Cyclic ADP-Ribose Does Not Regulate Sarcoplasmic Reticulum Ca ²⁺ Release in Intact Cardiac Myocytes

Cyclic ADP-ribose (cADPR), an intracellular second messenger known to mobilize Ca ²⁺ in sea urchin eggs, has been implicated in modulating Ca ²⁺ release in a variety of mammalian tissues. On the basis of studies of isolated cardiac sarcoplasmic reticulum (SR) vesicles and single SR Ca ²⁺ release channels, cADPR has also been proposed to be a modulator of SR Ca ²⁺ release in heart. In the present study, we directly examined the ability of cADPR to trigger SR Ca ²⁺ release and to modulate Ca ²⁺ -induced Ca ²⁺ release (CICR) in intact rat ventricular myocytes. Voltage-clamped myocytes were dialyzed with up to 100 μmol/L caged cADPR and 0.6 μmol/L calmodulin along with the Ca ²⁺ -sensitive dye fluo 3. A step increase in the cADPR concentration induced by flash photolysis of caged cADPR neither directly triggered SR Ca ²⁺ release nor modulated CICR in intact myocytes. In contrast, under similar conditions, extracellular application of caffeine (1 to 2.5 mmol/L) onto myocytes produced both effects. Under equivalent conditions, flash photolysis of caged cADPR-loaded sea urchin eggs resulted in large Ca ²⁺ transients. Further, the sustained presence of high cytosolic concentrations of either cADPR or its antagonist, 8-amino-cADPR, was ineffective in altering normal CICR in myocytes. These findings indicate that cADPR does not regulate SR Ca ²⁺ release in intact cardiac myocytes.