Dihydropyridine-sensitive and -insensitive voltage-operated calcium channels participate in the control of glucose-induced insulin release from human pancreatic β cells

Abstract

Calcium ion entry through voltage-operated calcium channels is a crucial step in the coupling of beta cell depolarization with insulin secretion. Various calcium channel subtypes have been shown to be coexpressed in single neurons and endocrine cells. Using the patch-clamp technique, we investigated the biophysical and pharmacological properties of calcium channels in freshly dispersed human pancreatic beta cells. Both low and high voltage activated currents were expressed, the two current types being easily distinguishable on the basis of biophysical criteria. The high voltage activated currents were not homogeneous: one component was affected by the dihydropyridine antagonist nitrendipine and the agonist Bay-K-8644; the other was insensitive to both dihydropyridines and omega-conotoxin GVIA. In line with this pharmacology, nitrendipine reduced and Bay-K-8644 increased glucose-induced insulin secretion from perifused human islets, whereas omega-conotoxin GVIA had no effect. However, about 20% of the glucose-induced insulin release was found to be resistant to high nitrendipine concentrations. These data show that human pancreatic beta cells express heterogeneous voltage-operated calcium channels, only one of which is dihydropyridine-sensitive (L type). The L type channels are clearly involved in the control of insulin secretion, but our data suggest that dihydropyridine- and omega-conotoxin GVIA-insensitive channels may also play a role in the stimulus-secretion coupling of human beta cells.

Voltage-Gated Ion Channels in Human Pancreatic β-Cells: Electrophysiological Characterization and Role in Insulin Secretion

“…Ca 2؉ currents and exocytosis. In agreement with previous studies (5,11), we show that L-type Ca 2ϩ channels are expressed in human ␤-cells. Blockade of L-type Ca 2ϩ channels using the selective antagonist isradipine leads to complete inhibition of glucose-induced insulin secretion.…”

“…Voltage-gated Ca 2ϩ channels are activated by coordinated fluctuations of the cell membrane potential (electrical activity), which are initiated by the glucose-induced closure of ATP-sensitive K ϩ channels (K ATP channels) (2,3) and dependent on voltagegated Na ϩ and K ϩ channels (4). Due to the limited availability of human islets, few electrophysiological studies of voltage-gated ion channels in human ␤-cells have been published, and the identity of the ␤-cells was not unequivocally established (5)(6)(7)(8), although 45% of normal human islets cells are non-␤-cells (9). In some earlier studies (10 -12), identification of ␤-cells was based on the presence of K ATP currents.…”

The human L-type calcium channel Cav1.3 regulates insulin release and polymorphisms in CACNA1D associate with type 2 diabetes

“…Various VGCC subclasses are described and the L-type class (L-VGCC) has been considered essential for insulin secretion [2]. However, which of the two main L-type isoforms (Ca v 1.2 and Ca v 1.3) is the more important appears to be species dependent and to differ between rodents and humans [3].…”

Up‐regulation of L‐ and non‐L, non‐N‐type Ca2+ channels by basal and stimulated protein kinase C activation in insulin‐secreting RINm5F cells

“…We also provide evidence that PKC-mediated phosphorylation is not involved in the G-protein-mediated noradrenergic modulation of Ca 2+ channels in RINm5F cells. channels are also implicated in insulin release from human 13 cells [10] and catecholamines secretion from chromaffin cells [11] and that they can be up-regulated by PKC activation in neurons [12,13], PKC-dependent phosphorylation of these channels in RINm5F cells may be relevant to the overall secretory activity of these cells.…”

“…We also provide evidence that PKC-mediated phosphorylation is not involved in the G-protein-mediated noradrenergic modulation of Ca 2+ channels in RINm5F cells. channels are also implicated in insulin release from human 13 cells [10] and catecholamines secretion from chromaffin cells [11] and that they can be up-regulated by PKC activation in neurons [12,13], PKC-dependent phosphorylation of these channels in RINm5F cells may be relevant to the overall secretory activity of these cells.Here we show that both L-and non-L, non-N-type Ca 2+ channels are effectively up-regulated by PKC-dependent phosphorylation under basal conditions. In agreement with this, stimulation of PKC by acute application of phorbol ester causes only minor up-regulation of both channel subtypes, with a net prevalence of the L-type.…”