L-type Ca2+ current and excitation-contraction coupling in single atrial myocytes from rainbow trout.
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 micrometer, width was 5.5 +/- 0.2 micrometer, and cell capacitance was 36.2 +/- 2.2 pF. With 25 microM 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 micromol Ca2+/l nonmitochondrial cell volume (microM). 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 microM to 5 mM increased ICa amplitude and charge by approximately 290%. Both isoproterenol and cAMP increased ICa by approximately 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.
Hove-Madsen
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Department of Physiology and Cell Biology, Faculty of Science, Universitat Autonoma de Barcelona, 08193 Cerdanyola, Barcelona, Spain.
Tort
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D000318:Adrenergic beta-Agonists / Q000494:pharmacology; D000818:Animals; D016275:Atrial Function / Q000502:physiology*; D002118:Calcium / Q000502:physiology*; D002469:Cell Separation; D002614:Chelating Agents / Q000494:pharmacology; D004533:Egtazic Acid / Q000494:pharmacology; D004553:Electric Conductivity; D004558:Electric Stimulation; D007545:Isoproterenol / Q000494:pharmacology; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000166:cytology; D017686:Oncorhynchus mykiss / Q000502:physiology*; D018408:Patch-Clamp Techniques
D000318:Adrenergic beta-Agonists; D002614:Chelating Agents; D004533:Egtazic Acid; D007545:Isoproterenol; D002118:Calcium