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First published online December 2, 2005
Journal of Experimental Biology 208, 4747-4756 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01967

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Biophysical properties of voltage-gated Na⁺ channels in frog parathyroid cells and their modulation by cannabinoids

Yukio Okada^1,*, Kotapola G. Imendra⁴, Toshihiro Miyazaki², Hitoshi Hotokezaka³, Rie Fujiyama¹, Jorge L. Zeredo¹, Takenori Miyamoto⁵ and Kazuo Toda¹

¹ Integrative Sensory Physiology, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Nagasaki 852-8588, Japan
² Oral Cytology and Cell Biology, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Nagasaki 852-8588, Japan
³ Orthodontics and Biomedical Engineering, Graduate School of Biomedical Sciences, Nagasaki University, Nagasaki, Nagasaki 852-8588, Japan
⁴ Department of Physiology, Faculty of Medicine, University of Rhuna, Galle, Sri Lanka
⁵ Department of Chemical and Biological Sciences, Faculty of Science, Japan Women's University, Bunkyo-ku, Tokyo 112-8681, Japan

^* Author for correspondence (e-mail: okada{at}net.nagasaki-u.ac.jp)

Accepted 2 November 2005

The membrane properties of isolated frog parathyroid cells were studied using perforated and conventional whole-cell patch-clamp techniques. Frog parathyroid cells displayed transient inward currents in response to depolarizing pulses from a holding potential of –84 mV. We analyzed the biophysical properties of the inward currents. The inward currents disappeared by the replacement of external Na⁺ with NMDG⁺ and were reversibly inhibited by 3 µmol l^–1 TTX, indicating that the currents occur through the TTX-sensitive voltage-gated Na⁺ channels. Current density elicited by a voltage step from –84 mV to –24 mV was –80 pA pF^–1 in perforated mode and –55 pA pF^–1 in conventional mode. Current density was decreased to –12 pA pF^–1 by internal GTPS (0.5 mmol l^–1), but not affected by internal GDPßS (1 mmol l^-1). The voltage of half-maximum (V_1/2) activation was –46 mV in both perforated and conventional modes. V_1/2 of inactivation was –80 mV in perforated mode and –86 mV in conventional mode. Internal GTPS (0.5 mmol l^–1) shifted the V_1/2 for activation to –36 mV and for inactivation to –98 mV. A putative endocannabinoid, 2-arachidonoylglycerol ether (2-AG ether, 50 µmol l^–1) and a cannabinomimetic aminoalkylindole, WIN 55,212-2 (10 µmol l^–1) also greatly reduced the Na⁺ current and shifted the V_1/2 for activation and inactivation. The results suggest that the Na⁺ currents in frog parathyroid cells can be modulated by cannabinoids via a G protein-dependent mechanism.

Key words: parathyroid, voltage-gated Na⁺ channel, G protein, activation, inactivation, cannabinoid, frog

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