Metabolic amplification of insulin secretion by glucose is independent of β-cell microtubules

Am J Physiol Cell Physiol. 2011 Mar;300(3):C697-706. doi: 10.1152/ajpcell.00329.2010. Epub 2010 Dec 22.

Abstract

Glucose-induced insulin secretion (IS) by β-cells is controlled by two pathways. The triggering pathway involves ATP-sensitive potassium (K(ATP)) channel-dependent depolarization, Ca(2+) influx, and rise in the cytosolic Ca(2+) concentration ([Ca(2+)](c)), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca(2+)](c). After exclusion of the contribution of actin microfilaments, we here tested whether amplification implicates microtubule-dependent granule mobilization. Mouse islets were treated with nocodazole or taxol, which completely depolymerized and polymerized tubulin. They were then perifused to measure [Ca(2+)](c) and IS. Metabolic amplification was studied during imposed steady elevation of [Ca(2+)](c) by tolbutamide or KCl or by comparing [Ca(2+)](c) and IS responses to glucose and tolbutamide. Nocodazole did not alter [Ca(2+)](c) or IS changes induced by the three secretagogues, whereas taxol caused a small inhibition of IS that is partly ascribed to a decrease in [Ca(2+)](c). When [Ca(2+)](c) was elevated and controlled by KCl or tolbutamide, the amplifying action of glucose was unaffected by microtubule disruption or stabilization. Both phases of IS were larger in response to glucose than tolbutamide, although triggering [Ca(2+)](c) was lower. This difference, due to amplification, persisted in nocodazole- or taxol-treated islets, even when IS was augmented fourfold by microfilament disruption with cytochalasin B or latrunculin B. In conclusion, metabolic amplification rapidly augments first and second phases of IS independently of insulin granule translocation along microtubules. We therefore extend our previous proposal that it does not implicate the cytoskeleton but corresponds to acceleration of the priming process conferring release competence to insulin granules.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Calcium Signaling / drug effects
  • Calcium Signaling / physiology
  • Cells, Cultured
  • Female
  • Glucose / metabolism*
  • Glucose / pharmacology
  • Insulin / metabolism*
  • Insulin Secretion
  • Insulin-Secreting Cells / drug effects
  • Insulin-Secreting Cells / metabolism*
  • Islets of Langerhans / drug effects
  • Islets of Langerhans / metabolism*
  • Metabolic Networks and Pathways / drug effects
  • Metabolic Networks and Pathways / physiology*
  • Mice
  • Mice, Inbred C57BL
  • Microtubules / drug effects
  • Microtubules / metabolism*
  • Organ Culture Techniques
  • Protein Transport / physiology

Substances

  • Insulin
  • Glucose