Conductance of the calcium channel in the membrane of snail neurones.

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Abstract
Isolated neurons from the snail H. pomatia were investigated under voltage clamp at 21-23.degree. C. The cells were internally dialyzed and the current through small electrically isolated patches of the membrane was measured. The area of the patches was 30-500 .mu.m2 (1/1000-1/100 of the cell surface). The internal resistance of the membrane patches was 109-1010 .OMEGA.. In order to obtain the maximum conductance of the Ca channels an external solution containing 130 mM-Ba2+ or Ca2+ and an internal solution containing Tris glutamate and 5 mM-EGTA [ethylene glycol bis(.beta.-aminoethyl ether) tetraacetate] were used. Fluctuations due to the activity of Ca channels were detected and analyzed. The power density spectra of Ba current fluctuations were calculated for conductance values from 3 to 30% of the maximum conductance in the frequency band 1-1000 Hz. They were fitted to a spectral density function of the Lorentz form. The half-power frequency of the spectra was 227 .+-. 60 Hz (SE). It did not reveal any distinct voltage dependence. The current flowing through a single Ca channel was calculated from the variance-to-mean relationship. Its value for the transfer of Ba2+ is iBa = 0.20 .+-. 0.02 pA (SE). Single channel current was not affected by the membrane potential (since the equilibrium potential was high) nor by inactivation. The maximum Ca inward current which flows through a single Ca channel is about 0.1 pA and corresponds to a conductance .gamma.Ca = 0.5 pS (calculated for an equilibrium potential of 200 mV). This estimate gives an upper limit to .gamma.Ca. The parameters of Ca channels modified by external EGTA were evaluated. Measurements were performed in an external solution containing 200 mM-Na+. The current carried by a single modified Ca channel is iNa = 1.0 .+-. 0.2 pA (SE) (.gamma.Na .apprxeq. 8.0 pS).