Tension activation and relaxation in frog atrial fibres

The effect of alkali-earth cations (Ca²⁺, Sr²⁺, Ba²⁺) on the excitation-contraction coupling events of the frog atrial fibres were studied using a double mannitol gap voltage clamp technique coupled with a mechano-electric transducer. Photoremoval of the suppressive effect of nifedipine on the calcium channels allowed to obtain rapid transient Ca²⁺, Sr²⁺ or Ba²⁺ ions current jumps. The effect on the amplitude of the associated contraction was proportional to the current jumps. These results together with portional to the current jumps. These results together with the correlation established between the estimated increase in the internal concentration of divalent cations and the amplitude of the phasic tension suggest that the essential source of divalent cations for activation of contraction is the extracellular space. Also Ba²⁺ ions reduced the tonic tension and strongly slowed the relaxation of the phasic component whereas Sr²⁺ exhibited smaller effects. Sr²⁺ ions could be more efficient than Ba²⁺ ions in substituting for Ca²⁺ ions in the Na⁺–Ca²⁺ exchange mechanism known to regulate these two mechanical events. The conclusions are that the order of effectiveness of these ions (Ca²⁺>Sr²⁺>Ba²⁺) is the same with regard to transarcolemmal exchange for Na⁺ ions, presumed uptake by a second relaxing system, activation of contraction, and inactivation of the slow inward current.