Ca²⁺ and Na⁺ permeability of high‐threshold Ca²⁺ channels and their volt age‐dependent block by Mg²⁺ ions in chick sensory neurones

Abstract

1. The Mg¥ block of Na¤ and Ca¥ currents through high-voltage activated (HVA; L_ and N_type) Ca¥ channels was studied in chick dorsal root ganglion neurones. 2. In low extracellular [Ca¥] (< 10¦Ì Ò) and with Naº and Cs¸as the main charge carriers (120 mÒ), 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 ìÒ Mg¥ to the bath caused a strong depression of inward Na¤ currents that was voltage and dose dependent (KD = 39 ìÒ in 120 mÒ Na¤ at −10 mV). The block was maximal at negative potentials (< −70 mV) and decreased with increasing positive potentials, suggesting that Mg¥ cannot escape to the cell interior. 4. Block of Ca¥ currents by Mg¥ was also voltage dependent, but by three orders of magnitude less potent than with Na¤ currents (KD = 24 mÒ in 2 mÒ Ca¥ at −30 mV). The high concentration of Mg¥ caused a prominent voltage shift of channel gating kinetics induced by surface charge screening effects. To compensate for this, Mg¥ block of inward Ca¥ currents was estimated from the instantaneous I-V relationships on return from very positive potentials (+100 mV). 5. Inward Na¤ and Ca¥ tail currents following depolarization to +90 mV were markedly depressed, suggesting that channels cleared of Mg¥ ions during strong depolarization are quickly re-blocked on return to negative potentials. The kinetics of re-block by Mg¥ was too fast (< 100 ìs) to be resolved by our recording apparatus. This implies a rate of entry for Mg¥ > 1·45 ² 10Ì Ò¢ s¢ when Na¤ is the permeating ion and a rate approximately 3 orders of magnitude smaller for Ca¥. 6. Mg¥ 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 Ca¥ inside the channel, micromolar amounts of Ca¥ 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 (KD = 0·7 ìÒ in 120 mÒ Na¤ at −20 mV). In this case, the kinetics of re-block could be resolved and gave rates of entry and exit for Ca¥ of 1·4 ² 10Ì Ò¢ s¢ and 2·95 ² 10 s¢, respectively. 8. The strong voltage dependence and weak current dependence of HVA channel block by divalent cations and the markedly different KD values of Na¤ and Ca¥ current block by Mg¥ can be well described by a previously proposed model for Ca¥ channel permeation based on interactions between the permeating ion and the negative charges forming the high-affinity binding site for Ca¥ inside the pore (Lux, Carbone & Zucker, 1990).

Induction of a Sodium-dependent Depolarization by External Calcium Removal in Human Sperm

“…It has been established that some calcium channels allow sodium influx in the absence of calcium that is blocked by magnesium (19,20). Thus, it was pertinent to explore whether the sodium-dependent depolarization induced by calcium removal was affected by magnesium.…”

“…Alternatively, the putative sodium channel may reveal the presence of a calcium channel in the plasma membrane that allows sodium permeation in the absence of external calcium. L-Type VDCC and transient receptor potential channels such as the SOCs allow large sodium currents when external calcium is brought to nanomolar and micromolar levels, respectively (19,20). In mature human sperm, there is a nifedipine-insensitive voltage-dependent calcium influx system, suggesting that these cells are endowed with VDCC (8,14).…”

Permeability characteristics of monovalent cations in atrial myocytes of the rat heart

“…It is also well known that the permeability of Ca¥ channels in the absence of external Ca¥ is altered and Ca¥ channels become permeable to monovalent cations (Matsuda, 1986). It has been reported that micromolar amounts of Ca¥ cause a strong depression of monovalent currents between −40 and 0 mV, which is effectively relieved at more positive potentials (Carbone et al 1997). It is thus possible that the outwardly rectifying currents recorded in Cs¤ and Na¤ solutions are attributable to cation permeability through Ca¥ channels.…”

Voltage-dependent modulation of L-type calcium currents by intracellular magnesium in rat ventricular myocytes

“…In regards to the first possible mechanism, Yamaoka and Seyama [5] showed that, at physiologic levels and lower concentrations, [Mg 2+ ] i did not change single channel Ba 2+ current amplitude in cell-attached patches. Using excised membrane patches in the presence of BayK 8644, Kuo & Hess [6] also reported that cytosolic Mg 2+ , at concentrations greater or equal to 5 mM, could block inwardly-directed monovalent cation flux through single L-type channels, but Carbone et al [7] showed that Mg 2+ is much less able to block divalent than monovalent cation fluxes through the same channel type. These data would seem to argue against the possibility that physiologically-relevant [Mg 2+ ] i produce significant block of Ca 2+ permeation through the Ltype Ca 2+ channel.…”