Li and Na currents through the cGMP-activated channels in excised, inside-out membrane patches from salamander rod outer segments were studied in the absence of divalent cations. The permeability ratio for these two ions determined from reversal potential measurements is close to 1 (Nunn 1987 J. Physiol. 394, 17P). However the ratio of the outward currents measured at +60 mV with 110 mN LiCl or 110 mM NaCl cytoplasmic solutions is about 0.3 (Menini 1990 J. Physiol. 424, 167-185). The amplitude of macroscopic currents is determined both by the mean number of open-channels and by the amplitude of the single channel current. To investigate if Li and Na ions have a different influence on the gating of the channel we have estimated single channel conductances from noise analysis. We studied stationary fluctuations of the membrane current as a function of cGMP concentrations at +60 mV. The relation between variance and mean current is well described by a parabola both in Li and in Na solutions. The ratio of single channel currents obtained from these relations is about 0.3. The power spectra of the fluctuations in Li and Na solutions are identical apart for a scaling factor. We conclude that the difference between Li and Na macroscopic currents is entirely due to the different permeation of these ions through the open pore and not to modifications of the gating of the cGMP-activated channel.
Na-to-Li substitution does not affect the gating of the cGMP-actived channel of retinal rods
Picco Cristiana;De Micheli Enrico;
1991
Abstract
Li and Na currents through the cGMP-activated channels in excised, inside-out membrane patches from salamander rod outer segments were studied in the absence of divalent cations. The permeability ratio for these two ions determined from reversal potential measurements is close to 1 (Nunn 1987 J. Physiol. 394, 17P). However the ratio of the outward currents measured at +60 mV with 110 mN LiCl or 110 mM NaCl cytoplasmic solutions is about 0.3 (Menini 1990 J. Physiol. 424, 167-185). The amplitude of macroscopic currents is determined both by the mean number of open-channels and by the amplitude of the single channel current. To investigate if Li and Na ions have a different influence on the gating of the channel we have estimated single channel conductances from noise analysis. We studied stationary fluctuations of the membrane current as a function of cGMP concentrations at +60 mV. The relation between variance and mean current is well described by a parabola both in Li and in Na solutions. The ratio of single channel currents obtained from these relations is about 0.3. The power spectra of the fluctuations in Li and Na solutions are identical apart for a scaling factor. We conclude that the difference between Li and Na macroscopic currents is entirely due to the different permeation of these ions through the open pore and not to modifications of the gating of the cGMP-activated channel.I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
