Fast and cAMP-Sensitive Mode of Ca2+-Dependent Exocytosis in Pancreatic β-Cells

Abstract

The fast component (mode 1) of Ca 2؉ -dependent exocytosis in pancreatic ␤-cells, unlike that in adrenal chromaffin cells, is regulated by cytosolic ATP in a concentration-dependent manner. This action of ATP is apparent within 3 min and does not require ATP hydrolysis; rather, it requires the production of cAMP by adenylate cyclase. Moreover, the effect of cAMP is ATP dependent, as revealed by the observation that the fast component of exocytosis is facilitated by ATP, even in the presence of a saturating concentration of cAMP (200 mol/l). Thus, the amplitude of mode-1 exocytosis depends quadratically on the cytosolic ATP concentration and is facilitated by ATP, even in the absence of an increase in the concentration of cAMP. Given that high glucose concentrations increase the cytosolic ATP concentration, glucose-induced insulin secretion likely involves this action of ATP on mode-1 exocytosis, together with its effect on ATP-dependent K ؉ channels. In contrast to the fast component of exocytosis, the slow component (mode 2) of this process is independent of cAMP and ATP and can account for the slow component of insulin secretion, which does not require these nucleotides. Diabetes 51 (Suppl. 1):S19 -S24, 2002

Deploying insulin granule–granule fusion to rescue deficient insulin secretion in diabetes

“…Interestingly, Hoppa et al did not observe a significant increase in primary exocytosis, shown in other reports to occur in response to GLP-1 potentiation [7,8]. In addition, granule-granule fusion kinetics varied according to the stimulating compound used, with GLP-1-stimulated release largely the result of sequential fusion [7,8] and carbacholstimulated release largely produced by multivesicular fusion [16].…”

“…In addition, granule-granule fusion kinetics varied according to the stimulating compound used, with GLP-1-stimulated release largely the result of sequential fusion [7,8] and carbacholstimulated release largely produced by multivesicular fusion [16]. This would suggest that downstream exocytotic fusion molecules acted upon by carbachol-evoked calcium (and PKC) signalling may be distinct from the exocytotic molecules coupled to GLP-1-evoked cAMP/PKA signalling.…”

“…However, in the initial elegant studies [5], insulin granules were shown to fuse, with some delay, to granules that had already fused with the plasma membrane, hence this type of compound fusion was termed sequential granule fusion; and this involved only two, or sometimes three, granules [6]. The incretin hormone glucagon-like peptide 1 (GLP-1), which is well known to greatly potentiate GSIS via cAMP and protein kinase A (PKA) pathways and is currently used clinically to treat diabetes, was shown to increase GSIS by increasing primary exocytosis and sequential exocytosis [7,8]. Another mode of granulegranule fusion is multivesicular exocytosis (also known as true compound exocytosis), whereby granules undergo complete or partial fusion inside the cell (homotypic fusion, Fig.…”

Mechanisms of action of glucagon-like peptide 1 in the pancreas

“…By contrast Takahashi and colleagues have presented experimental evidence for the importance of PKA in fast glucose-induced exocytosis (Kasai et al, 2002;Takahashi et al, 1999). The model they have developed is a rapid and reversible post-priming step in which ATP acts independently of its effects on [Ca 2+ ] or the K ATP channels but requires PKA.…”

“…Basal levels of cAMP (and presumably PKA) were sufficient to prime the β cell for this initial burst of vesicle release as FSK, even the presence of high ATP concentrations, did not augment insulin release whereas low concentrations did so. However, as Takahashi and colleagues point out, it is still not possible to rule out that a component of this Ca 2+ -dependent fast exocytosis could be dependent on another cAMP sensor such as Epac (Kasai et al, 2002).…”