Novel Digitalis-Like Factor, Marinobufotoxin, Isolated From Cultured Y-1 Cells, and Its Hypertensive Effect in Rats

Marinobufagenin and telecinobufagin have been identified as digitalis-like factors in mammals. In toads, marinobufagenin-related compounds, such as marinobufotoxin (MBT), have been isolated in some tissues but not in mammals, and its biological action has not been elucidated. Herein, we aimed to explore the possible production and/or secretion of MBT and the biological action in rats. First, the MBT in culture supernatant of the adrenocortical-originated cell line Y-1 was analyzed by high-performance liquid chromatography and sensitive ELISA for marinobufagenin-like immunoreactivity. Moreover, the structural information was obtained by mass spectrometry. To determine the biological action, MBT (9.6 and 0.96 μg/kg per day) was intraperitoneally infused via an osmotic minipump for 1 week. Blood pressure and renal excretion of marinobufagenin-like immunoreactivity were measured. Marinobufagenin-like immunoreactivity was found in Y-1 cell culture media, and the concentration increased until 24 hours. The structural analysis suggested that marinobufagenin-like immunoreactivities were marinobufagenin and MBT, and tandem mass spectrum analysis revealed them with the specific daughter ions. The highest sensitive ELISA-positive peak of marinobufagenin-like immunoreactivity in the media was MBT. Continuous administration of MBT in rats for 1 week significantly increased systolic blood pressure and renal excretion of marinobufagenin-like immunoreactivity compared with control rats (135±3.0 versus 126±2.0 mm Hg and 1.41±0.286 versus 0.34±0.064 ng/day, respectively). These data suggest that MBT, arginine-suberoyl ester of marinobufagenin, can be a novel digitalis-like factor with hypertensive action and is secreted from the adrenocortical cells.

The effects of digitalis glycosides on the ventricular fibrillation threshold in innervated and denervated canine hearts

Digitalis glycosides have been implicated in increased vulnerability to ventricular fibrillation in man. In order to investigate the genesis and occurrence of ventricular fibrillation in the presence of digitalis, the effects of both acetylstrophanthidin and ouabain on the ventricular fibrillation threshold (VFT) were studied in the open-chest anesthetized dog. The current required to induce ventricular fibrillation was determined by passing a train of 12 constant current pulses through epicardial electrodes during the vulnerable period of the cardiac cycle. It was found that an intravenous bolus infusion of acetylstrophanthidin (0.050-0.097 mg/kg) or ouabain (0.035-0.075 mg/kg) in intact innervated dog hearts raised the VFT from 40% to 260% above control values. Continuous infusions of acetylstrophanthidin to toxic levels also resulted in an elevated VFT. Vagotomy alone did not qualitatively change the effects of acetylstrophanthidin on VFT. However, following vagotomy and stellate sympathectomy, infusions of both toxic and subtoxic doses of acetylstrophanthidin resulted in a decrease in the VFT from 40 to 80% below control values. In denervated animals in which the peripheral ramifications of the left stellate ganglion nerves were stimulated, the VFT decreased below control values in the absence of acetylstrophanthidin, but during stellate stimulation in the presence of acetylstrophanthidin the VFT was increased above control values. These studies demonstrated that the increase in VFT by digitalis in the healthy, innervated heart was mediated via an associated increase in sympathetic activity; in the absence of neural influences digitalis decreased the VFT.

Comparison of serum digoxin level measurement with acetyl strophanthidin tolerance testing

Serum digoxin levels (SDL) were compared with tolerance for the rapidly acting cardiac aglycone, acetyl strophanthidin (AS). AS titration tests were performed on 133 patients with diverse cardiac disorders. All were receiving maintenance digoxin. Both exquisite AS sensitivity and tolerance for a 1.0 mg AS were associated with a wide range of SDL values. Concordance and discordance between the two methods in assessing degree of digitalization were evaluated by considering SDL of 1.4 ng/ml to be the mean value for patients without glycoside-induced cardiac arrhythmia. An SDL of < 1.5 ng/ml with tolerance for 1.0 mg AS and an SDL of > 1.4 ng/ml with sensitivity to 1.0 mg AS or less constituted concordant responses. An SDL of < 1.5 ng/ml with intolerance for 1.0 mg or less AS and an SDL of > 1.4 ng/ml with tolerance for 1.0 mg AS comprised discordant responses. In 60 of 144 (42%) AS titrations discordant results were observed. Severe pulmonic, coronary, and aortic valvular heart disease, as well as old age, contributed to unusual AS sensitivity. Titration with AS clarified pharmacologic quantification of SDL by providing insight into optimum therapeutic glycoside dose.

Plasma concentration and urinary excretion kinetics of acetyl strophanthidin

The pharmacokinetics of acetyl strophanthidin (AS) were studied in dogs and human subjects by the use of a newly developed radioimmunoassay. This method has a sensitivity of 0.1 ng of AS per ml and is applicable to direct measurement of AS in unextracted plasma, urine, or bile. After administration of a single intravenous (i.v.) dose of 1.0 mg of AS to 17-25-kg mongrel dogs, the principal exponential decline of plasma AS concentration began 20-60 min after the injection and had a mean half-life (T ½ ) of 83 ± 19 min ( sd ). Mean total urinary excretion of AS was 13.3 ± 4.8% of the i.v. dose and occurred with a mean T ½ of 79 ± 10 min. Biliary excretion of AS accounted for only 1.5-2.1% of the i.v. dose. After i.v. administration of 1.0 mg of AS to seven human subjects, the principal exponential decline of plasma AS concentration began 10-30 min after the infusion and had a mean T ½ of 2.3 ± 0.2 hours. Urinary excretion of AS, studied in two patients, accounted for an average of 21.8% of the i.v. dose and occurred with a mean T ½ of 2.4 hours. Thus the plasma level T ½ of AS in human subjects is about tenfold shorter than the 22-hour T ½ previously observed for the relatively short-acting cardiac glycoside ouabain, in agreement with the known brief duration of pharmacologic effects of acetyl strophanthidin.

Effects of acute digitalization on the pulmonary blood volume in patients with heart disease

Basic hemodynamic data and the pulmonary blood volume (PBV) were determined in 23 patients before and after intravenous administration of acetyl strophanthidin during right and left heart catheterization. These patients were divided into two groups depending upon whether or not digitalization produced a significant fall in left ventricular end-diastolic pressure (LV D ). Group I consisted of 11 patients who manifested a significant decrease in LV D , while in group II no change was noted after digitalization. In both groups, a significant rise in LV stroke volume, stroke work, and maximal rate of rise of LV systolic pressure (dp/dt) was recorded. In group I, a significant decrease in the pulmonary vascular distending pressure (P d ) was accompanied by a decrease in PBV. In contrast, no change in P d or PBV was noted in the patients in group II. Changes in pulmonary vascular resistance (PVR) were inconsistent. It is concluded that: (1) in patients of group I the concordant decrease in P d and PBV represented a passive effect on the pulmonary vascular bed mediated by improved LV performance and decrease in LV D ; (2) in patients of group II, despite evidence of significant positive inotropic effects on the left ventricle, the failure of LV D to fall prevented a decrease in P d and PBV; (3) no evidence of active vasomotor effects of acetyl strophanthidin in the lung could be demonstrated. Changes in PVR were capricious and not apparently useful in assessment of the presence or absence of pulmonary vasomotor effects induced by digitalis. The results of this study differ from the findings of other investigators in cardiac patients and experimental animals. Some of the differences may be attributed to different experimental methods and to species variation in vascular reactivity.

Also, the findings in this report contrast with those previously reported for isoproterenol and aminophylline, agents that produce both positive inotropic effects on the heart and active vasomotion in the pulmonary circulation.

Reflex cardiovascular effects of epicardial stimulation by acetylstrophanthidin in dogs

Application of 25 to 100 µg of acetylstrophanthidin to the epicardium of the left ventricle of anesthetized and unanesthetized dogs caused hypotension and bradycardia without signs of discomfort. The response developed after an average latency of 8 seconds and lasted up to 12 minutes. Cooling the cervical vagi to 8 to 10°C or prior application of 0.1% procaine hydrochloride to the epicardium of the heart blocked the response. The response was, therefore, a reflex; the sensory receptors were located in the surface layers of the left ventricle. Electrophysiological recordings from single and multifiber preparations of the right recurrent cardiac nerve showed that the receptors for this reflex were mechanoreceptors whose fibers belonged to the C group. The sinus bradycardia caused by digitalis glycosides may result partly from excitation of these receptors.