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Effects of magnesium on the isolated, blood-perfused atrial and ventricular preparations of the dog heart.

The effects of magnesium chloride were investigated on pacemaker activity and atrial contractility, using isolated, blood-perfused canine atrial and ventricular preparations with heparinized arterial blood led from the support dogs. Magnesium chloride injected directly into the sinus node artery produced dose-related negative chronotropic and inotropic effects in isolated right atria. In small doses (0.1-1 mg), magnesium chloride caused only a negative chronotropic effect without significant negative inotropic changes. The threshold dose for inducing the negative chronotropic response to magnesium chloride was approximately 0.3-1 mg, but that for the negative inotropic response 1-3 mg. The duration of the negative inotropic response was usually shorter than that of the negative chronotropic response. These negative effects were not inhibited by atropine which completely blocked the acetylcholine-induced effects. Magnesium chloride also produced a dose-dependent negative inotropic effect in the isolated, blood-perfused left ventricular preparation in relatively highdose ranges. Moreover, magnesium chloride produced an uniform depression of contraction amplitude at all frequencies (2-3.5 Hz) examined on the frequency-force relationship.