Surface charge and lanthanum block of calcium current in bullfrog sympathetic neurons

The density of surface charge associated with the calcium channel pore was estimated from the effect of extracellular ionic strength on block by La3+. Currents carried by 2 mM Ba2+ were recorded from isolated frog sympathetic neurons by the whole-cell patch-clamp technique. In normal ionic strength (120 mM N-methyl-D-glucamine, NMG), La3+ blocked the current with high affinity (IC50 = 22 nM at 0 mV). La3+ block was relieved by strong depolarization in a time- and voltage-dependent manner. After unblocking, open channels reblocked rapidly at 0 mV, allowing estimation of association and dissociation rates for La3+: k(on) = (7.2 +/- 0.7) x 10(8) M(-1) s(-1), k(off) = 10.0 +/- 0.5 s(-1). To assess surface charge effects, La3+ block was also measured in low ionic strength (12.5 mM NMG) and high ionic strength (250 mM NMG). La3+ block was higher affinity and faster by two- to threefold in 12.5 mM NMG, with little effect of 250 mM NMG. The data could be described by Gouy-Chapman theory with a surface charge density of approximately 1 e-/3000-4000 A2. These results indicate that there is a small but detectable surface charge associated with the pore of voltage-dependent calcium channels.