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 [Mirounga anguastrastes] hearts were studied using a single sucrose-gap voltage-clamp technqiue. A transient outward current (It0) 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 [epinephrine] 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 (> 5 mM) caffeine suppressed the activation of Ito, phasic tension and the 2nd 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 2nd component of the birefringence signal was bell-shaped. Activation of Ito evidently was related to triggered release of Ca2+ from the s.r. which then generated the phasic tension. An excitation-contraction coupling scheme was presented which incorporates these findings and suggests that Ito may be responsible for shorter action potentials found in atrial fibers.