J. Endocrinol. Invest. 13: 9-12, 1990
Total digoxin-like immunoreactive factor(s) in healthy population, uncomplicated term pregnancies and neonates1 N. Krivoy*, P. Jakobi, E. Paldi, and G. Alroy* Departments of 'Medicine A and Clinical Pharmacology Unit, and Gynecology/Obstetrics B, Rambam Medical Center and Faculty of Medicine, Technion, Haifa, Israel ABSTRACT. Free digoxin-like immunoreactive factor(s) (DLlF) which may have a homeostatic role, as documented in different physiological conditions, but is generally undetectable in plasma from normal population. Total digoxin-like immunoreactive factor(s) (protein bound and free) can be estimated after plasma is heated. In this study, total digoxin-like immunoreactive factor(s) as measured in plasma in a well defined control population and compared to healthy term pregnant women and neonates, categories known to be associated with increased free digoxin-like. immunoreactive factor(s) concentrations. The mean level of this fac-
tor(s) in the control group was 706 ± 129 pg digoxin equivalentlml (pg/ml) and was unaffected by age and sex. Significantly increased levels of total digoxin-like immunoreactive factor(s) were found in pregnant women and neonates (928 ± 127 and 1242 ± 367 pg/ml, respectively). We conclude that levels of total digoxin-like immunoreactive factor(s) are increased in term pregnancies and neonates, similarly to its free form. However total digoxin-like immunoreactive factor(s) is detected in the normal population as a plasma component, contrary to its free form, which is generally undetectable.
INTRODUCTION A wealth of information has accumulated concerning digoxin-like immunoreactive factor(s) (DLlF) in the recent years. It was observed that this factor(s) are found in tissues, biological fluids and in different pathophysiological conditions in humans (1-18). However, its exact physiological role, source(s) and chemical structure are still not established (1, 2, 7 -9). Recently, it was suggested that the adrenal gland (19), liver and kidney (20), placenta (5) and mammalian brain (21), could be the source(s) of DLiF in humans. DLiF can be measured in plasma by radioimmunoassay for digoxin. Although the RIA is non specific, it is considered as a rapid and reliable screening technique for DLiF detection (14). DLiF appears in plasma in its free form which can be analyzed directly, and also in a protein bound
form. DLiF measurements from heated plasma provide a very rapid estimate of the total (protein bound and free) levels of these factor(s) (1, 10). It was claimed that the variations in the different clinical conditions were due to increased amount of free DLlF, rather than higher total DLiF levels (15). Therefore, in most of the recent reports, only free DLiF levels were measured (3, 6, 12, 17). Free DLiF was reported as undetectable in several control groups (11, 16), but was detected in different clinical settings (3, 4, 7, 11-13). In most studies, the control groups were not clearly defined (eg: age and sex) and consisted of small number of patients (1,4, 10, 11 , 16). The aims of this study were to detect and define normal values of total DLiF in plasma in a well defined control population and to compare them with term pregnancies and neonates, categories known to be associated with increased free DLiF concentrations (5, 12, 13).
1 Supported in part by a Grant No. 180-0927 from the Technion (Israel Institute of Technology) Haifa, Israel.
MATERIALS AND METHODS
Key-words: Digoxin-like immunoreactive factor(s).
Experimental group Ninety-seven blood samples were analyzed for total DLiF. None of the experimental group were treated
Correspondence: N. Krivoy, MD., Medicine A and Clinical Pharmacology Un,t., Rambam Medical Center. Haifa, Israel.
Received December 7, 1988; accepted September t 5, 1989.
9
N. Krivoy, P. Jakobi, E. Paldi, et al.
(3, 4, 10-12). The antiserum did not crossreact with common steroids, known to be present in plasma such as hydrocortison, deoxycorticosterone, estradiol, androstenedione, 11-deoxycortisol, dehydroepiandrosterone, 17 -hydroxyprogesterone, progesterone, aldosterone or cortisol (4, 10). Unpaired t test was used for statistical analysis.
with digoxin or other drugs that are known to crossreact with the used radioimmunoassay (RIA) (4). The subjects were subdivided as follows. Control group: 24 young healthy students (12 males and 12 females) with a mean age of 19.3 ± 1.6 yr (± SO) and 11 healthy elderly subjects living in an old age home, with a mean age of 75.1 ± 8.5 yr (± SO), were examined. The older group showed normal liver and kidney functions, none of them had arterial hypertension or any endocrinological diseases. Pregnant women group: 32 term healthy pregnant women with a mean gestational age of 39.8 ± 0.9 weeks (± SO) and a mean age of 29.3 ± 3.3 yr (± SO), were selected for this study. Neonate group: 30 term neonates were examined, all of whom were spontaneously delivered to healthy mothers with uncomplicated pregnancies. Their mean weight was 30400 ± 316 g (± SO), the mean gestational age 39.7 ± 1.0 weeks (± SO), with an APGAR score> 9 at 5 min.
RESULTS Table 1 shows the OLIF plasma concentrations in the three study groups. The mean OLIF of the control group was 706 pg/ml. The elderly subjects showed a lower OLIF conc~ntration than the younger population (640 ± 125 vs. 737 ± 121 pg/ml, respectively) but not of statistical significance. The mean OLIF levels were similar in the male and female subgroups (740 ± 114 vs 733 ± 134 pg/ml, respectively). The mean OLIF concentrations in pregnant women were significantly higher than the control group. The mean OLIF levels in the neonate group were significantly higher than in the term pregnancy group or the controls. Of the pregnant group, no level of OLIF lower than the -1 SO of the control group was found, while in 84.3% (27/32) higher levels than the +1 SO of the control group were documented. Only 1/30 of the neonates (3.3%) showed levels of OLIF lower than -1 SO of the control group, while in 80% (24/30) higher levels than +1 SO of the control population were found, and in 66% (20/30) they were even higher than the +1 SO of the term pregnancies.
Laboratory methods Blood samples were collected in chilled-heparinized glass tubes, immediately centrifuged at 1000 g for 5 min and the supernatant recovered. Plasma specimens were separated and stored at -30 C until analyzed, for no more than 2 months. OLIF was measured in duplicates by RIA (RIANEN, New England Nuclear, Billerica, MA, USA) using the advised procedure described by the manufacturer. A few modifications were performed as described by Wijdicks et al. (11). Total OLIF was measured in plasma after predilution with deionized water 1:3 and heated at 82 C for 5 min. According to the dilution factor the lower limit of sensitivity was 150 pg/ml. The typical standard curve was composed from standards of 0, 50, 100, 200, 300, 500 and 1000 pg digoxin/ml which were prepared by dilution of 1000 pg/ml digoxin (stock solution) with zero standard. OLIF concentrations were expressed as pg digoxin equivalent/ml plasma (pg/ml). The resulting typical standard curve showed values for the normalized percentage of antigen bound to antibody of 100.0,96.6,9004,82.5,75.8,64.5 and 46.7, respectively. The between day coefficient of variation (CVs) in 24 replicates were 6.4%, 4.2% ans 2.4% at 300, 500 and 1000 pg/ml OLIF concentration, respectively, the intraassay CV in 42 replicates was 8%. The assay used (RIANEN) is the same used by others
DISCUSSION The presented data confirm that total (protein bound and free) OLIF is measurable in humans, as was suggested by a limited number of observations in the control groups documented in previous reports (1, 4, 10, 11, 16). However, in those studies the control populations were not well defined. According to the results of our study, total OLIF is a component
Table 1 - Total digoxin like immunoreactive factor(s) (OUF)
concentrations in plasma (pg/mf).
Population
Mean ± SD
Term-pregnancies (n ; 32) 928
± 129(') (.. ) ± 127(') (0'0)
710-1300
Term-neonates (n ; 30)
± 367(000) (00)
440-2080
Control (n ; 35)
706 1242
(.) p < 0.01; (.. ) p < 0.001; (... ) p < 0.05.
10
Range 470-980
Digoxin-like immunoreactive factor(s)
REFERENCES
of human plasma and is not affected significantly by age and sex, as was suggested by others using a different method for DLiF determination (18). These findings are in contrast with free DLlF, which is reported to be generally undetectable in healthy adult populations (11, 16). Free DLiF was reported to be detectable in third-trimester pregnant women and in neonates (1, 13, 17). It was claimed that the variations in the different clinical conditions are due to increased amount of free DLI F, rather than higher total DLiF (15). We found that total DLiF is also elevated in these conditions, as was previously suggested by another study (1 ). At present there is no standardization of DLiF measurement methods; therefore a quantitative comparison among the different studies is difficult and every laboratory must establish its own reference values. However, the mean value of total DLiF in our control group, 706 pg/ml (range 470-980 pg/ml) is comparable to the mean value of 615 pg/ml previously published (10). The possible role of DLiF in human physiology has not been established yet. It could be part of a regulatory homeostatic mechanism(s) (1, 2, 7) and may contribute to blood volume, electrolyte balance and blood pressure regulation (9). DLiF was shown to be increased in some forms of arterial hypertension (22). DLiF is thought to act on the central nervous system to enhance sympathetic nerve activity thereby indirectly increasing peripheral vascular resistance (2). Most of the recent studies could not demonstrate a possible etiological role of DLiF in the pathogenesis of preeclamptic toxemia (12, 13, 23). DLlF, together with the atrial natriuretic factor, norepinephrine, epinephrine and plasma renin activity, is involved in the regulation of fetal homeostasis (24). Clerico et al. (25) demonstrated a positive correlation between urinary DLiF concentration, K+ and creatinine in normal adults; the same author demostrated a good correlation between DLiF in urine and urinary creatinine and electrolytes in newborns (26): these data suggest that DLiF may play an important role in the regulation of fluid balance in newborns and pregnant women. Recently, increased total DLiF concentrations were reported after prolonged strenuous exercise, supporting the hypothesis that they might be associated also with cardiotrophic activity (10), a fact that may have physiological implications in the early neonatal period and in pregnancy (23).
1. Valdes Jr. R. Endogenous digoxin like immunoreactive factors: Impact on digoxin measurements and potential physiological implications. Clin. Chem. 31: 1525, 1985. 2. Kelly RA Endogenous cardiac glycoside like compounds. Hypertension 10 (S1 ): 187, 1987.
3. Ahmad S., Kenny M., Scribner B.H. Hypertension and digoxin like substances in the plasma of dialysis patients: Possible marker for natriuretic hormone. Clin. Physiol. Biochem. 4: 210, 1986. 4.
Shilo L., Adawi A., Solomon G., Shenkman L. Endogenous digoxin like immunoreactivity in congestive heart failure. Br. Med. J. 295: 415, 1987.
5. Beyers A.D., Spruyt L.L., Seifart H.I., Kriegler A., Parkin D.P., Van Jaarsveld P.P. Endogenous immunoreactive digitalis-like substance in neonatal serum and placental extracts. S. Afr. Med. J. 65: 878, 1984. 6. Vasdev S., Johnson E., Longerrich L., Prabhakar V.M., Gault M.H. Plasma endogenous digitalis-like factors in healthy individuals and in dialysis dependent and kidney transplant patients. Clin. Nephrol. 27: 169, 1987. 7. Graves SW. The possible role of digitalis like factors in pregnancy-induced hypertension. Hypertension 10 (S1): 184, 1987. 8. Shilo L., Weber M.M., Dolev S., Shapiro M., Shenkman L. Digoxin like immunoreactivity: Occurrence in three molecular forms and partial characterization. J. Clin. Endocrinol. Metab. 64: 1257, 1987. 9. Haddy F.J., Pamnami M.B. Evidence for a circulating endogenous Na-K pump inhibitor in low renin hypertension. Fed. Proc. 44:2789, 1985. 10. Valdes R. Jr., Hagberg J.M, Vaughan T.E., Lau BW.C., Seals D.R., Ehsani A.A. Endogenous digoxin like immunoreactivity in blood is increased during prolonged strenuous exercise. Life Sci. 42: 103, 1988. 11. Wijdicks E.F.M., Vermuelen M., Van Brummelen P., Den Boer N.C, Van Gijn J. Digoxin like immunoreactive substance in patients with aneurysmal subarachnoid haemorrhage. Br. Med. J. 294: 729, 1987.
11
N. Krivoy, P. Jakobi, E. Paldi, et al.
20. Valdes R. Jr., Becker S., Siegfried B.A. Measurement of endogenous digoxin-like immunoreactive factors in animal and human tissues. Clin. Chern. 31: 1015, 1985.
12. Phelps S.J., Cochran E.B., Gonzales-Ruiz A., Tolly E.A., Hammand KD., Sibai B.M. The influence of gestational age and pre-eclampsia on the presence and magnitude of serum endogenous digoxin-like immunoreactive substance(s). Arn. J. Obstet. Gynecol. 158: 34, 1988.
21. Carilli C.T., Berne M., Cantley L.C., Haupert G.T. Hypothalamic factor inhibits the (Na, K) ATPase from the extracellular surface. J. BioI. Chem. 260: 1027, 1985.
13. Gonzales-Ruiz A., Phelps S.J., Cochran E.B., Sibai B.M. Digoxin like immunoreactive substance in pregnancy. Am. J. Obstet. Gynecol. 157: 660,1987.
22. Giampietro 0., Clerico A., Gregori G., Bertolli S., Del Chicca M.G., Miccoli R., Lucchetti A., Cruschelli L., Navalesi R. Increased urinary excretion of digoxin-like immunoreactive substance by insulin-dependent diabetic patients: a linkage with hypertension? Clin. Chem. 34: 2418, 1988.
14. Clerico A., Ghione S., Del Chicca M.G., Balzan S. Problems in standardization of digitalis-like substance by means of competitive immunological methods. Clin. Chem. 33 340, 1987.
23. Jakobi P., Krivoy N., Weissman A., Paldi E. Digoxin-like immunoreactive factor in twin and pregnancy-associated hypertensive pregnancies. Obstet. Gynecol. 74 29,1989.
15. Valdes R. Jr., Graves S.w. Protein binding of endogenous digoxin immunoreactive factors in human serum and its variation with clinical condition. J. Clin. Endocrinol. Metab. 60: 1135, 1985.
24. Weiner C.P., Robillard J.E. Atrial natriuretic factor, digoxin-like immunoreactive substance, norepinephrine, epinephrine, and plasma renin activity in human fetuses and their alteration by fetal disease. Am. J. Obstet. Gynecol. 159: 1353, 1988.
16. Wickramasinghe L.S.P., Bansal S.K., Dillon RD.S. Digoxin like substance in serum of elderly patients. Age Ageing 15: 271, 1986. 17. Phelps S.J., Kamper CA, Bottorff M.B., Alpert B.S. Effect of age and serum creatinine on endogenous digoxin like substances in the infants and children. J. Pediatr. 110: 136, 1987. 18. Balzan S., Clerico A., Del Chicca M.G., Moritali U., Ghione S. Digoxin-like immunoreactivity in normal human plasma and urine as detected by a solid-phase radioimmunoassay. Clin. Chem. 30: 450, 1984.
25. Clerico A., Giampietro 0., Gregori G., Del Chicca M.G., Bertoli S., Miccoli R., Navalesi R. Excretion of digoxin-like immunoreactivity in urine of normal subjects: correlations with excretion of creatinine and electrolytes. Clin. Chern. 34: 554, 1988. 26. Clerico A., Boldrini A., Del Chicca M.G. Correlations between digoxin-like immunoreactivity and electrolytes values in urinary samples of newborns. BioI. Neonate 50: 27, 1986.
19. Shilo L., Shapiro M.S., Dolev S., Shenkman L. Endogenous digoxin-like material is of adrenal origin. Isr. J. Med. Sci. 23: 294, 1987.
12
Total digoxin-like immunoreactive factor(s) in healthy population, uncomplicated term pregnancies and neonates1 N. Krivoy*, P. Jakobi, E. Paldi, and G. Alroy* Departments of 'Medicine A and Clinical Pharmacology Unit, and Gynecology/Obstetrics B, Rambam Medical Center and Faculty of Medicine, Technion, Haifa, Israel ABSTRACT. Free digoxin-like immunoreactive factor(s) (DLlF) which may have a homeostatic role, as documented in different physiological conditions, but is generally undetectable in plasma from normal population. Total digoxin-like immunoreactive factor(s) (protein bound and free) can be estimated after plasma is heated. In this study, total digoxin-like immunoreactive factor(s) as measured in plasma in a well defined control population and compared to healthy term pregnant women and neonates, categories known to be associated with increased free digoxin-like. immunoreactive factor(s) concentrations. The mean level of this fac-
tor(s) in the control group was 706 ± 129 pg digoxin equivalentlml (pg/ml) and was unaffected by age and sex. Significantly increased levels of total digoxin-like immunoreactive factor(s) were found in pregnant women and neonates (928 ± 127 and 1242 ± 367 pg/ml, respectively). We conclude that levels of total digoxin-like immunoreactive factor(s) are increased in term pregnancies and neonates, similarly to its free form. However total digoxin-like immunoreactive factor(s) is detected in the normal population as a plasma component, contrary to its free form, which is generally undetectable.
INTRODUCTION A wealth of information has accumulated concerning digoxin-like immunoreactive factor(s) (DLlF) in the recent years. It was observed that this factor(s) are found in tissues, biological fluids and in different pathophysiological conditions in humans (1-18). However, its exact physiological role, source(s) and chemical structure are still not established (1, 2, 7 -9). Recently, it was suggested that the adrenal gland (19), liver and kidney (20), placenta (5) and mammalian brain (21), could be the source(s) of DLiF in humans. DLiF can be measured in plasma by radioimmunoassay for digoxin. Although the RIA is non specific, it is considered as a rapid and reliable screening technique for DLiF detection (14). DLiF appears in plasma in its free form which can be analyzed directly, and also in a protein bound
form. DLiF measurements from heated plasma provide a very rapid estimate of the total (protein bound and free) levels of these factor(s) (1, 10). It was claimed that the variations in the different clinical conditions were due to increased amount of free DLlF, rather than higher total DLiF levels (15). Therefore, in most of the recent reports, only free DLiF levels were measured (3, 6, 12, 17). Free DLiF was reported as undetectable in several control groups (11, 16), but was detected in different clinical settings (3, 4, 7, 11-13). In most studies, the control groups were not clearly defined (eg: age and sex) and consisted of small number of patients (1,4, 10, 11 , 16). The aims of this study were to detect and define normal values of total DLiF in plasma in a well defined control population and to compare them with term pregnancies and neonates, categories known to be associated with increased free DLiF concentrations (5, 12, 13).
1 Supported in part by a Grant No. 180-0927 from the Technion (Israel Institute of Technology) Haifa, Israel.
MATERIALS AND METHODS
Key-words: Digoxin-like immunoreactive factor(s).
Experimental group Ninety-seven blood samples were analyzed for total DLiF. None of the experimental group were treated
Correspondence: N. Krivoy, MD., Medicine A and Clinical Pharmacology Un,t., Rambam Medical Center. Haifa, Israel.
Received December 7, 1988; accepted September t 5, 1989.
9
N. Krivoy, P. Jakobi, E. Paldi, et al.
(3, 4, 10-12). The antiserum did not crossreact with common steroids, known to be present in plasma such as hydrocortison, deoxycorticosterone, estradiol, androstenedione, 11-deoxycortisol, dehydroepiandrosterone, 17 -hydroxyprogesterone, progesterone, aldosterone or cortisol (4, 10). Unpaired t test was used for statistical analysis.
with digoxin or other drugs that are known to crossreact with the used radioimmunoassay (RIA) (4). The subjects were subdivided as follows. Control group: 24 young healthy students (12 males and 12 females) with a mean age of 19.3 ± 1.6 yr (± SO) and 11 healthy elderly subjects living in an old age home, with a mean age of 75.1 ± 8.5 yr (± SO), were examined. The older group showed normal liver and kidney functions, none of them had arterial hypertension or any endocrinological diseases. Pregnant women group: 32 term healthy pregnant women with a mean gestational age of 39.8 ± 0.9 weeks (± SO) and a mean age of 29.3 ± 3.3 yr (± SO), were selected for this study. Neonate group: 30 term neonates were examined, all of whom were spontaneously delivered to healthy mothers with uncomplicated pregnancies. Their mean weight was 30400 ± 316 g (± SO), the mean gestational age 39.7 ± 1.0 weeks (± SO), with an APGAR score> 9 at 5 min.
RESULTS Table 1 shows the OLIF plasma concentrations in the three study groups. The mean OLIF of the control group was 706 pg/ml. The elderly subjects showed a lower OLIF conc~ntration than the younger population (640 ± 125 vs. 737 ± 121 pg/ml, respectively) but not of statistical significance. The mean OLIF levels were similar in the male and female subgroups (740 ± 114 vs 733 ± 134 pg/ml, respectively). The mean OLIF concentrations in pregnant women were significantly higher than the control group. The mean OLIF levels in the neonate group were significantly higher than in the term pregnancy group or the controls. Of the pregnant group, no level of OLIF lower than the -1 SO of the control group was found, while in 84.3% (27/32) higher levels than the +1 SO of the control group were documented. Only 1/30 of the neonates (3.3%) showed levels of OLIF lower than -1 SO of the control group, while in 80% (24/30) higher levels than +1 SO of the control population were found, and in 66% (20/30) they were even higher than the +1 SO of the term pregnancies.
Laboratory methods Blood samples were collected in chilled-heparinized glass tubes, immediately centrifuged at 1000 g for 5 min and the supernatant recovered. Plasma specimens were separated and stored at -30 C until analyzed, for no more than 2 months. OLIF was measured in duplicates by RIA (RIANEN, New England Nuclear, Billerica, MA, USA) using the advised procedure described by the manufacturer. A few modifications were performed as described by Wijdicks et al. (11). Total OLIF was measured in plasma after predilution with deionized water 1:3 and heated at 82 C for 5 min. According to the dilution factor the lower limit of sensitivity was 150 pg/ml. The typical standard curve was composed from standards of 0, 50, 100, 200, 300, 500 and 1000 pg digoxin/ml which were prepared by dilution of 1000 pg/ml digoxin (stock solution) with zero standard. OLIF concentrations were expressed as pg digoxin equivalent/ml plasma (pg/ml). The resulting typical standard curve showed values for the normalized percentage of antigen bound to antibody of 100.0,96.6,9004,82.5,75.8,64.5 and 46.7, respectively. The between day coefficient of variation (CVs) in 24 replicates were 6.4%, 4.2% ans 2.4% at 300, 500 and 1000 pg/ml OLIF concentration, respectively, the intraassay CV in 42 replicates was 8%. The assay used (RIANEN) is the same used by others
DISCUSSION The presented data confirm that total (protein bound and free) OLIF is measurable in humans, as was suggested by a limited number of observations in the control groups documented in previous reports (1, 4, 10, 11, 16). However, in those studies the control populations were not well defined. According to the results of our study, total OLIF is a component
Table 1 - Total digoxin like immunoreactive factor(s) (OUF)
concentrations in plasma (pg/mf).
Population
Mean ± SD
Term-pregnancies (n ; 32) 928
± 129(') (.. ) ± 127(') (0'0)
710-1300
Term-neonates (n ; 30)
± 367(000) (00)
440-2080
Control (n ; 35)
706 1242
(.) p < 0.01; (.. ) p < 0.001; (... ) p < 0.05.
10
Range 470-980
Digoxin-like immunoreactive factor(s)
REFERENCES
of human plasma and is not affected significantly by age and sex, as was suggested by others using a different method for DLiF determination (18). These findings are in contrast with free DLlF, which is reported to be generally undetectable in healthy adult populations (11, 16). Free DLiF was reported to be detectable in third-trimester pregnant women and in neonates (1, 13, 17). It was claimed that the variations in the different clinical conditions are due to increased amount of free DLI F, rather than higher total DLiF (15). We found that total DLiF is also elevated in these conditions, as was previously suggested by another study (1 ). At present there is no standardization of DLiF measurement methods; therefore a quantitative comparison among the different studies is difficult and every laboratory must establish its own reference values. However, the mean value of total DLiF in our control group, 706 pg/ml (range 470-980 pg/ml) is comparable to the mean value of 615 pg/ml previously published (10). The possible role of DLiF in human physiology has not been established yet. It could be part of a regulatory homeostatic mechanism(s) (1, 2, 7) and may contribute to blood volume, electrolyte balance and blood pressure regulation (9). DLiF was shown to be increased in some forms of arterial hypertension (22). DLiF is thought to act on the central nervous system to enhance sympathetic nerve activity thereby indirectly increasing peripheral vascular resistance (2). Most of the recent studies could not demonstrate a possible etiological role of DLiF in the pathogenesis of preeclamptic toxemia (12, 13, 23). DLlF, together with the atrial natriuretic factor, norepinephrine, epinephrine and plasma renin activity, is involved in the regulation of fetal homeostasis (24). Clerico et al. (25) demonstrated a positive correlation between urinary DLiF concentration, K+ and creatinine in normal adults; the same author demostrated a good correlation between DLiF in urine and urinary creatinine and electrolytes in newborns (26): these data suggest that DLiF may play an important role in the regulation of fluid balance in newborns and pregnant women. Recently, increased total DLiF concentrations were reported after prolonged strenuous exercise, supporting the hypothesis that they might be associated also with cardiotrophic activity (10), a fact that may have physiological implications in the early neonatal period and in pregnancy (23).
1. Valdes Jr. R. Endogenous digoxin like immunoreactive factors: Impact on digoxin measurements and potential physiological implications. Clin. Chem. 31: 1525, 1985. 2. Kelly RA Endogenous cardiac glycoside like compounds. Hypertension 10 (S1 ): 187, 1987.
3. Ahmad S., Kenny M., Scribner B.H. Hypertension and digoxin like substances in the plasma of dialysis patients: Possible marker for natriuretic hormone. Clin. Physiol. Biochem. 4: 210, 1986. 4.
Shilo L., Adawi A., Solomon G., Shenkman L. Endogenous digoxin like immunoreactivity in congestive heart failure. Br. Med. J. 295: 415, 1987.
5. Beyers A.D., Spruyt L.L., Seifart H.I., Kriegler A., Parkin D.P., Van Jaarsveld P.P. Endogenous immunoreactive digitalis-like substance in neonatal serum and placental extracts. S. Afr. Med. J. 65: 878, 1984. 6. Vasdev S., Johnson E., Longerrich L., Prabhakar V.M., Gault M.H. Plasma endogenous digitalis-like factors in healthy individuals and in dialysis dependent and kidney transplant patients. Clin. Nephrol. 27: 169, 1987. 7. Graves SW. The possible role of digitalis like factors in pregnancy-induced hypertension. Hypertension 10 (S1): 184, 1987. 8. Shilo L., Weber M.M., Dolev S., Shapiro M., Shenkman L. Digoxin like immunoreactivity: Occurrence in three molecular forms and partial characterization. J. Clin. Endocrinol. Metab. 64: 1257, 1987. 9. Haddy F.J., Pamnami M.B. Evidence for a circulating endogenous Na-K pump inhibitor in low renin hypertension. Fed. Proc. 44:2789, 1985. 10. Valdes R. Jr., Hagberg J.M, Vaughan T.E., Lau BW.C., Seals D.R., Ehsani A.A. Endogenous digoxin like immunoreactivity in blood is increased during prolonged strenuous exercise. Life Sci. 42: 103, 1988. 11. Wijdicks E.F.M., Vermuelen M., Van Brummelen P., Den Boer N.C, Van Gijn J. Digoxin like immunoreactive substance in patients with aneurysmal subarachnoid haemorrhage. Br. Med. J. 294: 729, 1987.
11
N. Krivoy, P. Jakobi, E. Paldi, et al.
20. Valdes R. Jr., Becker S., Siegfried B.A. Measurement of endogenous digoxin-like immunoreactive factors in animal and human tissues. Clin. Chern. 31: 1015, 1985.
12. Phelps S.J., Cochran E.B., Gonzales-Ruiz A., Tolly E.A., Hammand KD., Sibai B.M. The influence of gestational age and pre-eclampsia on the presence and magnitude of serum endogenous digoxin-like immunoreactive substance(s). Arn. J. Obstet. Gynecol. 158: 34, 1988.
21. Carilli C.T., Berne M., Cantley L.C., Haupert G.T. Hypothalamic factor inhibits the (Na, K) ATPase from the extracellular surface. J. BioI. Chem. 260: 1027, 1985.
13. Gonzales-Ruiz A., Phelps S.J., Cochran E.B., Sibai B.M. Digoxin like immunoreactive substance in pregnancy. Am. J. Obstet. Gynecol. 157: 660,1987.
22. Giampietro 0., Clerico A., Gregori G., Bertolli S., Del Chicca M.G., Miccoli R., Lucchetti A., Cruschelli L., Navalesi R. Increased urinary excretion of digoxin-like immunoreactive substance by insulin-dependent diabetic patients: a linkage with hypertension? Clin. Chem. 34: 2418, 1988.
14. Clerico A., Ghione S., Del Chicca M.G., Balzan S. Problems in standardization of digitalis-like substance by means of competitive immunological methods. Clin. Chem. 33 340, 1987.
23. Jakobi P., Krivoy N., Weissman A., Paldi E. Digoxin-like immunoreactive factor in twin and pregnancy-associated hypertensive pregnancies. Obstet. Gynecol. 74 29,1989.
15. Valdes R. Jr., Graves S.w. Protein binding of endogenous digoxin immunoreactive factors in human serum and its variation with clinical condition. J. Clin. Endocrinol. Metab. 60: 1135, 1985.
24. Weiner C.P., Robillard J.E. Atrial natriuretic factor, digoxin-like immunoreactive substance, norepinephrine, epinephrine, and plasma renin activity in human fetuses and their alteration by fetal disease. Am. J. Obstet. Gynecol. 159: 1353, 1988.
16. Wickramasinghe L.S.P., Bansal S.K., Dillon RD.S. Digoxin like substance in serum of elderly patients. Age Ageing 15: 271, 1986. 17. Phelps S.J., Kamper CA, Bottorff M.B., Alpert B.S. Effect of age and serum creatinine on endogenous digoxin like substances in the infants and children. J. Pediatr. 110: 136, 1987. 18. Balzan S., Clerico A., Del Chicca M.G., Moritali U., Ghione S. Digoxin-like immunoreactivity in normal human plasma and urine as detected by a solid-phase radioimmunoassay. Clin. Chem. 30: 450, 1984.
25. Clerico A., Giampietro 0., Gregori G., Del Chicca M.G., Bertoli S., Miccoli R., Navalesi R. Excretion of digoxin-like immunoreactivity in urine of normal subjects: correlations with excretion of creatinine and electrolytes. Clin. Chern. 34: 554, 1988. 26. Clerico A., Boldrini A., Del Chicca M.G. Correlations between digoxin-like immunoreactivity and electrolytes values in urinary samples of newborns. BioI. Neonate 50: 27, 1986.
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