Ion channel control of phasic insulin secretion

Sammanfattning: Glucose-stimulated insulin secretion exhibits a biphasic pattern. The mechanism underling biphasic insulin secretion is not fully understood, but consensus exists that an elevation in [Ca2+]i is required for both first- and second-phase insulin secretion. The molecular identity of the pancreatic β-cell L-type Ca2+ channel has not been established and it has variably been reported to be CaV1.2 (α1C) or CaV1.3 (α1D). Though the cellular background to the two phases of release remains unknown, it has been suggested to reflect the sequential release of distinct pools of granules, which vary with regard to release competence. This thesis investigated the role of different ion channels in insulin secretion.
β-cell-selective ablation of the CaV1.2 gene(βCaV1.2-/- mouse) decreased the whole-cell Ca2+ current by only ~45%, but almost abolished first-phase insulin secretion and resulted in systemic glucose intolerance. High-resolution capacitance measurements of exocytosis in single β-cells revealed that the loss of first-phase insulin secretion in the βCaV1.2-/- mouse was associated with the disappearance of a rapid component of exocytosis reflecting fusion of secretory granules physically attached to the CaV1.2 channel.
A 20% reduction in glucose-evoked insulin secretion was observed in CaV2.3-knockout (CaV2.3–/–) islets, close to the 17% inhibition by the R-type blocker SNX482. Genetic or pharmacological CaV2.3 ablation strongly suppressed second-phase secretion in vitro, as well as in vivo, whereas first-phase secretion was unaffected. Suppression of the second phase coincided with an 18% reduction in oscillatory Ca2+ signaling and a 25% reduction in granule recruitment after completion of the initial exocytotic burst in single CaV2.3–/– β-cells.
Intracellular ClC-3 chloride channels have been implicated in the process of making insulin granules release-competent, a process referred to as priming. Analysis of insulin secretion in vivo and in vitro as well as capacitance measurements revealed that the secretory response of ClC-3 deficient β-cells was reduced, but not abolished. The presence of ClC-3 in insulin granules was detected in a high-purification fraction of LDCVs obtained by phogrin-GFP labelling.
In conclusion: (1) CaV1.2 Ca2+ channels are required for first-phase insulin release and maintenance of systemic glucose tolerance. (2) CaV2.3 Ca2+ channels play an important role in second-phase insulin release. (3) ClC-3 chloride channels facilitate insulin secretion by enhancing properly acidification of insulin granules needed for granule priming.