SHORT COMMUNICATION
INTESTINAL ABSORPTION OF DIGOXIN AND INTERACTION WITH NIMODIPINE IN RATS
Fatma S. Kilic
, Özlem Batu, Baºar Sirmagul, Engin Yildirim, Kevser Erol
Department of Pharmacology, Medical Faculty, Osmangazi University, 26480 Meºelik, Eskiºehir, Turkey
Intestinal absorption of digoxin and interaction with nimodipine in rats.
F.S. KILIC, Ö. BATU, B. SIRMAGUL, E. YILDIRIM, K. EROL. Pol. J. Phar- macol., 2004, 56, 137–141.
It is known that digoxin, which is a liposoluble cardiac glycoside, is well absorbed from intestine. In the present study, the absorption rates of digoxin from rat duodenum and the proximal and terminal parts of small intestine were determined in vitro. The isolated everted duodenum and intestinal sacs were put into oxygenated Tyrode solution at 37°C. The Tyrode solution on the outer, mucosal side of intestinal segments contained 0.3 mM digoxin.
Samples from the internal serosal side of the intestinal sacs were taken at 30, 60 and 120 min after the start of the experiments. The concentration of di- goxin in the samples of fluid were determined using a radioimmunoassay method. The effect of nimodipine (0.1 and 0.2 mM) on digoxin absorption was also evaluated on the terminal segment of rat intestine. The interaction of nimodipine (0.5 mg/kg) and digoxin (0.2 mg/kg) was investigated in vivo when they were given perorally to rats. The duodenal absorption of digoxin was lower than in the small intestine. The highest absorption occured in the terminal segment of the small intestine. Nimodipine increased the absorption of digoxin from the terminal segment of intestine in vitro, while it did not af- fect the serum digoxin concentration in vivo.
Key words: digoxin, intestinal absorption, nimodipine ISSN 1230-6002
INTRODUCTION
The everted intestine sac, first described by Wilson and Wisemans, has been used to study drug absorption and metabolism [23]. This system is simple, quick, very reproducible and inexpensive.
It can provide information on the mechanism of drug absorption and can be used to test the effects of enhancers and formulations on absorption as well as the optimal sites in the small intestine [1].
Several studies have shown the potential inter- actions between digoxin and calcium channel an- tagonists [6, 7]. Digoxin is usually well absorbed from the gastrointestinal tract and its secretion is dependent on renal mechanisms, primarily glom- erular filtration [10]. An increase in blood digoxin levels and a decrease in digoxin clearance have been observed when calcium antagonists were added to digoxin therapy [6, 15, 19]. P-Glycopro- tein (P-gp), which is plasma membrane protein, has been demonstrated in many tissues [22]. P-gp in the intestine is known as an efflux transporter of cer- tain drugs including digoxin and was shown to limit its absorption [8]. Recently, it was also clari- fied that certain 1,4-dihydropyridine calcium an- tagonists were inhibitors of P-gp [9]. The oral bioa- vailability of digoxin is between 90% and 100%
[10]. The oral availability of nimodipine was re- ported to be approximately 10% [11].
In the present study, we aimed to investigate the absorption rate of digoxin from rat duodenum, proxi- mal and distal parts of small intestine in vitro and the possible influence of nimodipine on the intestinal ab- sorption of digoxin in vitro and in vivo in rats.
MATERIALS and METHODS Albino rats of both sexes weighing 200–250 g were used. They were fasted for 16 h with access to drinking water ad libitum. All experiments for ani- mal testing were approved by Osman Gazi Univer- sity School of Medicine, Animal Use and Care Committee.
Experiments in vitro
The rats were killed by cervical dislocation after being given an overdose of ether. Their abdominal wall was dissected and their duodenum, and 15 cm of the proximal and terminal parts of the small in- testine were removed. The parts of the intestine were turned inside out as described by Wilson and
Wiseman [23]. The everted parts of intestine were cut into 4–5 cm long segments and put into oxy- genated Tyrode solution (composition in mmol/l:
NaCl 136.9, KCl 2.68, CaCl2 1.8, MgCl2 1.05, NaHCO3 1.19, NaH2PO4 0.42 and glucose 0.55).
One end of each segment of the intestine was closed with a ligature while the opposite end was tied around the lower end of a plastic cannula. The cannulae and the intestinal sac were mounted in 10 ml isolated organ baths filled with Tyrode solution at 37°C and gassed with 95% O2and 5% CO2. The 0.3 ml of Tyrode solution was injected into the sac by using the cannulae. Two groups of experiments were carried out:
— Tyrode solution on the outer side containing 0.3mM digoxin
— Tyrode solution on the outer side containing 0.3mM digoxin and 0.1 or 0.2 mM nimodipine.
— The preparations were washed out every 15 min and the composition of Tyrode solution on the other side of intestinal sac was kept as de- scribed above. Samples from the internal side of intestinal sac were taken 30, 60, 120 min af- ter the start of experiments. One sample was taken from each intestinal segment. The experi- ments on the interactions of digoxin and nimo- dipine were carried out only on the terminal parts of the intestine.
Experiments in vivo
The 0.2 mg/kg of digoxin or 0.2 mg/kg of digoxin and 0.5 mg/kg of nimodipine were given orally to rats fasted for 16 h, using a plastic cannula. One milliliter of blood was taken from the heart of the animals anesthetized with ether 120 min after the administration of digoxin or the combination of di- goxin and nimodipine.
We determined the concentration of digoxin in samples by using commercially available radioim- munoassay kits (DPC).
Statistical analysis
Results were expressed as means ± SEM and sta- tistical significances were calculated by using the Student’s t-test and one-way analysis of variance.
RESULTS
The absorption of digoxin from the duodenal segment increased time-dependently while the ab- sorption from the distal segment declined time-
dependently (p < 0.05). The difference in the absorp- tion of digoxin between proximal and distal segments of intestine was not significant (p > 0.05). The duo- denal absorption of digoxin was lower than those of proximal and distal segments at 30 and 60 min (p
< 0.05). There were no differences between these segments in the absorption of digoxin at 120 min (Fig. 1).
Nimodipine increased time and/or dose-depen- dently (p < 0.05) the absorption of digoxin from the distal part of the intestine in vitro (Tab. 1). But there was no significant change in the interaction of di- goxin and nimodipine in vivo (Tab. 2).
DISCUSSION
We aimed to investigate the interaction of di- goxin and nimodipine in vitro and in vivo and the intestinal absorption rate of digoxin in vitro.
The everted intestinal sac was first used to ex- amine transport of sugar and amino acids. This technique is very simple, cheap and reproducible and has advantages as an in vitro system to study drug absorption [1] along the length of the gastro- intestinal tract. In the present study, we observed that there were differences in the absorption of di- goxin through the intestinal segments. The lowest rate of absorption was in the duodenal segment.
There was a significant difference in the absorp- tion of digoxin from distal segment between 60 and
120 min. The presents results show that there is a regional difference in the absorption rate of digoxin from the intestine of rat in vitro at 30 min. But the difference disappeared in 2 h. These results are not in accordance with observations indicating that the highest absorption rates of digoxin can be observed in the duodenum and jejunum, and the lowest in the colon in an in situ model in rats [17]. Autors of the- se studies reported that the P-gp-dependent exsorp- tion appeared to be the highest in the duodenum and jejunum and non existent in the colon, since nimodipine which is a P-gp inhibitor increased the absorption rate in the duodenum and jejunum but no significant effect was seen in the colon. So we used the distal segment to investigate the influence of nimodipine on the absorption of digoxin.
It is common to co-administer digoxin and cal- cium antagonists to control some cardiovascular symptoms. Calcium antagonists such as verapamil, nifedipine and diltiazem have been reported to in- crease the serum digoxin concentration [2, 13, 14].
Some possible mechanisms by which they may in- crease serum glycoside levels have been suggested:
1) by enhancing digoxin absorption, 2) by displac- ing digoxin from tissue binding sites, 3) by de- creasing renal excretion [5]. However, it was re- ported that certain calcium antagonists, such as
Groups 60 min
(pg/ml) 120 min
(pg/ml) Digoxin (0.3 mM)
(n = 10) 73.70 ± 4.91 82.70 ± 8.82
Digoxin (0.3 mM) + nimodipine (0.1 mM) (n = 8)
108.43 ± 9.05* 88.63 ± 6.37
Digoxin (0.3 mM) + nimodipine (0.2 mM) (n = 8)
99.88 ± 10.42* 123.00 ± 9.43*
Table 1.The effect of nimodipine on digoxin absorption from the distal segment of rat intestine in vitro
* p < 0.05 as compared to digoxin group
Groups 120 min
(pg/ml) Digoxin (0.2 mg/kg) (n = 8) 27.14 ± 5.6 Digoxin (0.2 mg/kg) + nimodipine
(0.5 mg/kg) (n = 8) 24.51 ± 4.31
Table 2.The interaction of digoxin and nimodipine in vivo
*** *
0 20 40 60 80 100 120 140
0 30 60 90 120 150
Time (min)
Concentration(pg/ml)
Duodenal Proximal s.
Distal s.
Fig. 1. The absorption rates of digoxin from the isolated rat duodenum and distal segments of small intestine (n = 10 at each point). ***p < 0.001 as compared to proximal and distal segments, *p < 0.05 as compared to proximal segment
verapamil, diltiazem, nifedipine and nicardipine in- hibited the intestinal absorption of digoxin in the guinea pig in vitro and in vivo [4]. The increased serum digoxin concentration by verapamil was par- tially explained by reduced renal excretion [13]. It was reported that 1,4-dihydropyridine calcium an- tagonists had inhibitory effects on cytochrome P450 (CYP) 3A4. Additionally, it was shown that the substrates and/or inhibitors of CYP3A4 and P-gp overlapped [9]. It was clarified that 1,4-dihy- dropyridine calcium antagonists have different in- hibitory potencies on the P-gp-mediated transport.
For example, nicardipine was found to be a potent antagonists of P-gp [9]. However, nifedipine was reported not to inhibit the P-gp-mediated transport [9]. It was also shown that the difference in the in vivo interactions of digoxin with nicardipine or nifedipine was in accordance with the inhibitory potency of the digoxin and the CYP3A4 activity.
Additionally, it was reported that the concentra- tion dependence of intestinal permeation in the ab- sorptive and secretory directions was consistent with saturability of a secretory transport system in vitro. It was also suggested that the cationic com- pounds (such as verapamil, chlorpromazine) were transported by P-gp-mediated efflux systems with different substrate specificities depending on the intestinal site [18].
The use of verapamil and diltiazem is thought to be limited in patients with heart disease because of their negative inotropic effects and interaction with digoxin [13–16]. Medium-dose of diltiazem when combined with digoxin was found clinically effective and safe to treat the patients with chronic atrial fibrilation [16]. But diltiazem has been re- ported to elevate serum digitalis concentration in the kidneys but not the absorption [14]. Oral nife- dipine or amlodipine administration was shown to give no clinically detectable changes in serum di- goxin concentrations in patients [20, 21], but some investigators reported that nifedipine significantly increased plasma digoxin concentration [3, 12].
The disparity in data obtained with different drugs suggested that the interaction with digoxin was not releated to the effects on calcium channel activity but depended on their chemical structures or sub- classification of each drug [20].
The present study indicates that the absorption rate of digoxin from the rat duodenal and intestinal segments was different and nimodipine increased the absorption of digoxin from the distal part of in-
testine in vitro. But nimodipine did not influence the serum digoxin concentration in vivo. However, further investigations are needed to clarify the me- chanism of interaction between nimodipine and di- goxin.
REFERENCES
1. Barthe L, Woodley J, Houin G: Gastrointestinal ab- sorption of drugs: methods and studies. Fundam Clin Pharmacol, 1999, 13, 154–168.
2. Belz GG, Aust PE, Munkes R: Digoxin plasma con- centrations and nifedipine. Lancet, 1981, 2, 844–845.
3. Belz GG, Doering W, Munkes R, Matthews J: Interac- tion between digoxin and calcium antagonists and an- tiarrhythmic drugs. Clin Pharmacol Ther, 1983, 33, 410–417.
4. Budihna MV, Strojan P: Ca++channel antagonists in- hibit the intestinal absorption of digoxin in the gui- nea-pig. Eur J Pharmacol, 1993, 230, 301–305.
5. De Cesaris R, Belestrazzi M, Chiarappa R, Ranieri G:
Increased digitalis-like activity in combined admini- stration of calcium antagonists and digitalis prepara- tions. Gazz Ital Cardiol, 1983, 13, 188–191.
6. De Vito JM, Friedman B: Evaluation of the pharmaco- dynamic and pharmacokinetic interaction between calcium antagonists and digoxin. Pharmacotherapy, 1986, 6, 73–82.
7. Echizen H, Eihelbaum M: Clinical pharmacokinetics of verapamil, nifedipine and diltiazem. Clin Pharma- cokinet, 1986, 11, 425–449.
8. Greiner M, Eichelbaum M, Fritz P, Kreichgauer HP, von Richter O, Zundler J, Kroemer HK: The role of intestinal P-glycoprotein in the interaction of digoxin and rifampin. J Clin Invest, 1999, 104, 147–153.
9. Katoh M, Nakajima M, Yamazaki H, Yokoi T: Inhibi- tory potencies of 1,4-dihydropyridine calcium antago- nists to P-glycoprotein-mediated transport: compari- son with the effects on CYP3A4. Pharm Res, 2000, 17, 1189–1197.
10. Kelly AR, Smith TW: Pharmacological treatment of heart failure. In: Goodman & Gilman’s The Pharma- cological Basis of Therapeutics, 9th edn., McGraw- Hill Co, New York, 1996, 809–838.
11. Kelly AR, Smith TW: Pharmacological treatment of heart failure. In: Goodman & Gilman’s The Pharma- cological Basis of Therapeutics, 9th edn., McGraw- Hill Co, New York, 1996, 1765.
12. Kirsh W, Hutt HJ, Dylewicz P, Gray KO, Ohnhaus EE: Dose-dependence of nifedipine-digoxin interac- tion. Clin Pharmacol Ther, 1986, 39, 35–39.
13. Klein HO, Lang R, Weiss E, Segni ED, Libhaber C, Guerrero J, Kaplinsky E: The influence of verapamil on serum digoxin concentration. Circulation, 1982, 65, 998–1003.
14. Oyama Y, Fuji S, Kanda K, Akino E, Kawasaki H, Nagata M, Goto K: Digoxin-diltiazem interaction. Am J Cardiol, 1984, 53, 1480–1481.
15. Reicher-Reiss H, Neufeld HN, Ebner FX: Calcium an- tagonists – adverse drug interactions. Cardiovasc Drugs Ther, 1987, 1, 403–409.
16. Roth A, Harrison E, Mitani G, Cohen J, Rahimtoda SH, Elkoyam U: Efficacy and safety of medium and high dose diltiazem alone and in combination with di- goxin for control of heart rate at rest and during exer- cise in patients with chronic atrial fibrilation. Circula- tion, 1986, 73, 316–324.
17. Sababi M, Borga O, Hultkvist-Bengtsson U: The role of P-glycoprotein in limiting intestinal regional ab- sorption of digoxin in rats. Eur J Pharm Sci, 2001, 14, 21–27.
18. Saitoh H, Aungst BJ: Possible involvement of multi- ple P-glycoprotein-mediated efflux systems in the transport of verapamil and other organic cations across rat intestine. Pharm. Res, 1995, 12, 1304–1310.
19. Schwartz JB: Digoxin and nifedipine. Am J Cardiol, 1985, 55, 31E–34E.
20. Schwartz JB: Effects of amlodipine on steady-state di- goxin concentrations and renal digoxin clerance. J Car- diovasc Pharmacol, 1988, 12, 1–5.
21. Schwartz JB, Raizner AE, Akers S: The effect of nife- dipine on serum digoxin concentrations in patients.
Am Heart J, 1984, 107, 669–673.
22. Thiebout T, Tsurno T, Hamada H, Gottesman MM, Postan I, Willingham MC: Cellular localization of the multidrug-resistance gene product P-glycoprotein in normal tissues. Proc Nat Acad Sci USA, 1987, 84, 7735–7738.
23. Wilson TH, Wiseman G: The use of sacs of everted small intestine for the study of the transference of sub- stance from the mucosal to the serosal surface, J Physiol, 1954, 123, 116–125.
Received: April 25, 2003; in revised form: November 20, 2003.
