Cell Calcium 35 (2004) 357–365
A store-operated mechanism determines the activity of the
electrically excitable glucagon-secreting pancreatic -cell
Yi-Jia Liu
1
, Elaine Vieira
1
, Erik Gylfe
∗
Department of Medical Cell Biology, Uppsala University Biomedical Centre, Husargatan 3, Box 571, SE-752 37 Uppsala, Sweden
Received 7 August 2003; received in revised form 3 October 2003; accepted 15 October 2003
Abstract
The glucagon-releasing pancreatic -cells are electrically excitable cells but the signal transduction leading to depolarization and secre-
tion is not well understood. To clarify the mechanisms we studied [Ca
2+
]
i
and membrane potential in individual mouse pancreatic -cells
using fluorescent indicators. The physiological secretagogue l-adrenaline increased [Ca
2+
]
i
causing a peak, which was often followed by
maintained oscillations or sustained elevation. The early effect was due to mobilization of Ca
2+
from the endoplasmic reticulum (ER) and
the late one to activation of store-operated influx of the ion resulting in depolarization and Ca
2+
influx through voltage-dependent L-type
channels. Consistent with such mechanisms, the effects of adrenaline on [Ca
2+
]
i
and membrane potential were mimicked by inhibitors of
the sarco(endo)plasmic reticulum Ca
2+
ATPase. The -cells express ATP-regulated K
+
(K
ATP
) channels, whose activation by diazoxide
leads to hyperpolarization. The resulting inhibition of the voltage-dependent [Ca
2+
]
i
response to adrenaline was reversed when the K
ATP
channels were inhibited by tolbutamide. However, tolbutamide alone rarely affected [Ca
2+
]
i
, indicating that the K
ATP
channels are nor-
mally closed in mouse -cells. Glucose, which is the major physiological inhibitor of glucagon secretion, hyperpolarized the -cells and
inhibited the late [Ca
2+
]
i
response to adrenaline. At concentrations as low as 3 mM, glucose had a pronounced stimulatory effect on Ca
2+
sequestration in the ER amplifying the early [Ca
2+
]
i
response to adrenaline. We propose that adrenaline stimulation and glucose inhibition
of the -cell involve modulation of a store-operated current, which controls a depolarizing cascade leading to opening of L-type Ca
2+
channels. Such a control mechanism may be unique among excitable cells.
© 2003 Elsevier Ltd. All rights reserved.
Keywords: Glucagon secretion; Store-operated Ca
2+
channel; Endoplasmic reticulum calcium sequestration; Endoplasmic reticulum calcium release;
Membrane potential
1. Introduction
Diabetes mellitus is characterized by hyperglycemia due
to insulin deficiency, which is caused by pancreatic -cell
destruction or disturbed secretion of the hormone. In both
types 1 and 2 diabetes, failure of glucose to inhibit the se-
cretion of blood glucose-elevating glucagon aggravates the
hyperglycemia [1–3]. Moreover, glucagon secretion is not
appropriately stimulated when blood glucose falls to low
concentrations, a potentially life-threatening condition in
insulin-treated diabetic subjects [4]. Whereas considerable
attention has been focussed on insulin secretion from the
-cell, much less is known about glucagon release from the
pancreatic -cell. It has been proposed that insulin [5,6],
∗
Corresponding author. Tel.: +46-18-4714428; fax: +46-18-4714059.
E-mail address: erik.gylfe@medcellbiol.uu.se (E. Gylfe).
1
Drs. Vieira and Liu contributed equally to the article.
GABA [7], or Zn
2+
[8] co-secreted with insulin from the
-cell, mediates glucose inhibition of glucagon secretion.
Although such mechanisms may contribute, they cannot ex-
plain why glucose inhibits glucagon secretion from purified
-cells [9], and why secretion is inhibited at lower sugar
concentrations than those stimulating the secretory activity
of the -cell [10].
Metabolism of glucose is essential both for stimulation
of insulin release [11] and inhibition of glucagon secretion
[12,13]. In the -cell the closure of the ATP-sensitive K
+
(K
ATP
) channels has a central role in stimulus-secretion
coupling, transducing the increase in ATP/ADP ratio ob-
tained with glucose metabolism into depolarization with re-
sulting opening of voltage-dependent L-type Ca
2+
channels
[11]. There are diverging opinions about the presence of
K
ATP
channels in the -cell and their possible involvement
in the regulation of glucagon secretion [6,14–18]. Since
glucose inhibits glucagon release, it is apparent that a fun-
0143-4160/$ – see front matter © 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ceca.2003.10.002