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)
The Journal of Physiology – Wiley
Published: Dec 1, 1987
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