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 (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 approximately 5 microM, 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 microM 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 microM. 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 approximately 10% for 1 microM and another 10% each in 10 and 100 microM 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 approximately 15 mV depolarized and slope increased approximately 2 mV for each of these La3+ concentrations. Cd2+ (1 mM), Zn2+ (200 microM), and Pb2+ (100 microM) 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 microM La3+. Time to peak IA was slowed in 10 and 100 microM La3+, whereas curves of normalized time constants of initial decay from peak IA versus Vc were right-shifted successively approximately 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.
Duan
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Békésy Laboratory of Neurobiology and Department of Zoology, University of Hawaii, Honolulu, Hawaii 96822, USA.
Cooke
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D000818:Animals; D003386:Brachyura; D002104:Cadmium / Q000494:pharmacology; D002478:Cells, Cultured; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D007811:Lanthanum / Q000494:pharmacology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009474:Neurons / Q000737:chemistry* / Q000166:cytology / Q000378:metabolism*; D009479:Neuropeptides / Q000378:metabolism; D018408:Patch-Clamp Techniques; D011188:Potassium / Q000378:metabolism*; D015221:Potassium Channels / Q000502:physiology*; D019789:Tetraethylammonium / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D009479:Neuropeptides; D015221:Potassium Channels; D002104:Cadmium; D013779:Tetrodotoxin; D019789:Tetraethylammonium; D007811:Lanthanum; D011188:Potassium