Properties of single calcium channels in the neuronal membrane

Abstract

The neurons of snail Helix pomatia were internally dialysed and the current through the small electrically isolated patches of the membrane was measured under voltage clamp. The area of the patches was between 30–300 μm2 (from 1/1000 to 1/100 of the cell surface). The analysis of current fluctuations was used to measure the characteristics of a single calcium channel. The measurements were performed under conditions ensuring the saturation of the calcium channels: the external solution contained 130 mM of Ba2+. The power density spectra of the barium current flucuations fitted a spectral density function of the Lorentz form which is characteristic of a single time constant process. The time constant obtained demonstrated no dependence on either the membrane voltage or on the acting conductance value. The value of the time constant was 0.7±0.2 ms [standard error (S.E.) of mean]. No distinct I/f noise component was found. The current flowing through a single calcium channel had the value (for the transfer of Ba2+ ions) 0.20±0.02 pA (S.E.). The single channel current did not depend on either the membrane potential (since the equilibrium potential was very high) or on the inactivation. The maximum calcium inward current which flows through a single calcium channel is aroud 0.1 pA and corresponds to a conductance of 0.5 pS.