Digitalis interaction

Potassium and digitalis interact in two ways. First, they inhibit each other s binding to Na+,K+ ATPase therefore, hyperkalemia reduces the enzyme-inhibiting actions of cardiac glycosides, whereas hypokalemia facilitates these actions. Second, abnormal cardiac automaticity is inhibited by hyperkalemia (see Chapter 14). Moderately increased extracellular K+ therefore reduces the effects of digitalis, especially the toxic effects. 

Additional chemotopographical information on the steroid binding subsite from the interaction energy changes by small substituents on the steroid skeleton Chemotopographical information derived from thermodynamic parameters of digitalis interaction with Na /K -ATPase... 

CHEMOTOPOGRAPHICAL INFORMATION DERIVED FROM THERMODYNAMIC PARAMETERS OF DIGITALIS INTERACTION WITH NaVK -ATPase... 

The answer is c. (Hardman, p 810.) Digitalis inhibits Na+,K+-ATPase and, hence, decreases myocyte Na pumping, resulting in a relative reduction of Ca expulsion from Na-Ca exchange. The consequent increase in free Ca in the cell causes an increased intensity of interaction between actin and myosin filaments and enhanced contractility... 

Drugs that may interact include ACE inhibitors, potassium-sparing diuretics, digitalis, and potassium-containing salt substitutes. 

Methimazole (Tapazole) [Antithyroid Agent] Uses Hyperthy-roidism, thyrotoxicosis, pr for thyroid surgery or radiation Action Blocks T3 T4 formation Dose Adults. Initial 15-60 mg/d PO tid Maint 5-15 mg PO daily Peds. Initial 0.4-0.7 mg/kg/24 h PO tid Maint V h- U h of initial dose PO daily w/ food Caution [D, +/-] Contra Breast-feeding Disp Tabs SE GI upset, dizziness, blood dyscrasias Interactions t Effects OF digitalis glycosides, metoprolol, propranolol X effects OF anticoagulants, theophylline X effects W/ amiodarone EMS None OD May cause N/V, HA, abd pain, fever, and pale skin symptomatic and supportive... 

Magnesium Poorly understood interacts with Na+,K+ ATPase, K+ and Ca2+ channels Normalizes or increases plasma Mg2+ Torsade de pointes digitalis-induced arrhythmias IV duration dependent on dosage Toxicity Muscle weakness in overdose... 

Calcium ion facilitates the toxic actions of cardiac glycosides by accelerating the overloading of intracellular calcium stores that appears to be responsible for digitalis-induced abnormal automaticity. Hypercalcemia therefore increases the risk of a digitalis-induced arrhythmia. The effects of magnesium ion appear to be opposite to those of calcium. These interactions mandate careful evaluation of serum electrolytes in patients with digitalis-induced arrhythmias. 

Drug Interactions Antineoplastlc agents Corticosteroids and corticotropin Cyclosporine-A Digitalis glycosides Flucytosine Imidazoles Leukocyte transfusions Nephrotoxic medications Skeletal muscle relaxants Zidovudine... 

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