Regulation of the gating of the sheep cardiac sarcoplasmic reticulum ca²⁺-release channel by luminal Ca²⁺

Abstract

We investigated the effects of changes in luminal [Ca²⁺] on the gating of native andpurified sheep cardiac sarcoplasmic reticulum (SR) Ca²⁺-release channels reconstituted intoplanar phospholipid bilayers. The open probability (P _o )of channels activated solely by cytosolic Ca²⁺ was greater at positive than negative holding potentials. Channels activatedsolely by 10 μm cytosolic Ca²⁺ exhibited no change in steady-stateP _o or in the relationship betweenP _o and voltage when the luminal[Ca²⁺] was increased from nanomolar to millimolar concentrations. In the absence of activating concentrationsof cytosolic Ca²⁺, the channel can be activated by the phosphodiesterase inhibitor sulmazole (AR-L 115BS). However, cytosolicCa²⁺-independent activation of the channel by sulmazole requires luminal Ca²⁺. In the presence ofsulmazole, at picomolar luminal [Ca²⁺] the channel remains completely closed. Increasing the luminal [Ca²⁺]to millimolar levels markedly increases the P _o via an increase in theduration of open events. The P _o and duration of the sulmazole-activated, luminalCa²⁺-dependent channel openings are voltage dependent. In the presence of micromolar luminal Ca²⁺, theP _o and duration of sulmazole-activated openings are greater atnegative voltages. However, at millimolar luminal [Ca²⁺], long openings are also observed at positive voltages and theP _o appears to be similar at positive and negative voltages. Our findings indicate thatthe regulation of channel gating by luminal Ca²⁺ depends on the mechanism of channel activation.