Subplasmalemmal Ca2+ measurements in mouse pancreatic beta cells support the existence of an amplifying effect of glucose on insulin secretion

Abstract

Aims/hypothesis: Glucose-induced insulin secretion is attributed to a rise of beta cell cytosolic free [Ca2+] ([Ca2+]c) (triggering pathway) and amplification of the action of Ca2+. This concept of amplification rests on observations that glucose can increase Ca 2+-induced insulin secretion without further elevating an imposed already high [Ca2+]c. However, it remains possible that this amplification results from an increase in [Ca2+] just under the plasma membrane ([Ca2+]SM), which escaped detection by previous measurements of global [Ca2+]c. This was the hypothesis that we tested here by measuring [Ca2+]SM. Methods: The genetically encoded Ca2+ indicators D3-cpv (untargeted) and LynD3-cpv (targeted to plasma membrane) were expressed in clusters of mouse beta cells. LynD3-cpv was also expressed in beta cells within intact islets. [Ca2+]SM changes were monitored using total internal reflection fluorescence microscopy. Insulin secretion was measured in parallel. Results: Beta cells expressing D3cpv or LynD3cpv displayed normal [Ca 2+] changes and insulin secretion in response to glucose. Distinct [Ca2+]SM fluctuations were detected during repetitive variations of KCl between 30 and 32-35 mmol/l, attesting to the adequate sensitivity of our system. When the amplifying pathway was evaluated (high KCl∈+∈diazoxide), increasing glucose from 3 to 15 mmol/l consistently lowered [Ca2+]SM while stimulating insulin secretion approximately two fold. Blocking Ca2+ uptake by the endoplasmic reticulum largely attenuated the [Ca2+]SM decrease produced by high glucose but did not unmask localised [Ca2+] SM increases. Conclusions/interpretation: Glucose can increase Ca2+-induced insulin secretion without causing further elevation of beta cell [Ca2+]SM. The phenomenon is therefore a true amplification of the triggering action of Ca2+. © 2010 Springer-Verlag.