Are the presynaptic membrane particles the calcium channels?

Abstract
The number of large intramembrane particles associated with sites of synaptic vesicle release at the squid giant synapse was determined and compared to the average maximal presynaptic Ca current to derive an estimate of the conductance each particle would have if it were a Ca channel. This value, 0.21 pS, compares favorably with conductances of Ca channels in other preparations, substantiating the idea that the large intramembrane particles, which are concentrated at active zones, represent Ca channels.