Surface Charge and Lanthanum Block of Calcium Current in Bullfrog Sympathetic Neurons

Abstract

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 2mM Ba2+ were recorded from isolated frog sympathetic neurons by the whole-cell patch-clamp technique. In normal ionic strength (120mM N-methyl-d-glucamine, NMG), La3+ blocked the current with high affinity (IC50=22nM at 0mV). La3+ block was relieved by strong depolarization in a time- and voltage-dependent manner. After unblocking, open channels reblocked rapidly at 0mV, allowing estimation of association and dissociation rates for La3+: kon=(7.2±0.7)×108M−1s−1, koff=10.0±0.5s−1. To assess surface charge effects, La3+ block was also measured in low ionic strength (12.5mM NMG) and high ionic strength (250mM NMG). La3+ block was higher affinity and faster by two- to threefold in 12.5mM NMG, with little effect of 250mM NMG. The data could be described by Gouy-Chapman theory with a surface charge density of ∼1 e−/3000–4000Å2. These results indicate that there is a small but detectable surface charge associated with the pore of voltage-dependent calcium channels.