1. The effects of the extracellular divalent cations barium, calcium, cadmium, cobalt, magnesium, manganese, nickel and zinc and the trivalent cation lanthanum on macroscopic sodium current (I(Na)) were characterized in enzymatically isolated single canine cardiac Purkinje cells under voltage clamp at 9-14-degrees-C. 2. All di(tri)valent cations produced depolarizing shifts in the conductance-voltage relationship. The order of efficacy, taken as the concentration required to produce a 5 mV shift in the mid-point of peak I(Na) conductance, from least to most effective was (mM): Ca2+ (2.97) almost-equal-to Mg2+ (2.67) almost-equal-to Ba2+ (1.93) > Co2+ (1.02) almost-equal-to Mn2+ (0.88) > Ni2+ (0.54) > La3+ (0.095) almost-equal-to Cd2+ (0.083) almost-equal-to Zn2+ (0.076). 3. Addition of di(tri)valent cations also produced depolarizing shifts in voltage-dependent availability. The order of efficacy from the least to most effective was (mM): Cd2+ (7.70) almost-equal-to Mg2+ (6.86) almost-equal-to Ba2+ (4.50) > Ca2+ (2.47) almost-equal-to Co2+ (1.87) almost-equal-to Mn2+ (1.24) almost-equal-to Ni2+ (1.20) > Zn2+ (0.300) > La3+ (0.060). 4.The Gouy-Chapman-Stern equations were used to evaluate di(tri)valent cation efficacy in binding to surface charges. Surface charge density was estimated as 0.72 sites nm-2, and it was assumed that Mg2+, the divalent cation that produced the smallest shift, screened but did not bind to surface charges. Based on voltage-dependent availability, K(D) from lowest to highest affinity were (mM): Ba2+ (2500) > Co2+ (1670) almost-equal-to Mn2+ (1430) almost-equal-to Ca2+ = Cd2+ = Ni2+ (1200) > Zn2+ (250) > La3+ (30). 5. All di(tri)valent cations also produced a concentration-dependent acceleration of I(Na) tail current relaxation. The addition of Ca2+ and La3+ produced acceleration of tail current relaxations that could be accounted for by the surface charge effects predicted from the shift in voltage-dependent availability. Cd2+, which produced almost no change in voltage-dependent availability, dramatically accelerated tail current relaxation. Zn2+, Ni2+, Mn2+ and Co2+ also produced greater acceleration of tail current relaxation that could be accounted for by surface charge effects. 6. Di(tri)valent cations delayed time to peak I(Na) in a concentration-dependent manner. The time to peak I(Na)-voltage relationship was well described by an exponential plus a constant, and di(tri)valent cations did not affect the slope factor or constant but shifted the relationship in the depolarizing direction. Similar to their effect on tail currents, addition of some di(tri)valent cations produced larger effects on time to peak I(Na) than expected from the shift of voltage-dependent availability. 7. Kinetic effects on I(Na) result from screening an binding to surface charges and from voltage-dependent block. Shifts in voltage-dependent availability are likely to best estimate the screening and binding effects of di(tri)valent cations. The rates of interactions of some di(tri)valent cations with the open channel appear to be slower than previously thought with resultant effects on the kinetics of I(Na).
