Selective Inhibition of Transient K+ Current by La3+ in Crab Peptide-Secretory Neurons

Abstract

Selective inhibition of transient K+ current by La3+ in crab peptide-secretory neurons. Although divalent cations and lanthides are well-known inhibitors of voltage-dependent Ca2+ currents ( I Ca), their ability to selectively inhibit a voltage-gated K+ current is less widely documented. We report that La3+ inhibits the transient K+current ( I A) of crab ( Cardisoma carnifex) neurosecretory cells at ED50 ∼5 μM, similar to that blocking I Ca, without effecting the delayed rectifier K+ current ( I K). 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 I K. 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 I A. La3+ (with TEA) reduced I A and maximum conductance ( G A) by ∼10% for 1 μM and another 10% each in 10 and 100 μM La3+. Normalized G A- V curves were well fit with a single Boltzmann function, with V 1/2 +4 mV and slope 15 mV in control; V 1/2 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 I A. Steady-state inactivation showed similar right shifts (from V 1/2 −39 mV) and slope increases (from 2.5 mV) in 10 and 100 μM La3+. Time to peak I A was slowed in 10 and 100 μM La3+, whereas curves of normalized time constants of initial decay from peak I A versus V c 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 I A by micromolar La3+ raises concerns about its use in studies of I Ca to evaluate contamination by outward current.