Inhibition of a voltage-dependent cation channel in sarcoplasmic reticulum vesicles by caesium studied by using a...

The effect of caesium on the cation transport system in sarcoplasmic reticulum vesicles has been analysed kinetically through Tris+ influx. The Tris+ influx was measured by following the change in K+ diffusion potential due to the mutual diffusion between K+ and Tris+ in the presence of valiomycin using a potential probe; 3,3'-dipropylthiadicarbocyanine iodide. The main results were as follows. (1) Tris+ influx increased when membrane potential became inside-negative. This suggests that Tris+ permeates through the channel which has a voltage-dependent gate. (2) Cs+ reacted with the cation transport system only from the outside of the vesicle and inhibited Tris+ influx. The inhibition follows a single-site titration curve with a voltage-dependent dissociation constant of 18 mM at -60 mV. The inhibition can be explained by assuming that Cs+ binds to a site located about 45% of the way through the membrane from the outside of the vesicle in the open state of the channel. These results are in good agreement with those reported by Coronado and Miller (Coronado, R. and Miller, C. (1979) Nature 288, 495-497), which were gained electrically by using sarcoplasmic reticulum vesicles incorporated into an artificial planar phospholipid bilayer.