Myocardial digoxin uptake in pressure overload right ventricular hypertrophy

Myocardial ouabain content and susceptibility to ouabain cardiotoxicity associated with circulatory volume overload in the dog

The influence of circulatory volume overload on the myocardial uptake of ouabain and on cardiotoxicity was studied in the unanaesthetised dog with aorto-caval fistula. One hour after tritiated ouabain (0-02 mg/kg IV) both ventricles and atria contained more ouabain than did those of normal dogs (left ventricle (LV), 166+/-23 (SD) ng/g vs. 97+/-19 ng/g, P less than 0-001) while concentrations in skeletal muscle, liver, kidney and plasma were not different in the two groups. In other experiments ouabain was infused to cardiotoxicity (7-5 microgram/kg followed by 3 microgram/kg/min). Cardiotoxicity occurred earlier in dogs with fistula than in normals (16-5+/-2-7 min vs. 24-1+/-2-4 min, P less than 0-001). Ouabain concentrations in myocardium were not different (LV, 434+/-58 ng/g, vs. 442+/-42 ng/g) while concentrations in liver and kidney were less in those with fistula (181+/-35 ng/g vs. 278+/-69 ng/g, P less than 0-001; 1422+/-189 ng/g vs. 2747+/-479 ng/g, P less than 0-001). Average content of skeletal muscle was also less, in proportion to administered dose. The increment in myicardial ouabain content associated with aorto-caval fistula appears to be physiologically active and hence is presumably specifically bound to the digitalis receptor. The observations in this model suggest the possibility of augmented cardiac glycoside uptake in some clinical cardiac diseases.

Myocardial digoxin uptake: dissociation between digitalis-induced inotropism and myocardial loss of potassium

The time course of myocardial uptake of digoxin, of increase in inotropic effect and of changes in myocardial potassium content were studied following a single intravenous dose of digoxin. Nineteen dogs with intact circulation were investigated by the use of a biopsy technique which allowed samplings before and 10, 30, 60, and 90 min after administration of digoxin. The myocardial concentration of digoxin was 196 X 10(-9) mol/kg 10 min after administration of digoxin. Uptake continued at a slower rate, maximum concentration being 293 X 10(-9) mol/kg at 60 minutes. The inotropic effect increased parallel with the uptake of digoxin; 10 min after digoxin, contractility was 127% of the control value and this increased to 139% at 90 minutes. Myocardial potassium content was slightly increased 10 min after digoxin, suggesting an initial stimulation of membrane Na+-K+ ATPase. A subsequent significant fall in the myocardial potassium content probably reflects ATPase inhibition. The temporal dissociation between the early onset of the positive inotropic effect and the delayed inhibition of membrane Na+-K+ ATPase indicates that inotropism of digitalis glycosides is not mediated by the same binding site as that responsible for inhibition of Na+-K+ ATPase.

Increase in myocardial digoxin content associated with circulatory volume overload in the dog

1. Tritiated (12alpha-3H) digoxin (0-05 mg/kg body weight) was administered intravenously to conscious dogs with circulatory volume overload induced by previous creation of aorto-caval fistulae. Dogs were killed after 5 min, 1, or 4 h, and the myocardium sampled. Digoxin was extracted and counted and results compared to those in normal dogs. 2. At each time, myocardial digoxin concentration of all cardiac chambers in test dogs was greater than normal. Plasma digoxin concentration measured 5 min after administration was greater in dogs with fistulae but the subsequent levels were not different. 3. Anaesthetized and open-chest dogs with fistulae studied 5 min after digoxin administration had greater myocardial concentrations than similarly studied normal dogs. Although myocardial concentrations of digoxin were higher in anaesthetized than in conscious dogs the group with fistulae had higher values than did the normal group, as was the case for unanaesthetized dogs. 4. The basis for the effect of fistula is probably multifactorial. Diminised peripheral blood flow and peripheral digoxin delivery and uptake, resulting initially in higher digoxin levels in plasma perfusing the myocardium, may play a role. Increased myocardial mechanical and metabolic activity almost certainly are important. Cardiac hypertrophy, cardiac failure per se and plasma electrolyte changes are probably not. 5. The results are consistent with previously demonstrated reduced digitalis tolerance in the dog with circulatory volume overload.