digoxin and acetylstrophanthidin

Glycoside-induced cardiac inotropy has traditionally been attributed to direct Na(+)-K(+)-ATPase inhibition, causing increased intracellular [Na(+)] and consequent Ca(2+) gain via the Na(+)-Ca(2+) exchanger (NCX). However, recent studies suggested alternative mechanisms of glycoside-induced inotropy: (1) direct activation of sarcoplasmic reticulum Ca(2+) release channels (ryanodine receptors; RyRs); (2) increased Ca(2+) selectivity of Na(+) channels (slip-mode conductance); and (3) other signal transduction pathways. None of these proposed mechanisms requires NCX or an altered [Na(+)] gradient. Here we tested the ability of ouabain (OUA, 3 microm), digoxin (DIG, 20 microm) or acetylstrophanthidin (ACS, 4 microm) to alter Ca(2+) transients in completely Na(+)-free conditions in intact ferret and cat ventricular myocytes. We also tested whether OUA directly activates RyRs in permeabilized cat myocytes (measuring Ca(2+) sparks by confocal microscopy). In intact ferret myocytes (stimulated at 0.2 Hz), DIG and ACS enhanced Ca(2+) transients and cell shortening during twitches, as expected. However, prior depletion of [Na(+)](i) (in Na(+)-free, Ca(2+)-free solution) and in Na(+)-free solution (replaced by Li(+)) the inotropic effects of DIG and ACS were completely prevented. In voltage-clamped cat myocytes, OUA increased Ca(2+) transients by 48 +/- 4% but OUA had no effect in Na(+)-depleted cells (replaced by N-methyl-d-glucamine). In permeabilized cat myocytes, OUA did not change Ca(2+) spark frequency, amplitude or spatial spread (although spark duration was slightly prolonged). We conclude that the acute inotropic effects of DIG, ACS and OUA (and the effects on RyRs) depend on the presence of Na(+) and a functional NCX in ferret and cat myocytes (rather than alternate Na(+)-independent mechanisms).

Topics: Animals; Bacterial Proteins; Calcium Signaling; Cardiac Glycosides; Cardiotonic Agents; Cats; Digoxin; Ferrets; Heart Ventricles; In Vitro Techniques; Membrane Potentials; Myocardial Contraction; Myocytes, Cardiac; Ouabain; Patch-Clamp Techniques; Ryanodine Receptor Calcium Release Channel; Sodium; Sodium-Calcium Exchanger; Streptolysins; Strophanthidin

Despite the historical use of cardiac glycosides, the data describing the electrophysiological characteristics of this class of drug are not fully clear. The present study reported the biphasic effect of cardiac glycosides, digoxin (1.25 microM) and acetylstrophanthidin (0.15 microM), on action potential duration in isolated Purkinje fibers by the conventional glass microelectrode technique. At the cycle lengths of 990, 690 and 490 msec., action potential duration lengthened within 10 min. and shortened after 10 min. of digoxin and acetylstrophanthidin administration. The biphasic effect was observed at a concentration of 4.0 mM [K(+)]o. However, at a higher [K(+)]o concentration of 5.4 mM, only the shortening effect on action potential duration was recorded. These results suggest that the biphasic effect of cardiac glycosides on action potential duration is related to the concentration of extracellular potassium and is not related to the stimulating cycle lengths.

Topics: Action Potentials; Animals; Anti-Arrhythmia Agents; Cardiac Glycosides; Cells, Cultured; Digoxin; Electrophysiology; Myocardial Contraction; Myocardium; Potassium; Purkinje Fibers; Sheep; Strophanthidin

New immunometric forms of immunoassay are much more flexible to use than competitive-format immunoassays for small molecular analytes. An example of the utility of this flexibility is the ability to wash the capture antibody after it has been exposed to analyte but before addition of the labeled reagent. This simple maneuver has a large impact on the specificity obtained from already highly specific assays. We also show that specificity can be further increased by means of our multiple binding assay approach, in which the final reading reflects analyte binding to two different primary capture monoclonal antibodies.

Topics: Antibodies, Monoclonal; Digitoxigenin; Digitoxin; Digoxigenin; Digoxin; Immunoassay; Sensitivity and Specificity; Strophanthidin

Chronic treatment of dogs with digoxin alone, quinidine alone and digoxin in combination with quinidine was initiated in dogs to assess changes in arrhythmogenic potential associated with the quinidine-induced increase in serum digoxin concentration observed during combined digoxin and quinidine treatment. The arrhythmogenic potential of digoxin was evaluated through the use of the acetylstrophanthidin (AcS) tolerance test. AcS was infused at a rate of 5 micrograms/kg/min until ventricular arrhythmias occurred during a drug-free period and during chronic treatment with digoxin, quinidine and digoxin plus quinidine. The dose of AcS required to initiate ventricular arrhythmias is inversely related to the arrhythmogenic potential of digoxin present at the time of AcS infusion. Administration of quinidine alone in two different dosage regimens produced serum quinidine concentrations of 5.99 +/- 1.18 and 2.99 +/- 0.43 micrograms/ml and significantly increased AcS tolerance, whereas digoxin alone, over a wide range of serum digoxin concentrations, significantly decreased AcS tolerance. This decrease in AcS tolerance was linearly related to the serum digoxin concentration. The addition of quinidine treatment to animals receiving digoxin resulted in a significant elevation in the steady-state serum digoxin concentration. However, the AcS tolerance determined during the elevated serum digoxin concentration induced by quinidine was greater than that determined during treatment with the same dose of digoxin alone. Thus, quinidine administration to animals receiving digoxin resulted in a significant increase in the steady-state serum digoxin concentration but did not increase the arrhythmogenic potential of digoxin over that observed during treatment with the same dose of digoxin alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Topics: Animals; Arrhythmias, Cardiac; Digoxin; Dogs; Drug Interactions; Quinidine; Strophanthidin

To determine whether digoxin-quinidine interaction alters the inotropic effect of the glycoside, the response of peak isometric force and maximal rate of force development (dF/dt) in isolated feline right ventricular papillary muscles to digoxin and quinidine alone, and in various combinations, was examined. The administration of 1.3 or 2.6 x 10(-5) M of quinidine after 2 x 10(-7) M of digoxin resulted in an increase in contractile performance in each animal. Although 1.3 x 10(-5) M of quinidine alone produced a positive inotropic effect, 2.6 x 10(-5) M of quinidine produced no such effect. Because myocardial digoxin content has been reported to decline after administration of quinidine these results suggest that the increase in contractile performance when quinidine was administered after digoxin is due to displacement of digoxin from less to more active myocardial sites. The administration of 2 or 4 x 10(-7) M of digoxin after 2.6 x 10(-5) M of quinidine resulted in a minimal increase in force and rate of force development. A similar inhibition of the inotropic effect of digoxin was found in rabbit papillary muscles pretreated with quinidine. Inhibition was not limited to digoxin because pretreatment of muscles with quinidine also inhibited the inotropic effect of acetylstrophanthidin. Thus, quinidine has diametrically opposite effects on digitalis-induced inotropy dependent on the sequence with which the drugs are administered.

Topics: Animals; Cats; Digoxin; Drug Interactions; Myocardial Contraction; Papillary Muscles; Quinidine; Strophanthidin

Topics: Animals; Cardiac Glycosides; Digitalis Glycosides; Digitoxin; Digoxin; Dogs; Infusions, Parenteral; Ouabain; Pharmacology; Research; Strophanthidin; Strophanthins