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Am J Physiol Regul Integr Comp Physiol 275: R2061-R2069, 1998;
0363-6119/98 $5.00
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Vol. 275, Issue 6, R2061-R2069, December 1998

L-type Ca2+ current and excitation-contraction coupling in single atrial myocytes from rainbow trout

Leif Hove-Madsen and Lluis Tort

Department of Physiology and Cell Biology, Faculty of Science, Universitat Autonoma de Barcelona, 08193 Cerdanyola, Barcelona, Spain

We have examined the contribution of L-type Ca2+ current (ICa) to the activation of contraction in trout atrial myocytes under basal and phosphorylating conditions. The average myocyte length was 197 ± 14 µm, width was 5.5 ± 0.2 µm, and cell capacitance was 36.2 ± 2.2 pF. With 25 µM EGTA in the patch pipette and a stimulation frequency of 0.125 Hz, ICa was 2.6 ± 0.4 pA/pF and it carried a total charge of 0.10 ± 0.01 pC/pF, giving rise to a contraction of 15.2 ± 2.8% of the resting cell length. With a cell volume of 2.4 ± 0.3 pl, the charge carried by ICa corresponded to 14.7 ± 2.2 µmol Ca2+/l nonmitochondrial cell volume (µM). This can account for only 30-40% of the Ca2+ binding to the myofilaments during a contraction. Increasing the stimulation frequency from 0.25 to 2 Hz decreased ICa amplitude and charge by 66 ± 5 and 80 ± 3%, respectively. Elevating the pipette EGTA concentration from 25 µM to 5 mM increased ICa amplitude and charge by ~290%. Both isoproterenol and cAMP increased ICa by ~230%. The total charge carried by the isoproterenol- or cAMP-stimulated current was increased by 170%. We conclude that the use of high-EGTA concentration may overestimate the total Ca2+ carried by ICa under physiological conditions. Furthermore, the results suggest that, in contrast to previous reports from other lower vertebrates, Ca2+ flux through L-type Ca2+ channels alone is not sufficient to fully activate contraction in trout atrial myocytes at room temperature.

isoproterenol; adenosine 3',5'-cyclic monophosphate; ethylene glycol-bis(beta -aminoethyl ether)-N,N,N',N'-tetraacetic acid; contraction


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