Intraerythrocytic sodium and potassium concentrations during acute and chronic digitalization

The influence of calcium antagonists on plasma digoxin concentration

Several investigators have independently discovered that the calcium antagonist, verapamil, causes a 60-80% increase in plasma digoxin. Pharmacokinetic studies indicate, that the elevated plasma digoxin level is due to verapamil-induced inhibition of both renal and extrarenal digoxin clearance. No significant changes in single-dose digoxin pharmacokinetics were observed during nifedipine coadministration . To elucidate the clinical relevance of this interaction, we investigated the influence of verapamil on digoxin-induced inotropism as assessed from systolic time intervals. In single-dose trials, the verapamil-induced elevation of plasma digoxin was associated with a more sustained reduction in left ventricular ejection time as compared to control. Correspondingly, the concentration-response relationship of digoxin inotropism was unaffected by verapamil. In-vitro studies showed that verapamil had no influence on the number of digitalis receptors on human lymphocytes. In accordance, verapamil enhanced the digoxin-induced elevation of intracellular sodium concentration possibly reflecting an increased receptor effect. The plasma digoxin elevation resulting from verapamil coadministration seems cardioactive with regard to both inotropism and toxic effects.

The diagnostic value of determination of intraerythrocytic sodium and potassium concentrations versus plasma digoxin concentration in digoxin intoxication

Plasma digoxin measurements have proved unserviceable as a means of differentiating between toxic and non-toxic patients. In order to assess the value of a biological effect of digoxin in this discrimination, intraerythrocytic sodium and potassium concentrations were determined in 55 chronically digitalized patients of whom 10 were digoxin-intoxicated according to ECG criteria. Digitoxicity was associated with elevated intraerythrocytic sodium concentration (mean +/- SEM 19.3 +/- 1.2 versus 11.3 +/- 0.3 mmol/l, p less than 0.001) and reduced intraerythrocytic potassium concentration (94.6 +/- 2.3 versus 100.0 +/- 0.6 mmol/l, p less than 0.001) compared to non-toxic patients. Mean (+/- SEM) plasma digoxin concentrations in the two groups were 3.14 +/- 0.41 and 1.57 +/- 0.09 nmol/l, respectively (p less than 0.001). When diagnosing toxicity in chronically digitalized patients, plasma digoxin and intraerythrocytic sodium determinations showed sensitivities of 60 and 100%, respectively. The predictive values of a positive test were 75% for plasma digoxin and 83% for intraerythrocytic sodium.

Assessment of the NMR visibility of intraerythrocytic sodium by flame photometric and ion-competitive studies

23Na nuclear magnetic resonance (NMR) studies of intraerythrocytic sodium concentrations were performed at 37 degrees C and the accuracy of the method was evaluated through concomitant emission flame photometric analysis. NMR visibility of intraerythrocytic sodium was estimated to be 75 +/- 3%. Verification of the presence of an NMR invisible intraerythrocytic sodium pool and assessment of the mechanisms explaining this invisibility was achieved by measurement of 23Na NMR signal intensities and T1 relaxation times of erythrocyte hemolysate during ion-competitive sodium release from association sites. Signal intensities after addition of small quantities of K+ and Mg2+ to the hemolysates were significantly larger than that expected from dilution of the sample, implying that the additions have resulted in the release of formerly invisible sodium. Furthermore, the intensities following the addition of K+ were significantly larger than those after the addition of Li+ and choline+. This signifies a specificity of the association sites at which sodium has decreased visibility. Increased T1 values following the additions confirmed the release of bound sodium.