Voltage‐dependent currents in isolated single Merkel cells of rats.

1. Merkel cells were dissociated enzymatically from the footpad epidermis of 10‐ to 20‐day‐old rats pretreated with fluorescent dye, quinacrine, for purposes of staining. The fluorescent Merkel cells had an elongated or elliptic shape in situ, yet the dissociated ones were round (7‐12 microns in diameter). 2. Electrical recordings were performed in the whole‐cell configuration using a conventional patch‐clamp technique. The mean resting membrane potential of fluorescent Merkel cells was ‐54.0 mV, the value being greater than the ‐26.1 mV of non‐fluorescent epidermal cells. No voltage‐dependent channel was observed in non‐fluorescent cells. 3. The Merkel cells had no Na+ spike in an external standard solution, but tetrodotoxin‐resistant long‐lasting action potentials were evoked by depolarization with injection of constant currents in an external solution containing Ba2+. 4. In Merkel cells under voltage clamp, depolarizing step pulses (800 ms) from a holding potential (VH) of ‐80 mV elicited predominantly outward K+ currents composed of transient and sustained components: the former was selectively inhibited by 4‐aminopyridine (4‐AP), while the latter was inhibited by both tetraethylammonium (TEA) and quinacrine. Quinacrine was more effective and selective than TEA in blocking the sustained K+ current but had no effect on the current at the low concentration (10(‐7) or 3 x 10(‐6) M) used for staining the Merkel cells. 5. The sustained outward K+ current (IKD) was activated at potentials more positive than ‐20 or ‐10 mV at a VH of ‐50 mV, at which potential the transient outward K+ channel was completely inactivated. The potential for half‐inactivation in the steady‐state inactivation curve for IKD was ‐33 mV. 6. The transient outward K+ current (IA) was activated at potentials more positive than ‐50 mV at a VH of ‐80 mV. The potential for half‐inactivation in the steady‐state inactivation curve for IA was ‐64 mV. 7. When the outward K+ currents were blocked by adding both TEA and 4‐AP, only a sustained inward Ca2+ current was observed. In an external solution containing 10 mM‐Ca2+, ICa was evoked by potentials more positive than ‐20 mV at a VH of ‐80 mV, and the maximum inward current appeared around +10 mV. Increases in external Ca2+ concentration ([Ca2+]o) induced a hyperbolic increase in ICa and shifted the current‐voltage (I‐V) relationship along the voltage axis in a more positive direction. Saturation of ICa occurred at about 25 mM [Ca2+]o.(ABSTRACT TRUNCATED AT 400 WORDS)