Cardiomyocytes express one or more subtypes of P2 receptors for extracellular
nucleotides.
P2 purinoceptors, which are activated by
nucleotides, are classified as P2X or P2Y: P2X receptors are
ligand-gated intrinsic
ion channels, and P2Y receptors are
G protein-coupled receptors. Extracellular
pyrimidine and
purine nucleotides are released from the heart during
hypoxia. Although the cardioprotective effects of
purines acting via
purinoceptors were studied intensively, the physiological role of
uracil nucleotide-responsive P2Y2, P2Y4, P2Y6, and P2Y14 receptors is still unclear, especially in the cardiovascular system. This study revealed that uridine-5'-triphosphate (
UTP) protected cultured rat cardiomyocytes during
hypoxia and explored the
UTP signaling pathway leading to this cardioprotection. We found that
UTP, but not
UDP or
uridine, significantly reduced cardiomyocyte death induced by
hypoxia. Incubation with
UTP for 1 h, before exposure to hypoxic conditions, protected the cells 24 h later. The cardioprotective effect of
UTP was reduced in the presence of the P2 antagonist
suramin. In addition,
UTP caused a transient increase of [Ca2+]i in cardiomyocytes. Pyridoxal-5'-phosphate-6-azophenyl-2,4-disulfonate (
PPADS) or
Reactive blue 2 (RB-2), other antagonists of P2 receptors, abolished the [Ca2+]i elevation caused by
UTP. We used various inhibitors of the Ca2+ signaling pathway to show that
UTP elevated levels of [Ca2+]i, originating from intracellular sources, via activation of
phospholipase C and the
IP3 receptor. Interestingly, these inhibitors of the Ca2+ signaling pathway did not prevent the immediate protective effect caused by
UTP. Although mitochondrial
KATP channels are involved in other preconditioning mediator pathways, the involvement of these channels in the cardioprotective effect induced by
UTP was ruled out, because
5-hydroxydecanoic acid (5-HD), a specific inhibitor of these channels, did not prevent the protection.