On the therapeutic mechanism of Mg2+ in digitoxic arrhythmias and the role of cardiac glycosides in Mg depletion.
In 9 (of 17 attempted) experiments (8 aborted due to premature tissue death), transmembrane electrical activity was recorded from canine false tendons superfused with Mg-free Tyrode's solution to simulate hypomagnesemia. Oscillatory after-potentials (OAP) developed similar to those seen after exposure to 2 X 10(-7) M ouabain, a short-lasting (0.5-1 time the duration of an action potential) OAP that often reached threshold superimposed on a long-lasting (3-4 times as long as the short) OAP. Both forms of OAP were shown to be Ca2+-dependent, as both were prevented by 10(-7) M nifedipine, but only the short OAP were abolished by nifedipine once they had appeared, while high (5 mM = 10 times normal) Mg2+ both prevented and terminated short and long OAP in digitoxic preparations. Results suggest that cardiac glycosides may deplete Mg from the myocardium. The mechanism of the therapeutic action of Mg2+ in digitalis intoxication is discussed in light of its involvement in Ca2+ sequestration by the sarcoplasmic reticulum.
D000818:Animals; D001145:Arrhythmias, Cardiac / Q000139:chemically induced / Q000188:drug therapy*; D002118:Calcium / Q000627:therapeutic use; D002301:Cardiac Glycosides / Q000009:adverse effects*; D004074:Digitoxin / Q000009:adverse effects*; D004285:Dogs; D005260:Female; D006321:Heart / Q000187:drug effects; D008274:Magnesium / Q000627:therapeutic use*; D008275:Magnesium Deficiency / Q000139:chemically induced*; D008297:Male; D009543:Nifedipine / Q000627:therapeutic use; D011690:Purkinje Fibers / Q000187:drug effects
D002301:Cardiac Glycosides; D004074:Digitoxin; D008274:Magnesium; D009543:Nifedipine; D002118:Calcium