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Changes of the energy profile of gramicidin A ionic channel dependent on the ratio of cations of different species in the flux passing through the channel.

The elucidation of the mode of diffusion and migration of ions in ionic channels spanning the cell plasma membranes and cell organelles is a special problem demanding joint efforts of physicists and biologists. A general approach to this problem is to investigate a number of ionic channel properties in a wide range of electrolyte conditions. Experiments concerning model bimolecular lipid membranes with incorporated channel-forming substances represent a method of choice in this area. Properties of ionic channels formed in lipid bilayers by pentadecapeptide — gramicidin A have been discussed in detail in recent reviews (Andersen 1984; Hladký and Haydon 1984; Urry 1985). To some extent cation selective gramicidin A channels may be considered an adequate model of the ionic channel in living cell plasma membranes, particularly K-channels of the giant axon and Ca activated K -channels (Hodgkin and Keynes 1955; Vestergaard-Bogind et al. 1985; Eskesen and Ussing 1986). Pronounced interaction of bidirectional fluxes of K ions is characteristic both of natural potassiumand model gramicidin A channels (Hodgkin and Keynes 1955; Schagina et al. 1978, 1980, 1983; Procopio and Andersen 1979; Begenishich and De Weer 1980). The interaction of bidirectional ion fluxes passing through channels is an important peculiarity of ionic movement in narrow pores. A degree of interaction of bidirectional fluxes in these channels may be determined experimentally by using tracer technique. The ratio of unidirectional fluxes of ions of i-th species is given by Ussing—Hodgkin—Keynes equation (Hodgkin and Keynes 1955);