Calcium Channel Permeation: A Field in Flux

In 1955, Alan Hodgkin and Richard Keynes published a study whose intent was to answer a rather technical question, but whose result triggered the modern analysis of ion channel permeation (Hodgkin and Keynes, 1955). They aimed to determine whether the K 1 fluxes in giant axons followed the “Ussing flux ratio criterion,” which was then considered to indicate whether flux through a membrane occurred passively through pores. The Ussing ratio states that passive unidirectional flux should be proportional to the activity of the compound on the side from which the flux occurs. Therefore, the ratio of inward:outward unidirectional K 1 fluxes in the axon was expected to equal the ratio of external:internal K 1 electrochemical activities. The K 1 fluxes in axons clearly failed the Ussing test, as Hodgkin and Keynes noted that the ratio of fluxes varied with the activity ratio raised to the 2.5th power. However, they argued that this indicated a different kind of pore: a long one that could hold multiple ions traversing the pore in single file. To corroborate their mathematical arguments, they mimicked their observations with the mechanical model shown in Fig. 1 A. Now, another dramatic paper (Doyle et al., 1998) has directly demonstrated multiple ions arranged in single file within a crystallized potassium channel pore (Fig. 1 B). A 12-Å length of this pore is so narrow that it provides a tight fit to a K 1 that is stripped of water molecules. This must be the channel’s selectivity filter. Two ions were found within the filter 7.5-Å apart. Thus, the multi-ion, single-file nature of the business region of the potassium channel pore is confirmed absolutely. The filter is electronegative, and the sources of the negativity are backbone carbonyl oxygens rather than amino acid side chains. An array of nearby a -helices appear poised to rigidly hold the selectivity filter structure so that it perfectly fits a K 1 ion ( z 3 Å), but cannot collapse to fit a Na 1 ion ( z 2 Å). This supports the classic “perfect fit” hypothesis for potassium channel selectivity (Mullins, 1959; Hille, 1973), which suggested that a 3–4-Å pore, deduced from permeation experiments, could fully solvate K 1 ions, but fail to do so for the smaller Na 1 ions. In contrast to potassium channels, permeation measurements of sodium channels (Hille, 1971) and calcium channels (McCleskey and Almers, 1985) suggest that their selectivity filters are far wider than their preferred ion. Sodium and calcium channels excel at distinguishing Na 1 from Ca 2 1 , two ions of identical diameter. These facts suggest that a rigid, perfect fit mechanism should not explain selectivity in sodium and calcium channels. Four decades apart, Hodgkin and Keynes (1955) and Doyle et al. (1998) form fundamental pillars to the field of ion channel permeation. They unequivocally demonstrate multi-ion, single-file permeation. Theoretical work between these two landmarks has explored the functional consequences of this mechanism.