Correlation between microsomal (Na+ + K+)-ATPase activity and [3H]ouabain binding to heart tissue homogenates

ATP plus Mg2+ plus Na+ supported [3H]ouabain binding to canine left ventricular tissue homogenates and microsomal (Na+ + K+)-ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity from the same tissue were measured. A linear relationship was found between the initial velocity of [3H]ouabain binding to tissue homogenates and microsomal (Na+ + K+)-ATPase activity from the same tissue in the presence and absence of in vivo bound digoxin. In vivo bound digoxin reduced both measurements. With tissue from digoxin-free hearts, a linear relationship was also obtained between the initial velocity and the maximum level of [3H]ouabain binding to tissue homogenate. Binding of [3H]ouabain to whole tissue homogenate is a convenient method for estimating (Na+ + K+)-ATPase activity in small left ventricular biopsy samples.

Age-related changes in ouabain pharmacology. Ouabain exhibits a different volume of distribution in adult and young dogs

To better understand why one must administer much higher doeses of digitalis glycosides to immature than to mature humans and animals, we studied ouabain pharmacokinetics in adult and young dogs. Consistent with reported observations that ouabain-binding, metabolism, and excretion do not change with age, we found no significant differences in the transfer coefficients in a linear two-compartment open model for ouabain pharmacokinetics following a bolus of 0.05 mg/kg. We did find, however, that young dogs had nearly twice the ouabain volumes of distribution per kilogram of body weight as adults (155.4 +/- 1.2 (SE) ml/kg vs. 80 +/- 0.6, P less than 0.0005) and that one could account for this difference with the fact that young dogs had nearly twice the plasma volume 108 +/- 9.8, vs. 68 +/- 7.3, P = 0.001) and interstitial fluid space (318 +/- 35 vs. 190 +/- 6.5, P = 0.006) as the adults. For the same dose per kilogram, left and right ventricular ouabain concentrations were inversely related to the volume of distribution, with the adults having significantly higher tissue levels and incidence of arrhythmias. One must give more ouabain to a young dog to get the same plasma concentration as in an adult because the mass of ouabain in rapid equilibrium with the plasma is diluted in a larger volume of distribution.

Investigation of the safe withdrawal period for propranolol in patients scheduled for open heart surgery

The time necessary for dissipation of radioactive labelled propranolol and its metabolites and the cardiac effects of this agent in the hearts of patients undergoing open-heart surgery were studied. Isoprenaline produced chronotropic and inotropic responses in atrial muscle in tissue bath studies which were normal 8 to 12 hours after withdrawing propranolol. After the administration of either 25 or 75 muCi of 14C-labelled propranolol, the myocardial tissue concentration declined to insignificant levels between 24 and 28 hours. We conclude that withdrawal of propranolol therapy 24 to 48 hours before cardiac surgery should be acceptable.

Studies on magnesium's mechanism of action in digitalis-induced arrhythmias

The mechanism by which magnesium affects digitalis-induced arrhythmias was studied in dogs with and without beta-receptor blockade. Digoxin was infused at a rate of 2.5mug/kg/min until ventricular tachycardia developed, then half the animals were given MgSO4, the other half saline. In animals given MgSO4, sinus rhythm was immediately re-established; in animals given saline, ventricular tachycardia persisted. In animals with beta-receptor blockade, MgSO4 was as effective in abolishing ventricular tachycardia as in those without beta-receptor blockade. We found no evidence that magnesium re-activated digoxin-inhibited (Na+, K+)-ATPase, altered myocardial or microsomal digoxin binding, or acted via the autonomic nervous system. Magnesium's direct effect on calcium and potassium fluxes across the myocardial cell membrane may be the mechanism of its antiarrhythmic action in digitalis-toxic arrhythmias.

Hemodynamics, renal function, plasma renin, and aldosterone in man after 5 to 14 days of bedrest

Continous bedrest for 5 to 14 days had no significant effect on resting heart rate, blood pressure or cardiac output in six normal men. Head-up tilt induced greater tachycardia in 5 of 6 patients after bed rest than in the control period. Propranolol diminished both the tachycardia and the incidence of hypotension and faintness in upright posture. Body weight, serum electrolytes and resting renal plasma flow, and glomerular filtration rate were unchanged by bedrest. Plasma volume fell, extracellular fluid volume increased, and plasma renin activity was significantly elevated following bedrest. Unusually large increases in plasma renin followed head-up tilt or administration of isoproterenol during bedrest, and after resuming normal activity. During bedrest, plasma aldosterone was often increased in the early morning. We conclude that after bedrest, upright posture evokes strong beta-adrenergic activity, with exaggerated metabolic and circulatory responses which can be reduced or abolished by the beta-adrenergic blocker, propranolol.

Dose response in vivo to digoxin in normo-and hyperkalaemia: associated biochemical changes

The effect of digoxin, at two different inotropic levels, was examined in normo- and hyperkalaemic dogs. For similar inotropic responses, normo- and hyperkalaemic dogs had similar levels of (Na+, K+)-ATPase inhibition and microsomal-bound digoxin.

Physiological distribution of digoxin in human heart

Using the recipient's human heart removed at cardiac transplantation, the distribution of digoxin at both the cellular and subcellular level has been studied. In the presence of diffuse histological myocardial abnormalities tissue digoxin is decreased, but the subcellular distribution, presumably reflecting binding to a possible receptor site, is uniform. When the histological abnormality is focal then digoxin distribution is uniform.These results suggest that in the presence of myocardial ischaemia plasma digoxin concentrations may not reflect total myocardial levels accurately.

The inotropic effects of digoxin in hyperkalemia. Relation to (Na+,K+)-ATPase inhibition in the intact animal

The effects of hyperkalemia on the inotropic properties of digoxin were studied in anesthetized mongrel dogs. With preload, afterload and heart rate maintained constant in dogs who received atropine and practolol to block cardiac reflexes, 0.08 mg/kg digoxin increased the first derivative of the left ventricular pressure curve (LV dp/dt) 53.1 ± 8.3% (SEM) in normokalemic animals but only 9.5 ± 2.5% in hyperkalemic animals at 60 min. This decrease in the inotropic effects of digoxin by hyperkalemia was accompanied by significant decreases in both total left ventricular digoxin concentration and microsomal-bound digoxin. (Na + , K + )-ATPase was inhibited 43.3 ± 2.4% in normokalemic animals but only 27.4 ± 2.5% in hyperkalemic animals ( P < 0.01), while (Mg ++ )-ATPase levels were similar in normokalemia and hyperkalemia. In animals not given digoxin, hyperkalemia had no effect on either LV dp/dt or (Na + , K + )-ATPase. There was a good correlation between the positive inotropic effects of digoxin and percent inhibition of (Na + , K + )-ATPase.

These studies provide further support for the importance of (Na + , K + )-ATPase inhibition and microsomal-bound digoxin in determining the inotropic effects of digoxin. Hyperkalemia alters the inotropic effects of digoxin by decreasing binding of digoxin to a microsomal cell fraction containing (Na + , K + )-ATPase activity.

These studies also provide support for the importance of reflex withdrawal of sympathetic tone in modifying the inotropic properties of digoxin in normokalemic and hyperkalemic animals.

Measurement of central venous oxygen saturation in patients with myocarial infarction

Central venous oxygen saturation (CVSO 2 ) was measured in 31 patients with myocardial infarction. CVSO 2 correlated well with the patients' clinical course. In those patients not in heart failure, mean ± sem for CVSO 2 was 70 ± 1%. When heart failure was present, CVSO 2 averaged 56 ± 1%. When both heart failure and shock were present, CVSO 2 averaged 43 ± 1%. In nine patients, serial determinations of arterial oxygen saturation and CVSO 2 were made. In 22 of 26 instances, either a fall in CVSO 2 was accompanied by an increase in the arteriovenous oxygen saturation difference or an increase in CVSO 2 was accompanied by a decrease in arteriovenous oxygen saturation difference. Serial measurements of CVSO 2 appear to be a useful method of monitoring changes in myocardial function in patients with myocardial infarction.