Block of N-type Calcium Channels in Chick Sensory Neurons by External Sodium

L-type Ca(2+) channels select for Ca(2+) over sodium Na(+) by an affinity-based mechanism. The prevailing model of Ca(2+) channel permeation describes a multi-ion pore that requires pore occupancy by at least two Ca(2+) ions to generate a Ca(2+) current. At [Ca(2+)] < 1 μM, Ca(2+) channels conduct Na(+). Due to the high affinity of the intrapore binding sites for Ca(2+) relative to Na(+), addition of μM concentrations of Ca(2+) block Na(+) conductance through the channel. There is little information, however, about the potential for interaction between Na(+) and Ca(2+) for the second binding site in a Ca(2+) channel already occupied by one Ca(2+). The two simplest possibilities, (a) that Na(+) and Ca(2+) compete for the second binding site or (b) that full time occupancy by one Ca(2+) excludes Na(+) from the pore altogether, would imply considerably different mechanisms of channel permeation. We are studying permeation mechanisms in N-type Ca(2+) channels. Similar to L-type Ca(2+) channels, N-type channels conduct Na(+) well in the absence of external Ca(2+). Addition of 10 μM Ca(2+) inhibited Na(+) conductance by 95%, and addition of 1 mM Mg(2+) inhibited Na(+) conductance by 80%. At divalent ion concentrations of 2 mM, 120 mM Na(+) blocked both Ca(2+) and Ba(2+) currents. With 2 mM Ba(2+), the IC(50) for block of Ba(2+) currents by Na(+) was 119 mM. External Li(+) also blocked Ba(2+) currents in a concentration-dependent manner, with an IC(50) of 97 mM. Na(+) block of Ba(2+) currents was dependent on [Ba(2+)]; increasing [Ba(2+)] progressively reduced block with an IC(50) of 2 mM. External Na(+) had no effect on voltage-dependent activation or inactivation of the channel. These data suggest that at physiological concentrations, Na(+) and Ca(2+) compete for occupancy in a pore already occupied by a single Ca(2+). Occupancy of the pore by Na(+) reduced Ca(2+) channel conductance, such that in physiological solutions, Ca(2+) channel currents are between 50 and 70% of maximal.