Conductance of the sodium channel in myelinated nerve fibres with modified sodium inactivation.

Abstract

SUMMARY1. Na current fluctuations in nodes of Ranvier were measured under voltage clamp conditions as described in the preceding paper (Conti, Hille, Neumcke, Nonner & Stampfli, 1976) and analysed in terms of power spectral density cAlculated for frequencies between 30 Hz and 5 kHz.2. External (I 5 g/ml.) Leiurus scorpion venom or Anemonia Toxin II (3 x 10-5 g/ml.) or internal 20 mm iodate were applied in order to remove or slow down inactivation in part of the Na channels. The treatment increased the steady-state Na current during the noise measurement oneto eight fold over that in normal fibres.3. Noise spectra were interpreted as the sum of 1/f noise and noise SNa(f) due to all-or-none, open-close transitions of single Na channels. The drug effects on the inactivation could be accounted for either by assuming two populations of channels, one with and one without inactivation, or by postulating a single population with modified inactivation characteristics.4. Except for an increase in amplitude, the fluctuation spectra SNa (f) were similar to the ones in normal nodes. Again, the time constants Tm obtained from the fit of the spectra agreed within a factor of 2 with the values of Tm found in the macroscopic Na currents. 5. From the fluctuation spectra, single Na channel conductances y of 5.4 + 0 4 pS (iodate), 6-7 + 0 5 pS (Leiurus) and 7 0 + 0-6 pS (Anemonia)were calculated. The value of y was not significantly voltage dependent. 6. Our observations indicate that inactivation of Na channels can be modified with at most small effects on the microscopic properties of the

Further Studies of Periodic Miniature Responses in Squid Giant Axons

“…This appears to explain why the results obtained by CONTI et al (1976) from the nodal membrane treated with scorpion venom are very different from those reported in this paper. CONTI et al (1975CONTI et al ( , 1976 and FISHMAN et al (1977) interpreted their results on the basis of the assumption that the axon membrane possesses two kinds of spatially separate ionic channels, sodium-and potassium-channels, and that TTX and TEA are specific blockers of these channels. Suppression of miniature responses by TTX may be taken as an indication of involvement of the sodium-channel in production of these periodic responses.…”

“…In fact, CONTI et al (1975CONTI et al ( , 1976 and FISHMAN et al (1975FISHMAN et al ( , 1977 carried out detailed studies of the smallest electric signals derived from the squid axon membrane under voltage clamp. It is interesting to note, however, that results obtained by these investigators are very different from what has been discussed in this paper.…”

“…were found to be capable of producing discrete miniature responses in the axon. Aminopyridine derivatives, which were studied recently by MEVES and PICHON (1977) and YEH et al (1976) using squid axons, and scorpion venoms, examined by CONTI et al (1976) were also found to be highly effective in evoking miniature responses. In many cases these responses appeared before there was any detectable change in the resting membrane potential of the axon.…”

Unitary current through sodium channel and anomalous rectifier channel estimated from transient current noise in the tunicate egg.

“…The density of Na channels in nerve fibres is considerably larger: 13-35/,um2 in the lobster giant axon (Moore, Narahashi & Shaw, 1967;Keynes, Ritchie & Rojas, 1971), 483-553//Zm2 in the squid giant axon (Levinson & Meves, 1975;Conti et al 1975, Keynes & Rojas, 1976, and 1000-2000/,tm2 in the frog myelinated nerve fibre (Conti et al 1976b;Sigworth, 1977). The unitary conductance, y, estimated for the Na channel is 7-35 pmho in 400 mM-Na sea water with 400 mM-K intracellular medium, and the Na permeability, PN&, of the single Na channel is 1 15 x 10-14 cm3 sec'.…”

The variance of sodium current fluctuations at the node of Ranvier

“…No nerve preparation, unfortunately, has yet shown a sufficiently low noise level to resolve single sodium channel currents; instead, indirect methods have been used. Based on the assumption of two conductance levels for the channel (one of which is zero), estimates for the single-channel conductance have already been obtained by several means, including the analysis of membrane current fluctuations (see Conti, Hille, Neumcke, Nonner & Stampfli, 1976b; and the review by DeFelice, 1977).…”