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)