Kinetics and voltage dependence of A-type currents on neonatal rat sensory neurons.

1. We have characterized three voltage-gated potassium currents on neonatal rat nodose neurons: a rapidly inactivating current (IAf), a slowly inactivating current (IAs), and a noninactivating current (IK). 2. Most neurons expressed all three currents. However, we found that a significant number of neurons had only one of the two A-currents. 3. IAf activates rapidly (tau = 1.0-1.5 ms at -10 mV) and inactivates in 10-30 ms. The activation and steady-state inactivation curves were fit with Boltzmann distributions of V' = -21, k = 12 mV and V' = -73, k = -8 mV, respectively. 4. IAs activates more slowly than IAf (tau = 5.4-9.2 ms at -10 mV) and inactivates with two components (150-300 ms; 1-3 s). The activation and inactivation curves are shifted approximately 20 mV more positive than those of IAf, with Boltzmann coefficients of V' = -2, k = 14 mV and V' = -51, k = -14 mV, respectively. 5. Of the three, IK activates most slowly (tau = 29.4-38.3 ms at -10 mV) and at more positive potentials than IAf or IAs (V' = 16, k = 12 mV). IK does not inactivate over tens of seconds. 6. In addition, we have identified the single channels that underlie IAf and IAs. These two channels, Af and As, have the same single-channel conductance, 22 pS, but different inactivation kinetics. 7. Furthermore, we show that there is an inverse relationship between the appearance of A-currents (IAf and IAs) and the appearance of IK, suggesting that these neurons coordinate the expression of these currents in their membranes.