Regulation of calcium in pancreatic α- and β-cells in health and disease

Abstract

The glucoregulatory hormones insulin and glucagon are released from the β- and α-cells of the pancreatic islets. In both cell types, secretion is secondary to firing of action potentials, Ca(2+)-influx via voltage-gated Ca(2+)-channels, elevation of [Ca(2+)](i) and initiation of Ca(2+)-dependent exocytosis. Here we discuss the mechanisms that underlie the reciprocal regulation of insulin and glucagon secretion by changes in plasma glucose, the roles played by different types of voltage-gated Ca(2+)-channel present in α- and β-cells and the modulation of hormone secretion by Ca(2+)-dependent and -independent processes. We also consider how subtle changes in Ca(2+)-signalling may have profound impact on β-cell performance and increase risk of developing type-2 diabetes.

Fig. 1

Stimulus-secretion coupling of human β-cells. (A) Glucose uptake via Glut1 leads to accelerated glucose metabolism, increased ATP production and closure of the ATP-regulated K⁺-channels (K_ATP-channels), consisting of the pore-forming subunit Kir6.2 and the sulphonylurea-binding protein SUR1. The increased membrane resistance (R_m↑) resulting from closure of the K_ATP-channels allows occasional spontaneous opening of T-type Ca²⁺-channels to depolarise the β-cell (Ψ↓) and this leads to regenerative opening of additional T-type Ca²⁺-channels and further membrane depolarization that culminates in rapid activation of L-type Ca²⁺-channels and voltage-gated Na⁺-channels during the upstroke of the action potential. The action potential culminates in the opening of P/Q-type Ca²⁺-channels and the associated Ca²⁺-influx triggers exocytosis of insulin granules. Opening of Ca²⁺-activated high-conductance K⁺-channels (BK) underlies action potential repolarization. (B) Glucose-induced electrical activity recorded from a β-cell in an intact islet in response to an elevation of glucose from 1 to 6 mM. Note oscillatory electrical activity. (C) Elevation of [Ca²⁺]_i in a β-cell within an intact human islets when in response to an elevation of glucose from 1 to 6 mM.

Fig. 2

Ca²⁺-signalling in human α-cells. (A) Schematic action potential in human α-cell. Contribution to action potential of T- (Ca[T]), L- (Ca[L]) and P/Q-type Ca²⁺-channels (Ca[P/Q]), TTX-sensitive Na⁺-channels (Na_V), transient A-type K⁺-current (K_V[A]) and delayed rectifying K⁺-current (K_V[DR]). P/Q-type Ca²⁺-channels, linked to glucagon exocytosis, open only at the peak of the action potential (highlighted in red) and if the peak voltage of the action potential is reduced (right), fewer P/Q-type Ca²⁺-channels will open with resultant suppression of glucagon secretion. Glucose may reduce spike height via membrane depolarization and this in turn leads to voltage-dependent inactivation of Na_V, K_V(A) and Ca(T) and under these conditions, action potential firing may depend only on Ca(L) and K_V(DR) channel activity. (B) Effects of increasing glucose from 1 to 6 mM on spontaneous [Ca²⁺]_i oscillations in an individual cell (assumed to be an α-cell) within an intact human pancreatic islet. Note that glucose has no major inhibitory effect during >10 min. (C) Effects of ω-agatoxin (200 nM) and isradipine (10 μM) on [Ca²⁺]_i measured in a cell spontaneously active at low glucose. Note small effect of ω-agatoxin and that subsequent addition of isradipine exerts a stronger inhibitory effect. Effect of ω-agatoxin irreversible so both P/Q and L-type Ca²⁺-channels are blocked following the addition of isradipine. Experiments in B–C were conducted by Dr CE Ward. Trace in C is taken from . (D) Differential roles (hypothetical) of L- and P/Q-type Ca²⁺-entry on release of glucagon-containing secretory granules and acetycholine-containing synaptic like microvesicles (SV). Note that L- and P/Q-type Ca²⁺-channels are spatially separated.

Fig. 3

Uneven Ca²⁺-channel distribution in β-cells. (A) On-cell recording of Ca²⁺-channel activity in a single mouse β-cell showing examples of patches containing no, 1, 2 or 3 channels. Values to the right indicate probability (in %) of the respective types of responses. Data from . (B) TIRF image of an isolated ‘control’ β-cell during a 50-ms depolarization from −70 mV to 0 mV. Spatial resolution was increased by the inclusion of 10 mM EGTA in intracellular medium (to restrict Ca²⁺-diffusion) and use of the low-affinity Ca²⁺-indicator Oregon Green 6F (30 μM). Changes in [Ca²⁺]_i are displayed in pseudocolours with black/blue and yellow/red corresponding to very low and high concentrations, respectively. Scale bars: 2 μm. The dotted line corresponds to the approximate footprint of the β-cell. (C) As in B but obtained in a β-cell treated for 72 h with 0.5 mM palmitate. Experiments in panels B–C were conducted by Dr MB Hoppa. Data from . (D) Schematic representation of Ca²⁺-channels and secretory granules in control cells. Secretory granules are tightly associated with voltage-gated Ca²⁺-channels. Stimulation with glucose leads to membrane depolarization, opening of voltage-gated Ca²⁺-channels, localized increases in [Ca²⁺]_i close to Ca²⁺-channels. This triggers exocytosis of the secretory granules. During long depolarizations (or when TEA was applied to broaden the action potentials), the active zones of elevated [Ca²⁺]_i extend further away from the Ca²⁺-channels leading to moderate (50%) further stimulation of secretion. (E) As in D but in palmitate-treated β-cells in which Ca²⁺-channels and secretory granules are not so tightly associated. Under these conditions, the [Ca²⁺]_i increases occur too far away from secretory granules to trigger their release. However, insulin secretion can be rescued when the duration of the depolarizations is increased (e.g. by TEA). Under these conditions, the size of the active zones is increased so that also granules not situated close to the Ca²⁺-channels undergo exocytosis.

Fig. 4

Ca²⁺-channels and glucagon exocytosis evoked by hypoglycaemia alone and in the presence of adrenaline. (A) Action potential generated under hypoglycaemic conditions leads to opening of non-L-type (presumably P/Q-type) Ca²⁺-channels and localized increases in [Ca²⁺]_i that trigger exocytosis of the few glucagon-containing secretory granules (SG) that happen to be situated close enough to these Ca²⁺-channels. During the action potentials, L-type Ca²⁺-channels also open but exocytosis of secretory granules is not triggered (spatial separation?). (B) In the presence of adrenaline, cAMP increases and activates the cAMP-sensor Epac2. Under these conditions, Ca²⁺-entry via L-type Ca²⁺-channels triggers Ca²⁺-induced Ca²⁺-release by activation of ryanodine receptor Ca²⁺-release channels (RyR3) in the sER and [Ca²⁺]_i rises throughout the α-cell triggering exocytosis of all release-competent granules.