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

Abstract

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.