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  • Title: Mechanisms of Cs+ blockade in a Ca2+-activated K+ channel from smooth muscle.
    Author: Cecchi X, Wolff D, Alvarez O, Latorre R.
    Journal: Biophys J; 1987 Nov; 52(5):707-16. PubMed ID: 2447963.
    Abstract:
    Large unitary conductance Ca2+-activated K+ channels from smooth muscle membrane were incorporated into phospholipid planar bilayers, and the blockade induced by internally and externally applied Cs+ was characterized. Internal Cs+ blockade is voltage dependent and can be explained on the basis of a Cs+ binding to a site that senses 54% of the applied voltage, with an apparent dissociation constant, Kd(0), of 70 mM. On the other hand, external Cs+ blocks the channel in micromolar amounts, and the voltage dependence of blockade is a function of Cs+ concentration. The fractional electrical distance can be as large as 1.4 at 10 mM Cs+. This last result suggests that the channel behaves as a multi-ion pore. At large negative voltages the I-V relationships in the presence of external Cs+ show an upturn, indicating relief of Cs+ block. External Cs+ blockade is relieved by increasing the internal K+ concentration, but can be enhanced by increasing the external K+. All the characteristics of external Cs+ block can be explained by a model that incorporates a "knock-on" of Cs+ by K+.
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