Presence of functional hyperpolarisation-activated cyclic nucleotide-gated channels in clonal alpha cell lines and rat islet alpha cells.
OBJECTIVE
The hyperpolarisation-activated cyclic nucleotide-gated (HCN) channels, discovered initially in cardiac and neuronal cells, mediate the inward pacemaker current (I (f) or I (h)). Recently, we have demonstrated the presence of HCN channels in pancreatic beta cells. Here, we aim to examine the presence and function of HCN channels in glucagon-secreting alpha cells.


METHODS
RT-PCR and immunocytochemistry were used to examine the presence of HCN channels in alpha cells. Whole-cell patch-clamp, calcium imaging and glucagon secretion experiments were performed to explore the function of HCN channels in alpha cells.


RESULTS
HCN transcripts and proteins were detected in alpha-TC6 cells and dispersed rat alpha cells. Patch-clamp recording showed hyperpolarisation-activated currents in alpha-TC6 cells, which could be blocked by HCN channel inhibitor ZD7288. Glucagon secretion RIA studies demonstrated that at both low and high glucose concentrations (2 and 20 mmol/l), ZD7288 significantly enhanced glucagon secretion in alpha-TC6 and IN-R1-G9 cell lines. Conversely, activation of HCN channels by lamotrigine significantly suppressed glucagon secretion at the low glucose concentration. Calcium imaging studies showed that blockade of HCN channels by ZD7288 significantly increased intracellular calcium in alpha-TC6 cells, while lamotrigine or the Na(+) channel blocker tetrodotoxin suppressed the effect of ZD7288 on intracellular calcium. Furthermore, we found the HCN channel inhibitors ZD7288 and cilobradine both significantly increased glucagon secretion from rat islets.


CONCLUSIONS
These results suggest a potential role for HCN channels in regulation of glucagon secretion via modulating Ca(2+) and Na(+) channel activities.
Zhang
Y
Y
Department of Physiology, University of Toronto, Toronto, ON, Canada.
Zhang
N
N
Gyulkhandanyan
A V
AV
Xu
E
E
Gaisano
H Y
HY
Wheeler
M B
MB
Wang
Q
Q
D000818:Animals; D002118:Calcium / Q000378:metabolism; D002478:Cells, Cultured; D054815:Cyclic Nucleotide-Gated Cation Channels / Q000235:genetics / Q000378:metabolism*; D004594:Electrophysiology; D005786:Gene Expression Regulation; D005934:Glucagon / Q000378:metabolism; D007515:Islets of Langerhans / Q000378:metabolism*; D008297:Male; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D012964:Sodium / Q000378:metabolism
D054815:Cyclic Nucleotide-Gated Cation Channels; D005934:Glucagon; D012964:Sodium; D002118:Calcium