Table 2.

The Effects of Extracellular [Na⁺] on Mg²⁺ Dissociation and Permeation

	k0 −Mg	ɛ	k0 pMg	λ
	s−1		s−1
[Na+]ex = 25 mM	6,909 ± 1,888	0.36 ± 0.08	675 ± 288	0.03 ± 0.03
[Na+]ex = 50 mM	5,860 ± 1,259	–	671 ± 283	–
[Na+]ex = 100 mM	4,417 ± 760	–	650 ± 262	–
[Na+]ex = 150 mM	3,513 ± 480	–	645 ± 249	–

k⁰ _−Mg is the rate constant for Mg²⁺ dissociation from the pore to the extracellular solution, and k⁰ _pMg is the rate constant for Mg²⁺ permeation into the intracellular solution in the absence of a membrane potential. The dissociation rate constant decreases as extracellular [Na⁺] increases because a bound Mg²⁺ cannot return to the extracellular solution when Na⁺ occupies an external binding site. ɛ is the electrical distance from the Mg²⁺ binding site to peak of the dissociation barrier, and is λ the electrical distance to the peak of the permeation barrier (see Fig. 6). We assume that these values do not change with the extracellular [Na⁺]. The values (±SD) were obtained by fitting globally (i.e., simultaneously to multiple data sets) to the data shown in Fig. 5 B.