title: A transient outward current related to calcium release and development of tension in elephant seal atrial fibres.
abstract: Membrane currents and development of tension in atrial trabeculae from elephant seal hearts were studied using a single sucrose-gap voltage-clamp technique. A transient outward current (Ito) was observed with kinetics, voltage and beat dependence, similar to those of tension. Ito had a bell-shaped voltage dependence similar to that of tension and the slow inward current (Isi). Ito, unlike Isi, showed beat dependence quite similar to developed tension. Increases in [Ca]o, frequency of stimulation, and addition of adrenaline enhanced Ito and developed tension. Ito was suppressed by addition of Mn2+, tetracaine, or by depolarizing pre-pulses (to -40 mV for 250 ms). Caffeine at low concentrations (1 mM) blocked beat dependence of Ito. At higher concentrations (greater than 5 mM) caffeine suppressed the activation of Ito, phasic tension, and the second component of the birefringence signal (related to Ca2+-releasing activity of the sarcoplasmic reticulum (s.r.]. Similar to Isi phasic tension and Ito, the voltage dependence of the second component of the birefringence signal was bell-shaped. Our studies suggest that activation of Ito is related to triggered release of Ca2+ from the s.r. which generates the phasic tension. An excitation-contraction coupling scheme is presented which incorporates these findings and suggests that Ito may be responsible for shorter action potentials found in atrial fibres.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D016275:Atrial Function; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000502:physiology*; D010872:Caniformia / Q000502:physiology*; D004837:Epinephrine / Q000494:pharmacology; D006321:Heart / Q000187:drug effects / Q000502:physiology*; D006325:Heart Atria / Q000187:drug effects; D066298:In Vitro Techniques; D008564:Membrane Potentials / Q000187:drug effects; D008955:Models, Cardiovascular; D009200:Myocardial Contraction; D046048:Seals, Earless / Q000502:physiology*; D013748:Tetracaine / Q000494:pharmacology; D013997:Time Factors
chemical_list: D013748:Tetracaine; D002110:Caffeine; D002118:Calcium; D004837:Epinephrine