Ca2+ and Na+ permeability of high-threshold Ca2+ channels and their voltage-dependent block by Mg2+ ions in chick sensory neurones

1. The Mg2+ block of Na+ and Ca2+ currents through high-voltage activated (HVA; L-and W-type) Ca2+ channels was studied in cl-rick dorsal root ganglion neurones. 2. In low extracellular [Ca2+] (<10(-8) M) and with Na-o(+) and Cs-i(+) as the main charge carriers (120 mM), HVA Na+ currents started to activate at -40 mV, reached inward peak values near 0 mV and reversed at about +40 mV. 3. Addition of 30-500 mu M Mg2+ to the bath caused a strong depression of inward Naf currents that was voltage and dose dependent (K-D = 39 mu M in 120 mM Na+ at -10 mV). The block was maximal at negative potentials (<-70 mV) and decreased with increasing positive potentials, suggesting that Mg2+ cannot escape to the cell interior. 4. Block of Ca2+ currents by Mg2+ was also voltage dependent, but by three orders of magnitude less potent than with Na+ currents (K-D = 24 mM in 2 mM Ca2+ at -30 mV). The high concentration of Mg2+ caused a prominent voltage shift of channel; gating kinetics induced by surface charge screening effects. To compensate for this, Mg2+ block of inward Ca2+ currents was estimated from the instantaneous I-V relationships on return from very positive potentials (+100 mV). 5. Inward Na+ and Ca2+ tail currents following depolarization to +90 mV were markedly depressed, suggesting that channels cleared of Mg2+ ions during strong depolarization are quickly re-blocked on return to negative potentials. The kinetics of re-block by Mg2+ was too fast (<100 mu s) to be resolved by our recording apparatus. This implies a rate of entry for Mg2+ > 1.45 x 10(8) M-1 s(-1) when Na+ is the permeating ion and a rate approximately 3 orders of magnitude smaller for Ca2+. 6. Mg2+ unblock of HVA Na+ currents at +100 mV was independent of the size of outward currents, whether Na+, Cs+ or NMG(+) were the main internal cations. 7. Consistent with the idea of a high-affinity binding site for Ca2+ inside the channel, micromolar amounts of Ca2+ caused a strong depression of Na+ currents between -40 and 0 mV, which was effectively relieved with more positive as well as with negative potentials (K-D = 0.7 mu M in 120 mM Na+ at -20 mV). In this case, the kinetics of re-block could be resolved and gave rates of entry and exit for Ca2+ of 1.4 x 10(8) M-1 s(-1) and 2.95 x 10(2) s(-1) respectively. 8. The strong voltage dependence and weak current dependence of HVA channel block by divalent cations and the markedly different K-D values of Na+ and Ca2+ current block by Mg2+ can be well described by a previously proposed model for Ca2+ channel permeation based on interactions between the permeating ion and the negative charges forming the high-affinity binding site for Ca2+ inside the pore (Lux, Carbone & Zucker, 1990).