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First published online March 9, 2004
Journal of Experimental Biology 207, 1369-1377 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00884

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Effect of ß-adrenergic stimulation on the relationship between membrane potential, intracellular [Ca²⁺] and sarcoplasmic reticulum Ca²⁺ uptake in rainbow trout atrial myocytes

Anna Llach^*,1, Jingbo Huang^*,2, Franklin Sederat², Lluis Tort¹, Glen Tibbits² and Leif Hove-Madsen^1,

¹ Unitat de Fisiologia Animal, Departamento de Biologia Celular, Fisiologia i Immunología, Facultat de Ciencies, Universitat Autònoma de Barcelona, 08193, Cerdanyola, Barcelona, España
² Cardiac Membrane Research Laboratory, Department of Kinesiology, Simon Fraser University, Burnaby, BC, Canada, V5A 1S6

Author for correspondence (e-mail: lhove{at}hsp.santpau.es)

Accepted 19 January 2004

Long depolarizations cause a steady tonic contraction and induce sarcoplasmic reticulum (SR) Ca²⁺-uptake in trout atrial myocytes. Simultaneous measurements of cytosolic [Ca²⁺] ([Ca²⁺]_i) and whole membrane current showed an elevated [Ca²⁺]_i throughout the depolarization. Rapid caffeine (Caf) applications at –80 mV before and after a long depolarization were used to determine SR Ca²⁺ loading and its dependency on membrane potential and [Ca²⁺]_i during depolarization. Following a 10 s depolarization, the maximal SR Ca²⁺ load was 597 µmol l^–1 and loading was half-maximal at –12 mV. The ß-adrenergic agonist isoproterenol (ISO) did not affect the maximal SR Ca²⁺ loading but shifted the potential for half-maximal loading by –26 mV. Following a 3 s depolarization, the maximal SR Ca²⁺ uptake rate (_max) was 418 µmol l^–1 s^–1 in control conditions. ISO did not affect _max, but significantly lowered the average free Ca²⁺ transient during the depolarization and shifted the K_0.5 for the relationship between SR Ca²⁺ uptake and [Ca²⁺]_i from 1.27 in control to 0.8 µmol l^–1 with ISO. Following repetitive 200 ms depolarizations, ISO increased the L-type Ca²⁺ current (I_Ca) amplitude by 91±29% and the peak Ca²⁺ transient by 41±10%, and decreased the half life of the Ca²⁺ transient from 151±12 to 111±6 ms. Using the relationship between [Ca²⁺]_i and SR Ca²⁺ uptake to calculate the total SR Ca²⁺ uptake during a Ca²⁺ transient elicited by a 200 ms depolarization, a significant increase in the SR Ca²⁺ uptake from 37±6 µmol l^–1 in control to 68±4 µmol l^–1 with ISO was seen. When normalized to the total Ca²⁺ transport the contribution of the SR was not significantly different in the absence (35±6%) or presence of ISO (41±4%). Exposure of cells to ISO and low extracellular [Ca²⁺] increased I_Ca by 67±40% (N=5) but significantly reduced SR Ca²⁺ uptake at membrane potentials above –30 mV. Together, these results suggest that (i) ISO has a stimulatory effect on the SR Ca²⁺ pump that may contribute to the faster decay of the Ca²⁺ transient, and (ii) the relative contribution of the SR to the Ca²⁺ removal during relaxation is not altered by ISO in trout atrial myocytes.

Key words: trout, excitation-contraction coupling, Na-Ca exchange, membrane current, teleost heart, caffeine