Extracellular NAD+ induces a rise in [Ca2+]i in activated human monocytes via engagement of P2Y1 and P2Y11 receptors

Extracellular nicotinamide adenine dinucleotide (NAD+) is known to increase the intracellular calcium concentration [Ca2+]i in different cell types and by various mechanisms. Here we show that NAD+ triggers a transient rise in [Ca2+]i in human monocytes activated with lipopolysaccharide (LPS), which is caused by a release of Ca2+ from IP3-responsive intracellular stores and an influx of extracellular Ca2+. By the use of P2 receptor-selective agonists and antagonists we demonstrate that P2 receptors play a role in the NAD+-induced calcium response in activated monocytes. Of the two subclasses of P2 receptors (P2X and P2Y) the P2Y receptors were considered the most likely candidates, since they share calcium signaling properties with NAD+. The identification of P2Y1 and P2Y11 as receptor subtypes responsible for the NAD+-triggered increase in [Ca2+]i was supported by several lines of evidence. First, specific P2Y1 and P2Y11 receptor antagonists inhibited the NAD+-induced increase in [Ca2+]i. Second, NAD+ was shown to potently induce calcium signals in cells transfected with either subtype, whereas untransfected cells were unresponsive. Third, NAD+ caused an increase in [cAMP]i, prevented by the P2Y11 receptor-specific antagonist NF157.