The regulation of cardiac calcium release channels by calsequestrin
Cardiac calsequestrin (Casq2) is a Ca2+ binding protein inside the lumen of the sarcoplasmic reticulum (SR) that binds Ca2+ and regulates the gating of the Ca2+ channel called the ryanodine receptor (RyR2). The role of Casq2 during cardiac contraction is still unclear. We hypothesize Casq2 serves as a lumenal Ca2+ sensor that increases RyR2 open probability as a function of [C2+] inside the SR. To better understand how Casq2 affects RyR2 open probability (Po), single channel electrophysiology experiments were performed using transgenic SR microsomes with (Casq2WT) and without (Casq2KO) Casq2. Increasing lumenal [Ca2+] from 2μM to 2mM produced a significant increase in Po and a decrease in the amplitude of the current in Casq2WT microsomes; though, it is uncertain whether this effect was due to a lumenal regulation by Casq2 or a cytosolic regulation by a feed-through activation. Increasing lumenal [Mg2+], a RyR2 cytosolic inhibitor, from 0 mM to 3 mM also produced an increase in RyR2 Po. An increase in lumenal Tb3+ [nM] activated RyR2s from Casq2WT but not from Casq2KO microsomes without modifications in the RyR2 current, indicating no feed-through activation. These results support the idea that Casq2 is involved in the lumenal regulation of RyR2.