Although divalent cations and lanthides are well-known inhibitors of
voltage-dependent Ca2+ currents (ICa),
their ability to selectively inhibit a voltage-gated K+
current is less widely documented. We report that La3+
inhibits the transient K+ current (IA) of
crab (Cardisoma carnifex) neurosecretory cells at
ED50 ~5 µM, similar to that blocking
ICa, without effecting the delayed rectifier
K+ current (IK). Neurons were
dissociated from the major crustacean neuroendocrine system, the
X-organ-sinus gland, plated in defined medium, and recorded by whole
cell patch clamp after 1-2 days in culture. The bath saline
included 0.5 µM TTX and 0.5 mM CdCl2 to eliminate
inward currents. Responses to depolarizing steps from a holding
potential of -40 mV represented primarily IK.
They were unchanged by La3+ up to 500 µM.
Currents from -80 mV in the presence of 20 mM TEA were shown to
represent primarily IA. La3+ (with
TEA) reduced IA and maximum conductance
(GA) by ~10% for 1 mM and
another 10% each in 10 and 100 µM La3+.
Normalized GA-V curves were well fit with a
single Boltzmann function, with V1/2 +4 mV and slope
15 mV in control; V1/2 was successively ~15 mV
depolarized and slope increased ~2 mV for each of these La3+
concentrations. Cd2+ (1 mM), Zn2+ (200
µM), and Pb2+ (100 µM) or removal of saline
Mg2+ (26 mM) had little or no effect on
IA. Steady-state inactivation showed similar
right shifts (from V1/2 -39 mV) and slope increases
(from 2.5 mV) in 10 and 100 µM La3+. Time to
peak IA was slowed in 10 and 100 µM
La3+, whereas curves of normalized time constants of initial
decay from peak IA versus VC were
right-shifted successively ~15 mV for the three La3+
concentrations. The observations were fitted by a Woodhull-type
model postulating a La3+-selective site that lies 0.26-0.34
of the distance across the membrane electric field, and both block of
K+ movement and interaction with voltage-gating mechanisms;
block can be relieved by depolarization and/or outward current.
The observation of selective inhibition of IA by
micromolar La3+ raises concerns about its use in studies of
ICa to evaluate contamination by outward
current.
(Click on the figure for an enlarged view.)
