title: Maximum heart rate in brown trout (Salmo trutta fario) is not limited by firing rate of pacemaker cells.
abstract: Temperature-induced changes in cardiac output (Q̇) in fish are largely dependent on thermal modulation of heart rate (fH), and at high temperatures Q̇ collapses due to heat-dependent depression of fHThis study tests the hypothesis that firing rate of sinoatrial pacemaker cells sets the upper thermal limit of fHin vivo. To this end, temperature dependence of action potential (AP) frequency of enzymatically isolated pacemaker cells (pacemaker rate, fPM), spontaneous beating rate of isolated sinoatrial preparations (fSA), and in vivo fHof the cold-acclimated (4°C) brown trout (Salmo trutta fario) were compared under acute thermal challenges. With rising temperature, fPMsteadily increased because of the acceleration of diastolic depolarization and shortening of AP duration up to the break point temperature (TBP) of 24.0 ± 0.37°C, at which point the electrical activity abruptly ceased. The maximum fPMat TBPwas much higher [193 ± 21.0 beats per minute (bpm)] than the peak fSA(94.3 ± 6.0 bpm at 24.1°C) or peak fH(76.7 ± 2.4 at 15.7 ± 0.82°C) (P < 0.05). These findings strongly suggest that the frequency generator of the sinoatrial pacemaker cells does not limit fHat high temperatures in the brown trout in vivo.
authors.affiliation: Department of Environmental and Biological Sciences, University of Eastern Finland, Joensuu, Finland.
authors.forename: Jaakko
authors.initials: J
authors.lastname: Haverinen
authors.affiliation: Department of Human and Animal Physiology, Lomonosov Moscow State University, Leninskiye Gory, Moscow, Russia; and.|Department of Physiology, Pirogov Russian National Research Medical University, Ostrovityanova, Moscow, Russia.
authors.forename: Denis V
authors.initials: DV
authors.lastname: Abramochkin
authors.affiliation: Department of Physiology, Pirogov Russian National Research Medical University, Ostrovityanova, Moscow, Russia.
authors.forename: Andre
authors.initials: A
authors.lastname: Kamkin
authors.affiliation: Department of Environmental and Biological Sciences, University of Eastern Finland, Joensuu, Finland; Matti.Vornanen@uef.fi.
authors.forename: Matti
authors.initials: M
authors.lastname: Vornanen
mesh_terms: D000200:Action Potentials / Q000502:physiology*; D000818:Animals; D001683:Biological Clocks / Q000502:physiology*; D001833:Body Temperature Regulation / Q000502:physiology*; D006329:Heart Conduction System / Q000166:cytology / Q000502:physiology; D006339:Heart Rate / Q000502:physiology*; D032383:Myocytes, Cardiac / Q000502:physiology*; D014337:Trout / Q000502:physiology*
keywords: cardiac pacemaker; fish heart; pacemaker action potentials; single pacemaker cells; thermal tolerance