Regulation of the gating of the sheep cardiac sarcoplasmic reticulum ca2+-release channel by luminal Ca2+
The properties of calcium-release channels of sheep cardiac muscle junctional sarcoplasmic reticulum (SR), have been investigated under voltage-clamp conditions following the fusion of isolated membrane vesicles with planar phospholipid bilayers. In the presence of activating calcium on the cytosolic side of the membrane, additions of the benzimidazole derivative sulmazole (AR-L 115BS) increased the open probability (P a ) of the channel reaching saturating values of 1.0 at 3mm sulmazole. The drug did not affect single-channel conductance and activation was readily reversible. Analysis of channel open and closed lifetimes suggested that low concentrations of sulmazole (0.1mm) may sensitize the channel to activating calcium, while at higher concentrations (1mm and above), calcium and sulmazole act synergistically to produce a unique gating scheme for the channel. Millimolar concentrations of sulmazole also stimulate a degree of channel opening at subactivating (60pm) calcium concentrations. Openings occurring under these conditions show very different kinetics to those of the calcium-activated channel but have an identical single-channel conductance and are modified by ATP, magnesium, ruthenium red and ryanodine in a similar manner to the calcium-activated channel. The release of calcium from the SR following the activation of the calcium-release channel by sulmazole may contribute to the positive inotropic action of this drug on mammalian cardiac muscle.