[Properties of the receptor for cardiac glycosides (author's transl)]

Release of adenine nucleotide metabolites by toxic concentrations of cardiac glycosides

In isolated perfused guinea-pig hearts the effect of toxic concentrations of cardiac glycosides on the release of the adenine nucleotide metabolites adenosine, inosine, hypoxanthine, xanthine, and uric acid was investigated. Digoxin concentrations of 0.03-1 mumol.l-1 produced moderate to severe tachyarrhythmias. Large amounts of metabolites were released by concentrations of 0.1 mumol.l-1, and higher. Occurrence of glycoside-induced ventricular fibrillation was associated with a particularly high release. Metabolite release was also obtained when fibrillation was elicited electrically in normal control hearts, or in hearts receiving simultaneously a marginally toxic digoxin concentration (0.03 mumol.l-1). Digoxin-induced tachyarrhythmias and metabolite release were almost completely prevented by a high potassium concentration in the coronary perfusion fluid (8.1 mmol.l-1). The antiarrhythmic effect was also obtained with lidocaine (60 mumol.l-1), but the release was only partially antagonized. Similar results concerning arrhythmias and metabolite release as with digoxin were obtained with ouabain. The findings suggest that the decrease in myocardial ATP observed in glycoside-intoxicated heart preparations is partly due to the loss of nucleotide precursor substances. Moreover, it appears likely that liberated adenosine in the interstitium of severely intoxicated heart preparations reaches pharmacologically effective concentrations.

A quantitative receptor assay for "digitalis-like" compounds in serum. Demonstration of raised concentrations in essential hypertension and correlation with arterial blood pressure

The influence of serum from patients with essential hypertension on [3H]ouabain binding to isolated (Na+ +K+)-ATPase and on the reactivity with digoxin-specific antibodies was investigated. [3H]Ouabain binding to (Na+ +K+)-ATPase was significantly decreased (P less than 0.001) by sera of 18 hypertensive patients (34.9 +/- 1.5 pmol/U) as compared with 22 normotensive controls (43.8 +/- 1.2 pmol/U). The factor, whose concentration is increased in the serum of patients with essential hypertension, competed with [3H]ouabain at isolated (Na+ +K+)-ATPase. Therefore, it was possible to quantify this "digitalis-like" factor with a receptor assay in ouabain equivalents. Three times higher mean serum levels were found in hypertensive patients (234.8 +/- 48.7 nM) than in normotensive controls (76.3 +/- 9.3 nM). Deproteinization of the sera by ultrafiltration through steroid-adsorbing membranes and by boiling of acidified sera for 10 min led to a significant reduction of the inhibitory activity and to the complete loss of a difference between the sera of normotensives and hypertensives. After deproteinization by boiling for 15 min, sera of normotensives showed levels of "digitalis-like" compounds of 16.53 +/- 2.15 nM and hypertensives of 41.65 +/- 8.41 nM (P less than 0.05). Though significantly elevated concentrations of "digitalis-like" factor were measured with the receptor assay, no significant increase of digoxin-like activity could be detected with digoxin-specific antibodies in untreated serum.

Magnesium concentration in saliva -- an indicator of digitalis toxicity. ?

In a prospective study salivary magnesium was measured by atomic absorption spectrophotometry in 168 patients on chronic digoxin therapy. Magnesium concentration in saliva was correlated with clinical data and plasma digoxin levels. A significant elevation in salivary magnesium concentration was caused by digoxin therapy (0.58 +/- 0.39 mmol/l, n = 93) in comparison to patients with no digitalis treatment (0.17 +/- 0.07 mmol/l, n = 35). Magnesium concentrations in saliva were significantly higher in toxic patients (1.1 +/- 0.68 mmol/l, n = 32) than in nontoxic patients. Possibly toxic patients showed a magnesium level of saliva of 0.63 +/- 0.39 mmol/l (n = 40). In 89% of the intoxicated patients salivary magnesium concentrations were higher than 1.0 mmol/l. The overlap of magnesium between toxic and nontoxic patients was less as compared to calcium and potassium concentrations in saliva. No changes were noted in serum magnesium levels. Magnesium concentration in saliva was influenced by chronic digoxin therapy only. No significant increase of magnesium in saliva was observed within 24 h after i.v. injection of 1.0 mg digoxin in four healthy volunteers. It is concluded that salivary magnesium concentration is a valid method for monitoring digoxin therapy in addition to plasma digoxin levels.

Pharmacokinetics of dihydrodigitoxin in the cat. A comparison with digitoxin

Pharmacokinetics of digitoxin (DGT) and its derivative hydrogenated at the unsaturated lactone ring have been studied using the 3H-labelled compounds in cats. Blood level data could be described by an open three compartment body model. The biological half-lives of dihydrodigitoxin (DH-DGT) and DGT are rather similar (0.12, 0.48, 20.4 h, and 0.11, 0.43, 31.8 h, respectively), whereas the volume of distribution of DH-DGT is about 1/10 from that of DGT (0.241 versus 2.581). Especially in myocardial tissue a significantly lower DH-DGT concentration was measured, indicating the loss of affinity to cardenolide binding sites by hydrogenation. Dihydrodigitoxinic acid was detected as the main metabolite of DH-DGT.

Relationship between dose and plasma level of digoxin and patient characteristics

The correlation between the daily dose of digoxin, its plasma level and clinical characteristics of 213 patients receiving beta-acetyldigoxin treatment has been evaluated. After logarithmic transformation of lognormally distributed variables multiple linear regression analysis was performed. Eight predictor variables were chosen: sex, age, height, weight, glycoside dose, creatinine and potassium level in serum and dose of spironolactone. The resulting correlation coefficient was 0.46, i.e. only 100 x r2 = 21.4% of the variance of the steady state digoxin plasma level could be interpreted with the aid of these variables. For only 4 of the 8 variables (age, glycoside dose, serum level of creatinine, and spironolactone dose) were partial coefficients of regression and of correlation significantly different from zero. Almost 80% of the variance could not be accounted for. This finding is in accordance with conclusions in the literature.

Correlation between inhibition of (Na+, K+)-membrane-ATPase and positive inotropic activity of cardenolides in isolated papillary muscles of guinea pig

Concentrations of 17 cardenolides, cardenolide glucuronides and sulfates producing half-maximal inhibition of (Na+, K+)-membrane-ATPase from different organs and animal species were determined in vitro. In addition the concentrations that increased the contractility of guinea pig isolated papillary muscles to a particular level were investigated. Comparisons between ATPase-inhibiting and positive inotropic cardiac activities showed extensive parallelism: the correlation coefficients after log/log transformation were between 0.92 and 0.97. The same close correlations are found if dissociation constants of cardenolide receptor complexes and concentrations causing 86Rb-uptake inhibition in human erythrocytes are examined. The concentrations necessary for inhibition of (Na+, K+)-membrane-ATPase of the guinea pig heart and the concentrations required to achieve a defined positive inotropic effect in guinea pig papillary muscle showed a log/log correlation coefficient of 0.97 (P less than 0.001). In both tests the potencies covered more than three orders of magnitude. The results support Repke's hypothesis on the digitalis receptor.

[Cardiac effects of antikaliuretic diuretics-clinical and biochemical investigation (author's transl)]

Recently direct myocardial effects of antikaliuretic diuretics with respect to contractility parameters and prevention of digitalis-induced arrhythmias were published. In order to test the value of these reports we measured the effect of potassium-canrenoate and triamterene on cardiac output and on digitalis-induced arrhythmias in patients during diagnostic and the therapeutic flow directed right heart catheterization (Swan-Ganz) in our intensive care unit. In addition the influence of these drugs on (Na+ + K+)-ATPase and on (3H)g-strophanthin binding to human cardiac cell membranes was investigated to gain information on the mechanism of their action. Triamterene (100-200 mg p.o.) was without any effect on cardiac output, the same was found true for potassium-canrenoate given in a single dose (200-1000 mg intravenously). However, when applied in two doses (200 mg i.v. and 60 min later 400 mg i.v.), potassium-canrenoate increased cardiac output by 11 percent (p less than 0.05). Only in 2 out of 14 patients potassium-canrenoate (200-400 mg i.v.) suppressed digitalis-induced ventricular ectopic beats. Canrenone, the active metabolite of potassium-canrenoate displaces [(3H)]g-strophanthin from its binding sites in human cardiac cell membranes and inhibits (Na+ + K+)-ATPase activity. These in vitro effects were measured at the same concentrations as found in vivo after "therapeutical" doses. The effects of triamterene in this respect were found only in extremely high concentrations. Our results imply that canrenone has cardiac glycoside-like effects in human cardiac cell membranes.

[The effect of potassium on ouabain-binding to human cardiac cell membranes (author's transl)]

The concentration of cardiac glycosides in the serum of an individual patient does not necessarily correlate strictly with the known effects or side effects of these drugs. It is good clinical experience for instance that an acutely decreased serum potassium concentration due to diuretic therapy or dialysis may cause severe digitalis-induced arrhythmias in spite of an unchanged serum glycoside level. On the other hand digitalis intoxication often is treated successfully by the application of potassium. These observations prompted us to investigate the specific binding of tritiated g-strophanthin to isolated human cardiac cell membranes in the presence of different potassium concentrations. Potassium in a concentration-dependent manner displaces ouabain from its binding sites by decreasing the affinity of the receptor for the drug. However, the number of receptor sites remains constant. When analysing the association and dissociation rates of the ouabain-receptor complex it was found that potassium inhibits the binding process only, without affecting the dissociation. At least up to a concentration of 5 mM the potassium induced displacement of ouabain from its binding sites seems to be competitive. These in vitro measurements of the strophanthin--receptor interactions agree with clinical observations and are helpful in their explanation.