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Abstract: In rat pancreatic β cells, arachidonic acid (AA) triggered intracellular Ca2+ release. This effect could be mimicked by eicosatetraynoic acid, indicating that AA metabolism is not required. The AA-mediated Ca2+ signal was not affected by inhibition of ryanodine receptors or emptying of ryanodine-sensitive store but was reduced by ∼70% following the disruption of acidic stores (treatment with bafilomycin A1 or glycyl-phenylalanyl-β-naphthylamide (GPN)). The action of AA did not involve TRPM2 channels or NAADP receptors because intracellular dialysis of adenosine diphosphoribose (ADPR; an activator of TRPM2 channels) or NAADP did not affect the AA response. In contrast, stimulation of IP3 receptors via intracellular dialysis of adenophostin A, or exogenous application of ATP largely abolished the AA-mediated Ca2+ signal. Intracellular dialysis of heparin abolished the ATP-mediated Ca2+ signal but not the AA response, suggesting that the action of AA did not involve the IP3-binding site. Treatment with the SERCA pump inhibitor, thapsigargin, reduced the amplitude of the AA-mediated Ca2+ signal by ∼70%. Overall, our finding suggests that AA mobilizes Ca2+ from the endoplasmic reticulum as well as an acidic store and both stores could be depleted by IP3 receptor agonist. The possibility of secretory granules as targets of AA is discussed. [Copyright &y& Elsevier]

Abstract: Arachidonic acid (AA) is generated in the pancreatic islets during glucose stimulation. We investigated whether AA activated extracellular Ca2+ entry in rat pancreatic β cells via a pathway that was independent of the activation of voltage-gated Ca2+ channels. The AA triggered [Ca2+]i rise did not involve activation of GPR40 receptors or AA metabolism. When cells were voltage clamped at −70mV, the AA-mediated intracellular Ca2+ release was accompanied by extracellular Ca2+ entry. AA accelerated the rate of Mn2+ quench of indo-1 fluorescence (near the Ca2+-independent wavelength of indo-1), reflecting the activation of a Ca2+-permeable pathway. The AA-mediated acceleration of Mn2+ quench was inhibited by La3+ but not by 2-APB (a blocker of capacitative Ca2+ entry), suggesting the involvement of arachidonate-regulated Ca2+ (ARC) channels. Consistent with this, intracellular application of the charged membrane-impermeant analog of AA, arachidonyl-coenzyme A (ACoA) triggered extracellular Ca2+ entry, as well as the activation of a La3+-sensitive small inward current (1.7pA/pF) at −70mV. Our results indicate that the activation of ARC channels by intracellular AA triggers extracellular Ca2+ entry. This action may contribute to the effects of AA on Ca2+ signals and insulin secretion in rat β cells. [Copyright &y& Elsevier]