title: Calcium block of single sodium channels: role of a pore-lining aromatic residue.
abstract: Extracellular Ca(2+) ions cause a rapid block of voltage-gated sodium channels, manifest as an apparent reduction of the amplitude of single-channel currents. We examined the influence of residue Tyr-401 in the isoform rNa(V)1.4 on both single-channel conductance and Ca(2+) block. An aromatic residue at this position in the outer mouth of the pore plays a critical role in high-affinity block by the guanidinium toxin tetrodotoxin, primarily due to an electrostatic attraction between the cationic blocker and the system of pi electrons on the aromatic face. We tested whether a similar attraction between small metal cations (Na(+) and Ca(2+)) and this residue would enhance single-channel conductance or pore block, using a series of fluorinated derivatives of phenylalanine at this position. Our results show a monotonic decrease in Ca(2+) block as the aromatic ring is increasingly fluorinated, a result in accord with a cation-pi interaction between Ca(2+) and the aromatic ring. This occurred without a change of single-channel conductance, consistent with a greater electrostatic effect of the pi system on divalent than on monovalent cations. High-level quantum mechanical calculations show that Ca(2+) ions likely do not bind directly to the aromatic ring because of the substantial energetic penalty of dehydrating a Ca(2+) ion. However, the complex of a Ca(2+) ion with its inner hydration shell, Ca(2+)(H(2)O)(6), interacts electrostatically with the aromatic ring in a way that affects the local concentration of Ca(2+) ions in the extracellular vestibule.
authors.affiliation: Department of Molecular Physiology and Biophysics, Institute of Hyperexcitability, Jefferson Medical College, Philadelphia, Pennsylvania, USA.
authors.forename: Vincent P
authors.initials: VP
authors.lastname: Santarelli
authors.forename: Amy L
authors.initials: AL
authors.lastname: Eastwood
authors.forename: Dennis A
authors.initials: DA
authors.lastname: Dougherty
authors.forename: Christopher A
authors.initials: CA
authors.lastname: Ahern
authors.forename: Richard
authors.initials: R
authors.lastname: Horn
mesh_terms: D000818:Animals; D001703:Biophysics / Q000379:methods*; D002118:Calcium / Q000737:chemistry*; D002412:Cations; D004594:Electrophysiology / Q000379:methods; D005461:Fluorine / Q000737:chemistry; D019791:Guanidine / Q000737:chemistry; D006801:Humans; D007477:Ions; D008968:Molecular Conformation; D009124:Muscle Proteins / Q000737:chemistry; D020033:Protein Isoforms; D026941:Sodium Channel Blockers / Q000737:chemistry*; D015222:Sodium Channels / Q000737:chemistry; D055672:Static Electricity; D013779:Tetrodotoxin / Q000737:chemistry
chemical_list: D002412:Cations; D007477:Ions; D009124:Muscle Proteins; D020033:Protein Isoforms; C466180:Scn4a protein, rat; D026941:Sodium Channel Blockers; D015222:Sodium Channels; D005461:Fluorine; D013779:Tetrodotoxin; D019791:Guanidine; D002118:Calcium