NAD derived second messengers: Role in spontaneous diastolic Ca2+ transients in murine cardiac myocytes

Strong β-adrenergic stimulation induced spontaneous diastolic Ca2+ transients (SCTs) in electrically paced murine cardiac myocytes [28]. To obtain further insights into the underlying mechanism, we developed a method for a simultaneous analysis, in which the free luminal Ca2+ concentration in the sarcoplasmic reticulum (SR) ([Ca2+]SR) and the free cytosolic Ca2+ concentration ([Ca2+]i) were measured in parallel in the same cell. Each spontaneous diastolic Ca2+ transient was exactly mirrored by a decrease of [Ca2+]SR. Since antagonism of the Ca2+ mobilizing second messenger nicotinic acid adenine dinucleotide phosphate (NAADP) was shown to block SCTs in single cardiac myocytes [28], we analyzed the effect of the novel ADP-ribosyl cyclase inhibitor SAN4825 on both cytosolic and intra-luminal Ca2+ transients upon strong β-adrenergic stimulation. A strong antagonist effect of SAN4825 on SCTs at low micromolar concentrations was observed. Our results suggest that the underlying mechanism of spontaneous diastolic Ca2+ transients observed upon strong β-adrenergic stimulation is sensitization of type 2 ryanodine receptor by the Ca2+ releasing activity of the products of ADP-ribosyl cyclase activity.