NAADP receptors are present and functional in the heart

Alongside the well-studied inositol 1,4,5 trisphosphate and ryanodine recep tors, evidence is gathering that a new intracellular release mechanism, gat ed by the pyridine nucleotide nicotinic acid adenine dinucleotide phosphate (NAADP), is present in numerous organisms, ranging from plant to mammalian cells (reviewed in [1]), Most cells have been shown to express at least tw o Ca2+-release mechanisms controlled by different messengers, and this can lead to redundancy, convergence, or divergence of responses. One exception appears to be muscle and heart contractile tissues. Here, it is thought tha t the dominant intracellular channel is the ryanodine receptor, while IF, r eceptors are poorly expressed and their role appears to be negligible, We n ow report that NAADP receptors are functional and abundant in cardiac micro somes. NAADP binds specifically and with high affinity (130 pM and 4 nM) to two sites on cardiac microsomes and releases Ca2+ with an apparent EC50 of 323 +/- 14 nM, Furthermore, binding experiments show that this receptor di splays both positive and negative cooperativity, a peculiarity unique among intracellular Ca2+ channels. Therefore, we show that the heart possesses m ultiple mechanisms to Increase the complexity of Ca2+ signaling and that NA ADP may be integral in the functioning of this organ. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.