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First published online December 2, 2005
Journal of Experimental Biology 208, 4577-4584 (2005)
Published by The Company of Biologists 2005
doi: 10.1242/jeb.01930

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Long-lasting potassium channel inactivation in myoepithelial fibres is related to characteristics of swimming in diphyid siphonophores

Isao Inoue^1,2,*, Izuo Tsutsui^1,3 and Quentin Bone^1,4

¹ Ine Marine Laboratory of National Institute for Physiological Sciences, Ine, Kyoto 626-0424, Japan
² Institute for Enzyme Research, Tokushima University, Tokushima 770-8503, Japan
³ Laboratory of Biology, Graduate School of Commerce and Management, Hitotsubashi University, Kunitachi, Tokyo 186-8601, Japan
⁴ Marine Biological Association of UK, Plymouth, PL1 2PB, UK

^* Author for correspondence (e-mail: iinoue{at}ier.tokushima-u.ac.jp)

Accepted 13 October 2005

Diphyid siphonophores swim using bursts of propulsive jets, which are produced by contractions of a monolayer of subumbrellar myoepithelial fibres lining the nectophore. This swimming behaviour is characterised by successive increases in the force generating the jets during the initial jets of the burst. Action potentials that generate the contractions propagate throughout the myoepithelial layer: both their amplitude and duration successively increase during the first part of the burst. To investigate the ionic mechanism of this action potential augmentation, single myoepithelial cells were enzymatically dissociated and whole-cell voltage clamped. Na⁺, Ca²⁺ and K⁺ currents were recorded under different internal and external salt compositions. The Na⁺ current was blocked by a relatively high concentration (4 µmol l^–1 or higher) of tetrodotoxin (TTX), indicating that the Na⁺ channel belongs to a group of TTX-resistant Na⁺ channels. The Ca²⁺ current was blocked by nifedipine (10 µmol l^–1) and Co²⁺ (5 mmol l^–1), indicating that the Ca²⁺ channel is L-type. The K⁺ current possessed a unique property of long-lasting inactivation. The K⁺ current fully inactivated during a depolarisation to +30 mV with a time-constant of 9 ms, and the time constant of recovery from inactivation at –70 mV was 13.2 s. This long-lasting inactivation of the K⁺ channel was the major factor in the augmentation of both action potentials and contractions of the myoepithelial sheet during the initial part of the burst.

Key words: diphyid, behaviour, striated muscle, K⁺ channel, inactivation, Diphyes chamissonis