Glucose stimulates somatostatin secretion in pancreatic δ-cells by cAMP-dependent intracellular Ca2+ release

Abstract

Somatostatin secretion from pancreatic islet δ-cells is stimulated by elevated glucose levels, but the underlying mechanisms have only partially been elucidated. Here we show that glucose-induced somatostatin secretion (GISS) involves both membrane potential-dependent and -independent pathways. Although glucose-induced electrical activity triggers somatostatin release, the sugar also stimulates GISS via a cAMP-dependent stimulation of CICR and exocytosis of somatostatin. The latter effect is more quantitatively important and in mouse islets depolarized by 70 mM extracellular K+, increasing glucose from 1 mM to 20 mM produced an ∼3.5-fold stimulation of somatostatin secretion, an effect that was mimicked by the application of the adenylyl cyclase activator forskolin. Inhibiting cAMP-dependent pathways with PKI or ESI-05, which inhibit PKA and exchange protein directly activated by cAMP 2 (Epac2), respectively, reduced glucose/forskolin-induced somatostatin secretion. Ryanodine produced a similar effect that was not additive to that of the PKA or Epac2 inhibitors. Intracellular application of cAMP produced a concentration-dependent stimulation of somatostatin exocytosis and elevation of cytoplasmic Ca2+ ([Ca2+]i). Both effects were inhibited by ESI-05 and thapsigargin (an inhibitor of SERCA). By contrast, inhibition of PKA suppressed δ-cell exocytosis without affecting [Ca2+]i. Simultaneous recordings of electrical activity and [Ca2+]i in δ-cells expressing the genetically encoded Ca2+ indicator GCaMP3 revealed that the majority of glucose-induced [Ca2+]i spikes did not correlate with δ-cell electrical activity but instead reflected Ca2+ release from the ER. These spontaneous [Ca2+]i spikes are resistant to PKI but sensitive to ESI-05 or thapsigargin. We propose that cAMP links an increase in plasma glucose to stimulation of somatostatin secretion by promoting CICR, thus evoking exocytosis of somatostatin-containing secretory vesicles in the δ-cell.

Cyclic AMP links glucose stimulation to somatostatin secretion in δ-cells

“…If cAMP promotes CICR, one would expect to see [Ca 2+ ] i spikes that are unrelated to membrane potential. This was indeed what Denwood et al (2019) observed. Increases in cAMP sometimes induced “spontaneous” [Ca 2+ ] i spikes even when the membrane potential was clamped at −70 mV.…”

“…CICR is enhanced by glucose but it has been unclear how the sugar acts in this process. The present study ( Denwood et al, 2019 ) pinpoints the intracellular messenger cAMP as the amplifying factor in glucose-stimulated somatostatin secretion.…”

“…The secretion of somatostatin is stimulated by glucose and other nutrients ( Rorsman and Huising, 2018 ), as well as paracrine factors ( van der Meulen et al, 2015 ; Adriaenssens et al, 2016 ; Vergari et al, 2019 ), but because of the scarcity of δ-cells and difficulties of accessing them for physiological investigations the understanding of the control mechanisms for somatostatin secretion is still rather limited (see Rorsman and Huising [2018] for a review of δ-cell physiology). In this issue of the Journal of General Physiology , Denwood et al (2019) from the Rorsman laboratory present an extensive experimental effort that provides new insights into the mechanisms by which glucose stimulates somatostatin secretion from δ-cells.…”

“…To further clarify the relationship between cAMP and Ca 2+ , Denwood et al (2019) recorded [Ca 2+ ] i in δ-cells in which the Ca 2+ reporter protein GCaMP3 was transgenically expressed. When the cells were maintained at a substimulatory glucose concentration and the membrane was depolarized with an excessive K + concentration, there was only a transient [Ca 2+ ] i elevation.…”

“…cAMP regulates secretion via both PKA and the guanine nucleotide exchange factor Epac2 ( Seino and Shibasaki, 2005 ). To elucidate cAMP effector protein involvement, Denwood et al (2019) used both pharmacological activators and inhibitors as well as cells from Epac2-deficient mice. PKA and Epac2 inhibition had largely overlapping effects, but there were also differences.…”