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Cation interactions within the cyclic GMP‐activated channel of retinal rods from the tiger salamander.
Author(s) -
Zimmerman A L,
Baylor D A
Publication year - 1992
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1113/jphysiol.1992.sp019112
Subject(s) - chemistry , depolarization , reversal potential , membrane potential , permeation , biophysics , divalent , analytical chemistry (journal) , membrane , patch clamp , chromatography , biochemistry , biology , receptor , organic chemistry
1. The ionic dependence of current through the 3',5'‐cyclic guanosine monophosphate (cyclic GMP)‐activated channels of salamander rods was studied in excised inside‐out membrane patches from isolated outer segments. Voltage‐clamp experiments on transducing rods were performed so that the channels in intact cells could be compared with those in excised patches. 2. The reversal potential of the cyclic GMP‐induced patch current was close to the Na+ equilibrium potential when the concentration of NaCl on the cytoplasmic surface of a patch was varied at constant external NaCl concentration. Fitting the Goldman‐Hodgkin‐Katz equation indicated that the apparent ratio of permeabilities for Na+ and Cl‐ was at least 50. This confirms a previous report that the channel's Na+ permeability is much larger than its Cl‐ permeability. 3. Na+ currents through the channel did not obey the independence principle. The outward patch current at large positive potential began to saturate with increasing concentrations of internal Na+, as if permeation required Na+ to bind to a site with an apparent dissociation constant around 180 mM. 4. In symmetrical NaCl solutions containing very low concentrations of divalent cations the current‐voltage relation measured from excised patches 50 microseconds after switching the voltage showed mild outward rectification. By 1 ms the rectification was more pronounced. The rectification at 50 microseconds is attributed to voltage dependence of Na+ permeation. The additional rectification at later times is attributed to voltage dependence of the channel's probability of being open, depolarization favouring the open state. 5. In symmetrical Mg2+ solutions the cyclic GMP‐induced patch currents were smaller and the outward rectification was more pronounced. 6. Addition of Mg2+ or Ca2+ to an internal Na+ solution blocked the cyclic GMP‐induced Na+ current through the channels, as if by occupying a single binding site with an affinity in the 0.1‐2 mM range. Block by Mg2+ was voltage dependent, suggesting that the binding site was within the channel's transmembrane electric field. Raising the Mg2+ concentration on the external surface of the patch increased the apparent dissociation constant of block by internal Mg2+, as expected if external and internal Mg2+ compete for the same binding site. 7. Block by internal Ca2+ had an opposite and weaker voltage dependence than block by internal Mg2+. 8. In symmetrical solutions containing both Na+ and Mg2+ the outward rectification was more pronounced than in solutions containing Na+ alone. In solutions thought to be close to physiological the outward patch current increased e‐fold for a depolarization of 24‐30 mV.(ABSTRACT TRUNCATED AT 400 WORDS)