Ionic pores, gates, and gating currents

119

118

SUMMARY1. Na channel reversal potentials were studied in perfused voltage clamped squid giant axons. The concentration dependence of ion selectivity was determined with both external and internal changes in Na and ammonium concentrations.2. A tenfold change in the internal ammonium activity results in a 42 mV shift in the reversal potential, rather than the 56 mV shift expected from the Goldman, Hodgkin, Katz equation for a constant PNa/P~N 14 ratio. However, changing [Na]0 tenfold at constant internal [NH4] gives approximately the expected 56 mV shift. Therefore, the apparent channel selectivity depends upon the internal ammonium concentration but not the external Na concentration.3. With ammonium outside and Na inside, the calculated permeability ratio is nearly constant, regardless of the permeant ion concentration.4. Internal Cs ions can alter the Na/K permeability ratio. 5. The results are considered in terms of a three-barrier, two-site ionic permeation model.

Section: Concentration-dependent Na Channel Selectivity 235supporting

confidence: 74%

Sodium channel permeation in squid axons. I: Reversal potential experiments.

Begenisich¹,

Cahalan²

1980

119

118

“…However, there are useful theories which suggest how we may envisage this process (Hodgkin & Huxley, 1952;Hille, 1970;Stevens, 1972;Armstrong, 1975). Suppose that ions are carried through specific conductance channels, thus generating a current.…”

Section: Discussionmentioning

confidence: 99%

The regulation of the calcium conductance of cardiac muscle by adrenaline.

Réuter¹,

Scholz²

1977

465

138

SUMMARY1. The effect of adrenaline on the Ca-dependent slow inward current, I8, of mammalian cardiac muscle has been investigated by the voltageclamp method. The mechanism of the increase in the conductance, g8, was analysed on the basis of a kinetic scheme (Hodgkin & Huxley, 1952) applicable to this system. 2. The rate constants, ad and fld, of activation of g8 were not influenced by adrenaline, although the limiting conductance, g ,was greatly increased.3. Reduction of [Ca]o from 1-8 to 0-2 mm decreased the amplitude of inward tail currents when 98 was fully activated; however, the relative decrease of the current amplitude was the same with and without adrenaline. The reversal potential, ER, of I. was not changed by the drug.This indicates that the catecholamine has no influence on the selectivity of these conductance channels.4. An increase in the number of functional conductance channels by adrenaline is discussed as a possible mechanism for the increase in 9.

“…However, distortions by substantial differences in distance between recording and active sites or in the properties of the intervening membrane cannot be excluded. (Hille, 1974;Narahashi, 1974;Armstrong, 1975;Kleinhaus & Prichard, 1975;Heyer & Lux, 1976a, b) as well as the alga Nitella (Koppenhofer, 1972). In all cases TEA was known or supposed to act by blocking a K current.…”

Section: Extracellular Teamentioning

confidence: 99%

Close relation between TEA responses and Ca‐dependent membrane phenomena of four identified leech neurones

Kleinhaus¹,

Prichard²

1977

SUMMARY1. Tetraethylammonium chloride (TEA) was applied to four kinds of identified neurones in leech segmental ganglia, namely, the sensory cells responding to touch (T), pressure (P) and noxious (N) stimuli and the Retzius cell (R).2. TEA prolonged the action potentials of these cells to characteristically different degrees, in the order R > N > P > T, regardless of exposure time. This result was the same whether TEA was presented to the whole ganglion via the bathing fluid or injected iontophoretically into the soma of the cell under study.3. TEA in Na-free solution caused the behaviour of the N cell membrane to be dominated by a Ca-dependent, Mn-blockable event identical in every respect except smaller size to the previously described behaviour of the R cell under the same conditions. The P cell displayed a still smaller event of the same kind, but none was detectable in the T cell.4. In the absence of both TEA and Na, when Ca was the only extracellular cation available to carry current, active membrane responses to depolarization were present in the R cell (previous study) and the N cell; such responses were minimal in the P cell and absent from the T cell.5. Differences among the four cells in density of a divalent cation conductance mechanism are the simplest explanation for these observations, though a more complex explanation based on multiple, pharmacologically distinct K conductances is not excluded by our data.