Regulation of calcium release by calcium inside the sarcoplasmic reticulum in ventricular myocytes

To study the effects of changes in sarcoplasmic reticulum (SR) intraluminal Ca2+ on the Ca2+ release mechanism, we correlated the activity of single cardiac ryanodine receptor (RyR) channels, monitored in planar bilayers, with the properties of spontaneous elementary Ca2+ release events (sparks) in intact ventricular myocytes, monitored by scanning confocal microfluorimetry. Under both normal conditions and Ca2+ overload, induced by elevation of extracellular [Ca2+], Ca2+ sparks represented single populations of events. During Ca2+ overload, the frequency of sparks increased from 0.8 to 3.1 events per second per 100 mu m line scanned, and their amplitude increased from 100 nM to 400 nM. The duration of the Ca2+ sparks, however, was not altered. Changes in the properties of Ca2+ sparks were accompanied by only an approximate to 30% increase in the SR Ca2+ content, as determined by emptying the intracellular Ca2+ stores using caffeine. When single Ca2+ release channels were incorporated into lipid bilayers and activated by cytoplasmic Ca2+ (approximate to 100 nM) and ATP (3 mM), elevation of Ca2+ on the luminal side from 20 mu M to 0.2-20 mM resulted in a 1.2-fold to 7-fold increase, respectively, in open probability (P-o). This potentiation of P-o was due to an increase in mean open time and frequency of events. The relative effect of luminal Ca2+ was greater at low levels of cytoplasmic [Ca2+] than at high levels of cytoplasmic [Ca2+], and no effect of luminal Ca2+ was observed to occur in channels activated by 0.5-50 mu M cytoplasmic Ca2+ in the absence of ATP. Our results suggest that SR Ca2+ release channels are modulated by SR intraluminal Ca2+. These alterations in properties of release channels may account for, or contribute to, the mechanism of spontaneous Ca2+ release in cardiac myocytes.