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First published online August 3, 2006
Journal of Experimental Biology 209, 3091-3100 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02321

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Sarcolemmal ion currents and sarcoplasmic reticulum Ca²⁺ content in ventricular myocytes from the cold stenothermic fish, the burbot (Lota lota)

Holly A. Shiels^1,*, Vesa Paajanen² and Matti Vornanen²

¹ Faculty of Life Sciences, University of Manchester, 2.18c Core Technology Facility, 46 Grafton Street, Manchester, M13 9NT, UK
² Department of Biology, University of Joensuu, PO Box 11, 80101 Joensuu, Finland

View larger version (17K): [in a new window]   Fig. 1. Representative recordings of ventricular action potentials and associated contractions of the burbot ventricle at 4°C and 11°C. Scale bars are for recordings at both temperatures. Force of contraction is in arbitrary units.

View larger version (23K): [in a new window]   Fig. 2. The two major K+ currents in burbot ventricular myocytes. (A) Background inward rectifier K+ current, IK1. Values are means ± s.e.m. from 6-8 cells at 4°C (black circles) and 11°C (white circles) measured at the beginning of 500 ms square wave pulses that were elicited from the holding potential of -80 mV to voltages between -120 and 40 mV in 20 mV steps (inset). (B) Delayed rectifier K+ current, IKr, measured as an outward tail current at -40 mV after 5 s depolarising pulses between -80 and 80 mV (inset). Representative recordings at 4°C and mean values (± s.e.m.) from 5-10 cells at 4°C (black circles) and 11°C (white circles). *Density of IKr is increased significantly by acute warming to 11°C (Student's t-test, P<0.005).

View larger version (18K): [in a new window]   Fig. 3. Na+-Ca2+ exchange current (INCX) measured as the Ni2+-sensitive current during the hyperpolarising phase of the ramp pulse in ventricular myocytes from the burbot heart. (A) Values are means ± s.e.m. from 8 cells at 4°C and 11°C when the intracellular solution contained 20 mmol l-1 BAPTA. *Density of INCX is increased significantly by acute warming to 11°C (Student's t-test, P<0.005). (B) Mean values (± s.e.m.) from 9 cells at 4°C only. The pipette solution contained a lower level of Ca2+ buffering (0.025 mmol l-1 EGTA). Insets show voltage ramp protocols.

View larger version (28K): [in a new window]   Fig. 4. L-type Ca2+ current in burbot ventricular myocytes measured with 5 mmol l-1 EGTA in the pipette solution. (A) Representative recording at 11°C with and without stimulation by isoprenaline (10 µmol l-1). Values are means ± s.e.m. from 6-12 cells at 4°C (B) and 11°C (C) under control conditions and in the presence of 10 µmol l-1 isoprenaline (Iso). *Density of ICa is increased significantly by 10 µmol l-1 isoprenaline (Student's t-test, P<0.01). (D) Steady-state activation and inactivation relationships ICa at 4°C only. (E) The ICa window current (product of activation and inactivation curves in D) at 4°C.

View larger version (18K): [in a new window]   Fig. 5. SR Ca2+ content in burbot ventricular myocytes. Representative recordings of inward Na+-Ca2+ exchange currents (INCX) in response to caffeine (10 mmol l-1, designated by the white bar) in burbot ventricular myocytes at 4°C (A) and 11°C (B). The insets show the corresponding time integral of INCX at each temperature. (C) Mean values ± s.e.m.; N=119 myocytes at 4°C and 20 myocytes at 11°C, respectively. NS, values were not significantly different (Student's t-test, P>0.05). SR Ca2+ content is expressed as the charge carried by the integral (pC) normalized to cell capacitance (pF). In the text, values for charge (pC) are converted into µmol Ca2+ (see Materials and methods) to facilitate comparisons with literature values.

View larger version (24K): [in a new window]   Fig. 6. SR Ca2+ release in burbot myocytes does not affect the inactivation kinetics of ICa. Representative recordings of ICa at 4°C (A) or 11°C (B) on the first pulse after caffeine application (SR empty) and the 25th pulse after caffeine application (steady-state SR Ca2+ content). Recordings are superimposed. Tau of inactivation was calculated by fitting a single exponential function to the decaying portion of ICa. For clarity, only the fit for 25th pulse is shown at each temperature (red lines). (C) Values are means ± s.e.m. from 9 myocytes at 4°C and 16 myocytes at 11°C, 1, 5, 10 and 25 pulses after caffeine application. *Inactivation of ICa was faster at 11°C than at 4°C (Student's t-test, P<0.05).