Pharmacological characterization of NAADP-induced Ca²⁺ signals in starfish oocytes

The recently discovered second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) is central to the onset of intracellular Ca²⁺ signals induced by several stimuli, including fertilization. The nature of the Ca²⁺ pool mobilized by NAADP is still controversial. Depending on the cell type, NAADP may target either an acidic compartment with lysosomal properties or ryanodine receptors (RyRs) on endoplasmic reticulum. In addition, NAADP elicits a robust Ca²⁺ influx into starfish oocytes by activating a Ca²⁺-mediated current across the plasma membrane. In the present study, we employed the single-electrode intracellular recording technique to assess the involvement of either acidic organelles or RyRs in NAADP-elicited Ca²⁺ entry. We found that neither drugs which interfere with acidic compartments nor inhibitors of RyRs affected NAADP-induced depolarization. These data further support the hypothesis that a yet unidentified plasma membrane Ca²⁺ channel is the target of NAADP in starfish oocytes.