Kinetic and pharmacological properties distinguishing three types of calcium currents in chick sensory neurones.

Abstract

1. Calcium currents in cultured dorsal root ganglion (d.r.g.) cells were studied with the whole‐cell patch‐clamp technique. Using experimental conditions that suppressed Na+ and K+ currents, and 3‐10 mM‐external Ca2+ or Ba2+, we distinguished three distinct types of calcium currents (L, T and N) on the basis of voltage‐dependent kinetics and pharmacology. 2. Component L activates at relatively positive test potentials (t.p. greater than ‐10 mV) and shows little inactivation during a 200 ms depolarization. It is completely reprimed at a holding potential (h.p.) of ‐60 mV, and can be isolated by using a more depolarized h.p. (‐40 mV) to inactivate the other two types of calcium currents. 3. Component T can be seen in isolation with weak test pulses. It begins activating at potentials more positive than ‐70 mV and inactivates quickly and completely during a maintained depolarization (time constant, tau approximately 20‐50 ms). The current amplitude and the rate of decay increase with stronger depolarizations until both reach a maximum at approximately ‐40 mV. Inactivation is complete at h.p. greater than ‐60 mV and is progressively removed between ‐60 and ‐95 mV. 4. Component N activates at relatively strong depolarizations (t.p. greater than ‐20 mV) and decays with time constants ranging from 50 to 110 ms. Inactivation is removed over a very broad range of holding potentials (h.p. between ‐40 and ‐110 mV). 5. With 10 mM‐EGTA in the pipette solution, substitution of Ba2+ for Ca2+ as the charge carrier does not alter the rates of activation or relaxation of any component. However, T‐type channels are approximately equally permeable to Ca2+ and Ba2+, while L‐type and N‐type channels are both much more permeable to Ba2+. 6. Component N cannot be explained by current‐dependent inactivation of L current resulting from recruitment of extra L‐type channels at negative holding potentials: raising the external Ba2+ concentration to 110 mM greatly increases the amplitude of L current evoked from h.p. = ‐30 mV but produces little inactivation. 7. Cadmium ions (20‐50 microM) virtually eliminate both N and L currents (greater than 90% block) but leave T relatively unaffected (less than 50% block). 200 microM‐Cd2+ blocks all three components. 8. Nickel ions (100 microM) strongly reduce T current but leave N and L current little changed. 9. The dihydropyridine antagonist nifedipine (10 microM) inhibits L current (approximately 60% block) at a holding potential that inactivates half the L‐type channels.(ABSTRACT TRUNCATED AT 400 WORDS)