On the theory of ion transport across the nerve membrane. II. Potassium ion kinetics and cooperativity (with x = 4).

We use a tetrahedral model of four interacting protein subunits to represent the K(+) channel or gate in the squid nerve membrane. The kinetic predictions, with varying degrees of cooperativity, are compared with experimental observations, especially those of Hodgkin and Huxley (J. Physiol. 117, 500, 1952) and of Cole and Moore (Biophys. J. 1, 1, 1960). The tentative conclusion reached is that if there is any cooperativity present it must be rather weak. There is no indication here that cooperativity improves the Hodgkin-Huxley assumption of independent "subunits". Other related models will be discussed in Part III. We also find evidence against the suggestion that there is cooperativity between K(+) channels arranged in patches of a two-dimensional lattice.