The Journal of Neurophysiology Vol. 81 No. 4 April 1999, pp. 1848-1855
Copyright ©1999 by the American Physiological Society

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

Békésy Laboratory of Neurobiology and Department of Zoology, University of Hawaii, Honolulu, Hawaii 96822

Duan, Shumin and Ian M. Cooke. 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.