Direct Positive Chronotropic and Inotropic Effects of Ouabain in the Isolated and Blood-Perfused Canine Atrium

Using isolated, blood-perfused canine atrial preparations the effects of ouabain on sinus rate and contraction were investigated in 20 preparations. A continuous infusion of ouabain (1μg/min) usually induced a significant increase in sinus rate but occasionally a slight but no significant decrease followed by a clear increase, and then finally decrease followed by atrial arrest. On the inotropy, ouabain induced only distinct positive inotropic effect from the earlier stage, and the time to the maximum positive inotropic effect was much earlier than that to the maximum chronotropic effect. The positive chronotropic effect of norepinephrine was not affected by ouabain in the dose for inducing marked positive inotropic effect. On the other hand, the positive inotropic effect of norepinephrine was suppressed in percent changes by ouabain but not abolished. The negative chronotropic and inotropic effects of ACh were not significantly affected by ouabain in the dose for inducing marked positive inotropic effect. During atrial arrest by ouabain infusion, distinct staircase phenomenon disappeared. In this condition, atrial muscle responded more readily to higher freqencies of electric pacing (above 2Hz) than to lower frequencies (under 1Hz).