Inhibition of a cardiac sarcoplasmic reticulum chloride channel by tamoxifen

Anion and cation channels present in the sarcoplasmic reticulum (SR) are believed to be necessary to maintain the electroneutrality of SR membrane during Ca²⁺ uptake by the SR Ca²⁺ pump (SERCA). Here we incorporated canine cardiac SR ion channels into lipid bilayers and studied the effects of tamoxifen and other antiestrogens on these channels. A Cl^- channel was identified exhibiting multiple subconductance levels which could be divided into two primary conductance bands. Tamoxifen decreases the time the channel spends in its higher, voltage-sensitive band and the mean channel current. The lower, voltage-insensitive, conductance band is not affected by tamoxifen, nor is a K⁺ channel present in the cardiac SR preparation. By examining SR Ca²⁺ uptake, SERCA ATPase activity, and SR ion channels in the same preparation, we also estimated SERCA transport current, SR Cl^- and K⁺ currents, and the density of SERCA, Cl ^-, and K⁺ channels in cardiac SR membranes.