Reconstitution of a voltage-activated calcium conducting cation channel from brain microsomes

Many aspects of nerve cell function are controlled by cytosolic Ca2+. Intracellular organelles can sequester the cation and release it in a regulated fashion through specific ion channels including ryanodine-sensitive and inositol trisphosphate (InsP3)-activated intraneuronal Ca2+ channels. We have now used the planar bilayer technique to characterize a distinct high-conductance Ca2+ channel from brain microsomal membranes, which was not found in synaptic plasma membranes. Channel conductance in 50 mM CaCl2 is approximately 100 pS. The channel is permeable to Ca2+, Ba2+, K+ and Cs+, but it is not ideally cation-selective (PCs+:PCl- = 4:1). It opens in bursts in a steeply voltage-dependent manner, with maximal activation around zero mV. Channel activity is unaffected by caffeine or ryanodine (both of which modify the gating of ryanodine-sensitive Ca2+ channels), or by InsP3 or heparin (which act on InsP3-sensitive Ca2+ channels). omega-conotoxin GVIA, ruthenium red, amiloride and procaine all block the channel, the latter two by interacting with one or more negatively-charged binding sites in the voltage gradient within the channel pore. We suggest the channel may have a role in intracellular Ca(2+)-signalling, possibly linked to the operation of intracellular Ca2+ stores.