CaV2.3 calcium channels control second-phase insulin release

Abstract

Concerted activation of different voltage-gated Ca 2+ channel isoforms may determine the kinetics of insulin release from pancreatic islets. Here we have elucidated the role of R-type Ca V 2.3 channels in that process. A 20% reduction in glucose-evoked insulin secretion was observed in Ca V 2.3-knockout (Ca V 2.3 -/-) islets, close to the 17% inhibition by the R-type blocker SNX482 but much less than the 77% inhibition produced by the L-type Ca 2+ channel antagonist isradipine. Dynamic insulin-release measurements revealed that genetic or pharmacological Ca V 2.3 ablation strongly suppressed second-phase secretion, whereas first-phase secretion was unaffected, a result also observed in vivo. Suppression of the second phase coincided with an 18% reduction in oscillatory Ca 2+ signaling and a 25% reduction in granule recruitment after completion of the initial exocytotic burst in single Ca V 2.3 -/-β cells. Ca V 2.3 ablation also impaired glucose-mediated suppression of glucagon secretion in isolated islets (27% versus 58% in WT), an effect associated with coexpression of insulin and glucagon in a fraction of the islet cells in the Ca V 2.3 -/-mouse. We propose a specific role for Ca V 2.3 Ca 2+ channels in second-phase insulin release, that of mediating the Ca 2+ entry needed for replenishment of the releasable pool of granules as well as islet cell differentiation. IntroductionSystemic glucose tolerance is orchestrated by the regulated release of insulin and glucagon from the β and α cells of the pancreatic islets of Langerhans. The α and β cells are electrically excitable and use electrical signals to couple changes in blood glucose concentration to stimulation or inhibition of hormone release. In both cell types, influx of extracellular Ca 2+ through voltage-gated Ca 2+ channels with resultant elevation of intracellular Ca 2+ concentration ([Ca 2+ ] i ) triggers exocytosis of the hormone-containing secretory granules. Like other electrically excitable cells, both α and β cells contain several types of voltage-gated Ca 2+ channel (1, 2). Assigning physiological functions to the respective Ca 2+ channels is central to the understanding of electrical and secretory activities in these cells.Voltage-gated Ca 2+ channels are divided into 3 subfamilies: (a) L-type high voltage-activated (HVA) Ca 2+ channel family that comprises the Ca V 1.1, 1.2, 1.3, and 1.4 channels and is inhibited by dihydropyridines (DHPs) (1, 3, 4); (b) non-L-type HVA channels Ca V 2.1 (P/Q-type), 2.2 (N-type), and 2.3 (R-type) that are sensitive to ω-agatoxin IVA and ω-conotoxin GVIA and SNX482, respectively (1, 4, 5); and (c) the low voltage-activated (LVA) T-type Ca 2+ channel family (Ca V 3.1, 3.2, and 3.3). The latter subtype differs electrophysiologically from the HVA Ca 2+ channels in opening transiently already upon modest depolarization (6, 7) and fulfilling important roles in pacemaker cells (8).

Exploring the function and pharmacotherapeutic potential ­of voltage-gated Ca2+channels with gene-knockout models

“…165 An unexpected finding was that-in addition to LTCCs-Ca V 2.3 channels play a major role for insulin secretion. In pancreatic β-cells Ca V 2.3 channels contribute only a quarter of total I Ca , 166 but they strongly support the second (slow) phase of insulin secretion as shown in pancreatic perfusion experiments. 166 Accordingly SNX-482 strongly inhibited the second, but not the first phase of insulin secretion.…”

CaV2.3 channel and PKCλ: new players in insulin secretion

“…The work by Jing et al (9) confirms that the Ca V 2.3 channel is present in mouse β cells and that in Ca V 2.3-knockout (Ca V 2.3 -/-) mice, impaired glucose homeostasis is caused by defective insulin secretion. This is in agreement with previous work from this group demonstrating that SNX482 significantly diminishes β cell Ca V currents, causing a reduction in β cell exocytosis (11).…”

“…This is strongly supported by the fact that a massive decrease in insulin release occurred selectively at the second phase from either the Ca V 2.3 -/-islets or islets exposed to SNX482 (9). Recently, it has been hypothesized that there is 1 rate-limiting step between the 2 phases of insulin secretion (16).…”

“…The work by Jing et al (9) draws our attention to several important points. First, unlike the β cell Ca V 1.2 and Ca V 1.3 channels, which are tightly coupled to the exocytotic machinery (13)(14)(15), the Ca V 2.3 channel seems to be distant from the areas in the β cell where insulin-containing granules fuse with the plasma membrane and release their contents.…”

Of rodents and men: Species-specific glucose regulation and type 2 diabetes research

“…Human β-cells express functionally important Ca 2+ -channels that are not thought to be as critical in the mouse β-cell (Braun et al, 2008). For example, R-type Ca 2+ channels, which play a critical role in mouse β-cells by modulating the kinetics of insulin release and glucose-mediated suppression of glucagon secretion, are not expressed at detectable levels in human β-cells (Jing et al, 2005;Rorsman and Braun, 2013). The expression of voltage-gated delayed rectifier potassium channels that regulate exocytosis and delayed phase ionic current in humans also differs in rodents.…”