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

1. Isolated neurones from the snail Helix pomatia were investigated under voltage clamp at 21-23 degrees C. The cells were internally dialysed and the current through small electrically isolated patches of the membrane was measured. The area of the patches was 30-500 micrometers 2 (1/1000-1/100 of the cell surface). The internal resistance of the membrane patches was 10(9)-10(10) omega. 2. In order to obtain the maximum conductance of the calcium channels an external solution containing 130 mM-Ba2+ or Ca2+ and an internal solution containing Tris glutamate and 5 mM-EGTA were used. Fluctuations due to the activity of calcium channels have been detected and analysed. 3. The power density spectra of barium 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 +/- Hz (S.E.). It did not reveal any distinct voltage dependence. 4. The current flowing through a single calcium channel was calculated from the variance-to-mean relationship. Its value for the transfer of Ba2+ ions is iBa = 0.20 +/- 0.02 pA (S.E.). Single channel current was not affected by the membrane potential (since the equilibrium potential was high) nor by inactivation. The maximum calcium inward current which flows through a single calcium 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. 5. The parameters of calcium channels modified by external EGTA have been evaluated. Measurements were performed in an external solution containing 200 mM-Na+. The current carried by a single modified calcium channel is iNa = 1.0 +/- 0.2 pA (S.E.) (gamma Na approximately 8.0 pS).