An in-depth look into a well-known herbal drug: Fingerprinting, isolation, identification, and content estimation of saponins in different Strophanthus seeds

Seeds of Strophanthus species are known as a source of rapid-acting cardenolides. These water-soluble glycosides are listed as the sole critical constituents of this raw herbal drug. A non-standard cardioprotective medication with ouabain-containing oral remedies has become popular in Europe as a result of the withdrawal of corresponding registered drugs from the market. However, the bioequivalence of pure ouabain solutions, tinctures, and home-made extracts from Strophanthus seeds is unknown. Thus, this study aimed to update the information on the composition of Strophanthus seeds used for this purpose. The distribution of two main saponins and about 90 previously unreported compounds, tentatively identified as saponins in eleven Strophanthus species, was systematically evaluated by ultra-high-performance liquid chromatography mass spectrometry (UHPLC-MS) and -MS/MS. Seeds of S. gratus were selected to isolate the dominant unreported triterpenoids, bidesmosides of echinocystic and oleanolic acid. Their structures were established by HRMS, MS/MS, as well as by NMR techniques. The total saponin content, estimated by UHPLC-MS, was up to 1%. The detected saponins could influence the peroral bioavailability of hardly absorbable Strophanthus cardenolides and exhibit their own activity. This finding may be relevant when Strophanthus preparations (containing both saponins and cardiac glycosides) are used, particularly when homemade preparations are administered.

Specificity and reversibility of the training effects on the concentration of Na+, K+-ATPase in foal skeletal muscle

The purpose of the present study was to determine whether training and detraining affect the Na+,K+-ATPase concentration in horse skeletal muscles, and whether these effects are specific for the muscles involved in the training programme. Twenty-four Dutch Warmblood foals age 7 days were assigned randomly to 3 groups: Box (box-rest without training), Training (box-rest with training: short-sprint) and Pasture (pasture without training). Exercise regimens were carried out for 5 months and were followed by 6 months of detraining. Five of the foals in each group were subjected to euthanasia at age 5 months and the remaining foals at 11 months. Muscle samples were collected from the deep part of the gluteus medius, semitendinosus and masseter muscles. The Na+,K+-ATPase concentration was quantified by [3H]ouabain binding. In the Training group, the concentration of Na+,K+-ATPase in gluteus medius and semitendinosus muscle, but not in masseter muscle, showed a relative increase of 20% (P<0.05) as compared to Box foals. After detraining for the subsequent 6 months, the concentration of Na+,K+-ATPase in semitendinosus muscle remained the same, while that in gluteus medius muscle was reduced by 10%. It is concluded that: 1) short-sprint training for 5 months induced an increase of the Na+,K+-ATPase concentration in gluteus medius and semitendinosus muscles of the foal. Interestingly, this effect persisted during the 6 months of the detraining period. Whether the higher Na+,K+-ATPase concentration due to training of young foals leads to a better athletic performance when they become mature still needs to be established; 2) the factors that initiate an increase in Na+,K+-ATPase concentration following training are likely to be located in the muscle itself and 3) the training effect may last for several months after returning to normal activity, especially in muscles containing a high percentage of fast-twitch fibres.

[Comparison between anti-ouabain egg yolk(IgY) and rabbit antibody(IgG) in enzyme-linked immunosorbent assay]

AIM

To improve specificity and accuracy of endogenous ouabain measurement assay.

METHODS

Anti-ouabain polyclonal antibody egg yolk (IgY) and anti-ouabain rabbit antibody (IgG) were prepared respectively. In the presence of two kinds of antibody, then the specificity and accuracy of enzyme-linked immunosorbent assay (ELISA) were compared.

RESULTS

The ELISA, in the presence of IgY, provided a sensitivity of the average intraassay coefficient of variation(CV) was 2.03%, and the inter-assay CV was 2.34% respectively. In contrast, IgG were 2.83% and 3.29%. No significant interferences were observed with hydrocortisone and dexamethasone. There was 3.45% vs. 5.95%, 3.20% vs. 5.20% of crossreaction with cedilanid and digoxin.

CONCLUSION

The specificity and accuracy of ELISA, in which IgY was used, were more better than IgG.

[Preparation and application of anti-ouabain IgY antibody]

OBJECTIVE

To prepare highly specific anti-ouabain polyclonal antibody for detecting endogenous ouabain in tissues.

METHODS

Ouabain-BSA compound was used to immunize hens, and the eggs were collected one week after the first immunization. The IgY antibodies in the egg yolk were separated and purified by PEG-6000 Method, and analyzed by 12% SDS-PAGE and enzyme-linked immunosorbent assay (ELISA) for titration. The IgY antibodies obtained were applied subsequently in ELISA and immunohistochemistry.

RESULTS

The IgY titer increased rapidly after the second immunization, with the highest titer of 1:10240 that lasted for at least 4 weeks. Competitive ELISA for IgY detection showed an average intraassay coefficient of variation (CV) of 2.03% and an inter-assay CV of 2.34%. Immunohistochemistry visualized the location of the endogenous ouabain mainly in the cytoplasm of the zona reticularis of rat adrenal cortex.

CONCLUSION

Immunization of hens allows efficient preparation of IgY antibody which can be used in routine immunoassays.

Organ hypertrophic signaling within caveolae membrane subdomains triggered by ouabain and antagonized by PST 2238

In addition to inhibition of the Na-K ATPase, ouabain activates a signal transduction function, triggering growth and proliferation of cultured cells even at nanomolar concentrations. An isomer of ouabain (EO) circulates in mammalians at subnanomolar concentrations, and increased levels are associated with cardiac hypertrophy and hypertension. We present here a study of cardiac and renal hypertrophy induced by ouabain infused into rats for prolonged periods and relate this effect to the recently described ouabain-induced activation of the Src-EGFr-ERK signaling pathway. Ouabain infusion into rats (15 microg/kg/day for 18 weeks) doubled plasma ouabain levels from 0.3 to 0.7 nm and increased blood pressure by 20 mm Hg (p < 0.001), cardiac left ventricle (+11%, p < 0.05), and kidney weight (+9%, p < 0.01). These effects in vivo are associated with a significant enrichment of alpha1, beta1, gammaa Na-K ATPase subunits together with Src and EGFr in isolated renal caveolae membranes and activation of ERK1/2. In caveolae, direct Na-K ATPase/Src interactions can be demonstrated by co-immunoprecipitation. The interaction is amplified by ouabain, at a high affinity binding site, detectable in caveolae but not in total rat renal membranes. The high affinity site for ouabain is associated with Src-dependent tyrosine phosphorylation of rat alpha1 Na-K ATPase. The antihypertensive compound, PST 2238, antagonized all ouabain-induced effects at 10 microg/kg/day in vivo or 10(-10)-10(-8) m in vitro. These findings provide a molecular mechanism for the in vivo pro-hypertrophic and hypertensinogenic activity of ouabain, or by analogy those of EO in humans. They also explain the pharmacological basis for PST 2238 treatment.

Identification of endogenous ouabain in culture supernatant of PC12 cells

OBJECTIVE

Ouabain-like factor (OLF), assayed as ouabain-like immunoreactivity (OLI), is thought to represent an endogenous digitalis-like factor. We found increased plasma OLI during the surgical removal of a pheochromocytoma. The elution volume of the OLI extracted from plasma and the pheochromocytoma tissue was the same as that for authentic ouabain, using reverse phase high-performance liquid chromatography. The present study was performed to characterize OLF from the culture supernatant of a rat pheochromocytoma cell line, PC12 cells.

DESIGN

OLI from culture supernatant and chromatographic fractions were assayed by a sensitive enzyme-linked immunosorbent assay for ouabain. PC12 cells, subcultured in RPMI 1640 with 10% horse serum and 5% fetal bovine serum, were washed, and then cultured in Iscove's modified Dulbecco's medium (Life Technologies, Rockville, Maryland, USA) with 0.4% bovine serum albumin (without serum). Progesterone was added to augment the production or secretion of OLI. The conditioned medium was acidified to dissociate the binding protein, and OLI was purified by five steps of octadecylsilane (ODS) column chromatography. The structural identity of this OLI was determined by liquid chromatography and mass spectrometry (LC/MS).

RESULTS

OLI in the culture medium increased after addition of progesterone in a dose-dependent manner. The concentration in the culture medium was approximately double of that in homogenized PC12 cells. After five rounds of ODS column chromatography, approximately 100 ng of OLI was purified from 21 of culture supernatant, without fetal calf serum, in the presence of progesterone. The molecular size of purified OLI was found to be identical to authentic ouabain, based on analysis by LC/ MS.

CONCLUSION

Mammalian cells originating from a rat pheochromocytoma cell line were found to produce and/or secrete OLF by the addition of progesterone.

Liquid chromatography mass spectrometric analysis of ouabainlike factor in biological fluid

Ouabainlike factor (OLF), assayed as ouabainlike immunoreactivity (OLI), is a probable endogenous digitalislike factor (EDLF). Liquid chromatography/mass spectrometry (LC/MS) is one of the most highly sensitive tools for obtaining structural information regarding low-molecular weight materials in a target compound, and to measure the concentrations of these materials. We have previously reported that OLI can be isolated from the culture supernatant of the rat pheochromocytoma cell line, PC12, by several reverse-phase chromatography and LC/MS techniques. The present study was performed to characterize OLF from biological fluids such as plasma and culture supernatant of PC12 cells by LC/MS. The previous applications of LC/MS to OLI in plasma have been limited to structural identification at the final stages of isolation, in which the starting volume of plasma has been over 10 I. In the present study, we tried to minimize the volume of plasma, and to develop a new preclearing step to gain adequate LC/MS characterization using MS/MS analysis. The plasma was acidified, and OLI was purified by ODS column chromatography. OLI in chromatographic fractions from plasma was assayed by a sensitive enzyme-linked immunosorbent assay for ouabain. After Sep-Pak treatment and two rounds of ODS column chromatography, OLI was identified from 80 ml of plasma. The structure of the purified OLI was identical to authentic ouabain and digoxin, as assessed by LC/MS. In conclusion, we identified the chemically or structurally clarified ouabain and digoxin as the circulating form in plasma by LC/MS.

A brain Na+, K+-ATPase inhibitor (endobain E) enhances norepinephrine release in rat hypothalamus

We have shown that synaptosomal membrane Na+, K+-ATPase activity is stimulated or inhibited by norepinephrine according to the presence or absence of a brain soluble fraction. Gel filtration of such soluble fraction has allowed the separation of two fractions, peaks I and II, able to stimulate and inhibit Na+, K+-ATPase activity, respectively. Peak II behaves much like ouabain, which has suggested the term endobain. From peak II, a subfraction termed II-E (endobain E), which highly inhibits Na+, K+-ATPase, has been separated by anionic exchange chromatography in a Synchropack AX-300 column. We determined the in vitro effect of endobain E obtained from rat cerebral cortex on neuronal norepinephrine release by incubating rat hypothalamic tissue in the presence of [3H]norepinephrine. Neuronal norepinephrine release was quantified as the factor above basal [3H]norepinephrine released to the medium at experimental and three post-experimental periods. Endobain E was found to increase norepinephrine release in a concentration-dependent fashion, reaching 200%, equivalent to the effect achieved with 400 microM ouabain. Ouabain effect persisted along three post-experimental periods whereas that of endobain E remained only during the first post-experimental period. These results led us to conclude that endobain increases norepinephrine release in hypothalamic neurons at the presynaptic nerve ending level, an effect resembling that of ouabain. It is postulated that endobain E may enhance catecholamine availability in the synaptic gap, leading to an increase in noradrenergic activity.

Ouabain-like immunoreactivity in the medulla oblongata of rats

An isomer of ouabain, the ouabain-like compound (OI,C), may participate in the regulation of body fluid volume and vascular tone. Forebrain regions, especially the hypothalamus, are reported to be sites of OLC action in the central nervous system. The medulla oblongata is another critical area involved in central cardiovascular regulation. We reported that the microinjection of either monoclonal antibody to ouabain T8B11 or Fab fragment of digoxin-specific antibody into the rostral ventrolateral medulla significantly decreased mean arterial pressure and renal sympathetic nerve activity in anesthetized normotensive rats (TERUYA et al.: J. Clin. Invest. 99: 2791-2798, 1997). Using T8B11, we examined the ouabain-like immunoreactivity in the medulla oblongata of normotensive rats. In periodate-lysine-paraformaldehyde fixed tissues, ouabain-like immunoreactive neurons were detected in the nuclei and regions in the medulla oblongata including the ventrolateral medulla, ventromedial medulla, nucleus ambiguus, caudal raphe nuclei, nucleus of solitary tract, and dorsal motor nucleus of the vagus. When an Fab fragment of digoxin-specific antibody was used as a first antibody, the digoxin-like immunoreactive neurons were distributed in almost the same pattern as those observed with the use of T8B11. In the brain fixed with the "three-step" procedure developed by YAMADA et al. (1987), which was used in a previous ouabain immunohistochemical study of the hypothalamus, ouabain-like immunoreactivity in the medulla oblongata was much weaker in intensity and less restricted in distribution than that in the hypothalamus. These findings suggest that ouabain-like immunoreactivities are present in the medulla oblongata with a manner of distribution different from that seen in the hypothalamus. Some ouabain-immunopositive nuclei and regions in the medulla oblongata, especially the rostral ventrolateral medulla, may be other OLC action sites.

Endogenous ouabain secretion in man is not regulated by ACTH

It has been suggested that endogenous ouabain-like substance (OLS) is of adrenal origin and the secretion of OLS may be ACTH dependent. To determine if OLS is influenced by the pituitary-adrenal axis, we studied the effect of adrenal stimulation (0.25 mg Synacthen) and suppression (1 mg Dexamethasone) on two separate groups of nine subjects. Serum OLS was measured by a radioimmunoassay (RIA) developed in our lab, and cortisol and ACTH were measured by commercial assay kits. Dexamethasone significantly (P< 0.001) suppressed serum cortisol and ACTH concentrations, without effecting endogenous OLS concentration (0.64+/-0.17 vs 0.85+/-0.18nmol/l). Synacthen increased the concentration of cortisol in serum (p < 0.001) over the test period; OLS concentration, again, remained unchanged (0.45+/-0.04 vs 0.43+/-0.05 nmol/l). In further studies, serum concentrations of cortisol and OLS were compared between left (LAV) and right (RAV) adrenal veins with that from the inferior vena cava (IVC). Concentration of cortisol in the LAV and RAV was five-fold greater than that in IVC. However, there was no difference in OLS concentration at the corresponding sites. In addition, serum OLS concentrations in patients having undergone bilateral adrenalectomy or diagnosed with Addison's disease (0.62+/-0.19 nmol/l) were similar to concentrations in healthy subjects (0.67+/-0.21 nmol/l). Examination of bovine adrenal, liver, kidney, heart and human placenta demonstrated that OLS content of bovine adrenal was comparable with other tissues analysed. HPLC studies of human serum and bovine adrenal gland produced identical elution profiles, resolving a single peak which coincided with the retention time observed for standard ouabain. We conclude that the adrenal is unlikely to be the source of endogenous OLS, the secretion of which appears to be independent of ACTH.

[Increased concentration of Na,K-pumps in skeletal muscles of patients with chronic obstructive lung disease. Significance of magnesium depletion and treatment with glucocorticoids]

Patients with COLD may develop Mg depletion due to inadequate nutrition or treatment with diuretics and beta 2-agonists. In 36 consecutive COLD patients skeletal muscle concentrations of Mg and K were reduced by 22% and 14%, respectively, compared to 23 age- and sex-matched controls (p < 0.001). Patients receiving diuretics showed a further reduction of muscle Mg (-31%) and K (-27%) compared to controls. The mean concentration of Na,K pumps was increased by 31% (p < 0.001), while a more pronounced increase (+61%) was seen in 12 intensive care patients receiving high dosages of glucocorticoids. Thus muscle concentrations of Mg and K are reduced in COLD patients and are associated with an upregulation of the Na,K-pump concentration. It is plausible that this upregulation may be caused by glucocorticoid treatment. The clinical benefits of glucocorticoids may therefore in part be due to an increased activity and capacity of the Na,K-pump and thereby in a possible enhancement of muscle force.

Effect of high salt intake on plasma and tissue concentration of endogenous ouabain-like substance in the rat

The effect of high salt intake on serum concentration and tissue distribution of ouabain-like substance (OLS) was examined in rats. Sprague-Dawley rats (n=8) were placed on a high salt diet by the inclusion of 1.8% sodium chloride in drinking water for 7 days and a 'control' group (n=8) was maintained on normal drinking water during the study period. Serum and tissue OLS was measured by radioimmunoassay after solid phase extraction. High salt intake significantly increased serum OLS concentration (1.43 +/- 0.06 vs 1.14 +/- 0.05 nmol/L; mean +/- SEM, P=0.002). In both groups, the adrenal showed significantly (p < 0.001) higher OLS content compared to liver, kidney, heart and brain. HPLC of rat serum extract resolved a major peak with a retention time identical to that of standard ouabain, further confirming the nature of OLS. We conclude that high salt intake increases endogenous production of OLS, which appears to originate from the adrenal gland in the rat.

Ouabain-like factor quantification in mammalian tissues and plasma: comparison of two independent assays

The resolution of controversies that concern the detectability of an endogenous ouabain-like factor (OLF) in mammalian tissues and plasma was approached by the application of a standardized method for its extraction and quantification. Two independent assays were used to quantify the OLF: (1) a radioimmunoassay, which used a polyclonal anti-ouabain antiserum, and (2) a radioenzymatic assay based on the inhibition of dog kidney Na+,K+-ATPase. Plasma and tissues were obtained from the Milan hypertensive strain (MHS) and the Milan normotensive strain (MNS) of rats and from healthy human volunteers. Results indicate that (1) a single high-performance liquid chromatography (HPLC) fraction identical to that of ouabain was identified by both assay methods in the rat hypothalamus and hypophysis and in both rat and human plasma; (2) dilution curves of OLF and standard ouabain were parallel and with a similar Kd, both in radioimmunoassay (3 nmol/L) and ATPase assay (14 nmol/L); (3) after HPLC, OLF was similarly quantified by the two methods in the hypothalamus, hypophysis, adrenals, and plasma of rats and in human plasma; (4) OLF was present in larger amounts in the hypothalamus, hypophysis, and plasma of MHS rats than that of MNS rats; (5) the HPLC fraction of human plasma was quantified similarly by both assays (range, 60 to 150 pmol/L); (6) recovery of standard ouabain in pre-HPLC plasma extracts was approximately 90%; and (7) pre-HPLC OLF concentrations in human plasma ranged between 0.05 and 0.75 nmol/L. Rat cerebral tissues and both rat and human plasma contained measurable amounts of OLF, which were quantified similarly by radioimmunoassay and ATPase assay, both before and after HPLC fractionation. The increased MHS tissue and plasma levels of OLF are in keeping with the pathogenetic role of this factor in MHS hypertension.

Endogenous digoxin-like immunoactivity and diabetes mellitus: facts and hypotheses

Substances with digoxin- and ouabain-like immunoactivity (DLIA) are specific inhibitors of Na(+)-K(+)-ATPase which increase the total amount of intracellular stored calcium (Ca2+i). In diabetic patients, DLIA levels have been reported to be increased. Although this increase is probably secondary to sodium retention and volume expansion (included in diabetic subjects by hyperinsulinemia and/or diabetic nephropathy), the question arises of whether it has pathophysiological consequences: namely, whether substances with DLIA, via their effect on Na(+)-K(+)-ATPase activity and Ca2+i stores, could in diabetic subjects facilitate development of hypertension and/or modulate insulin sensitivity or insulin secretion. Clinical findings of correlations of DLIA to blood pressure, insulin levels and to degree of insulin resistance, together with experimental findings of decreased Na(+)-K(+)-ATPase activity, increased Ca2+i and decreased Mg2+i in both diabetic and hypertensive subjects, support these hypotheses. However, the issue of whether or not these relations are causative and whether or not defects in intracellular milieu are primary or secondary to non-insulin-dependent diabetes mellitus has not been resolved yet. Moreover, pathogenesis of both diabetes mellitus and hypertension is multifactorial and includes many other factors. Therefore, further efforts should be made to elucidate the exact role of substances with DLIA in diabetes mellitus.

Elevation of ouabainlike compound levels with hypertonic sodium chloride load in rat plasma and tissues

A major biologically active endogenous digitalis-like factor in the mammalian body may be an isomer of ouabain (ouabainlike compound, OLC). However, the exact role of OLC in sodium homeostasis is still unclear, and acute isotonic volume expansion does not enhance the secretion of OLC. We tested the hypothesis that OLC may be more important in the response to acute hypertonic NaCl load rather than isotonic volume expansion. We injected intraperitoneally 2 mL of 20% NaCl solution into male Wistar rats (n=34) and measured OLC levels in plasma, hypothalamus, pituitary, and adrenal at baseline (n=10) and 1, 2, and 4 hours (n=8 for each). In response to hypertonic NaCl loading, plasma Na-K ratio was elevated at 2 and 4 hours (P<.01). OLC levels in pituitary increased (P<.01) at 1 hour. Thereafter, plasma OLC levels increased at 2 and 4 hours (P<.05; basal, 75+/-11 pmol/L [+/-SEM]; 1 hour, 55+/-11; 2 hours, 130+/-24; 4 hours, 156+/-20). Concomitantly, OLC levels in adrenal increased at 2 and 4 hours (P<.01; basal, 1.7+/-0.2 pmol/g; 1 hour, 4.5+/-0.9; 2 hours, 5.0+/-0.7; 4 hours, 6.8+/-2.2). A significant correlation was observed between OLC levels in plasma and adrenal (P<.05). Plasma Na-K ratio positively correlated with OLC levels in plasma (r=.51, P<.01) and adrenal (r=.48, P<.01). Similar injection of physiological saline solution or hypertonic sucrose solution in physiological saline did not increase OLC levels in plasma and tissues. These findings indicate the elevation of OLC levels in plasma, pituitary, and adrenal in response to acute hypertonic NaCl load in rats and suggest that OLC may be involved in the response to the hypernatremic state.

Solubilization of a complex of tryptic fragments of Na,K-ATPase containing occluded Rb ions and bound ouabain

The nonionic detergent C12E10 (polyoxyethylene 10-laurylether) has been used to solubilize a complex of tryptic fragments of Na, K-ATPase containing occluded Rb ions and bound ouabain. The aim was to define which fragments are required to maintain Rb occlusion. The experiments utilize "19 kDa membranes" consisting of a 19 kDa and several smaller tryptic fragments (8-11.7 kDa) of the alpha subunit, which include trans-membrane segments M7/M10 and the pairs M1/M2, M3/M4, and M5/M6 [Capasso, J. M., et al (1992) J. Biol. Chem. 267, 1150-1158]. The beta subunit is partially split into a 16 kDa fragment and a glycosylated approximately 50 kDa fragment. Cation occlusion and ouabain binding are intact. After preincubation of "19 kDa membranes" with Rb (5 mM) and then ouabain (10 mM), 90-100% of occluded Rb was solubilized by C12E10 at 0 degrees C. All fragments of the alpha and beta subunits, and also the gamma subunit, were cosolubilized by C12E10, and were observed to sediment together on a sucrose density gradient as a complex containing occluded Rb ions. The soluble complex consists of a monomer containing one copy of each fragment, as indicated by size-exclusion HPLC, as well as estimates of specific Rb occlusion (20.0 +/- 1.2 nmol/mg of protein). In the absence of Rb ions and ouabain, the complex was unstable. Whereas the 19 kDa fragment (M7-M10) and beta subunit remained associated, the smaller fragments, containing M5/M6 and M3/M4 and M1/M2, and the subunit dissociated. Observations on the thermal inactivation of Rb occlusion, and effect of pH and ionic strength, suggest that the soluble complex is stabilized by multiple interactions, both within the lipid bilayer and in hydrophilic domains (e.g., salt bridges).

Effects of intracerebroventricular administration of 6-hydroxydopamine on ouabain-like immunoreactivity in plasma and the hypothalamo-pituitary axis in rats

OBJECTIVE

To examine the role of central mechanisms on the production and release of an ouabain-like factor, the effects of intracerebroventricular injections of 6-hydroxydopamine on the tissue content and on the plasma level of the ouabain-like factor were determined in rats.

METHODS

The vehicle (0.1% ascorbic acid in 0.9% saline) and 6- hydroxydopamine (250 micrograms/rat) were injected into the left lateral ventricle in ether-anaesthetized Wistar rats. Hypothalamus, pituitary, adrenal and venous blood was sampled 24h and 7 days later. The procedure was repeated using another rat group 7 days later. Characteristics of immunoreactive ouabain-like factor were determined by a combination of high-performance liquid chromatography and a highly sensitive enzyme-linked immunosorbent assay for ouabain. The level of the ouabain-like factor in these tissues and in plasma extracts measured by the enzyme-linked immunosorbent assay was compared between the two groups receiving 6-hydroxydopamine and the vehicle.

RESULTS

Twenty-four hours after the intracerebroventricular injections of 6-hydroxydopamine, the ouabain-like factor level in the pituitary, hypothalamus and plasma had decreased significantly, whereas the ouabain-like factor level in the adrenal had not changed. The content of noradrenaline in the hypothalamus was also decreased markedly 7 days later and the content of ouabain-like factor in the pituitary remained low. On liquid chromatography the elution pattern of the ouabain-like factor in plasma and in tissue extracts coincided with that of authentic ouabain.

CONCLUSIONS

Intracerebroventricular treatments with 6-hydroxydopamine elicited decreases in ouabain-like factor contents in the pituitary, the hypothalamus and the plasma. These results suggest that the production and release of ouabain-like factor are closely associated with the brain, particularly the hypothalamus-pituitary axis, and that noradrenergic or dopaminergic neurons, or both, play a key role in this mechanism.

Stress-induced elevation of ouabainlike compound in rat plasma and adrenal

Recent observations demonstrate the presence of neurosteroids and their rapid increase in response to acute stress. In view of a steroidal nature of ouabainlike compound, we tested the hypothesis that ouabainlike compound may participate in a homeostatic response to acute stress. Male Wistar rats were subjected to acute stress by swimming in water (22 degrees C) for 10 minutes. The levels of ouabainlike compound in plasma, hypothalamus, pituitary, and adrenal at 10, 40, and 70 minutes (n = 8 for each) after the end of swim stress were compared with nonstressed control levels (n = 10). Ouabainlike compound was measured by a radioimmunoassay for ouabain. Plasma levels of corticosterone and catecholamines were also measured. Plasma corticosterone concentrations increased rapidly at 10 minutes (P < .01) and then declined. A trend for a rise in plasma catecholamines was found at 10 minutes. Adrenal levels of ouabainlike compound concomitantly increased at 10 minutes (P < .01, control: 58.9 +/- 5.9 pmol ouabain equivalents per gram; 10 minutes: 92.5 +/- 4.8; 40 minutes: 47.3 +/- 9.6; 70 minutes: 45.1 +/- 6.3). In contrast, the response of plasma ouabainlike compound was slow and doubled at 40 minutes (P < .01, control: 115 +/- 12 pmol ouabain equivalents per liter; 10 minutes: 132 +/- 23; 40 minutes: 226 +/- 53; 70 minutes: 117 +/- 16). Ouabainlike compound levels in hypothalamus and pituitary remained unaltered. These findings suggest that ouabainlike compound may function as a stress hormone.

Quantification of the Na+/K(+)-pump in solubilized tissue by the ouabain binding method coupled with high-performance gel chromatography

Membrane-bound Na+/K(+)-ATPase purified from dog kidney outer medulla was solubilized with octaethylene glycol n-dodecyl ether (C12E8) and incubated with [3H]ouabain in the presence of NaCl. ATP and MgCl2 for 10 min at 0 degrees C. The resulting enzyme was separated, by high-performance gel chromatography executed at 0.2 degrees C. Mainly into its (alpha beta)2-diprotomer and alpha beta-protomer, which both bound stoichiometrically to [3H]ouabain. The amounts of ouabain that bound to the tissue itself and its microsomes could be estimated in the same way, as [3H]ouabain was found to bind only to the diprotomer and protomer they possessed. The amounts of ouabain that bound to them in the solubilized state were at least 5-times higher than those that did so when they were non-solubilized, suggesting that the surfactant rendered the enzyme accessible to ouabain. When the solubilized tissue (138 mg ml-1 wet tissue) was reacted with ouabain in the presence of 0.1 M NaCl and 4.8 mM MgCl2 for 10 min at 0 degrees C, maximal ouabain binding was attained in the presence of 18.3 microM [3H]ouabain, 1.2 mM ATP and 3 to 5 mg ml-1 C12E8, which was common to the outer medulla and human colon cancer cells. The present method enabled the pump number in protein and tissue samples in the range 7.2 x 10(-9) (purified pump) to 1.5 x 10(-12) (cancer tissue) mol/mg protein to be estimated within 2 h.

Brain 'ouabain' mediates sympathetic hyperactivity in congestive heart failure

In congestive heart failure (CHF), endogenous compounds with ouabainlike activity (OLA) may contribute to the maintenance of the circulatory homeostasis by peripheral as well as central effects. In the present study, we assessed changes in peripheral (plasma and left ventricle) and central (pituitary, hypothalamus, pons, and cortex) OLA in two animal models of CHF and determined whether brain OLA mediates sympathetic hyperactivity in CHF. Cardiomyopathic hamsters with their controls were studied at 9 months of age for tissue OLA. Rats were studied 4 weeks after acute coronary artery ligation for tissue OLA and sympathetic activity. In both models, left ventricular end-diastolic pressure was markedly increased. CHF was associated with significant increases in both plasma and tissue OLA in both models. In the brain, the most marked (twofold to threefold) increases occurred in the hypothalamus. In vitro, all OLA measured could be blocked by antibody Fab fragments (Digibind). Conscious rats with CHF showed elevated plasma catecholamines and enhanced responses of mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) to air stress and to intracerebroventricular (ICV) injection of the alpha 2-adrenergic receptor agonist guanabenz compared with sham-operated rats. ICV administration of the Fab fragments did not change resting RSNA or responses to air stress at 1 hour. However, 18 hours after injection of the Fab fragments, resting RSNA levels had significantly decreased compared with the control values, and plasma catecholamine levels had decreased to control values.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes of erythrocyte ouabain maximum binding after birth in neonates--in relation to erythrocyte sodium and potassium concentrations

Erythrocyte ouabain maximum binding (Vmax level) represents the number of molecules of Na-K ATPase on the erythrocyte membrane. Erythrocyte Vmax levels and sodium potassium concentrations were studied in 38 mature and 57 premature infants, including 40 very low birth weight infants, 41 children and 16 adults, in order to clarify the development of erythrocyte Na-K ATPase related to mineral balance and thermogenesis. Vmax levels were the highest (22.9 +/- 8.9 nmol/l cells) for premature infants; correlated negatively with gestational age (r = -0.77, P < 0.01). Vmax levels correlated positively with the ratio of erythrocyte potassium to sodium concentration (IC-K/Na) at < 20 nmol/l cells of Vmax levels (r = 0.72, P < 0.01), but not at > or = 20 nmol/l cells (r = 0.23). Vmax levels in very low birth weight infants, especially < 28 weeks postconception, decreased gradually with postnatal days. Ln(Vmax) levels correlated negatively with postconceptional age (r = -0.75, P < 0.01). Under 28 weeks postconception, Vmax levels were high, but not correlated with IC-K/Na. It is probable that individual Na-K ATPase molecules may be less active, and the large numbers of Na-K ATPase molecules may compensate for the individual lower activity.

Ouabain is secreted by bovine adrenocortical cells

Ouabain is a specific inhibitor of the sodium pump. This steroid has been found in the mammalian circulation in significant amounts and may be of adrenal origin. Secretion of ouabain from adrenal cells has been little studied and the purpose of the present work was to determine the adrenal distribution of ouabain, aldosterone and cortisol, and to characterize the effects of ACTH and angiotensin II on the secretion of these steroids in primary cultures of bovine adrenocortical cells. In fresh bovine adrenals, the cortical to medullary ratios for aldosterone, cortisol and ouabain were 14, 4.25 and 2.5, respectively. All three steroids were detected in elevated amounts in the conditioned medium of primary cultures of adrenocortical cells. Reverse phase HPLC of the secreted ouabain immunoreactivity showed it was isopolar with commercial ouabain. In the presence of 10 nM ACTH or angiotensin II, the secretion of all three steroids increased significantly with similar time courses. The stimulated secretion of ouabain exceeded the intracellular content of this steroid in either control or activated cells by 3-5 fold. The amount of angiotensin II stimulated ouabain secretion was greater from cells incubated in larger volumes. These results show that ouabain is enriched in the bovine adrenal cortex, and is secreted by primary cultures of these cells. The secretion of ouabain is increased by ACTH and angiotensin II, is due to either de novo synthesis or transformation of an intracellular precursor that is not overtly immunoreactive, and is feedback regulated by either ouabain itself or a cosecreted factor. These cells may be useful to study stimulus-secretion coupling and the biosynthetic pathway of ouabain.

Is the circulating ouabain-like compound ouabain?

The evidence is very strong for a circulating inhibitor of the sodium, potassium ATPase in volume-expanded hypertension. Recently, this inhibitor was isolated from human plasma and identified as ouabain. We are reporting our results using a very specific and sensitive immunoassay for ouabain with which we were unable to detect or able to detect only very low levels of circulating immunoreactive ouabain. Immunoassay of 5 mL of human and rat plasma, incubation fluid from bovine and human adrenal cell cultures extracted using a C-18 solid phase column, and HPLC separation did not detect a peak corresponding to ouabain. This procedure could easily detect authentic ouabain added to these extracts at a concentration slightly below that reported to be present by others. The extract from the adrenal cultures had clearly detectable sodium, potassium ATPase using an assay based on inhibition of tritiated ouabain binding to human red cells. Extraction of bovine adrenals detected a very small amount of immunoassayable ouabain which did not elute at a time corresponding to that of ouabain. This study indicates that the postulated sodium, potassium ATPase inhibitor that circulates in plasma is not ouabain, but it is likely to be structurally similar to ouabain, as it appears to cross-react with some antibodies against ouabain.

Is ouabain an authentic endogenous mammalian substance derived from the adrenal?

Ouabain has recently been reported to be an endogenous mammalian substance released by the adrenal cortex and present in normal plasma. We have attempted to confirm and extend this observation. Using a ouabain radioimmunoassay developed in this laboratory, we fractionated by high-performance liquid chromatography (HPLC) normal human plasma from healthy volunteers to determine the presence of ouabain immunoreactivity and compare this immunoreactivity with authentic ouabain. In most subjects no ouabain immunoreactivity that coeluted with authentic ouabain was observed. Some subjects had ouabain-immunoreactive material present at low levels, but it was largely attributable to cross-reactivity with diverse substances found not to be ouabain. Similar results were obtained after analysis of plasma collected from 10 patients entering a medical intensive care unit. Studies of serum-free medium conditioned by bovine adrenocortical cells showed some ouabain immunoreactivity. To determine whether this material might be a steroid product of cholesterol side-chain cleavage, we performed chemical blockade of steroidogenesis, which effectively suppressed progesterone production by these cells but had no consistent effect on ouabain immunoreactivity in this medium. Stimulation of steroidogenesis with 22-R-OH-cholesterol in bovine adrenocortical cells did not produce any increase in the ouabain immunoreactivity present in conditioned medium. Subsequent HPLC studies of ouabain immunoreactivity in bovine adrenocortical cell-conditioned medium indicated that authentic ouabain did not account for most of the ouabain immunoreactivity in serum-free medium. Studies with bovine adrenocortical cells incubated in a minimal salt and glucose medium indicated a small peak of immunoreactivity that may correspond to authentic ouabain.(ABSTRACT TRUNCATED AT 250 WORDS)

Purification and properties of endogenous ouabain-like substances from hemofiltrate and adrenal glands

Chick embryo heart cells in culture were found to contain two cell types differing in their [Ca2+]i. Elevation of [Ca2+]i in cells with 99.4 +/- 7.1 nM [Ca2+]i was induced half-maximally at 2 x 10(-9) M ouabain and of cells with 27.9 +/- 4.4 nM [Ca2+]i at 4 x 10(-8) M ouabain. The localization of a sodium pump with differing sensitivity for ouabain in different cell types of the heart raised the question of the existence of endogenously formed ouabain. From 6,000 I hemofiltrate of uremic patients, a substance of 582 Da cross-reacting with ouabain antibodies was purified 42,000-fold. The substance was slightly more hydrophobic on a C-18 reversed-phase high-performance liquid chromatography (HPLC) than ouabain was. It inhibited 86Rb+ uptake into red blood cells and raised like ouabain [Ca2+]i in both types of chick heart cells. From pig adrenal glands, a substance inhibiting 86Rb+ uptake into erythrocytes was purified by affinity chromatography on a column containing antibodies against ouabain. A major part of the eluted substances chromatographed on a C-18-HPLC column as ouabain did, but a minor part was somewhat more hydrophobic. It is presumed that more than a single endogenous ouabain-like factor exists in mammals.

Increase in Na+/K+ pump numbers in vivo in healthy volunteers taking oral lithium carbonate and further upregulation in response to lithium in vitro

1. We have measured [3H]-ouabain binding to lymphocyte membranes in eight healthy volunteers before and after they had taken lithium carbonate for 14 days in doses which maintained the serum lithium concentration in the range 0.5-1.0 mmol 1-1. 2. There was a statistically significant increase in the [3H]-ouabain binding capacity of the lymphocyte membranes (reflecting the number of Na+, K+-ATPase molecules) after 14 days of lithium administration in vivo. This suggests that a failure to increase pump numbers after similar exposure to lithium in vivo in patients with manic-depressive psychosis is a primary abnormality associated with the disease. 3. In vivo lithium administration did not alter the normal adaptive (upregulatory) response of lymphocyte Na+, K+-ATPase to standard pharmacological challenges, involving in vitro incubation for 3 days with lithium chloride (8 mmol 1-1) or sodium ethacrynate (1 mumol 1-1). 4. We have previously found that there is an impaired response of the Na+, K+-ATPase to these in vitro stimuli in patients with manic-depressive psychosis, and our present data suggest that this abnormality is attributable to the disease itself and not to in vivo lithium therapy. 5. The data also suggest that the increase in vivo Na+/K+ pump activity which we have previously described in healthy volunteers after 21 days of lithium administration is at least partly due to an increase in Na+/K+ pump numbers.

New strategies to establish human monoclonal antibodies

In order to establish human monoclonal antibodies to any sort of antigens efficiently, we have made following two approaches. Our first approach is to improve cell fusion frequency. By improving our previous method for production of human hybridomas, we obtained higher frequency (1/700 vs. 1/5500) compared with our previous method by adding irradiated myeloma cells to culture of fusion cells and modifying the selective medium. Our second approach is to use a SCID-hu mouse for immunization. Since the injection of human PBL can result in the stable long-term reconstitution of a human immune system in SCID mouse, we tried to immune SCID-hu mouse with KLH. In the serum of immunized SCID-hu mouse, we obtained human IgG antibodies to KLH. Additionally, we succeeded in establishing human B lymphoblastoid cell lines which produced antibodies specific to KLH. These methods will open new prospects for the detection and therapy of cancer.

Mass spectrometric analysis of cardiac glycosides by the desorption/ionization technique potassium ion ionization of desorbed species

The analysis of cardiac glycosides by the desorption/ionization (D/I) mass spectrometric technique potassium ion ionization of desorbed species (K+IDS) is presented. K+IDS mass spectra of digitonin, digoxin, digoxigenin, digitoxin and ouabain are discussed to demonstrate the capabilities of this D/I method. The K+IDS analysis consists of two steps: thermal desorption of neutral molecules representative of the analyte, followed by gas-phase addition of K+ ions to these species. Structural and molecular weight information of the cardiac glycosides is obtained with the K+IDS technique. The most intense peak in the K+IDS mass spectrum of an analyte, M, is frequently the [M]K+ ion. Interpretation of the K+IDS mass spectra is simple, since one thermal degradation mechanism dominates. This mechanism is a 1,2-elimination process. A variation of the original K+IDS technique, performed by changing the ionizing metal from K+ to Na+ (i.e. Na+IDS), is presented for the analysis of digoxin. The Na+IDS mass spectrum of digoxin contains more structural information than the K+IDS mass spectrum of that compound. This may lead to a means of controlling the types of information obtainable with this D/I technique by varying the cation that is thermionically generated. K+IDS analyses can be performed rapidly, no sample derivatization is necessary, no matrix is required and little instrument modification is necessary.

Relation of left ventricular function and Na,K-pump concentration in suspected idiopathic dilated cardiomyopathy

The possible relation between Na-K-pump concentration and left ventricular (LV) function was studied in 24 patients with suspected idiopathic dilated cardiomyopathy. This was done by measurement of 3H-ouabain binding to biopsies obtained during left-sided heart catheterization. In all patients light microscopy of biopsies was compatilel with dilated cardiomyopathy. Nineteen patients had impaired LV function as defined by NYHA/WHO and a Na,K-pump concentration of 331 +/- 19 pmol/g wet weight, whereas 5 patients had normal LV function and a Na,K-pump concentration of 559 +/- 62 pmol/g wet weight (p less than 0.001). The correlation between Na,K-pump concentration and ejection fraction was highly significant n = 24, r = 0.81, p less than 0.001). There was no correlation between volume fraction of collagen tissue and Na,K-pump concentration in the biopsies (n = 24, r = -0.08, p less than 0.80), indicating that the decrease in Na,K-pump concentration with dilated cardiomyopathy is not the simple outcome of increased fibrosis in the myocardium. The results indicate that the decrease in Na,K-pump concentration may be of importance for myocardial dysfunction and suggest a simple biochemical assessment of dilated cardiomyopathy by measurement of 3H-ouabain binding.

Enhancing effect of bucolome on biliary excretion of ouabain and digoxin in the rat

The effect of bucolome (1-cyclohexyl-5-n-butyl-2,4,6-trioxoperhydropyrimidine) on the biliary excretion of intravenously administered ouabain and digoxin was examined in rats. The intravenously administered [3H] ouabain (0.1 mg/100 g b wt) was more rapidly excreted in the bile in female rats than in males. Intraperitoneal administration of bucolome induced a two fold increase in bile flow rate, accompanied by a significant increase in biliary recovery of ouabain during first 10 min (by 100% in males and 50% in females) resulting in significantly higher 60-min total biliary recoveries in both sexes. In female rats 80% of radioactivity was recovered in the bile as an unmetabolized form after 10 min of intravenous administration of [3H] digoxin. Biliary excretion of the parent drug, digoxin, was also enhanced by intraperitoneal administration of bucolome. Although the underlying mechanism(s) remains unknown, it was shown that biliary excretion of both ouabain and digoxin was enhanced by bucolome, as was previously shown for ouabain in male rats.

Further biochemical characterization of an Na+ pump inhibitor purified from human urine

An increase in endogenous Na+,K+-ATPase inhibitor(s) with digitalis-like properties has been reported in chronic renal insufficiency, in Na+-dependent experimental hypertension and in some essential hypertensive patients. The present study specifies some properties and some biochemical characteristics of a semipurified compound from human urine having digitalis-like properties. The urine-derived inhibitor (endalin) inhibits Na+,K+-ATPase activity and [3H]-ouabain binding, and cross-reacts with anti-digoxin antibodies. The inhibitory effect on ATPases of endalin is higher on Na+,K+-ATPase than on Mg2+-ATPase and Ca2+-ATPase. The mechanism of endalin action on highly purified Na+,K+-ATPase was compared to that of ouabain and was similar in that it reversibly inhibited Na+,K+-ATPase activity; it inhibited Na+,K+-ATPase non-competitively with ATP; its inhibitory effect was facilitated by Na+; K+ decreased its inhibitory effect on Na+,K+-ATPase; it competitively inhibited ouabain binding to the enzyme; its binding was maximal in the presence of Mg2+ and Pi; it decreased the Na+ pump activity in human erythrocytes; it reduced serotonin uptake by human platelets; and it was diuretic and natriuretic in rat bioassay. The endalin differed from ouabain in only three aspects: its inhibitory effect was not really specific for Na+,K+-ATPase; its binding to the enzyme was undetectable in the presence of Mg2+ and ATP; it was not kaliuretic in rat bioassay. Endalin is a reversible and partial specific inhibitor of Na+,K+-ATPase, its Na+,K+-ATPase inhibition closely resembles that of ouabain and it could be considered as one of the natriuretic hormones.

Analysis of a Maasai arrow poison

An arrow poison prepared by traditional methods from Acokanthera schimperi in the Maasai plains of Kenya was shown to contain acolongifloroside K as its major active principle, as well as smaller amounts of ouabain and acovenoside A. The major cardenolide is the first identified "ouabain equivalent" from an East African arrow poison.

An endogenous ouabain-like factor associated with hypertensive pregnant women

Levels of a ouabain-like factor (OLF) were measured in amniotic fluid from 49 undigitalized third trimester pregnant women by means of its cross-reactivity in a digoxin RIA and its inhibition of ouabain-sensitive [Na,K]ATPase. The results from these 2 assays were significantly correlated (P less than 0.05). Of the women included in this study, 25 had blood pressures considered normal for their gestational age, while 24 had developed during their current pregnancy blood pressures judged to be elevated. When levels of OLF in the amniotic fluids from the normotensive and hypertensive pregnant women were compared, significantly higher levels were present in the hypertensive group for both assays (P less than 0.002). Further, there was a significant correlation between the diastolic blood pressures of these women at the time of amniocentesis and the amniotic fluid OLF levels determined by either assay (P less than 0.002). These results are consistent with OLF having a role in hypertensive complications of pregnancy.

Affinity-column-mediated immunoenzymometric assays: influence of affinity-column ligand and valency of antibody-enzyme conjugates

We describe an affinity-column-mediated, enzyme-linked immunometric assay that is highly sensitive and adaptable to automation. Digoxin is the model test analyte. A comparison of digoxin with its analog, ouabain, for use as the immobilized ligand on the affinity column showed ouabain to be superior. We also report the effect of column elution rate. Antibody-enzyme conjugates prepared with the monovalent Fab'-fragment and the divalent F(ab')2-fragment coupled to beta-galactosidase are compared in terms of their overall assay performance. Although the monovalent Fab'--beta-galactosidase conjugate yields a more sensitive assay and dose-response curves that are linear over a wider range, the divalent F(ab')2--beta-galactosidase conjugate provides an assay with adequate sensitivity and extremely good precision, and is generally easier to synthesize reproducibly. This fully automated, rapid, and precise assay for digoxin is compatible with the Du Pont aca discrete clinical analyzer.

Centrally mediated enhancement of ouabain cardiotoxicity by angiotensin II in dogs

The effect of angiotensin II (Ang II) infusion on the doses of ouabain required to induce ventricular arrhythmias and death was investigated in anesthetized dogs. Tritiated ouabain was infused alone i.v. (1 microgram/kg per min) to establish the control toxic doses and the level of ouabain in the heart at death. When Ang II and ouabain were co-infused i.v. the doses of ouabain needed to induce arrhythmias and death were significantly reduced. Spinal transections performed at C-1 prior to drug infusions prevented the effect of Ang II to enhance the toxicity of ouabain. Thus, the action of Ang II to augment ouabain toxicity appeared to be related to its effects on the sympathetic nervous system. To confirm that Ang II was acting within the brain rather than at peripheral sites of the sympathetic system, Ang II was infused into the vertebral artery of spinal cord intact dogs. The infusion rate of Ang II needed to produce a significant enhancement of ouabain toxicity was much lower when given into the vertebral artery than into the femoral vein. These data indicate that Ang II enhances the cardiotoxicity of ouabain via an action produced within the brain and mediated by the sympathetic nervous system.

Characterization of ouabain receptor in neuronal tissue: evidence for endogenous ouabain-like compound

This study shows that [3H]ouabain binds specifically to a single, saturable binding site located on rat brain membranes with an affinity constant of 6.21 X 10(-8) M. As expected from studies on the mechanics of the Na+,K+-ATPase, sodium increased while potassium and lithium decreased ouabain binding. The occupation of other neurotransmitter receptors did not affect [3H]ouabain binding. Based on its ability to compete with [3H]ouabain binding and to inhibit Na+,K+-ATPase, it is suggested that rat brain extract contains an endogenous ouabain-like compound. The results are discussed with respect to the possibility that the ouabain receptor is a physiological regulatory site of the Na+,K+-ATPase activity.

Ouabain binding to isolated frog gastric mucosa

Inhibition of gastric acid secretion by the cardiac glycoside, ouabain, is considerably reduced by elevating external K+ as first shown by Davenport (Proc. Soc. Exp. Biol. Med. 110: 613-615, 1962). To determine the possible role of K+ in this effect, we measured [3H]ouabain binding in isolated bullfrog gastric mucosa. Uptake of the labeled drug showed two components: one that saturated at 0.36 pmol ouabain per milligram wet weight and one that was linear with the external ouabain concentration. The former component is considered to represent specific binding to Na+-K+-ATPase; activity of this enzyme system in mucosal homogenates was 0.2 mumol per milligram protein per hour. Increase of K+ in the nutrient bathing solution from 3 to 30 mM, or replacement of Na+ by K+ in the secretory bathing solution, largely reversed inhibition of acid secretion by ouabain but did not affect maximum specific binding. The results fit a model in which Na+-K+-ATPase is normally required in oxyntic cells to maintain a high K+ level, which in turn supports exchange of K+ for H+ at the apical surface.

Myocardial ouabain content and susceptibility to ouabain cardiotoxicity associated with circulatory volume overload in the dog

The influence of circulatory volume overload on the myocardial uptake of ouabain and on cardiotoxicity was studied in the unanaesthetised dog with aorto-caval fistula. One hour after tritiated ouabain (0-02 mg/kg IV) both ventricles and atria contained more ouabain than did those of normal dogs (left ventricle (LV), 166+/-23 (SD) ng/g vs. 97+/-19 ng/g, P less than 0-001) while concentrations in skeletal muscle, liver, kidney and plasma were not different in the two groups. In other experiments ouabain was infused to cardiotoxicity (7-5 microgram/kg followed by 3 microgram/kg/min). Cardiotoxicity occurred earlier in dogs with fistula than in normals (16-5+/-2-7 min vs. 24-1+/-2-4 min, P less than 0-001). Ouabain concentrations in myocardium were not different (LV, 434+/-58 ng/g, vs. 442+/-42 ng/g) while concentrations in liver and kidney were less in those with fistula (181+/-35 ng/g vs. 278+/-69 ng/g, P less than 0-001; 1422+/-189 ng/g vs. 2747+/-479 ng/g, P less than 0-001). Average content of skeletal muscle was also less, in proportion to administered dose. The increment in myicardial ouabain content associated with aorto-caval fistula appears to be physiologically active and hence is presumably specifically bound to the digitalis receptor. The observations in this model suggest the possibility of augmented cardiac glycoside uptake in some clinical cardiac diseases.

Concentration of digoxin, methyldigoxin, digitoxin and ouabain in the myocardium of the dog following coronary occulsion

26 mongrel dogs were given a single dose of 0.03mg/kg tritium-labelled digoxin, beta-methyldigoxin, digitoxin or ouabain 2 hrs or 95 hrs following experimental coronary occlusion. Examination of the epicardial ECG was performed by moving from intact to ischemic or necrotic zones. 60 min after glycoside administration the animals were sacrificed and tissue samples from the marked heart muscles areas and from the skeletal muscle were analysed for glycoside content. The early glycoside uptake in acute ischemic or necrotic myocardium was diminished independently of the physicochemical properties of the glycoside. Significantly higher glycoside concentrations (ng/g wet weight) were measured in the injured myocardium 3 hrs after coronary occlusion than 96 hrs afterward (p less than 0.005). The values in acute ischemic myocardium varied considerably. This nonhomogeneity of glycoside uptake in the acute ischemic heart muscle may partly explain the increased sensitivity to glycosides in myocardial infarction. The decline of glycoside concentration correlates with the alterations in the epicardial ECG. The cardiac effects of cardenolides 60 min after intravenous administration was caused by the unchanged glycoside. In contrast to the myocardium, glycoside accumulation could not be found in the skeletal muscle. The concentrations of digoxin, beta-methyldigoxin and digitoxin in the skeletal muscle were significantly higher than the concentration of ouabain, which was rapidly eliminated via the urine.

The reactivity of derivatives of digoxin and digitoxin as measured by the Na-K-atpase displacement assay and by radioimmunoassay

Derivatives of digoxin and digitoxin were measured by the Na-K-ATPase displacement assay and by radioimmunoassay and the data compared with biological potency of these compounds. There was a slightly higher affinity of digoxigenin-bis-digitoxoside as compared with digoxin using digoxin-specific antiserum and considerably less affinity of digoxigenin and dihydrodigoxin than the parent compound in the same system. A similar trend was observed for the derivatives of digitoxin using digoxin-specific antiserum. The recovery on extraction of some of the derivatives of digoxin and digitoxin differed from that of the parent compounds in the ATPase assay. The potency of the derivatives of these drugs in displacing 3H ouabain also differed from the parent compoundmboth the recovery on extraction and the potency for displacing ouabain must be considered in the estimation of the contribution of the derivatives of digoxin or digitoxin to the result of Na-K-ATPase assay. Quantitative information of the metabolites of digoxin and digitoxin in normal and pathological conditions is needed to properly interpret assay data obtained either by radioimmunoassay or by ATPase assay.