Interactions of divalent cations with single calcium channels from rat brain synaptosomes

Voltage-dependent calcium channels from a rat brain membrane preparation ('synaptosomes') were incorporated into planar lipid bilayers. The effects of calcium, barium, strontium, manganese, and cadmium ions on the amplitudes and kinetics of single channel currents were examined. The order of single channel conductances was g(Ba) > g(Ca) = g(Sr) > g(Mn), which was the inverse of the order of the mean channel open times: T(Mn) > T(Ca) = T(Sr) > T(Ba). In contrast, the identity of the charge carrier had little or no effect on the mean closed times of the channel. Manganese, in the absence of other permeant ions, can pass through single channels [g(Mn) = 4 pS]. However, when added to a solution that contained another type of permeant divalent cation, manganese reduced the single channel current in a voltage-dependent manner. Cadmium, a potent blocker of macroscopic 'ensemble' calcium currents in many preparations, reduced the current through an open channel in a manner consistent with Cd ions both not being measurably permeant and interacting with a single site. The permeant ions competed with cadmium for this site with the following order: Mn > Sr = Ca > Ba. These results are consistent with the existence of no less than one divalent cation binding site in the channel that regulates ion permeation.