Rapid detection of oleander poisoning using fluorescence polarization immunoassay for digitoxin. Effect of treatment with digoxin-specific Fab antibody fragment (ovine)

Poisoning from the oleander plant is common. Taking advantage of the high cross-reactivity of oleandrin, the major cardiac glycoside found in the oleander plant, we demonstrated that the serum digitoxin assay can be successfully used for the rapid diagnosis of oleander poisoning. Digitoxin is rarely used for treatment of cardiac disorders in the United States and has a therapeutic range of 19.7 to 39.3 nmol/L. In a typical oleander poisoning, serum oleandrin concentrations may reach 174 mmol/L or more. A serum specimen supplemented with 174 mmol/L of oleandrin containing no digitoxin showed an apparent digitoxin concentration of 1,272.1 nmol/L, a very high value compared with the range of the serum digitoxin assay, which is 2.6 to 104.8 nmol/L. Moreover, the response of the serum digitoxin assay with serum specimens containing various concentrations of oleandrin (and no digitoxin) is linear. Therefore, the oleandrin concentration in serum can be calculated from the apparent digitoxin concentration to access the severity of poisoning. Recently, the usefulness of the digoxin-specific Fab antibody fragment in the treatment of oleander poisoning has been described; however, no laboratory test was performed to demonstrate the progress of therapy. We demonstrated that the digoxin-specific Fab antibody can bind oleandrin in vitro, thus reducing the pharmacologically active free oleandrin. Because Fab and oleandrin bound to Fab are absent in the protein-free ultrafiltrates, monitoring the activity of free oleandrin in the ultrafiltrates can be used for monitoring the effectiveness of therapy.

Digoxin intoxication in a patient with end-stage renal disease: efficacy of digoxin-specific Fab antibody fragments and peritoneal dialysis

Digoxin intoxication is a serious medical problem, and impairment of renal function is a common risk factor for toxicity. Digoxin specific antibody fragments (Fab) is the most effective treatment available for severe digitalis intoxication. The use of Fab therapy in a patient with renal disease is considered as effective as in patients with normal renal function, although the increased risk of rebound digoxin toxicity mandates a longer period of observation. In patients with kidney failure, neither digoxin nor Fab can be removed efficiently from the systemic circulation by hemodialysis or continuous arteriovenous hemofiltration. Knowledge about the clearance of both compounds by peritoneal dialysis is limited. The authors describe a patient with end stage renal disease who was treated with Fab and peritoneal dialysis for life threatening digoxin intoxication. Like other forms of dialysis, peritoneal dialysis, even when performed in an intensive schedule, is not associated with an enhanced clearance of digoxin.

Pharmacokinetics of digoxin-specific Fab: effects of decreased renal function and age

AIMS

To study the influence of age and renal function on digoxin-specific Fab (DS-Fab) pharmacokinetics.

METHODS

Sixteen patients (35-91 years) with creatinine clearance ranging from 10.6 to 122.1 ml min(-1) who had been admitted to hospital with severe digoxin or digitoxin self-poisoning were treated with DS-Fab (80 to 800 mg). Plasma DS-Fab concentrations were determined by radioimmunoassay.

RESULTS

The mean (s.d.) distribution and elimination half-lives, apparent volume of distribution and total body clearance were 1.1 +/- 0.4 h, 20.2 +/- 7.3 h, 13.1 +/- 5.8 l, and 17.6 +/- 10.8 ml min(-1), respectively. Interindividual variability of DS-Fab total body clearance was linked linearly with the decrease in creatinine clearance or with the increase in age and DS-Fab distribution volume was not dependent on creatinine clearance or age.

CONCLUSIONS

The data suggest that DS-Fab should be given to elderly and renal-impaired patients at doses similar to those given to younger or normal renal function patients.

Isolation and characterization of a digoxin-like immunoreactive substance from human urine by affinity chromatography

A series of observations has suggested that one or more digoxin-like immunoreactive substances (DLIS) in biological fluids is able to cross-react with the antidigoxin antibody. Whether this substance is the endogenous inhibitor of Na+/K+ ATPase has not been well established. The aim of this study was to identify and characterize DLIS from human urine. Treated urine from healthy men was run on an affinity chromatography column at a flow rate of 1 mL/min in which the ligand was an antibody (antiserum) to digoxin. Eluates from affinity chromatography were applied onto analytical reversed-phase HPLC. The active material was eluted with a linear gradient of acetonitrile (from 350 to 650 mL/L) and water. A second step in HPLC was carried out isocratically with 280 mL/L acetonitrile in water. We found a single peak showing cross-reactivity with antidigoxin antibody as measured by RIA. It showed the same retention time as that of a digoxin calibrator. This highly purified substance is able to displace [3H]ouabain from dog kidney-derived Na+/K+ ATPase, to inhibit Na+/K+ ATPase activity as measured by the 86Rb+ uptake in red blood cells and by coupled enzyme assay. Our results are consistent with the hypothesis that DLIS, as isolated by this particular digoxin antibody, is a single substance and an inhibitor of Na+/K+ ATPase.

[A specific antidote for the treatment of digitalis poisoning in uremic patients]

Two chronic haemodialyzed patients with digitalis intoxication are reported. One of them took digoxin 0.25 mg three times daily for an unknown period and the other took digitoxin 0.1 mg twice daily for two weeks. The symptoms of intoxication were mainly concealed by uremic syndrome. The diagnosis was established by noticed sinus bradycardia, first- and second-degree atrioventricular block in ECG and the determination of sera levels of glycosides (serum digoxin concentration was 7.36 ng/ml, serum digitoxin concentration was 46.5 ng/ml) in both cases. Considering the probable long elimination period of digitalis and the potentially life-threatening situation the patients were given digoxin-specific antibody (Fab) fragments with potassium replacement therapy. The symptoms disappeared within a few hours after therapy, side effects and rebound toxicity did not develop. In connection with these cases the aim of this report is to publish a method which can reverse the life-threatening digitalis intoxication in patients suffering from renal failure as well. As to the above method, the authors have not found any similar case reports in the Hungarian medical literature.

Role of ouabain-like compound in the rostral ventrolateral medulla in rats

To determine whether ouabain-like compound (OLC) exerts modulatory influences on the activity of vasomotor neurons in the rostral ventrolateral medulla (RVLM), we examined the effects of microinjecting ouabain, digoxin-specific antibody Fab fragments, and mAb against ouabain on the rat RVLM. Microinjection of ouabain into the unilateral RVLM of anesthetized normotensive rats elicited dose-dependent increases in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). The pressor and sympathoexcitatory effects of ouabain in the RVLM were reversed by microinjections of an M2 muscarinic antagonist, gallamine, or digoxin-specific antibody Fab fragments. Furthermore, a prior microinjection in the RVLM of gallamine, digoxinspecific antibody Fab fragments, or kainic acid or intravenous injection of hexamethonium all prevented the pressor and sympathoexcitatory effects induced by a subsequent microinjection of ouabain. Microinjections of either digoxinspecific antibody Fab fragments or gallamine per se significantly decreased baseline MAP and RSNA. Injection of digoxin-specific antibody Fab fragments attenuated the effects of a subsequent injection of gallamine. Microinjection of mAb against ouabain, but not nonspecific IgG, also significantly decreased baseline MAP and RSNA. These results suggest that OLC in the RVLM contributes to the tonic activity of vasomotor neurons in anesthetized normotensive rats, and the action of OLC in the RVLM is at least partly mediated by M2 muscarinic mechanisms.

Effects of digoxinlike immunoreactive substances and digoxin FAB antibodies on the new digoxin microparticle enzyme immunoassay

Digoxin-like immunoreactive substance (DLIS) is known to interfere with fluorescence polarization immunoassay (FPIA) (Digoxin II, Abbott Laboratories) and falsely elevates the total digoxin concentrations. Digoxin FAB antibody (Digibind) is also known to affect digoxin results by FPIA assay. The authors studied the effects of DLIS and Digibind on a new microparticle enzyme immunoassay (MEIA) for digoxin recently introduced by Abbott Laboratories, compared with the standard FPIA method and chemiluminescence assay (ACS-digoxin, Ciba-Corning). They studied 30 volume-expanded patients (term pregnancy, liver and renal disease) for the presence of DLIS. None of these patients received digoxin. They observed measurable DLIS concentrations in 12 of 30 patients by the FPIA assay and in only 1 patient by both MEIA and ACS assays. The concentration of DLIS in that patient was 0.31 ng/ml of digoxin equivalent by the MEIA assay, 0.36 ng/ml by the ACS assay, and 1.15 ng/ml by the FPIA assay. When they supplemented serum containing digoxin with low to high concentrations of digibind (0.5, 1.0, 2.0 and 4.0, 10, and 20 micrograms/ml), and measured digoxin concentrations by FPIA, MEIA, and ACS assays, they observed lower than expected values of total digoxin. However, when they supplemented serum containing no digoxin with high concentration of digibind (5.0, 10.0 and 20.0 micrograms/ml) and supplemented protein-free ultrafiltrates with digoxin, they observed expected digoxin concentrations in the ultrafiltrates by all three assays, indicating that the ultrafiltrates are essentially free of digibind.

Hemodynamic effects of the Fab fragment of digoxin antibody (digibind) in corticotropin (ACTH)-induced hypertension

To determine whether the immune Fab fragment of digoxin antibody (digibind) attenuates established corticotropin (ACTH) hypertension, rats were given ACTH sham control (0.1 mL normal saline, twice daily, subcutaneously, n = 18) or ACTH treatment (2.5 microg/kg in 0.1 mL normal saline, twice daily, subcutaneously; n = 27) for 10 days. Acute hemodynamic effects of digibind (30 mg/kg, intravenous bolus injection) were examined after 10 days of sham control or ACTH treatment. Rats were divided into 7 groups: digibind (30 mg/kg, in 1 mL 0.9% NaCl intravenous bolus injection) plus sham (n = 6) or ACTH (n = 8), sham digibind (1 mL 0.9% NaCl intravenous bolus) plus ACTH (n = 7), digibind vehicle (sorbitol 1.8 mg in 1 mL 0.9% NaCl, intravenous bolus) plus sham (n = 6), preimmune sheep IgG (30 mg/kg in 1 mL 0.9% NaCl intravenous bolus) plus sham (n = 6) or ACTH (n = 6) and preimmune sheep IgG (Fab)2 fragment (30 mg/kg in 1 mL 0.9% NaCl intravenous bolus) plus ACTH (n = 6). ACTH increased systolic blood pressure (SBP) from 118 +/- 2 to 132 +/- 3 mm Hg on treatment day 10. BP was unchanged in sham treated rats. The acute administration of digibind decreased MAP (-14 +/- 3 mm Hg, P <.001) in ACTH hypertensive rats, but not in ACTH sham control normotensive rats (+2 +/- 3 mm Hg). Blood pressure reached a minimum after 14 +/- 3 min and the effect lasted more than 30 min. No significant change of blood pressure was found in ACTH treated rats receiving sham (0.9% NaCl) digibind injection (+2 +/- 2 mm Hg). However, both preimmune sheep IgG and IgG (Fab)2 fragment caused a decrease of blood pressure in both sham or ACTH treated rats. Although these data that digibind decreases BP in ACTH but not sham treated rats are consistent with the notion that digitalis-like substances may play a role in ACTH induced hypertension, the evidence that both preimmune sheep IgG and IgG (Fab)2 fragments also decreased blood pressure in rats suggests caution in interpretation of studies that employ digibind preparations.

In vitro scanning saturation mutagenesis of an antibody binding pocket

We have combined PCR mutagenesis with in vitro transcription/translation and ELISA for the rapid generation and characterization of antibody mutants. The PCR products are used directly as the template for the in vitro transcription/translation reactions and because no cloning steps are required, the in vitro saturation mutagenesis of one residue can be completed in duplicate within a week by a single investigator. In vitro scanning saturation mutagenesis was used to analyze the role and plasticity of six key contact residues (H:Tyr-33, H:Asn-35, H:Tyr-50, H:Trp-100, L:Val-94, and L:Pro-96) in the binding pocket of a single chain Fv antibody derived from the 26-10 monoclonal antibody. A total of 114 mutant antibodies were produced; all 19 substitutions at each of the 6 chosen positions. The mutants were analyzed for binding to digoxin, digitoxin, digoxigenin, and ouabain resulting in the generation of a comprehensive data base of 456 relative affinity values. Excellent agreement between the relative affinity values obtained with in vitro synthesized mutant antibodies and equilibrium affinity data obtained with previously reported purified mutant monoclonal antibodies was observed. Approximately 75% of the single amino acid mutants exhibited significant binding to one or more of the digoxin analogs. Mutations that alter and, in some cases, reverse specificity for the different digoxin analogs were identified. In vitro scanning saturation mutagenesis represents a new tool for protein structure-function and engineering studies and can be interfaced with laboratory automation so that an even higher throughput of protein mutants can be constructed and analyzed.

Treatment of toad venom poisoning with digoxin-specific Fab fragments

Toxicity from toad venom poisoning is similar to digoxin toxicity and carries a high mortality rate. We report on six previously healthy men who developed vomiting and bradycardia after ingesting a purported topical aphrodisiac. Each patient had positive apparent digoxin levels and the first four patients died of cardiac dysrhythmias. The last two patients recovered following treatment with digoxin Fab fragments. We analyzed samples of the purported aphrodisiac and found that it was identical to Chan Su, a Chinese medication made from toad venom. To our knowledge, this is the first reported use of digoxin Fab fragments to treat toad venom poisoning.

Functional antagonism by a monoclonal antibody to digoxin in a test system of cultured rat heart myocytes

A monoclonal antibody with a high affinity for digitoxin (KA = 0.50 nM) and digoxin (KA = 0.55 nM) was produced by somatic cell fusion. This antibody, designated 2A3(47), displayed little cross reactivity with other glycosides. In cultured rat heart myocytes, 2A3(47), antagonized the positive chronotropic effect exerted by digitoxin but did not alter that of ouabain. Our results suggest that this monoclonal antibody may prove to be useful in treating digoxin and digitoxin intoxication.

Interspecies scaling of clearance and volume of distribution for digoxin-specific Fab

Digoxin-specific Fab are recognized to be effective in the treatment of acute cardiac glycoside poisoning but no pharmacokinetic studies have been performed in human volunteers. We thus propose an allometric approach among three mammalian species to predict Fab pharmacokinetic parameters in humans. Plasma disposition of digoxin-specific Fab was studied at a 20 mg/kg i.v. dose in mice, rats, and rabbits. Fab plasma concentration was determined by a sensitive and specific radioimmunoassay. Allometric equations showed that the pharmacokinetic parameters (distribution volumes (Vc (ml) = 0.084 W0.96; Vdss (ml) = 0.24 W0.96; Vd beta (ml) = 0.55 W0.96, r2 = 1), total body clearance (Cltot (ml/hr) = 0.61 W0.67, r2 = 0.999), and terminal half-life (t 1/2 beta (hr) = 0.63 W0.29) correlated with body weight. The Fab plasma concentration-time data plotted as a complex Dedrick relationship were superimposable. Using these allometric techniques, Vdss, Vd beta, Cltot and t 1/2 beta were calculated as 10.75 liter, 24.64 liter, 17.9 ml/min, and 16 hr, respectively, for a human subject of 70 kg body weight. These values are in accordance with those previously described in digoxin-Fab-treated patients (body weight = 61 +/- 3 kg, Vd beta = 24.9 +/- 3. 7 liter; Cltot = 20.8 +/- 2.1 ml/min; t 1/2 beta = 14.3 +/- 1.8 hr). Results indicate that the primary Fab pharmacokinetic parameters can be reasonably estimated in man using pharmacokinetic data from three animal species.

Dose-dependent reversal of digoxin-inhibited activity of an in vitro Na+K+ATPase model by digoxin-specific antibody

We investigated the potency of digoxin-specific Fab fragments to reverse digoxin-induced Na+K+ATPase inhibition in rat brain microsomes according to (a) the extent of initial inhibition of Na+K+ATPase and (b) the neutralizing dose of antibody. Mathematical analysis of the digoxin concentration-Na+K+ATPase inhibition curve supports the existence of 2 digoxin sensitive Na+K+ATPase isoforms. The IC50 was 1.3 x 10(-4) M and 2.5 x 10(-8) M for the low (alpha 1) and high (alpha 2) digoxin affinity isoenzyme, respectively. The reversal of digoxin-induced Na+K+ATPase inhibition was dependent on the digoxin-specific Fab concentration. The maximal effect was observed when the Fab:digoxin ratio was stoichiometrical and addition of an excess of antibodies did not result in a complete reversal of inhibition at the 4 digoxin concentrations studied. This simple and rapid in vitro model will be a useful tool to predict the efficacy of a new generation of antibodies.

Deglycosylated products of endogenous digoxin-like immunoreactive factor in mammalian tissue

Digoxin-like immunoreactive factor (DLIF) from adrenal cortex is an endogenous molecule with structural features remarkably similar to those of digoxin, a plant-derived cardiac glycoside (Shaikh, I. M., Lau, B. W. C., Siegfried, B. A., and Valdes, R., Jr. (1991) J. Biol. Chem. 266, 13672-13678). Two characteristic structural and functional features of digoxin are a lactone ring and three digitoxose sugars attached to a steroid nucleus. Digoxin is known to undergo deglycosylation during metabolism in humans. We now demonstrate the existence of several naturally occurring deglycosylated components of DLIF in human serum. The components are identified as DLIF-genin, DLIF-mono, and DLIF-bis, corresponding to the aglycone, and the aglycone with one and two sugars, respectively. Similar components are produced by acid-induced deglycosylation of DLIF isolated from bovine adrenal cortex. The elution pattern and sequence of DLIF-deglycosylation was identical to that of digoxin suggesting identical sugar stoichiometry. However, analysis of these newly discovered congeners by reverse-phase chromatography, spectrophotometry, antibody reactivity, and kinetics of deglycosylation, demonstrates that subtle structural and physical differences do exist when compared to digoxin. DLIF was chromatographically distinct from digoxin, and interestingly, the mobility of the DLIF-genin was shifted toward increased polarity relative to digoxigenin. DLIF and DLIF-bis, -mono, and -genin congeners have absorbance maxima at 216 nm, whereas digoxin and its congeners absorb at 220 nm. Reaction with specific antibodies directed at the lactone portion of these molecules shows DLIF and its deglycosylated congeners to be 10(3)-fold less reactive than digoxin. Kinetics of sugar removal suggests that DLIF is 8-fold more susceptible to deglycosylation than is digoxin. Two less polar DLIF components produced from the DLIF-genin have lambdamax at 196 nm and are 4-fold less immunoreactive than DLIF. Our data suggest that subtle structural differences exist between DLIF and digoxin at or near the lactone ring as well as in the nature of the sugars. The presence of deglycosylated congeners of DLIF in human serum, including the less polar components, suggests in vivo deglycosylation of these factors. This is the first demonstration of the existence of naturally occurring deglycosylated derivatives of DLIF and establishes the likelihood of active metabolism of DLIF in mammals.

Simultaneous isolation of endogenous digoxin-like immunoreactive factor, ouabain-like factor, and deglycosylated congeners from mammalian tissues

DLIF (digoxin-like immunoreactive factor) and OLF (ouabain-like factor) are endogenous steroid-like ligands (approximately 781 and 595 Da, respectively) with molecular and structural properties similar to the plant-derived cardiac glycosides, digoxin and ouabain. We developed a purification method with a sufficiently wide range of extraction solubility to separate compounds with polarities spanning those of ouabain and digoxin. This technique provides a rapid, reliable, and efficient method for simultaneously isolating DLIF, OLF, and several naturally existing deglycosylated congeners, including three deglycosylated species of DLIF (DLIF-genin, DLIF-mono, and DLIF-bis) and one deglycosylated species of OLF (OLF-genin). Separation is achieved using acid extraction, C-18 reverse-phase HPLC chromatography, and signal detection using two antibodies, one specific for digoxin and one for ouabain. The average extraction efficiency is 400 pmol digoxin equivalent (range 300-500) and 42 pmol ouabain equivalent (range 37-50) per gram of adrenal cortex for DLIF and OLF, respectively. The relative molar immunoreactivity of DLIF is 10(3)-fold less than that of digoxin, whereas that of OLF is unity compared to ouabain, suggesting that OLF is structurally more similar to ouabain than DLIF is to digoxin. Of interest is the presence of a compound reacting with both digoxin and ouabain antibodies. This unique immunoreactive species is liekly to have structural similarity to both digoxin and ouabain and thus may represent a metabolic link between DLIF and OLF.

[Severe digoxin intoxication in a 15-year-old girl treated with Fab antidigoxin]

A 15-year-old-female admitted after ingesting 5 milligrams of digoxin, presented atrial tachycardia with 2.0 degree atrioventricular block and frequent ventricular premature complexes. Serum digoxin determination at admission was 16 ng/ml. Two hours following the administration of 2 amp of Fab antidigoxin (160 milligrams) the arrhythmias disappeared and remained asymptomatic until discharge.

The affinity of binding of digoxin to ovine anti-digoxin Fab (DIGIBIND) preparations

A fundamental study was performed to assess the binding characteristics of an ovine anti-digoxin-Fab preparations, DIGIBIND, for digoxin, using equilibrium dialysis. The average mass of the antibody fragment was determined by matrix-assisted laser-desorption mass spectrometry to be 46,200 Da. The main conclusions are that the binding affinity and capacity are similar for several batches, produced over a period of several years. The average median affinity for digoxin of five batches determined by Sips analysis was 3.4 x 10(11) M-1 (range 1.95-6.15 x 10(11) M-1) (corresponding to dissociation constants, Kd, in the range 1.63-5.13 pM), and the index of heterogeneity was 0.78 +/- 0.03.

Effect of antibody specificity on results of selected digoxin immunoassays among various clinical groups

We examined the specificity of three automated digoxin immunoassays (Abbott TDxFLx Digoxin II assay, Baxter-Dade Stratus II Digoxin assay, and Ciba Corning ACS Digoxin assay) applied without modification to (a) sera from 229 digoxin-free patients in 12 cohorts associated with nonspecific or endogenous digoxin-like immunoreactive factor (DLIF) interference, and (b) drug-free serum supplemented with the major metabolites and analogs of digoxin. We observed three patterns of apparent digoxin results among the DLIF samples: one common to kidney and liver failure patients, where TDx and Stratus assays showed significant positive results; one common to newborns and cord blood, where only the TDx assay had significant interference; and one from cardiac surgery patients, where the Stratus assay alone showed interference. Of the three assays, the ACS had the least interference from DLIF. The assays also behaved differently with respect to cross-reactivity with digoxin metabolites, digitoxin, and digitoxin metabolites. The ACS assay again had the least analog or metabolite cross-reactivity. The three methods agreed well on digoxin-positive specimens, with a mean bias of <0.15 microgram/L digoxin for each and discrepancies (defined as >3 SD between the assay pairs compared) of only 3-5%.

Plasma marinobufagenin-like and ouabain-like immunoreactivity during saline volume expansion in anesthetized dogs

OBJECTIVES

This study investigated effects of acute plasma volume expansion on plasma levels and urinary output of two endogenous Na,K-ATPase inhibitors, marinobufagenin-like and ouabain-like immunoreactive substances.

METHODS

Plasma volume was expanded for 3 h via intravenous saline infusion in three groups of anesthetized dogs--nontreated (n = 5); pretreated with rabbit antidigoxin (n = 5); and pretreated with rabbit antimouse (control) antibody (n = 4).

RESULTS

Plasma marinobufagenin-like immunoreactivity increased to 11.87 +/- 3.16 nmol.l-1 (vs. 0.30 +/- 0.16 nmol.l-1) within 10 min of volume expansion, in parallel with a 15% increase in LVdP/dt, then decreased to 2.21 +/- 0.59 nmol.l-1, and in 90 min increased to 11.8 +/- 2.8 nmol.l-1, in parallel with the maximal natriuretic response. Plasma concentrations of ouabain-like immunoreactive material were increased after 90 min of saline infusion (0.019 +/- 0.004 nmol.l-1 vs. 0.139 +/- 0.056 nmol.l-1). Pretreatment of the animals with antidigoxin antibody blocked the positive inotropic and reduced natriuretic response to volume expansion, and decreased the urinary release of marinobufagenin-like, but not ouabain-like, material.

CONCLUSIONS

These results show the presence of marinobufagenin-like immunoreactive substance in dog plasma and suggest that mammalian EDLF may have a bufodienolide nature. Endogenous marinobufagenin-like immunoreactive substance, which is likely to cross-react with antidigoxin antibody, is involved in the natriuretic and positive inotropic responses to plasma volume expansion.

Role of endogenous cardiac glycosides in the spontaneously hypertensive rat--antagonism by active immunization

The effects of simultaneous active immunization against two cardiac glycoside drugs, digoxin and proscillaridin, have been examined in young spontaneously hypertensive and Wistar-Kyoto rats. Control animals were immunized with protein carrier only. Animals were studied from 5 weeks to 13 weeks of age. Effectiveness of immunization to produce antibody responses was assessed at the end of the study by estimating the titer of antibodies in plasma against both of the antigens. Robust antibody responses were obtained. Immunization had no effect on the normal growth of these animals. Further, immunization against cardiac glycosides did not change blood pressure in either strain of animals. Blood pressure in the SHR increased as anticipated as the weanling animals grew to maturity. These studies indicate that active immunization against cardiac glycosides does not alter blood pressure in the SHR in spite of strong evidence for increased levels of endogenous cardiac glycosides in this strain.

Reversal of sodium pump inhibitor induced vascular smooth muscle contraction with digibind. Stoichiometry and its implications

The possibility that a circulating sodium pump inhibitor contributes to the pathogenesis of volume-dependent hypertension via an action on vascular smooth muscle (VSM) is supported by multiple lines of investigation, but remains controversial. We had two goals in this study. The first was to compare the pattern of contractile response of rabbit aorta induced by two candidates, ouabain and a labile sodium pump inhibitor that we have identified in the peritoneal dialysate of volume-expanded hypertensive patients with chronic renal failure. Our second goal was to examine the ability of Digibind, a Fab fragment of antisera directed against digoxin, to reverse VSM contraction induced by both agents. Ouabain induced a concentration-dependent contraction, which was delayed in onset, was gradual, and reached a stable plateau after many hours. The labile sodium pump inhibitor induced a qualitatively similar series of responses. Digibind rapidly reversed the contractile responses to both sodium pump inhibitors, with a rate of relaxation that matched that induced by physical removal of the pump inhibitor from the bath. For ouabain, the Digibind:ouabain stoichiometry was highly predictable. When Digibind was present in a molar concentration equivalent to that of ouabain, or less, it had no effect. When the Digibind concentration was twice that of ouabain, complete relaxation occurred. Although the concentration:VSM response relationship for ouabain was steep, the concentration:effect interaction with Digibind was even more steep. The molar concentration of Digibind required to reverse the effects of the labile endogenous inhibitor from peritoneal dialysate was consistently lower than that for ouabain, which is compatible with either greater potency of the labile factor in VSM or greater affinity for Digibind. These findings are compatible with a role for one or more endogenous sodium pump inhibitors as the determinant of vascular smooth muscle tone in the volume-sensitive hypertension of renal disease.