Kinetics and selectivity of a low‐voltage‐activated calcium current in chick and rat sensory neurones.

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Abstract
1. Using the whole-cell recording mode of the patch-clamp technique, we have investigated kinetic and selectivity properties of a low-voltage-activated (l.v.a.) Ca2+ current in chick and rat dorsal root ganglion (d.r.g.) neurones. 2. L.v.a currents were activated at about -50 mV and reached maximum amplitudes between -30 and -20 mV with averages of -0.16 nA in chick and -0.3 nA in rat d.r.g. cells with 5 mM-extracellular Ca2+. Between -60 and -20 mV, the time to peak, tp, of this current decreased with increasing membrane depolarizations. An e-fold change of tp required a 14 mV potential change in chick and a 17 mV change in rat d.r.g. cells at 22.degree. C. 3. Between -50 and +20 mV inactivaiton of this current was fast, single exponential and voltage dependent. In rat, the time constant of inactivation, Th, was smaller and less voltage dependent than in chick. 4. The amplitude of these currents increased by a factor of 5-10, when the extracellular Ca2+ concentration was changed from 1 to 95 mM. Amplitudes and kinetic parameters of the currents showed typical shifts along the voltage axis. No correlation between Ca2+ current amplitudes and activation-inactivation kinetics was found, suggesting that the reaction rates which control these processes are not dependent on Ca2+ entry. 5. Recovery from inactivation was voltage dependent and developed with a time constant, tr, in the order of 1 s. Tr was nearly halved by changing the potential from -80 to -120 mV. 6. Tail currents associated with membrane repolarization were also voltage dependent and developed exponentially. Their time constant decreased by a factor of 3 when the potential was changed from -60 to -100 mV. 7. A second and more prominent Ca2+ current was activated at potentials positive to -20 mV (high-voltage-activated Ca2+ currents, h.v.a.), masking the time course of l.v.a. currents. Between -20 and 0 mV, time to peak of the entire current increased by a factor of 2 but decreased again at higher membrane potentials. Inactivation also became significantly slower in this potential range. 8. The contribution of the h.v.a. component to the total membrane current was markedly reduced using a high intracellular Ca2+ concentration, [Ca2+]1, or internal fluoride salts. This made it possible to study the kinetic parameters and the I-V characteristics of the l.v.a. current more precisely over a wider potential range (-50 to +30 mV). 9. L.v.a. and h.v.a. currents were blocked reversibly by 5 mM-Ni2+ or 100 .mu.M-Cd2+. Replacement of Ca2+ with Ba2+ reduced the l.v.a. currents by about one-fifth, but roughly doubled the amplitude of h.v.a. currents. Sr2+, on the other hand, increased the amplitude of both l.v.a. and h.v.a. components by one-fifth. Mg2+ currents through this channel could not be resolved.