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A study of stretch‐activated channels in the membrane of frog oocytes: interactions with Ca2+ ions.
Author(s) -
Taglietti V,
Toselli M
Publication year - 1988
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.1988.sp017417
Subject(s) - pipette , chemistry , membrane potential , depolarization , reversal potential , membrane , conductance , patch clamp , biophysics , analytical chemistry (journal) , permeability (electromagnetism) , ion channel , electrophysiology , chromatography , biochemistry , receptor , medicine , biology , mathematics , combinatorics
1. We have carried out patch‐clamp measurements on a cationic channel in the plasma membrane of the frog oocyte, which can be specifically activated by membrane stretch. The kinetics of this channel also display a distinct dependence upon membrane potential, the probability of the channel being open increasing with membrane depolarization. 2. When the patch‐clamp pipette filling solution was standard Ringer solution, the single‐channel current‐voltage (I‐V) relationship was linear, the elementary conductance being 38 pS and the reversal potential +7 mV, suggesting very poor selectivity of the channel for the various cations. 3. The I‐V relationship was highly non‐linear having a strong inward‐going rectification when Ca2+‐free solutions were used to fill the patch pipette. These solutions also resulted in a selective, inward cationic permeability through the membrane, with K+ being more permeable than Na+ greater than Li+ greater than Ba2+ greater than Ca2+. 4. Though permeant through the stretch‐activated channel, Ca2+ inhibited in a concentration‐dependent manner the currents carried by other cations. La3+ (0.1 mM) was also an effective channel blocker. 5. The inward current carried by individual cations at a given membrane potential increased with increasing external cation concentration up to a saturating level, this level being maximal for K+ and minimal for Ca2+. Also the half‐saturating concentration was maximal for K+ and minimal for Ca2+ at all membrane potentials. 6. In the presence of a constant Ca2+ concentration (50 microM) increasing [K+] did not change the absolute level at which the current saturated; however the half‐saturating K+ concentration was greatly increased, indicating competitive inhibition between Ca2+ and K+ for the same site. 7. The data are consistent with a model based on Eyring rate theory for current conduction through ionic channels, in which we assume that the ions capable of entering the channel compete for a binding site that they must first occupy before proceeding on. The possible energy profile of the stretch‐activated channel was defined by optimizing the model parameters to obtain the best fit of the experimental data. Ca2+ was found to have a smaller dissociation constant and much longer occupancy time than Na+ or K+, thus accounting for its lower permeability and inhibitory effect on current conduction by other cations through the stretch‐activatable channel.