Abstract—Previous studies of arterial smooth muscle have shown that inositol 1,4,5-trisphosphate (IP3) and cyclic ADP-ribose mobilize Ca2+from the sarcoplasmic reticulum. In contrast, little is known about Ca2+mobilization by nicotinic acid adenine dinucleotide phosphate, a pyridine nucleotide derived from &bgr;-NADP+. We show here that intracellular dialysis of nicotinic acid adenine dinucleotide phosphate (NAADP) induces spatially restricted “bursts” of Ca2+release that initiate a global Ca2+wave and contraction in pulmonary artery smooth muscle cells. Depletion of sarcoplasmic reticulum Ca2+stores with thapsigargin and inhibition of ryanodine receptors with ryanodine, respectively, block the global Ca2+waves by NAADP. Under these conditions, however, localized Ca2+bursts are still observed. In contrast, xestospongin C, an IP3receptor antagonist, had no effect on Ca2+signals by NAADP. We propose that NAADP mobilizes Ca2+via a 2-pool mechanism, and that initial Ca2+bursts are amplified by subsequent sarcoplasmic reticulum Ca2+release via ryanodine receptors but not via IP3receptors.