BLOCK OF N-TYPE CALCIUM CHANNELS IN CHICK SENSORY NEURONS BY EXTERNALSODIUM

L-type Ca2+ channels select for Ca2+ over sodium Na+ by an affinity-ba sed mechanism. The prevailing model of Ca2+ channel permeation describ es a multi-ion pore that requires pore occupancy by at least two Ca2ions to generate a Ca2+ current. At [Ca2+] < 1 mu M, Ca2+ channels con duct Na+. Due to the high affinity of the intrapore binding sites for Ca2+ relative to Na+, addition of mu M concentrations of Ca2+ block Na + conductance through the channel. There is little information, howeve r, about the potential for interaction between Na+ and Ca2+ for the se cond binding site in a Ca2+ channel already occupied by one Ca2+. The two simplest possibilities, (a) that Na+ and Ca2+ compete for the seco nd binding site or (b) that full time occupancy by one Ca2+ excludes N a+ from the pore altogether, would imply considerably different mechan isms of channel permeation. We are studying permeation mechanisms in N -type Ca2+ channels. Similar to L-type Ca2+ channels, N-type channels conduct Na+ well in the absence of external Ca2+. Addition of 10 mu M Ca2+ inhibited Na+ conductance by 95%, and addition of 1 mM Mg2+ inhib ited Na+ conductance by 80%. At divalent ion concentrations of 2 mM, 1 20 mM Na+ blocked both Ca2+ and Ba2+ currents. With 2 mM Ba2+, the IC5 0 for block of Ba2+ currents by Na+ was 119 mM. External Li+ also bloc ked Ba2+ currents in a concentration-dependent manner, with an IC50 of 97 mM. Na+ block of Ba2+ currents was dependent on [Ba2+]; increasing [Ba2+] progressively reduced block with an IC50 of 2 mM. External Na had no effect on voltage-dependent activation or inactivation of the channel. These data suggest that at physiological concentrations, Naand Ca2+ compete for occupancy in a pore already occupied by a single Ca2+. Occupancy of the pore by Na+ reduced Ca2+ channel conductance, s uch that in physiological solutions, Ca2+ channel currents are between 50 and 70% of maximal.