Simultaneous Measurement of Cytoplasmic and SR Calcium during Modulation of Ryanodine Receptor Open Probability in Dog Ventricular Myocytes

We have previously found that, in rat ventricular myocytes, increasing ryanodine receptor open probability using low concentrations of caffeine had no maintained effect on the amplitude of systolic Ca. On application of caffeine, following an initial increase, systolic Ca returned to control levels in around 20 s and it was argued that this was due to a concurrent decrease in SR Ca. In the present study, we sought to obtain direct evidence for the involvement of a decrease of SR Ca in this transient response.SR Ca was measured directly with Mag-Fura-2. Application of 0.5 mM caffeine initially caused a 197 % increase in the amplitude of systolic Ca. This was associated with a 874 % increase in the amplitude of SR Ca loss and a 328 and 178 % increase in the rate of systolic cytoplasmic Ca removal and SR Ca replenishment respectively. In sustained caffeine exposure, all these parameters returned to levels comparable to control, typically within 1 - 2 beats. During caffeine exposure, SR Ca content rapidly decreased within 1 - 2 beats to a new steady state level. All measured parameters recovered to control levels on removal of caffeine.These data show, on RyR potentiation, following the initial increase, the secondary decrease of systolic Ca is due to a decrease in SR Ca.