depolarization or repolarization results in inhibition of glucagon secretion. We propose that voltage-dependent inactivation of these channels underlies the inhibition of glucagon secretion by tolbutamide and glucose.

Show MeSH
Related in: MedlinePlus