Aims: 

Ryanodine receptor (RyR) is the major intracellular Ca²⁺release channel required for excitation-contraction coupling in cardiac muscle. The crystallographic structure of this channel is currently unavailable. Thus, the architecture of the conductive pore involved in ion handling can be probed only indirectly by examining the permeability properties of the channel. All available lines of evidence are compatible with the proposal that the conduction pathway of the RyR channel is occupied by only one ion at a time. The purpose of our study was to re-examine this conclusion under asymmetrical ionic conditions that have not yet been tested.

Methods: 

RyR channels isolated from the rat heart were reconstituted into planar lipid membrane. The zero-current potential was determined from the current-voltage relationship obtained under various asymmetrical ionic conditions.

Results: 

The zero-current potential showed clear concentration dependence when Li⁺ or Na⁺ was present at the cytosolic and Ca²⁺at the luminal side of the channel. In one set of experiments, concentration ratios [Li⁺]/[Ca²⁺] and [Na⁺]/[Ca²⁺] were held constant. Importantly, the concentration dependence of zero-current potential was abolished when the concentration ratio was lowered from 12 to 1. Furthermore, the zero-current potential did not show extremes with varying Ba²⁺/Ca²⁺molar ratio at the luminal face of the channel.

Conclusion: 

In the light of the barrier model of ion permeation through channels, the concentration dependence of the zero-current potential is one of the characteristics predicted for multi-ion channels. Thus, our results weaken the hypothesis of the single-ion nature of the RyR channel conductive pathway.