Mechanisms of extracellular divalent and trivalent cation block of the sodium current in canine cardiac Purkinje cells.

Abstract

1. Single canine cardiac Purkinje cells were internally perfused and voltage clamped with a large-bore perfusion pipette for measurement of sodium ionic current (INa) in the absence and presence of extracellular group IIA divalent cations (Mg2+, Ba2+ and Ca2+), transition divalent cations (CO2+, Mn2+ and Ni2+), group IIB divalent cations (Cd2+ and Zn2+), and the trivalent cation La3+. 2. Open channel block of cardiac INa by external Ca2+, assessed from instantaneous INa-voltage (I-V) relationships, has been well described by a two-barrier, one-well model with a dissociation constant at 0 mV, KB(0), of 37 mM and an electrical distance, z' = delta, of 0.34. At the most negative test potentials there was less block of INa than predicted by the model, but correction of INa for the contribution of Na+ channel gating current (Ig) to the peak current improved the fit by the model. 3. The divalent cations Ba2+, Mg2+, CO2+ and Mn2+ produced voltage-dependent, open channel block of INa, which by the two-barrier, one-well model predicted a similar z' about one-third into the membrane field. The relative efficacy for voltage-dependent block was CO2+ > Mn2+ > Ca2+ > Mg2+ > Ba2+ with respective KB(0)s of 11, 13, 37, 43 and 61 mM. 4. Cd2+, Zn2+ and La3+ produced block of INa at low concentrations that was nearly voltage independent with z' < or = 0.13. Fits of single-site binding curves to peak INa in response to step depolarizations at positive test potentials gave the following apparent KD values: Zn2+ 0.14 mM, Cd2+ 0.27 mM and La3+ 0.50 mM. 5. In the presence of Cd2+, INa tail current relaxations were much faster than could be accounted for by Cd2+ binding to and/or screening of extracellular surface charges. Fits of the data to a model that assumed voltage-dependent open channel block during the tail current relaxations estimated the KB(0) for Cd2+ to be 0.80 mM. 6. Both z' and KB(0) for Ni2+ from fits of the two-barrier, one-well model to instantaneous I-V relationships varied as a function of [Ni2+], consistent with the hypothesis that Ni2+ blocked with similar affinity at a voltage-dependent and a voltage-independent site. At [Ni2+] > or = 5 mM, KB(0) was 7.6 mM and z' was 0.21.(ABSTRACT TRUNCATED AT 400 WORDS)