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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

View larger version (29K): [in a new window]   Fig. 1. Schematic illustrations of the anterior nectophore of Chelophyes appendiculata Eschscholtz (A) and that of Diphyes chamissonis Huxley (B). The scale bar indicates 1 cm. The subumbrellar myoepithelium sheet lining the nectophore is shaded, and a portion is shown enlarged below. (C) The entire colony of C. appendiculata shows fishing stem and tentacles only partially extended.

View larger version (23K): [in a new window]   Fig. 2. Burst of action potentials and associated contractions in the myoepithelium sheet. (A) Series of spontaneous action potentials from subumbrellar myoepithelium evoked by a single stimulus, showing a successive increase in both the amplitude and duration. The dotted line indicates 0 mV. (B) Trace of strain gauge output, showing that the force of contractions of the myoepithelium sheet became stronger as both amplitude and duration of the action potentials increased. The external solution was ASW.

View larger version (18K): [in a new window]   Fig. 3. Na+ currents at different voltages. (A) Pulse protocol of the whole-cell voltage clamp. Step depolarisations from –50 mV to +50 mV in 10 mV steps every 5 s were applied from a holding potential of –70 mV. (B) Whole-cell membrane currents in response to the step depolarisations. Eleven traces are superimposed. The bath solution was N-Ca-free-ASW and the pipette solution was Cs-asp. Cell membrane capacitance (Cm)=124.4 pF.

View larger version (16K): [in a new window]   Fig. 4. Ca2+ currents at different voltages and the effect of nifedipine. (A) Whole-cell membrane currents in response to the step depolarisations (same pulse protocol as in Fig. 3A except that the pulse duration was increased to 25 ms). Eleven traces are superimposed. The bath solution was 4Ca-ASW containing 4 µmol l–1 TTX, and the pipette solution was Cs-asp. Cell membrane capacitance (Cm)=109.0 pF. (B) Current–voltage (I–V) relationship of the peak currents obtained from eight experiments. Values are means ± S.D. (C) Superposition of five current records at +10 mV pulses before (1, 2) and after (3–5) application of 10 µmol l–1 nifedipine. Records 1 and 2 were obtained 3 and 6 min after the whole-cell configuration was made, and records 3, 4 and 5 were obtained 2, 4 and 6.5 min after the nifedipine application. Nifedipine was applied after record 2 was taken. Cm=138.2 pF.

View larger version (12K): [in a new window]   Fig. 5. K+ currents at different voltages. (A) Whole-cell membrane currents in response to the step depolarisations (same pulse protocol as in Fig. 3A except that the pulse duration was increased to 25 ms). Eleven traces are superimposed. The bath solution was N-Ca-free-ASW containing 4 µmol l–1 TTX and the pipette solution was K-asp. Cell membrane capacitance (Cm)=132.6 pF. (B) Membrane currents associated with depolarisations to +30 mV from three different holding potentials of –40 mV (square), –70 mV (circle) and –90 mV (triangle). (C) Current–voltage (I–V) relationships of the peak currents at three different holding potentials of –40 mV (open squares), –70 mV (open circles) and –90 mV (open triangles).

View larger version (8K): [in a new window]   Fig. 6. Changes in Na+, Ca2+ and K+ currents during 10 repetitive depolarising pulses with a 200 ms interval. (A) Upper panel: pulse protocol of the repetitive stimulations. Step depolarisations from the holding potential of –70 mV to +30 mV. Lower panel: superposition of 10 traces of Na+ currents in response to step depolarisations. The bath solution was N-Ca-free-ASW, and the pipette solution was Cs-asp. Cell membrane capacitance (Cm)=68.0 pF. (B) Superposition of 10 traces of Ca2+ currents. The pulse duration was 25 ms. The bath solution was 4Ca-ASW containing 4 µmol l–1 TTX and the pipette solution was Cs-asp. Cm=64.9 pF. (C) Superposition of 10 traces of K+ currents. The pulse duration was 25 ms. Note that K+ current was observed only during the first pulse. The bath solution was N-Ca-free-ASW containing 4 µmol l–1 TTX, and the pipette solution was K-asp. Cm=187.9 pF.

View larger version (11K): [in a new window]   Fig. 7. Effect of Ca2+ current on K+ current. (A) Superposition of two current traces associated with depolarisations to +30 mV with a 200 ms interval. The pulse duration was 25 ms. (B) K+ current component obtained by subtracting the second current record from the first current record in A. The bath solution was 3Ca-ASW containing 4 µmol l–1 TTX, and the pipette solution was K-asp. Cell membrane capacitance (Cm)=110.4 pF.

View larger version (12K): [in a new window]   Fig. 8. Time course of recovery of K+ current from inactivation. (A) Upper panel: protocol of the double pulses stimulation. Two-step depolarisations from the holding potential of –70 mV to +30 mV with the interval of t were applied. Lower panel: K+ currents in response to double-pulse stimulation (t=10 s). Amplitude of the 2nd K+ current (right) was 50% of the 1st current (left). (B) Relationship between the recovery of K+ current amplitude and t. The curve was fitted with an equation of experimental decay, I(t)/I0=1–exp(–t/), where I(t)/I0 is the relative amplitude of K+ current after an interval time of t (s), and is the time constant in s for K+ current recovery. The value of is 13.2 s. The bath solution was N-Ca-free-ASW containing 4 µmol l–1 TTX, and the pipette solution was K-asp. Cell membrane capacitance (Cm)=143.2 pF.

View larger version (18K): [in a new window]   Fig. 9. Relationship between action potential augmentation and K+ channel inactivation. (A) Superposition of initial six action potentials in Fig. 3 in Chelophyes appendiculata. (B) Superposition of 10 K+ currents associated with repetitive short (5 ms) depolarising pulses to +10 mV at 200 ms intervals in Diphyes chamissonis. The bath solution was N-Ca-free-ASW containing 10 µmol l–1 TTX, and the pipette solution was K-asp. Cell membrane capacitance (Cm)=201.2 pF. (C) Decay of peak K+ current values during repetitive depolarisations relative to the 1st current obtained from five experiments. Values are means ± S.D. The curve was drawn by fitting the points with a single exponential function. The time constant was calculated to be 0.46 s.