Voltage Dependence Of Recovery Research Articles

The voltage dependence of recovery from use-dependent block by QX-222 separates mechanisms for drug egress in the cardiac sodium channel

In neuronal sodium channels of squid giant axons, recovery from QX-222 block is slowed by hyperpolarization. However, in ventricular cells, hyperpolarization speeds recovery. Previously, we showed that isoform-specific residues in the external side of the cardiac sodium channel isoform (D1P-loop C373 and D4S6 T1752) influence use-dependent block (UDB) by lidocaine. To determine whether these isoformspecific residues contribute to the contrasting voltage-dependent recovery observed in ventricular myocytes, we measured recovery rates from UDB by QX-222 at holding potentials of 120, 140, 160 and 180 mV for wild-type cardiac channel (WT), the mutants C373Y (CY) and T1752V (TV), and C373Y/T1752V (CY/TV). Unlike neuronal channels, cardiac sodium channels recovered from QX block faster at hyperpolarized potentials. All mutations slowed QX-222 recovery, with the greatest rate reduction observed for the double mutant, indicating that the isoform-specific residues define external drug paths. The recovery rates varied linearly with voltage over the range tested, and we used the slopes of rate versus voltage plots to quantify voltage dependence. The TV mutation caused reduction in recovery rates without changing the slope, indicating that the mutation closed a voltage-independent egress path. The CY mutation, however, flattened the slope and reduced the voltage dependence of recovery. In addition, the reduction in rate caused by CY/TV is less than the sum of those for CY and TV, suggesting that the impacts of these two residues are interrelated. Therefore, we propose that the isoform-specific residues C373 and T1752 change recovery from UDB by distinct mechanisms but determine a common drug egress path.

U-Type Inactivation of Kv3.1 and Shaker Potassium Channels

We previously concluded that the Kv2.1 K + channel inactivates preferentially from partially activated closed states. We report here that the Kv3.1 channel also exhibits two key features of this inactivation mechanism: a U-shaped voltage dependence measured at 10 s and stronger inactivation with repetitive pulses than with a single long depolarization. More surprisingly, slow inactivation of the Kv1 Shaker K + channel ( Shaker BΔ6–46) also has a U-shaped voltage dependence for 10-s depolarizations. The time and voltage dependence of recovery from inactivation reveals two distinct components for Shaker. Strong depolarizations favor inactivation that is reduced by K 0 + or by partial block by TEA o, as previously reported for slow inactivation of Shaker. However, depolarizations near 0 mV favor inactivation that recovers rapidly, with strong voltage dependence (as for Kv2.1 and 3.1). The fraction of channels that recover rapidly is increased in TEA o or high K 0 + . We introduce the term U-type inactivation for the mechanism that is dominant in Kv2.1 and Kv3.1. U-type inactivation also makes a major but previously unrecognized contribution to slow inactivation of Shaker.

Sodium currents in isolated rat CA1 neurons after kindling epileptogenesis

Cellular excitability of CA1 neurons from a kindled focus in the rat hippocampus is persistently increased. The changes in the underlying voltage-dependent sodium current were characterized under whole-cell voltage-clamp conditions. We compared sodium currents in acutely isolated CA1 neurons from kindled rats with those in matched controls, one day and five weeks after cessation of kindling stimulations. The sodium current in CA1 neurons was tetrodotoxin sensitive and inactivated completely with two time-constants. In 97 cells from control rats, the current evoked at −20 mV consisted of a fast-inactivating component of 3.8±0.2 nA which decayed with a time-constant of 1.0±0.1 ms, and a slow-inactivating component of 1.2±0.1 nA with a time-constant of 3.6±0.1 ms. The potential of half-maximal inactivation was −72.2±1.0 mV for the fast-inactivating component and −63.2±1.0 mV for the slow-inactivating component. The time-constant of recovery at −80 mV was 14.1±0.4 ms for the fast-inactivating component and 9.3±0.4 ms for the slow-inactivating component. One day after kindling, the voltage dependence of inactivation of the slow-inactivating and the fast-inactivating component was shifted in the depolarizing direction (3.2±1.3 and 3.0±1.3 mV, respectively). The voltage dependence of recovery from inactivation was shifted in the same direction. Five weeks after kindling, the shift in voltage dependence of inactivation was (3.3±1.2 and 2.9±1.2 mV, respectively) and was accompanied by a 20% increase in sodium current amplitude. The voltage-dependent activation was not different after kindling. The changes in sodium current inactivation will increase the number of channels available for activation and may enhance the maximum firing rate. This implies that the changes in sodium current inactivation will contribute to the enhanced excitability of pyramidal neurons observed after kindling.

The activation gate of the sodium channel controls blockade and deblockade by disopyramide in rabbit Purkinje fibres

1. The effect of disopyramide on the maximum upstroke velocity (Vmax) and the sodium current of rabbit cardiac Purkinje fibres was studied with the two-microelectrode voltage-clamp technique. 2. In the absence of stimulation the drug did not cause block at membrane potentials ranging from -100 to -65 mV. Use-dependent block of Vmax was most pronounced at -75 mV. At hyperpolarized membrane potentials development of use-dependent block was faster than at depolarized membrane potentials. The time course of development of use-dependent block was not significantly influenced by the duration of the depolarizing pulse. These results strongly suggest that disopyramide predominantly blocks activated sodium channels. 3. The relative decrease of the sodium current at the beginning of a 2 s depolarizing clamp to -45 mV was almost the same as at the end, implying a rapid blockade of activated sodium channels. The Hill plots were linear with slopes ranging from 0.978 to 1.08 indicating a first order reaction; the dissociation constant for activated channels was 70 microM. 4. Recovery of Vmax from use-dependent block during rest was strongly voltage-dependent, the time constant of recovery increasing upon hyperpolarization. When the fraction of charged molecules was reduced by changing the pH of the external solution, the voltage-dependence of recovery was decreased. In contrast, recovery of Vmax for a change in holding potential from -80 to -95 mV was very fast during repetitive stimulation. 5. It is concluded that disopyramide blocks the sodium channel during activation and is trapped in the channel when the activation gate closes.