Regulation of the Gating of the Sheep Cardiac Sarcoplasmic Reticulum Ca(2+)-release Channel by Luminal Ca2+

We investigated the effects of changes in luminal [Ca2+] on the gating of native and purified sheep cardiac sarcoplasmic reticulum (SR) Ca(2+)-release channels reconstituted into planar phospholipid bilayers. The open probability (Po) of channels activated solely by cytosolic Ca2+ was greater at positive than negative holding potentials. Channels activated solely by 10 microM cytosolic Ca2+ exhibited no change in steady-state Po or in the relationship between Po and voltage when the luminal [Ca2+] was increased from nanomolar to millimolar concentrations. In the absence of activating concentrations of cytosolic Ca2+, the channel can be activated by the phosphodiesterase inhibitor sulmazole (AR-L 115BS). However, cytosolic Ca(2+)-independent activation of the channel by sulmazole requires luminal Ca2+. In the presence of sulmazole, at picomolar luminal [Ca2+] the channel remains completely closed. Increasing the luminal [Ca2+] to millimolar levels markedly increases the Po via an increase in the duration of open events. The Po and duration of the sulmazole-activated, luminal Ca(2+)-dependent channel openings are voltage dependent. In the presence of micromolar luminal Ca2+, the Po and duration of sulmazole-activated openings are greater at negative voltages. However, at millimolar luminal [Ca2+], long openings are also observed at positive voltages and the Po appears to be similar at positive and negative voltages. Our findings indicate that the regulation of channel gating by luminal Ca2+ depends on the mechanism of channel activation.