Interactions among Cations in Current Conduction through the Stretch-Activated Channel of the Frog Oocyte
An increasing literature is being accumulating about stretch-activated (s.a.) channels. They were found first in chick skeletal muscle by the group of F. Sachs (Guharay and Sachs, 1984, 1985), and later on in many other structures (Hamill, 1983; Cooper et al., 1986; Methfessel et al., 1986; Yang et al., 1986; Falke et al., 1987; Gustin et al., 1987; Lansman et al., 1987; Ove, 1987). S.a. channels, unlike voltage- or chemically-activated channels, are primarily gated by mechanical distortion of the cell membrane. By using the improved patch-clamp technique in the cell-attached configuration, we are studying the s.a. channels present in the plasma membrane of the frog oocyte. An interesting property of this cation channel was evidenced in a previous report (Toselli et al., 1987): the current carried in the inward direction through the s.a. channel obviously increases when Ca2+is removed from the standard Ringer. This was investigated thouroughly in the present experiments. Briefly, we find that the cations that can enter the channel do not move independently of one another: the Goldman-Hodgkin-Katz equations therefore do not adequately describe the modalities of ion permeation. Conversely, the experimental observations are in agreement with the expectations of a model representing the s.a. channel as a membrane pore with a single binding site accepting one ion at a time, and displaying different affinities for the various cations.
KeywordsPatch Pipette Membrane Patch Single Channel Current Standard Ringer Single Binding Site
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