How some ion channels display both extreme selectivity and rapid flux remains a central topic of modern research in quantitative biology. Two papers in this issue (Dang and McCleskey, 1998; Kiss et al., 1998) independently present a class of new models that attempt to explain new data gathered over the past few years from careful experiments on mutant channels.
The classical squid axon K+ channel experiment that suggested multi-ion behavior in channels, or the “long pore effect,” was prompted by the flux-ratio criterion (Ussing, 1949; Hodgkin and Keynes, 1955; Hille, 1992). Unidirectional fluxes of radioactive tracers were measured under varying electrochemical gradients for K+. The ratio of inward to outward tracer fluxes depended exponentially on a power n of the electrochemical driving force, (E–EK). This experiment is usually explained by stating that an individual K+ ion...
