Department of Physiology
University of Minnesota
Streaming potentials arise from a coupling between ion flux and fluid movement. For example, suppose a fluid flow is produced in a cation selective ion channel by an applied osmotic pressure difference. The fluid flow will induce a drag force on the cations. If the membrane is under open circuit conditions (electrical current = 0), then an electrical potential must be developed that just balances the drag force so that the cation flux is zero. The classical theory of streaming potentials was developed for pores whose diameter was much larger than the ion radius (Kruyt, 1952, Colloid Science, Vol. 1, p. 204). In addition to the large pore radius requirement, the classical theory makes some very restrictive assumptions about the channel (e.g. a uniform constant surface charge) which are not satisfied by biological channels. At the opposite extreme, it can be shown that, for channels that are so narrow that the ion and water cannot pass around each other ("single-file condition", a very general theory can be derived that relates the streaming potential to the number of water molecules in the channel (Levitt, 1984, Cur. Topics in Memb. & Transp. V. 21, p. 181). Biological ion channels usually have relative wide mouth regions connected in series to a short "ingle-file"region. In order to interpret experimental streaming potential measurements on biological ion channels, it is usually assumed that the contribution of the mouth regions is negligible so that the potential arises only from the narrow single-file region. There is third theory that provides direct predictions of the streaming potential contributions of the wide mouth regions of biological ion channels (Levitt, J. Chem. Phys. 1990, V. 92, p. 6953). This result is based on a novel continuum type theory that is applicable to a wide range of pore sizes, varying from the classical limit down to biological ion channels.