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

Abstract

Glucose-induced insulin secretion (IS) by β-cells is controlled by two pathways. The triggering pathway involves ATP-sensitive potassium (KATP) channel-dependent depolarization, Ca2+influx, and rise in the cytosolic Ca2+concentration ([Ca2+]c), which triggers exocytosis of insulin granules. The metabolic amplifying pathway augments IS without further increasing [Ca2+]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 [Ca2+]cand IS. Metabolic amplification was studied during imposed steady elevation of [Ca2+]cby tolbutamide or KCl or by comparing [Ca2+]cand IS responses to glucose and tolbutamide. Nocodazole did not alter [Ca2+]cor IS changes induced by the three secretagogues, whereas taxol caused a small inhibition of IS that is partly ascribed to a decrease in [Ca2+]c. When [Ca2+]cwas 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 [Ca2+]cwas 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.