Naunyn-Schrniedeberg's Naunyn-Schmiedeberg's Arch. Pharmacol. 296, 139- 142 (1977)
Archivesof
Pharmacology
9 by Springer-Verlag1977
Analysis of the Actions of Cimetidine and Metiamide on Gastric Acid Secretion in the Isolated Guinea Pig Gastric Mucosa S V E N E R I K S J O S T R A N D 1, B I R G I T T A R Y B E R G 1, a n d L A R S O L B E 2 1 Department of Pharmacology, AB H/issle, Fack, S-43120 MSlndal 2 Department of Surgery II, Sahlgren's Hospital, S-40033 G6teborg, Sweden
Summary. C u m u l a t i v e dose-response curves for histam i n e were d e t e r m i n e d o n acid secretion f r o m the isolated g u i n e a pig gastric mucosa. T w o H2-receptor a n t a g o n i s t s - m e t i a m i d e a n d c i m e t i d i n e - b e h a v e d like competitive a n t a g o n i s t s to h i s t a m i n e o n gastric acid secretion in vitro. T h e isolated g u i n e a pig gastric m u c o s a seems to be a suitable in vitro m o d e l for a n a l y s i n g the a c t i o n of c o m p o u n d s o n receptors i n v o l v e d in acid secretion. Key words: G u i n e a pig G a s t r i c acid secretion Hz-receptor-antagonists.
Isolated s t o m a c h Histamine - Histamine
INTRODUCTION C i m e t i d i n e is the latest s u b s t a n c e in a series of H2receptor a n t a g o n i s t s ( B r i m b l e c o m b e et al., 1975). P h a r m a c o l o g i c a l d a t a indicate that this c o m p o u n d is a c o m p e t i t i v e a n t a g o n i s t at h i s t a m i n e receptors in the g u i n e a pig heart a n d rat uterus, as has also b e e n s h o w n for m e t i a m i d e (Black et al., 1973). The latter c o m p o u n d has also been s h o w n to act as a competitive a n t a g o n i s t o n gastric acid secretion in vitro (Parsons, 1975) a n d in vivo (Parsons, 1973). T h e m a m m a l i a n gastric m u c o s a isolated in a n o r g a n b a t h secrets acid a n d r e s p o n d s to chemical stimuli such as h i s t a m i n e (Parsons, 1975; Spencer, 1973; W a n et al., 1974; H o l t o n a n d C u r w a i n , 1973). I n the p r e s e n t study the g u i n e a pig isolated gastric m u c o s a has b e e n used. This p r e p a r a t i o n is particularly suitable as the s t o m a c h wall is very t h i n (Spencer, 1973) a n d also due to the high sensitivity to h i s t a m i n e in this species.
Send offprint requests to: S. E, Sj6strand at the above address
T h e a i m o f the present study was to d e t e r m i n e the dose-response curve for h i s t a m i n e a n d investigate the i n t e r a c t i o n of c i m e t i d i n e a n d m e t i a m i d e o n the histam i n e dose-response.
MATERIALS AND METHODS Female guinea pigs weighing 300-350 g were used. The animals were starved for 24 h but given water. The guinea pigs were killed by a blow on their heads and the stomach was rapidly removed. Two pieces of the secretory portion of the stomach were cut out. On each piece the outer muscular layer was stripped off. The stomach preparations were then mounted on the end of funnel shaped plastic containers with the mucosal surface facing inwards. The secretory area was 2 cm2. In each experiment two guinea pigs were used thus giving 4 isolated mucosa preparations running parallel. Each preparation was immersed in separated organ baths containing 30 ml of a serosal solution having the following ionic composition (mM): Na + 140.9; K + 5.8; Ca2+ 2.6; C1- 125.8; SO42- 1.2; HCO~ 24.9; HPO~- 1.2. The solution contained 25.0 mM glucose and was gassed with 95 ~o 02 and 5 ~ COz. To the mucosat part was added 3 - 5 ml of a solution with the same composition as the serosal solution except that NaHCO3 and KH2PO4 were replaced by equimolar amounts of NaCI and KC1. The mucosal solution contained 10.0 mM glucose and was gassed with 100~ Oz. All experiments were performed at 36~C. Acid production was determined by continuous titration using a Radiometer pH-stat (TTT2, TTT60) and an Autoburette (ABU13) for recording the amount of 0.025 M NaOH required to maintain. the mucosal solution at pH 3.8. In each experiment the spontaneous secretion was followed for about 1 h. During this time the spontaneous secretory rate was stabilized. Histamine was added to the serosal bath repeated times for obtaining a cumulative dose-response. Between each drug administration the secretory level was allowed to reach a steady state level for about 20 rain. In experiments carried out for dissociation constant estimations only two different doses of histamine were used, chosen to give about 50 ~ of maximal response. The two antagonists used, metiamide and cimetidine, were dissolved in the nutrient solution and added to the serosai bath together with histamine at a dose giving a combined agonist-antagonist response close to 50 ~ of the maximal histamine response.
140
Naunyn-Schmiedeberg's Arch. Pharmacol. 296 (1977)
Several doses of the antagonists were tested in each mucosal preparation. The receptor dissociation constants (KB) were calculated according to procedures described in the Appendix. Metiamide and cimetidine were gifts kindly supplied by Smith, Kline & French Laboratories, England.
Inversion and elimination gives: Ks -
[a] [A]AB
[AloB
(7)
1
The significance of KB is illustrated by: KB = [B] APPENDIX
[A]AB
gives [A]oB -- 2. Accordingly KB corresponds to the
According to conventional theories there is a receptor R which can react with an agonist A forming a complex RA resulting in an effect EA. It can also react with an antagonist B forming a complex RB which prevents R from reacting with A and therefore B blocks the effect EA. The formation of RB does not cause any effect EB. The equilibrium of the law of mass action can be applied:
KA. [RA] = [A]. [R] = [ A J . ( R t - [ R A ] - [RB])
(1)
KB. [RB ] = (B]. [R ] = [B ]. (Rt - [RA] - [RB 1)
(2)
where Rt is the total concentration of R in mol/1. [] gives the current concentration of A, B, R, RA, RB. Division of (1) with (2) gives:
[RB] = [RA].
KA" [B]
concentration of the blocking agent B at which the concentration of the stimulating agent must be doubled in order to achieve the same effect.
RESULTS A low spontaneous hydrogen ion secretion from the mucosal preparation was recorded. About 30 min after mounting the preparation there was a steady state level of acid output, the mean rate in 52 control experiments being 0.527 + 0.019 pM/2 cm 2. 10 min -1. Cumulative dose-response curves for histamine were constructed showing an S-shaped form (Fig. 1), with a maximum response being obtained at about 4 x 1 0 - 5 M histamine. The histamine dose-response curve was found to be very steep. There was a linear
(3)
K , . [A]
LIMo{ H+/IO Min,2cm 2
Applying (3) with (~) gives: KA " [RA]
= [A] ( R t -
KA [B]
[RA]
-
[RA])-
KB [A]
200.
(4) 1.80,
9 Control 9 MetiamJde IosM
:
9 Metiamide 5xlgSM
Solving for RA in the absence of B ([RA]oB) gives:
[RA]oB Rt
1
(5)
KA I + - [A]oB where [A]oB is [A] in the absence of B.
1
KA 1 + - [A]o,
[RA]AI~
Rt
tOO "tJ
Q80 0.60 040 0.20
. .
giving:
1
KA 1 + - - + [A]AB
140 1.20
We assume that the effect E emerges from the value [RA]/Rt, i.e. at the same rate of covering A on the receptors. The effect EA is measured at a certain concentration [A]oB. After adding B and new doses of A, the effect EAB is measured. By interpolation [A]A~ is obtained giving an effect EAB which equals Ea.
[RA]oB EA = EAB gives : - Rt
1.60
KA [B] Ks [A]AB
where [RA]AB is [RA] in the presence of B.
3
(6)
6 10 20 40 100 200xl(j6M Log concentration of histamine
Fig. 1. Histamine cumulative log-dose response curves from isolated guinea pig gastric mucosa; without antagonist and after equilibrium with metiamide. The number of experiments in each point are for control 13--22, for metiamide 1 0 - 5 M 5 - 1 5 and for metiamide 5 x 10-SM 4 - 9 . Spontaneous acid secretion subtracted at each point
S. E. Sj6strand et al. : Inhibition of Gastric Acid Secretion in vitro by H2-Blockers
14]
Cimetidine
Metiamide 3.0
1.5
~
1,0
2.0-
1.0
0.5
Log KB
Log K a
Log[B]
Log [B]
Fig. 2. Antagonism of histamine by metiamide on isolated guinea pig gastric mucosa. The plotted values (taken from Table 1) are derived from Eq. (7) and show a linear correlation
Fig. 3. Antagonism of histamine by cimetidine on isolated guinea pig gastric mucosa. The plotted values (taken from Table 1) are derived from Eq. (7) and show a linear correlation
Table 1. Dissociation constants (Ks) for metiamide and cimetidine estimated on isolated guinea pig gastric mucosa using histamine as agonist
showed that the KB at different doses of metiamide or cimetidine were not significantly different. The K~values for the two antagonists showed that the K~ for metiamide was about 10 times higher than that for cimetidine.
Metiamide
Cimetidine
Dose x 105 M
K~ a x 105 M
n
2.0 4.0 8.0 12.0 18.0 24.0
1.57 1.42 1.46 1.52 1.48 1.54
5 4 4 3 4 4
__+ 0.13 __+ 0.18 + 0.15 __+ 0.08 __+ 0.05 __+ 0.15
Dose x 106 M
KB a
2.0 20.0 200.0
1.54 + 0.19 1.41 __+ 0.12 1.55 __+ 0.10
x 106 M 11 8 8
K~ + SEM
pattern between 6 • 10 -6 and 2 x 10 .5 M. The responses to repeated single doses in this range were fairly constant. The antagonism of histamine by metiamide and cimetidine on the isolated guinea pig gastric mucosa estimated from displacement of dose-response curves, has been analysed (Figs. 1, 2 and 3). The influence on the histamine response by the two compounds was recorded as a parallel displacement to the right of the dose-response curves for histamine with unchanged maximal responses. Two-point dose-response curves were made in each preparation allowing intrapolation used for calculations of the receptor dissociation constant (K~) according to Eq. (7) (Table 1). Analysis of variance
DISCUSSION The experimental results of the present study show that histamine produces dose-response related acid secretion from the isolated guinea pig gastric mucosa. The cumulative dose-response curves showed a maximum at about 4x 10-5M compared to 3 x 10-~M in similar rat preparations (Parsons, 1975) indicating that the guinea pig mucosa preparation is more sensitive to histamine than the rat mucosa. This may partly be due to differences in the preparation techniques. Metiamide has produced a parallel displacement to the right of the histamine dose-response curve, indicating competitive receptor antagonism in the isolated rat stomach (Parsons, 1975). Data for metiamide in the present study are in full agreement with those reported by Parsons (1975). Cimetidine was also found to produce a parallel displacement to the right of the histamine dose-acid secretory curve in the isolated guinea pig gastric mucosa. In fact the two antagonists metiamide and cimetidine produced dose related displacement of histamine dose-response curves without significantly affecting the slope or maxinmm and demonstrated
142
Naunyn-Schmiedeberg's Arch. Pharmacol. 296 (1977)
a linear correlation between antagonist log concentration and log
[A]AB
[AJoB
1
from Eq. (7) (Figs. 2 and 3) suggesting a competition of antagonists with histamine for a common site. Thus the present data suggest that cimetidine behaves as a competitive antagonist to histamine on gastric acid secretion in vitro in agreement with the findings that cimetidine acts as a competitive antagonist to histamine in other organs (Brimblecombe et al., 1975). The affinity for cimetidine was about 10 times higher than that for metiamide in the gastric tissue preparation (the dissociation constant KB 10 times lower). However, the affinity constant for metiamide in other tissue preparations (heart and uterus: KB = 7 - - 9 x 10 -7 M, Black et al., 1973) is of about the same magnitude as the affinity constant for cimetidine in the gastric tissue preparation (Table 1) and other tissue preparations (Brimblecombe et al., 1975). Then the observed difference between the affinity constants for cimetidine and metiamide in the gastric tissue preparation does not necessarily reflect a higher affinity to histamine receptors for cimetidine than for metiamide but may merely express a facilitated penetration of the much more water soluble cimetidine
to the histamine receptors in this particular preparation. REFERENCES Black, J. W., Duncan, W. A. M., Emmet, J. C., Ganellin, C. R., Hesselbo, T., Parsons, M.E., Wyllie, J.H.: Metiamide-an orally active histamine Hz-receptor antagonist. Agents and Actions 3, 133-137 (1973) Brimbleemnbe, R. W., Duncan, W. A. M., Durant, C. J., Emmet, J. ~., Ganellin, C. R., Parsons, M. E. : Cimetidine- a non-thiourea H2-receptor antagonist. J. int. reed. Res. 3, 86-92 (1975) Holton, P., Curwain, B.P.: Evidence for the specific action of burimamide and metiamide on the H2-receptors involved in gastric secretion. International Symposium on Histamine HzReceptor Antagonists, pp. 225--230. London: Smith, Kline & French Lab. Ltd. 1973 Parsons, M. E. : The evidence that inhibition of histamine-stimulated gastric secretion is a result of the blockade of histamine H2-receptors. International Symposium on Histamine H2Receptor Antagonists, pp. 207-218. London: Smith, Kline & French Lab. Ltd. 1973 Parsons, M. E. : Studies on gastric acid secretion using an isolated whole mammalian stomach in vitro. J. Physiol. (Lond.) 247, 35P-36P (1975) Spencer, J. : Gastric secretion in the isolated stomach of the guinea pig. Proc. of the Physiol. Soc. Nov., pp. 1--3 (i973) Wan, B. Y.C., Assem, E.-S. K., Schild, H. O.: Inhibition of in vitro stimulated gastric acid secretion by a histamine Hz-receptor antagonist, metiamide. Europ. J. Pharmacol. 29, 83-88 (1974)
Received August 20~Accepted October 13, 1976
Archivesof
Pharmacology
9 by Springer-Verlag1977
Analysis of the Actions of Cimetidine and Metiamide on Gastric Acid Secretion in the Isolated Guinea Pig Gastric Mucosa S V E N E R I K S J O S T R A N D 1, B I R G I T T A R Y B E R G 1, a n d L A R S O L B E 2 1 Department of Pharmacology, AB H/issle, Fack, S-43120 MSlndal 2 Department of Surgery II, Sahlgren's Hospital, S-40033 G6teborg, Sweden
Summary. C u m u l a t i v e dose-response curves for histam i n e were d e t e r m i n e d o n acid secretion f r o m the isolated g u i n e a pig gastric mucosa. T w o H2-receptor a n t a g o n i s t s - m e t i a m i d e a n d c i m e t i d i n e - b e h a v e d like competitive a n t a g o n i s t s to h i s t a m i n e o n gastric acid secretion in vitro. T h e isolated g u i n e a pig gastric m u c o s a seems to be a suitable in vitro m o d e l for a n a l y s i n g the a c t i o n of c o m p o u n d s o n receptors i n v o l v e d in acid secretion. Key words: G u i n e a pig G a s t r i c acid secretion Hz-receptor-antagonists.
Isolated s t o m a c h Histamine - Histamine
INTRODUCTION C i m e t i d i n e is the latest s u b s t a n c e in a series of H2receptor a n t a g o n i s t s ( B r i m b l e c o m b e et al., 1975). P h a r m a c o l o g i c a l d a t a indicate that this c o m p o u n d is a c o m p e t i t i v e a n t a g o n i s t at h i s t a m i n e receptors in the g u i n e a pig heart a n d rat uterus, as has also b e e n s h o w n for m e t i a m i d e (Black et al., 1973). The latter c o m p o u n d has also been s h o w n to act as a competitive a n t a g o n i s t o n gastric acid secretion in vitro (Parsons, 1975) a n d in vivo (Parsons, 1973). T h e m a m m a l i a n gastric m u c o s a isolated in a n o r g a n b a t h secrets acid a n d r e s p o n d s to chemical stimuli such as h i s t a m i n e (Parsons, 1975; Spencer, 1973; W a n et al., 1974; H o l t o n a n d C u r w a i n , 1973). I n the p r e s e n t study the g u i n e a pig isolated gastric m u c o s a has b e e n used. This p r e p a r a t i o n is particularly suitable as the s t o m a c h wall is very t h i n (Spencer, 1973) a n d also due to the high sensitivity to h i s t a m i n e in this species.
Send offprint requests to: S. E, Sj6strand at the above address
T h e a i m o f the present study was to d e t e r m i n e the dose-response curve for h i s t a m i n e a n d investigate the i n t e r a c t i o n of c i m e t i d i n e a n d m e t i a m i d e o n the histam i n e dose-response.
MATERIALS AND METHODS Female guinea pigs weighing 300-350 g were used. The animals were starved for 24 h but given water. The guinea pigs were killed by a blow on their heads and the stomach was rapidly removed. Two pieces of the secretory portion of the stomach were cut out. On each piece the outer muscular layer was stripped off. The stomach preparations were then mounted on the end of funnel shaped plastic containers with the mucosal surface facing inwards. The secretory area was 2 cm2. In each experiment two guinea pigs were used thus giving 4 isolated mucosa preparations running parallel. Each preparation was immersed in separated organ baths containing 30 ml of a serosal solution having the following ionic composition (mM): Na + 140.9; K + 5.8; Ca2+ 2.6; C1- 125.8; SO42- 1.2; HCO~ 24.9; HPO~- 1.2. The solution contained 25.0 mM glucose and was gassed with 95 ~o 02 and 5 ~ COz. To the mucosat part was added 3 - 5 ml of a solution with the same composition as the serosal solution except that NaHCO3 and KH2PO4 were replaced by equimolar amounts of NaCI and KC1. The mucosal solution contained 10.0 mM glucose and was gassed with 100~ Oz. All experiments were performed at 36~C. Acid production was determined by continuous titration using a Radiometer pH-stat (TTT2, TTT60) and an Autoburette (ABU13) for recording the amount of 0.025 M NaOH required to maintain. the mucosal solution at pH 3.8. In each experiment the spontaneous secretion was followed for about 1 h. During this time the spontaneous secretory rate was stabilized. Histamine was added to the serosal bath repeated times for obtaining a cumulative dose-response. Between each drug administration the secretory level was allowed to reach a steady state level for about 20 rain. In experiments carried out for dissociation constant estimations only two different doses of histamine were used, chosen to give about 50 ~ of maximal response. The two antagonists used, metiamide and cimetidine, were dissolved in the nutrient solution and added to the serosai bath together with histamine at a dose giving a combined agonist-antagonist response close to 50 ~ of the maximal histamine response.
140
Naunyn-Schmiedeberg's Arch. Pharmacol. 296 (1977)
Several doses of the antagonists were tested in each mucosal preparation. The receptor dissociation constants (KB) were calculated according to procedures described in the Appendix. Metiamide and cimetidine were gifts kindly supplied by Smith, Kline & French Laboratories, England.
Inversion and elimination gives: Ks -
[a] [A]AB
[AloB
(7)
1
The significance of KB is illustrated by: KB = [B] APPENDIX
[A]AB
gives [A]oB -- 2. Accordingly KB corresponds to the
According to conventional theories there is a receptor R which can react with an agonist A forming a complex RA resulting in an effect EA. It can also react with an antagonist B forming a complex RB which prevents R from reacting with A and therefore B blocks the effect EA. The formation of RB does not cause any effect EB. The equilibrium of the law of mass action can be applied:
KA. [RA] = [A]. [R] = [ A J . ( R t - [ R A ] - [RB])
(1)
KB. [RB ] = (B]. [R ] = [B ]. (Rt - [RA] - [RB 1)
(2)
where Rt is the total concentration of R in mol/1. [] gives the current concentration of A, B, R, RA, RB. Division of (1) with (2) gives:
[RB] = [RA].
KA" [B]
concentration of the blocking agent B at which the concentration of the stimulating agent must be doubled in order to achieve the same effect.
RESULTS A low spontaneous hydrogen ion secretion from the mucosal preparation was recorded. About 30 min after mounting the preparation there was a steady state level of acid output, the mean rate in 52 control experiments being 0.527 + 0.019 pM/2 cm 2. 10 min -1. Cumulative dose-response curves for histamine were constructed showing an S-shaped form (Fig. 1), with a maximum response being obtained at about 4 x 1 0 - 5 M histamine. The histamine dose-response curve was found to be very steep. There was a linear
(3)
K , . [A]
LIMo{ H+/IO Min,2cm 2
Applying (3) with (~) gives: KA " [RA]
= [A] ( R t -
KA [B]
[RA]
-
[RA])-
KB [A]
200.
(4) 1.80,
9 Control 9 MetiamJde IosM
:
9 Metiamide 5xlgSM
Solving for RA in the absence of B ([RA]oB) gives:
[RA]oB Rt
1
(5)
KA I + - [A]oB where [A]oB is [A] in the absence of B.
1
KA 1 + - [A]o,
[RA]AI~
Rt
tOO "tJ
Q80 0.60 040 0.20
. .
giving:
1
KA 1 + - - + [A]AB
140 1.20
We assume that the effect E emerges from the value [RA]/Rt, i.e. at the same rate of covering A on the receptors. The effect EA is measured at a certain concentration [A]oB. After adding B and new doses of A, the effect EAB is measured. By interpolation [A]A~ is obtained giving an effect EAB which equals Ea.
[RA]oB EA = EAB gives : - Rt
1.60
KA [B] Ks [A]AB
where [RA]AB is [RA] in the presence of B.
3
(6)
6 10 20 40 100 200xl(j6M Log concentration of histamine
Fig. 1. Histamine cumulative log-dose response curves from isolated guinea pig gastric mucosa; without antagonist and after equilibrium with metiamide. The number of experiments in each point are for control 13--22, for metiamide 1 0 - 5 M 5 - 1 5 and for metiamide 5 x 10-SM 4 - 9 . Spontaneous acid secretion subtracted at each point
S. E. Sj6strand et al. : Inhibition of Gastric Acid Secretion in vitro by H2-Blockers
14]
Cimetidine
Metiamide 3.0
1.5
~
1,0
2.0-
1.0
0.5
Log KB
Log K a
Log[B]
Log [B]
Fig. 2. Antagonism of histamine by metiamide on isolated guinea pig gastric mucosa. The plotted values (taken from Table 1) are derived from Eq. (7) and show a linear correlation
Fig. 3. Antagonism of histamine by cimetidine on isolated guinea pig gastric mucosa. The plotted values (taken from Table 1) are derived from Eq. (7) and show a linear correlation
Table 1. Dissociation constants (Ks) for metiamide and cimetidine estimated on isolated guinea pig gastric mucosa using histamine as agonist
showed that the KB at different doses of metiamide or cimetidine were not significantly different. The K~values for the two antagonists showed that the K~ for metiamide was about 10 times higher than that for cimetidine.
Metiamide
Cimetidine
Dose x 105 M
K~ a x 105 M
n
2.0 4.0 8.0 12.0 18.0 24.0
1.57 1.42 1.46 1.52 1.48 1.54
5 4 4 3 4 4
__+ 0.13 __+ 0.18 + 0.15 __+ 0.08 __+ 0.05 __+ 0.15
Dose x 106 M
KB a
2.0 20.0 200.0
1.54 + 0.19 1.41 __+ 0.12 1.55 __+ 0.10
x 106 M 11 8 8
K~ + SEM
pattern between 6 • 10 -6 and 2 x 10 .5 M. The responses to repeated single doses in this range were fairly constant. The antagonism of histamine by metiamide and cimetidine on the isolated guinea pig gastric mucosa estimated from displacement of dose-response curves, has been analysed (Figs. 1, 2 and 3). The influence on the histamine response by the two compounds was recorded as a parallel displacement to the right of the dose-response curves for histamine with unchanged maximal responses. Two-point dose-response curves were made in each preparation allowing intrapolation used for calculations of the receptor dissociation constant (K~) according to Eq. (7) (Table 1). Analysis of variance
DISCUSSION The experimental results of the present study show that histamine produces dose-response related acid secretion from the isolated guinea pig gastric mucosa. The cumulative dose-response curves showed a maximum at about 4x 10-5M compared to 3 x 10-~M in similar rat preparations (Parsons, 1975) indicating that the guinea pig mucosa preparation is more sensitive to histamine than the rat mucosa. This may partly be due to differences in the preparation techniques. Metiamide has produced a parallel displacement to the right of the histamine dose-response curve, indicating competitive receptor antagonism in the isolated rat stomach (Parsons, 1975). Data for metiamide in the present study are in full agreement with those reported by Parsons (1975). Cimetidine was also found to produce a parallel displacement to the right of the histamine dose-acid secretory curve in the isolated guinea pig gastric mucosa. In fact the two antagonists metiamide and cimetidine produced dose related displacement of histamine dose-response curves without significantly affecting the slope or maxinmm and demonstrated
142
Naunyn-Schmiedeberg's Arch. Pharmacol. 296 (1977)
a linear correlation between antagonist log concentration and log
[A]AB
[AJoB
1
from Eq. (7) (Figs. 2 and 3) suggesting a competition of antagonists with histamine for a common site. Thus the present data suggest that cimetidine behaves as a competitive antagonist to histamine on gastric acid secretion in vitro in agreement with the findings that cimetidine acts as a competitive antagonist to histamine in other organs (Brimblecombe et al., 1975). The affinity for cimetidine was about 10 times higher than that for metiamide in the gastric tissue preparation (the dissociation constant KB 10 times lower). However, the affinity constant for metiamide in other tissue preparations (heart and uterus: KB = 7 - - 9 x 10 -7 M, Black et al., 1973) is of about the same magnitude as the affinity constant for cimetidine in the gastric tissue preparation (Table 1) and other tissue preparations (Brimblecombe et al., 1975). Then the observed difference between the affinity constants for cimetidine and metiamide in the gastric tissue preparation does not necessarily reflect a higher affinity to histamine receptors for cimetidine than for metiamide but may merely express a facilitated penetration of the much more water soluble cimetidine
to the histamine receptors in this particular preparation. REFERENCES Black, J. W., Duncan, W. A. M., Emmet, J. C., Ganellin, C. R., Hesselbo, T., Parsons, M.E., Wyllie, J.H.: Metiamide-an orally active histamine Hz-receptor antagonist. Agents and Actions 3, 133-137 (1973) Brimbleemnbe, R. W., Duncan, W. A. M., Durant, C. J., Emmet, J. ~., Ganellin, C. R., Parsons, M. E. : Cimetidine- a non-thiourea H2-receptor antagonist. J. int. reed. Res. 3, 86-92 (1975) Holton, P., Curwain, B.P.: Evidence for the specific action of burimamide and metiamide on the H2-receptors involved in gastric secretion. International Symposium on Histamine HzReceptor Antagonists, pp. 225--230. London: Smith, Kline & French Lab. Ltd. 1973 Parsons, M. E. : The evidence that inhibition of histamine-stimulated gastric secretion is a result of the blockade of histamine H2-receptors. International Symposium on Histamine H2Receptor Antagonists, pp. 207-218. London: Smith, Kline & French Lab. Ltd. 1973 Parsons, M. E. : Studies on gastric acid secretion using an isolated whole mammalian stomach in vitro. J. Physiol. (Lond.) 247, 35P-36P (1975) Spencer, J. : Gastric secretion in the isolated stomach of the guinea pig. Proc. of the Physiol. Soc. Nov., pp. 1--3 (i973) Wan, B. Y.C., Assem, E.-S. K., Schild, H. O.: Inhibition of in vitro stimulated gastric acid secretion by a histamine Hz-receptor antagonist, metiamide. Europ. J. Pharmacol. 29, 83-88 (1974)
Received August 20~Accepted October 13, 1976
