Frog node

Strichart It is something that is seen in frog node. With normal Na gradients this is something that is seen as channels are driven through the reversal potential to yield outward current. I wondered whether this might not be due to the possibihty that the last closed state — or the pre-open state, if you will— can go to an inactivated state, a pathway which may be relatively favoured for the smaller depolarizations, but larger depolarizations may inactivate channels faster from the open state, an inactivation state that may be more reversible. 

Meves H, Rubly, Watt DD 1987 Gating current experiments on frog nodes of Ranvier treated with Centruroidessadpturatus toxins or aconitine. Pfliigers Arch 409 381-393... 

In frog nodes of Ranvier very similar phenomena were observed. 

Fig. 4. Slow currents. Is, in arbitrary units ( . .), during a two-pulse experiment on a frog node of Ranvier. A first 6.3-s pulse to V=57 mV during which Is increased with ts,on=l-7 s was followed by a second pulse of 11.5 s duration (trace 2 9.6 s) to various potentials -10,7,14, and 28 mV for traces 1,2,3, and 4 with tail time constants, Xs,off, of 1.0,2.0,2.5, and 3.1 s, respectively the tail amplitude decreased with the driving force on Na. The fast peak Iwa does not show at the slow time base. 60 iAf veratridine, 20.4 C holding potential Vh=-10 mV, all potentials relative to the resting potential Is 1 a.u.=2.5 nA, set at ls=0 for Vh. (Unpublished experiment of W. Ulbricht and M. Stoye-Herzog)...

Meeder T, Ulbricht W (1987) Action of benzocaine on sodium channels of frog nodes of Ranvier treated with chloramine-T. Pfliigers Arch 409 265-273 Meves H (1966) The effect of veratridine on internally perfused giant axons. Pflugers Arch 290 211-217... 

Hille [85], using frog nodes, showed that the sodium channel was also permeant to some small organic molecules such as guanidinium, hydrazine and hy-droxylamine but was impermeable to others such as methylamine. Only the latter is incapable of hydrogen bonding with possible fixed anionic sites in the membrane. 

Much has been written of the veratrine alkaloids derived from plants of the Lilliaceae family. Many early studies were made with natural mixtures of plant alkaloids, and as different alkaloids have differing activities the results are com plex and confusing. From studies with a sin e alkaloid, veratridine [134], it was found with squid axon and frog nodes that the transient increase in sodium permeability was followed by a second, dow-developing permeabUity increase whidi did not undergo inactivation. In consequence, many excitable membranes show repetitive depolarisations in response to a sin e stimulus. The alkaloid is more active when applied to the inside of the membrane, and the cation appears... 

Two stable states in frog node of Ranvier in 20 0 in 0.5 M potassium chloride solutions are observed by Stampfli [46]. Similar phenomena are also observed in the case of squid action membrane [46, 47]. It should be noted that the biological phenomena could be correlated with the generation of action potential. The value of conductance of membrane when filled with the solutions in the two steady states is given in Table 8.3. 

Following such straightforward approach-, estimates of 4 pS for Yk(10) and of 7.7 pS for were obtained in frog nodes. A... 

A less direct estimate of and is obtained from the amplitude of the power spectra of current fluctuations. Basically, power spectra contain more information than the simple variance, as shown by eqn.(7). The additional information is very useful to ascertain to what extent the measured current fluctuations can be attributed to the flickering of ion specific channels between open and closed states rather than to other noise sources. This control was particularly desirable in the early studies of nerve membrane noise, which revealed the presence of large 1/f spectral components of still unclear origin (16). Indeed, the first unequivocal characterizations of sodium and potassium channel noise in the squid axon membrane (13) and of sodium channel noise in frog nodes (14,15) were obtained from the fitting of measured spectra with the superposition of Lorentzian-like spectra plus 1/f components. From the low frequency asymptote and the cut-off frequency of the Lorentzian components estimates of Y =12 pS and were obtained for the ionic channels of quid... 

Berg et al. (17) in frog nodes, is likely affected by larger errors because it was based on voltage fluctuation measurements (8) and it was calculated using "standard Hodgkin-Huxley (HH) parameters (18) rather than macroscopic relaxation data from the same nodes. 

Potassium current fluctuation spectra in frog nodes (19) yield Yjr=2.9 pS in close agreement with the value obtained from single variance measurements (10). Table 1 summarizes estimates of y and obtained from fluctuation analysis. 

A compromise between the two methods has been recently used to evaluate y in frog nodes (20). Fluctuation spectra, obtained after subtracting TTX insensitive components, were analyzed and shown to fit a simple sum of relaxation spectra. The noise variance was simultaneously and independently measured as the mean square of the fluctuations. It was then corrected for bandwidth limitations according to the extrapolation of the spectra to infinite frequency, and inserted in eqn.(5), to yield i j, after subtraction of the TTX insensitive contribution. estimated in this way, was found... 

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