Modulation of CICR has no maintained effect on systolic Ca2+: simultaneousmeasurements of sarcoplasmic reticulum and sarcolemmal Ca2+ fluxes in rat ventricular myocytes

1. The effects of modulating Ca2+-induced Ca2+ release (CICR) in single car diac myocytes were investigated using low concentrations of caffeine (< 500 mu M) in reduced external Ca2+ (0.5 mM). Caffeine produced a transient pot entiation of systolic [Ca2+](i) (to 800 % of control) which decayed back to control levels. 2. Caffeine decreased the steady-state sarcoplasmic reticulum (SR) Ca2+ con tent. As the concentration of caffeine was increased, both the potentiation of the systolic Ca2+ transient and the decrease in SR Ca2+ content were in creased. At higher concentrations, the potentiating effect decayed more rap idly but the rate of recovery on removal of caffeine was unaffected. 3. A simple model in which caffeine produces a fixed increase in the fracti on of SR Ca2+ which is released could account qualitatively but not quantit atively for the above results. 4. The changes in total [Ca2+] during systole were obtained using measureme nts of the intracellular Ca2+ buffering power. Caffeine initially increased the fractional release of SR Ca2+ This was followed by a decrease to a lev el greater than that under control conditions. The fraction of systolic Ca2 + which was pumped out of the cell increased abruptly upon caffeine applica tion but then recovered back to control levels. The increase in fractional loss is due to the fact that, as the cytoplasmic buffers become saturated, a given increase in systolic total [Ca2+] produces a larger increase in fre e [Ca2+] and thence of Ca2+ efflux. 5. These results confirm that modulation of the ryanodine receptor has no m aintained effect on systolic Ca2+ and show the interdependence of SR Ca2+ c ontent, cytoplasmic Ca2+ buffering and sarcolemmal Ca2+ fluxes. Such analys is is important for understanding the cellular basis of inotropic intervent ions in cardiac muscle.