NAADP controls cross-talk between distinct Ca2+ stores in the heart

In cardiac muscle the sarcoplasmic reticulum (SR) plays a key role in the control of contraction, releasing Ca2+ in response to Ca2+ influx across the sarcolemma via voltage- gated Ca2+ channels. Here we report evidence for an additional distinct Ca2+ store and for actions of nicotinic acid adenine dinucleotide phosphate (NAADP) to mobilize Ca2+ from this store, leading in turn to enhanced Ca2+ loading of the SR. Photoreleased NAADP increased Ca2+ transients accompanying stimulated action potentials in ventricular myocytes. The effects were prevented by bafilomycin A (an H+-ATPase inhibitor acting on acidic Ca2+ stores), by desensitizing concentrations of NAADP, and by ryanodine and thapsigargin to suppress SR function. Bafilomycin A also suppressed staining of acidic stores with Lysotracker Red without affecting SR integrity. Cytosolic application of NAADP by means of its membrane permeant acetoxymethyl ester increased myocyte contraction and the frequency and amplitude of Ca2+ sparks, and these effects were inhibited by bafilomycin A. Effects of NAADP were associated with an increase in SR Ca2+ load and appeared to be regulated by beta-adrenoreceptor stimulation. The observations are consistent with a novel role for NAADP in cardiac muscle mediated by Ca2+ release from bafilomycin-sensitive acidic stores, which in turn enhances SR Ca2+ release by increasing SR Ca2+ load.