The effect of small cations on the delayed rectifier and the resting potassium conductance of frog sartorius muscle.

The Effect of Small Cations on the Delayed Rectifier and the Resting Potassium Conductance of Frog Sartorius Muscle. By Louise Ann Gay Standard electrophysiological techniques were used to examine the effects of hydrogen ions, thallous and alkali metal ions on the electrical properties of frog sartorius muscle. A reduction in external pH from 9.2 to 5.2 slowed the rising and falling phases of the action potential. In voltage-clamped fibres, a similar pH change shifted the delayed potassium conductance to more positive membrane potentials, but had little or no effect on the maximum delayed conductance. At pH 5-2, the delayed current turned on more slowly than at pH 7.2 and the threshold for the sodium current was shifted to more positive potentials. These results are consistent with the titration of fixed charges at the membrane surface. Reversal potential measurements were used to investigate the selectivity of delayed potassium channels. Calculated mean permeability ratios were PLi/PK = 0.024, PNa/PK = 0.030, PRb/PK = 0.95 and PCs/PK =011. A voltage-clamp technique was used to investigate the effects of caesium and thallium on potassium currents in resting muscle. Caesium blocked the resting potassium conductance in a concentration- and voltage-dependent manner. At a given membrane potential, the blockade was reduced by raising internal potassium, but was largely independent of the external potassium concentration. In fibres hyperpolarized under constant current conditions in the presence of caesium, oscillations in the membrane potential were recorded. In potassium- free solutions, the major effect of thallium was to remove the time- and sodium-dependent permeability change which gives rise to a negative slope in the steady-state current-voltage relations of fibres hyperpolarized in normal Ringer. It is concluded that potassium crosses the resting membrane by way of multi-ion channels containing two or more ion binding sites. A method is described which demonstrates that at least part of the slow conductance decrease which occurs when muscle fibres are hyperpolarized is due to a depletion of potassium from the transverse tubules.