Naunyn-Schmiedeberg's

Archivesof Pharmacology

Naunyn-Schmiedeberg's Arch. Pharmacol. 292, 7 3 - 7 7 (1976)

9 by Springer-Verlag 1976

Methylxanthines and Intestinal Drug Absorption E. Beubler and F. Lembeck Institut fiir experimentelle und klinische Pharmakologie der Universitfit Graz, Universitfitsplatz 4, A-8010 Graz, Austria

Summary. 1. Jejunal loops of anaesthetized rats were filled with buffered isotonic solutions of tritiated water (HTO), urea, antipyrine and salicylic acid at pH 6 - 8 . The venous outflow and the appearence rate of the substances in the intestinal venous blood were determined. Blood pressure was kept constant by adjustable supply of blood from donor rats throughout the experiment. 2. The absorption of urea, antipyrine and salicylic acid was, in concentrations from 0.001 to 1.0 mg/ml found to be directly proportional to the intraluminal concentration. 3. Theophylline and caffeine (2 mg/ml), when injected into the lumen, increased the blood flow to 188 ~o and 166 ~ of controls. 4. The theophylline induced increase in blood flow caused an enhancement in the absorption of antipyrine to 153 ~, HTO and urea to 135~ and salicylic acid to 123~o of controls. 5. Caffeine influenced the absorption of HTO and salicylic acid similar to theophylline. Key words: Theophylline - Caffeine blood flow - Intestinal absorption.

Intestinal

INTRODUCTION A considerable number of investigations about the influence of drugs on intestinal absorption have been carried out. As reviewed by Caspary (1975) drugs like laxatives, diuretics and cardiac glycosides inhibit the absorption of various substances. Little is known about drugs which enhance the absorption of water, drugs or nutrients. Atropine has been found to increase the absorption of water in dogs (Blickenstaff and Lewis, 1952). The parasympatholytic substance propanthelin increases the Send offprint requests to." E. Beubler at the above address.

absorption of digoxin only when the latter is given in tablets but not in solution (Manninen et aI., 1973). The elevated blood level of paracetamol induced by the antiemetic drug metoclopramid has been explained by its intestinal motility increasing action (Nimmo et al., 1973). EDTA leads to an increased intestinal absorption of heparin-like substances and of mannitol and inulin (Windsor and Cronheim, 1961; Schanker and Johnson, 1961). I.v. infusion of isoprenaline increases the intestinal blood flow and the absorption of 85Kr in the cat (Svanvik, 1973). Gibaldi and Feldmann (1970) reviewed the increase of the absorption of various drugs by the application of surfactants. Because of controversal results about the influence of caffeine on drug absorption (Schmidt and Fanchamps, 1974; Siegers, 1973; Siegers el al., 1972; Lippert, 1965), we studied the influence of blood flow enhancing doses of methylxanthines on the absorption of some model substances. The model substances were tritiated water and urea as small, hydrophylic, unionized molecules, antipyrine at pH 8 as a lipophylic, not dissociated substance and salicylic acid at pH 6 as an almost completely dissociated lipophylic compound. METHODS Sprague Dawley rats (breeder: Chemie Linz AG.) of both sexes and weighing 250 4- 20 g were used after being deprived of food for 20 hrs prior to the experiments. The intestinal absorption was studied by using the method of Winne (1966) except that the blood pressure of the rat was kept constant at 6 0 - 85 mm Hg throughout the experiment. A jejunal loop of about 7 cm length was fitted with polyethylene tubes on the oral and aboral end, the descending jejunal vein was punctured and the blood collected. The loss of blood was compensated by an adjustable supply of heparinized blood from donor rats. The venous outflow from the loop was collected in periods of 5 min. At the begi~ming of each 5 min period the loop was filled with 0.25 ml of buffered solutions of the model substances. At the end of the period the loop was emptied and refilled again. Emptying and refilling lasted 15 sec. The following buffered solutions were applied to the loop:

74 a) During the first three control periods (C1, C2, C3) isotonic Soerensen phosphate-buffer (pH 6 - 8 ) , containing the ~4C-labelled model-compounds. b) During the next experimental periods (El, E2, E3) the same solutions, but also containing theophylline or caffeine. c) During the last three periods (C4, C5, C6) the same solutions as injected in a). By this procedure controls and methylxanthine treatment could be studied on the same loop. The absorption of the following model-substances was investigated: a) Tritiated water (HTO) (Packard, specific activity 2.63 9 106 dpm/g), applied in a concentration of 40 gCi/ml buffer (pH 7). b) Urea and 14C-urea (0.5 ~tCi/ml, pH 7). c) Antipyrine and ~4C-antipyrine (0.1 gCi/ml, pH 8). d) Salicylic acid and 14C-salicyclic acid (0.5 gCi/ml, pH 6), all diluted in isotonic Soerensen phosphate buffer. For the determination of radioactivity in the venous effluent, each 5-rain blood sample was weighed and ceutrifuged. 0.1 ml plasma was transferred into 1 ml Soluene-100 and, after addition of 10 ml toluene-dioxan-alcohol scintillator, the radioactivity was measured in a Packard scintillation counter, Model 3380. The quench correction was made by an internal standard. Blood flow (ml 9 rain -I . g-~) was calculated from the weight of the collected Mood and the wet tissue weight of the loop. Absorption (gg. min - i . g - l ) was calculated from blood flow and the appearance of tritiated water or i4C-activity in the venous effluent. Normal absorption was calculated from the controls (C1, C2, C3) in the different rats. The alternation of the absorption was statistically confirmed by Student's t-test between the control period and the period with the highest absorption.

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A. Absorption of the Model Substances a) Antipyrine. The absorption of antipyrine was investigated at concentrations of 0.001, 0.1, 1.0, 2.0 and 10.0 mg/ml in the lumen. The results showed that in a large concentration range (104) the appearance of antipyrine in the venous effluent of the intestine was directly proportional to the concentration of this drug in the lumen (correlation coefficient r = 0.9994, P < 0.01, Fig. 1). b) Salicylic Acid. Salicylic acid was administered in concentrations of 0.001, 0.01 and 1.0 mg/ml. Again the absorption was directly proportional to the intraluminal concentration (r = 0.9991, P -- 0.01, Fig. 1). c) Urea. At the intraluminal urea-concentrations of 0.002 and 2.0 mg/ml the absorption was found to be proportional to the intraluminal concentration. (r = 0.9943, P < 0.01, Fig. 1). B. a) Influence of Theophylline on Intestinal Blood Flow. Theophylline (2 mg/ml) increased the blood flow in the course of three periods from 0.624 ml min-i . g - t (J00~) to 1.175ml - rain -i 9 g-1 (188~). During the following control periods the flow returned to 0.694ml. rain t . g-~. A concentration of 0.5 mg/ml theophylline increased the flow only to 133 ~o (Fig. 2). 9

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b) Influence of Caffeine on Intestinal Blood Flow. Caffeine (2.0 mg/ml) increased the intestinal blood flow to 166%. This is also reversible during the following control periods (Fig. 2). C. Influence of Theophylline on the IntestinaI Absorption of the Model Substances The intestinal absorption was studied by applying solutions of model substances (HTO, urea, antipyrine, salicylic acid) in several concentrations with and with-

Methylxanthines and Intestinal Drug Absorption

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Fig. 3. Increase in absorption with 2.0 mg/ml theophylline. (Abscissa) As in Fig. 2; (Ordinate) (a) Increase in blood flow with theophylline. Increase in absorption of (b) tritiated water (n = 18) (i 1); 0.002 mg/ml urea (n = 3) (O ); 2.0 mg/ml urea (n - 3) ( o - - - O); (c) 0.00I mg/ml antipyrine (n = 4) (A A); 1.0 mg/ml antipyrine (n = 7) ( A - - A); (d) 0.001 rag/m] salicylic acid (n = 3) (O O) ; 0.01 mg/ml salicylic acid (n = 3) (O . . . . . O); 1.0 mg/ml salicylic acid (n = 3) (O . . . . O). All values are refered to C3 = 100%. * P < 0.1, ** P < 0.05, *** P < 0.01

out theophylline (2 mg/ml) into the lumen. A comparison of the percentual changes in blood flow and in absorption with and without theophylline is summarized in Fig. 3. a) Tritiated Water (HTO). The absorption of HTO rose almost parallel to the increase in blood flow rate, from 100 % (control) to about 140 ~ during the E1 period; thereafter, in spite of the further increase in blood flow, it dropped (Fig. 3 b). b) Urea. Absorption of urea (0.002 mg/ml) rose with the blood flow rate to 133~. When the blood flow decreased, also the absorption decreased. At the higher concentration (2 mg/ml) the absorption rose only to 1 2 2 ~ (Fig. 3b).

c) Antipyrine. The change in the absorption ofantipyrine (0.001 and 1.0 mg/ml) with theophylline was determined. After the first dose of theophylline the absorption of antipyrine increased from control (100~) to 153%, whereas the blood flow rose to 143 ~. A further increase could not be attained during the E2 and E3 period. Rather, the absorption dropped while the blood flow rate still rose. The same could be observed with the higher concentration of antipyrine: the absorption increased approximately to 135~ (Fig. 3c). d) Salicylic Acid. Absorption of salicylic acid (0.001, 0.01 and 1.0mg/ml) increased under the influence of theophylline but to a smaller degree than antipyrine, never exceeding 123 ~o (Fig. 3 d). D. Caffeine The influence of caffeine was only studied on the absorption of HTO and on salicylic acid (1 mg/ml). The changes were fairly similar to those with theophylline (Fig. 4). With all investigated substances the absorption seemed to rise most markedly at the lowest concentrations. A significant difference between the concentrations could only be found with antipyrine (P < 0.05).

76

DISCUSSION

a) Absorption of the Model Substances It was shown that the absorption of the model substances is directly proportional to the concentration in the lumen in a large concentration range (103 or 104). This linear relation indicates that absorption of the model substances follows the rules of diffusion (Schanker et al., 1958; Schanker, 1959; Schedl and Clifton, 1961) and excludes an active transport mechanism. On a molar basis the absorption of antipyrine and urea was almost equal, that of salicylic acid 1.5 times higher.

b) Increase in Intestinal Blood Flow by Theophylline The vasodilatation by theophylline is explained by its inhibition of the phosphodiesterase (Lugnier et al., 1972). The concentration of cAMP increases in mesenteric arteries of the dog during inhibition of the phosphodiesterase by papaverine (Shepherd et al., 1973) as well as in the isolated mucosa of the guinea pig by theophylline (Karppanen et al., 1974). Therefore an attempt was made to enhance intestinal blood flow by perfusion of theophylline or caffeine. A concentration of 2 mg/ml theophylline in the intestinal lumen increased the blood flow to 188 ~. Caffeine in the same dosage enhanced blood flow to 166~o. The amount of venous outflow as measured does, however, not differentiate between the total and the mucosal blood flow. This must be considered since Svanvik et al. (1973) showed in the cat that isoprenaline increased blood flow in the villi to a greater extend than the total intestinal blood flow. A different method of studying the intestinal absorption in relation to the intestinal blood flow was used by Ochsenfahrt and Winne (1969). They regulated the intestinal blood flow by increasing or lowering the blood pressure of the rat by varying the supply of blood from donor rats.

c) The Influence of Theophylline on Drug Absorption Several investigators have found a relationship between intestinal blood flow and drug absorption. Goldberg and Fine (1945) observed a diminished intestinal absorption of glucose and water in haemorrhagic shock. The absorption of glucose and xylose was found to be reduced when mesenteric blood flow was diminished. Increased blood flow, however, did not increase the absorption (Williams et aI., 1964). Our results show that absorption of all of the model substances used in this study was increased when blood flow was enhanced by methylxanthines. The influence of blood flow on the absorption was most pronounced with antipyrine. Less markedly influenced

E. Beubler and F. Lembeck

were tritiated water, urea and salicylic acid. The increase in absorption during the first period with theophylline (El period) was nearly parallel to the increase in blood flow with all model substances. In spite of further increase in blood flow during the E2 and E3 period, absorption of the model substances did not increase further except for tritium water in the experiment with caffeine. Rather a declining tendency can be seen, which could not be confirmed statistically in the single experiments. This could be explained as follows: a) The action of theophylline first takes place at the site of absorption by increasing blood flow in the villi, which leads to an enhanced absorption of the model substances. As soon as theophylline affects the blood flow also in deeper layers, a "steal phenomenon" could result: the total blood flow but not the blood flow in the villi increases further and therefore no additional increase in absorption of drugs occurs. b) Another possibility could be a delayed opening of shunts in deeper layers which would not affect the blood flow in the villi but enhance the total blood flow (Delenay, 1969; Renkin, 1971; Zweifach, 1974). The opening of shunts could lead to a compensatory decrease of the blood flow in the villi and therefore to a decrease in absorption rate. c) It cannot be excluded that theophylline affects the absorption by another mechanism beside the action on blood flow. Ochsenfahrt and Winne (1969), Winne and Remischowsky (1971) and Winne (1972) found an increase in absorption of the same substances of only 0 to 20 ~ , when the blood flow was passively increased by an increased blood pressure. This contrasts with our experiments in which, by means of theophylline, the increase in absorption was higher in all the experiments, i.e. in the range of 2 3 - 50 ~. The experiments of Svanvik (1973) who found that the blood flow through the villi is almost independent of the perfusion pressure, may offer an explanation for this discrepancy. Isoprenaline also increases the blood flow in the villi much more than the total blood flow. The findings of Winne (1966) that the vasoconstriction by noradrenaline diminished the absorption of HTO more than a comparable reduction of the flow attained by lowering the perfusion pressure (Winne, 1972) might be interpreted in the same way.

REFERENCES Blickenstaff, D. D., Lewis, L. J. : Effect of atropine on intestinal absorption. Amer. J. Physiol. 170, 1 7 - 2 3 (1952) Caspary, W. F.: Die Beeinftussung der Resorptionsleistungen der Darmmucosa durch Pharmaka. Arzneimittel-Forsch. (Drug Res.) 25, 489-498 (1975)

Methylxanthines and Intestinal Drug Absorption Delanay, J.P.: Arteriovenous anastomotic blood flow in the mesenteric organs. Amer. J. Physiol. 216, 1556-1561 (1969) Gibaldi, M., Feldmann, S.: Mechanisms of surfactant effects on drug absorption. J. pharm. Sci. 59, 579-589 (1970) Goldberg, M., Fine, J. : Traumatic shock. XI. Intestinal absorption in hemorrhagic shock. J. clin. Invest. 24, 445-450 (1945) Karppanen, H. O., Neuvonen, P. J., Bieck, P. R., Westermann, E. : Effect of histamine, pentagastrin and theophylline on the production of cyclic AMP in isolated gastric tissue of the guinea pig. Naunyn-Schmiedeberg's Arch. Pharmacol. 284, 15-23 (1974) Lippert, T. H., Samaha, M.: Die Beeinflussung der analgetischen Wirkung des Salicylamids durch Coffein. Arzneimittel-Forsch. (Drug Res.) 15, 1079-1080 (1965) Lugnier, C., Bertrand, Y., Stoclet, J. C.: Cyclic nucleotide PDE inhibition and vascular smooth muscle relaxation. Europ. J. Pharmacol. 19, 134-136 (1972) Manninen, V., Apajaiahti, A., Melin, J., Karesoja, M.: Altered absorption of digoxin in patients given propantheline and metoclopramide. Lancet 1973I, 398- 399 Nimmo, J., Heading, R. C., Tothill, P., Prescott, L. F.: Pharmacological modifications of gastric emptying. Brit. reed. J. 1973I, 587- 589 Ochsenfahrt, H., Winne, D. : Der Einftul3 der Durchblutung auf die Resorption von Arzneimitteln aus dem Jejunum der Ratte. Naunyn-Schmiedebergs Arch. Pharmak. 264, 55-75 (1969) Renkin, E. M. : Nutritional shunt-flow hypothesis in skeletal muscle circulation. Circulat. Res. 28, (Suppl. I) 2 1 - 2 5 (1971) Schanker, L. S. : Absorption of drugs from the rat colon. J. Pharmacol. exp. Ther. 126, 283-290 (1959) Schanker, L. S., Johnson, J. M.: Increased intestinal absorption of organic compounds in the presence of EDTA. Biochem. Pharmacol. 8, 421-422 (1961) Sck/anker, L. S., Tocco, D.J., Brodie, B.B., Hogben, C. A. M.: Absorption of drugs from the rat small intestine. J. Pharmacol. exp. Ther. 123, 81-88 (1958) Schedl, H. P., Clifton, J. A. : Small intestinal absorption of steroids. Gastroenterology 41, 491-499 (1961)

77 Schmidt, R., Fanchamps, A. : Effect of caffeine on intestinal absorption of ergotamine by man. Europ. J. clin. Pharmacol. 7, 213- 216 (1974) Shepherd, A. P., Mao, C. C., Jacobson, E.D., Shanbour, L. L. : The role of cAMP in mesenteric vasodilatation. Microvasc. Res. 6, 332--341 (1973) Siegers, C. P. : Effects of caffeine on the absorption and analgesic efficacy of paracetamol in rats. Pharmacology 10, 19 - 27 (1973) Siegers, C. P., Strubelt, O., Bach, G.: Inhibition by caffeine of ethanol absorption in rats. Europ. J. Pharmacol. 20, 18I-187 (1972) Svanvik, J. : Mucosal blood circulation and its influence on passive absorption in the small intestine. Acta physiol, scand., Suppl. 385 (1973) Williams, J. H., Mager, M., Jacobsen, E. D. : Relationship of mesenteric blood flow to intestinal absorption of carbohydrates. J. Lab. clin. Med. 63, 853-863 (1964) Windsor, E., Cronheim, G. E.: Gastro intestinal absorption of heparin and heparinoids. Nature (Lond.) 190, 263-264 (1961) Winne, D. : Einflul3 einiger Pharmaka auf die Darmdurchblutung und die Resorption tritiummarkiertem Wassers aus dem D/inndarm der Ratte. Naunyn-Schmiedebergs Arch. Pharmak. exp. Path. 254, 199-224 (1966) Winne, D. : The influence of blood flow and water net flux on the absorption oftritiated water from the jejunum of the rat. NaunynSchmiedeberg's Arch. Pharmacol. 272, 417-436 (1972) Winne, D., Remischowsky, J. : Der Einflul3 der Durchblutung auf die Resorption von Harnstoff, Methanol und Athanol aus dem Jejunum der Ratte. Naunyn-Schmiedebergs Arch. Pharmak. 268, 392-416 (1971) Zweifach, W.: Quantitative studies of microcirculatory structure and function. I. Analysis of pressure distribution in the terminal vascular bed in cat mesentery. Circulat. Res. 34, 843 857 (1974)

Received July 19/Accepted September 30, 1975

Methylxanthines and intestinal drug absorption.

Naunyn-Schmiedeberg's Archivesof Pharmacology Naunyn-Schmiedeberg's Arch. Pharmacol. 292, 7 3 - 7 7 (1976) 9 by Springer-Verlag 1976 Methylxanthin...
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