Effects of extracellular adenine nucleotides on the electrical, ionic and secretory events in mouse pancreatic β‐cells

Abstract

1 The mechanisms whereby extracellular adenine nucleotides modulate pancreatic #-cell function were studied with mouse islets stimulated by 15 mm glucose.2 Adenosine 5'-triphosphate (ATP) and adenosine 5'-diphosphate (ADP) (100pM) inhibited insulin release, 45Ca efflux and 86Rb efflux from islet cells, and decreased electrical activity in fl-cells. These changes were rapid but small and transient. 3 x,#-Methylene ADP caused a rapid and sustained inhibition of insulin release, 45Ca efflux and 86Rb efflux from islet cells. It also produced a slight hyperpolarization of the #-cell membrane, with sustained modification of the pattern but only transient decrease of the intensity of the electrical activity. In the absence of extracellular Ca2, a,#-methylene ADP increased 45Ca and 86Rb efflux without changing insulin release. Most effects of a,#-methylene ATP were qualitatively similar but quantitatively smaller than those of the ADP-analogue. 4 Adenylylimido-diphosphate (AMP-PNP) slightly increased 45Ca and 86Rb efflux and potentiated insulin release in the presence of extracellular Ca22. However, its effects on electrical activity in fl-cells were qualitatively similar to those of the a,#-methylene analogues. 5 The small effects of ATP and ADP could result from their degradation into adenosine. a,#-Methylene ADP appears to increase K+ permeability of the fl-cell membrane and to produce a second, intracellular, effect which largely contributes to the inhibition of insulin release. Another recognition site, with higher affinity for triphosphate derivatives, could mediate the small stimulatory effects of AMP-PNP.

ADP mediates inhibition of insulin secretion by activation of P2Y13 receptors in mice

“…ADP stimulates insulin secretion from beta cells via the Gα qcoupled P2Y 1 receptor and inhibits insulin release via the Gα i -coupled P2Y 13 receptor [30]. This finding may explain the sometimes contradictory results from previous studies [9][10][11][12][13] where ADP has been shown to both increase [9][10][11] and decrease [12,13] insulin release via P2Y receptors. ADP acting on P2Y 1 receptors has been reported to increase pulsatility of insulin release, but not Ca 2+ rhythmicity in intact pancreas [16].…”

“…3 Expression of mouse ADP receptors P2Y 1 , P2Y 12 and P2Y 13 in pancreatic islets and beta cells from the mouse (n=3). Only trace amounts of P2Y 12 were detected. Black columns, whole islets; white columns, isolated beta cells The addition of 16.6 mmol/l glucose, but not mannitol, resulted in a significant increase (p<0.05) in ATP release that was completely abolished by the addition of the Ca 2+ -channel blocker nifedipine (20 µmol/l).…”

“…The net pharmacological effects of extracellular ATP are difficult to predict since ATP by itself can stimulate P2Y 2 , P2Y 4 (in rodents), P2Y 11 and P2X 1 -P2X 7 receptors present on the cell surface [6]. Furthermore, ATP is rapidly degraded by ectonucleotidases to ADP [7], which can act on P2Y 1 , P2Y 12 and P2Y 13 receptors. ADP is then further degraded by ecto-5′-nucleotidase to adenosine, which in turn can activate four different adenosine receptors [8].…”

“…A large number of studies indicate that extracellular ATP and ADP have a key role in regulating insulin secretion by purinoreceptors [9][10][11][12][13]. Insulin-secretory granules contain 3.5 mmol/l ATP and 5 mmol/l ADP [14], and with a novel biosensor it has been demonstrated that glucose stimulation releases ATP from a single pancreatic beta cell to a local extracellular ATP concentration exceeding 25 µmol/l [4].…”

“…Beta cells are activated by ADP and can also propagate this signal via intermittent release of ATP [15]. However, ADP has also been shown to decrease insulin release [12,13]. These discrepancies are probably in part due to involvement of several different purinoreceptors [13,[15][16][17][18][19][20][21][22].…”