Laser photolysis of caged calcium: Rates of calcium release by nitrophenyl-EGTA and DM-nitrophen

Nitrophenyl-EGTA and DM-nitrophen are Ca2+ cages that release Ca2+ when cleaved upon illumination with near-ultraviolet light. Laser photolysis of nitrophenyl-EGTA produced transient intermediates that decayed biexponentially with rates of 500,000 s(-1) and 100,000 s(-1) in the presence of saturating Ca2+ and 290,000 s(-1) and 68,000 s(-1) in the absence of Ca2+ at pH 7.2 and 25 degrees C. Laser photolysis of nitrophenyl-EGTA in the presence of Ca2+ and the Ca2+ indicator Ca-orange-5N produced a monotonic increase in the indicator fluorescence, which had a rate of 68,000 s(-1) at pH 7.2 and 25 degrees C. Irradiation of DM-nitrophen produced similar results with somewhat slower kinetics. The transient intermediates decayed with rates of 80,000 s(-1) and 11,000 s(-1) in the presence of Ca2+ and 59,000 s(-1) and 3,600 s(-1) in the absence of Ca2+ at pH 7.2 and 25 degrees C. The rate of increase in Ca2+-indicator fluorescence produced upon photolysis of the DM-nitrophen: Ca2+ complex was 38,000 s(-1) at pH 7.2 and 25 degrees C. In contrast, pulses in Ca2+ concentration were generated when the chelator concentrations were more than the total Ca(2+)concentration. Photoreleased Ca2+ concentration stabilized under these circumstances to a steady state within 1-2 ms.