The effects of acetyl-strophanthidin on the kinetics of potassium and Rb86 in the myocardium of dogs

Origin of acetyl strophanthidin-induced ventricular arrhythmias

To examine the origin of digitalis-induced ventricular tachycardia (VT), acetyl strophanthidin (AS) (25 mug/min) was perfused into a limited zone of myocardium in intact anesthetized dogs through a catheter placed fluoroscopically in the left anterior descending artery without ischemia. A second catheter in the great cardiac vein sampled venous effluent from this region. His and left bundle branch depolarizations were recorded and bipolar intramural electrograms from endocardial and epicardial sites within the anterior descending region were obtained. No conduction alterations preceded arrhythmia. Cardiac venous K+ rose from 3.3 +/- to 4.4 +/- 0.2 meq/liter (P less than 0.001), indicating egress from the perfused zone. 10 animals (Group 1) were sacrificed 2 min after onset of VT while 11 (Group 2) continued until fibrillation (4-14 min). All showed normal (endocardial leads to epicardial) transmural depolarization during sinus rhythm, but 10/21 demonstrated reversal, usually late during VT, including 8/11 in Group 2. Epicardial activation preceded fascicular activation and QRS. Recordings from the border and circumflex regions in 10 additional dogs (Group 3) demonstrated activation reversal only in the border zone. Myocardial K+ was reduced (mean 63 +/- 1 mueq/g) and Na+ increased (mean 41 +/- 2 mueq/g) in the perfused zone (nonperfused circumflex area K+ 72 +/- 1, Na+ 33 +/- 1 mueq/g, P less than 0.001 for both); changes were similar in inner and outer ventricular wall. In related experiments, subepicardial injections of AS induced activation reversal within the immediate area, similar to recordings during coronary infusion. Reversed transmural activation with early epicardial depolarization suggest VT arises within myocardium; electrolyte gradients between adjacent regions may be causative.

Glucagon in prevention and abolition of ouabain-induced ventricular tachycardia in normokalemic and hypokalemic dogs

A recent study suggested that glucagon has antiarrhythmic properties in addition to its positive inotropic effect. The present study was designed to evaluate the efficacy of glucagon in preventing and converting ouabain-induced ventricular tachycardia (VT) in normokalemic and hypokalemic dogs. Paired studies were made in ten normokalemic dogs and in eight dogs rendered hypokalemic by diet and hydrochlorothiazide. All dogs were pretreated with glucagon, and the amount of ouabain required to produce VT was measured. Each dog served as its own control in a second study in which normal saline was substituted for glucagon. When VT was produced during the control study, the response to a bolus injection of glucagon was evaluated. (1) Pretreatment with glucagon delayed the appearance of VT in hypokalemic dogs but not in normokalemic dogs. (2) Glucagon, by bolus injection, converted VT to sinus rhythm in 78% of the normokalemic dogs and in 100% of the hypokalemic dogs. (3) The mechanisms by which glucagon exerts its antiarrhythmic effect are not clearly defined.

Gradients in the regional rates of myocardial rubidium-86 clearance in tranquilized dogs

In order to determine what gradients in local rates of myocardial blood flow are present in the resting hemodynamic state, the regional rates of myocardial clearance of 86 Rb were measured in dogs tranquilized with chlorpromazine. The clearance rate in the left ventricle corresponded to an average coronary flow of 11.7 ml/g per 10 min. Estimated right ventricular flow was 7.1 ml/g per 10 min and left atrial flow 7.9 ml/g per 10 min. High rates of 86 Rb uptake were noted in the atrial appendages and papillary muscles. Uptake in serially sectioned cores from both ventricles was consistently highest near the endocardial surface and decreased toward the epicardium. Selective injections of 86 Rb into the aorta or vena cava demonstrated that isotope penetrated the myocardium from the cavities of both ventricles. In the right ventricle, high subendocardial 86 Rb clearance was not found unless the isotope had been present within its cavity. In the left ventricle, in contrast, more 86 Rb was taken up by the subendocardial zones, even when all of the tracer was distributed by coronary arterial blood. The results support both diffusion of 86 Rb and perfusion of the myocardium by intracavitary blood as mechanisms responsible for the uptake of 86 Rb from the heart cavities.

Influence of potassium depletion on myocardial concentration of tritiated digoxin

The influence of potassium deficiency on myocardial 3 H-digoxin concentration in the mouse was studied. Tritiated digoxin, 400 µg/kg, was injected intraperitoneally into mice, which were killed at varying time intervals, and the 3 H-digoxin was extracted. One-half of the mice were made potassium deficient by a diet free of potassium.

In control animals, myocardial 3 H-digoxin was maximal by 30 min and declined through 24 hours. There was no significant difference between the myocardial 3 H-digoxin of the control and potassium-depleted groups at ½ 1, 2, 6, or 16 hours, but at 24 hours a significantly higher concentration of digoxin was present in the potassium-deficient mouse hearts (23.6 ± 5.2 mµg/g) than in the control group (3.1 ± 1.5 mµg/g). Potassium-depleted mice with total renal failure, produced by bilateral ligation of the renal pedicle, showed increased myocardial 3 H-digoxin levels at 20 hours as compared with mice with renal failure but without potassium deficiency.

It is concluded that potassium deficiency may lead to increased concentrations of cardiac digoxin. The findings in the anuric animals suggest that diminished renal function produced by potassium depletion is not the sole mechanism of this retention of cardiac 3 H-digoxin.