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Effect of nizatidine and ranitidine on the D-tubocurarine neuromuscular blockade in the toad rectus abdominis muscle
Pharmacological Research. Vol. 29, No. 2,1994
155
EFFECT OF NIZATIDINE AND RANITIDINE ON THE D-TUBOCURARINE NEUROMUSCULAR BLOCKADE IN THE TOAD RECTUS ABDOMINIS MUSCLE G. KOUNENIS, M. KOUTSOVITI-PAPADOPOULOU Department of Pharmacology, Aristotelian University,
and V. ELEZOGLOU
Faculty of Veterinary Medicine, 540 06 Thessaloniki, Greece
Received in final form 24 September 1993
SUMMARY The influence of varying concentrations of the Hz-receptor antagonists nizatidine and ranitidine on the acetylcholine- and carbachol-induced contractures on the toad rectus abdominis muscle, as well as the possible interaction between the above Hz-receptor antagonists and D-tubocurarine were studied. Nizatidine and ranitidine at a concentration of 3.2~10~~ mol 1-l augmented, and at 3.2~10~~ mol 1-l inhibited, the acetylcholine-induced contractures on the toad rectus abdominis muscle, while at concentrations from 3.2~10~~ to 3.2~10~~ molll’ they inhibited the carbachol-induced contractures, in a concentration-dependent manner. In addition, nizatidine and ranitidine at a concentration of 3.2~10-~ mol ll’ reversed the D-tubocurarine blocking activity on the acetylcholine-induced contractures, but at a concentration of 3.2~10~~ mol 1-l they augmented it. These findings provide evidence that the above Hz-receptor antagonists produce either cholinesterase inhibition or neuromuscular blockade, depending on their concentration. Thus, the D-tubocurarine neuromuscular blocking at high activity is potentiated concentrations of nizatidine and ranitidine. while it is reversed at lower ones. KEY WORDS: nizatidine, abdominis muscle.
ranitidine,
acetylcholine,
carbachol,
o-tubocurarine,
toad
rectus
INTRODUCTION The Hz-receptor antagonists nizatidine and ranitidine have a potency in reducing gastric acid production and have been prescribed for the treatment of peptic ulcer disease [ 1, 21, as well as before induction of anaesthesia to minimize aspiration pneumonitis [3]. On the other hand, it was shown that ranitidine and another Hzreceptor antagonist cimetidine at high concentrations inhibit the acetylcholineinduced contractures on the toad rectus abdominis muscle, while at relatively lower concentrations they augment them [4]. The above inhibition has been attributed to their neuromuscular blocking activity [4], and the augmentation to their anticholinesterase activity [5-71. It has also been reported that cimetidine 104346 18/94/020 155-07/$08.00/0
0 1994 The Italian Pharmacological Society
156
Pharmacologral
Rrseurc~h, Vol. 29, No. 2. 1994
potentiates the blocking activity caused by o-tubocurarine in the anaesthetized cat [8] and by atracurium in the anaesthetized rat [9]. On the basis of the above findings it was decided to investigate: the influence of varying concentrations of the Hz-receptor antagonists nizatidine and ranitidine on the acetylcholineand carbachol-induced contractures of the toad rectus abdominis muscle, and the possible interaction between the above H,-receptor antagonists and the nondepolarizing blocking agent n-tubocurarine.
MATERIALS
AND METHODS
Preparation of the toad rectus ahdominis muscle The toad (Bufo melanostictus) was decapitated after stunning and was pithed using a pithing needle. Two rectus abdominis muscles were obtained from each toad. The preparations were mounted in 20 ml organ baths (Hugo Sachs Electronik KG, Germany) containing Ringer solution of the following composition (in mmol I-‘): NaCl, 111; KCl, 1.9; CaCl*, 1.2; NaHC03, 2.4; NaH2P04, 0.064; dextrose 1.1. The solution in the organ baths was bubbled with 95% 02--5% CO1 gas and maintained at a temperature of 20°C. A resting tension of 500 mg was applied to the preparations which then were allowed to stabilize for a period of 30 min before any compound addition. During this period the preparations were washed with fresh Ringer solution every 10 min. The isotonic responses of the preparations were recorded on a physiograph recorder (desk model, type DMP-4A, Narco Co., USA) via isotonic myograph transducers (Narco Co., USA). Drugs The following compounds were used: nizatidine (Eli Lilly, USA), ranitidine hydrochloride (Glaxo, UK), acetylcholine chloride (E. Merck, Germany), carbachol chloride (Sigma Chemical Co., USA) and o-tubocurarine (Sigma Chemical Co., USA). The solutions of the above compounds were freshly prepared, before each experiment, using Ringer solution as a solvent. Concentration-response curves After the 30 min stabilization period, the preparations were exposed to cumulatively increasing concentrations of acetylcholine ( 10~7-10~4 mol ll’) or carbachol (3.2x10-‘-3.2~10-~ mol I-‘) to obtain full concentration-response curves and then to acetylcholine or carbachol 3 min after pretreatment with nizatidine or ranitidine (3.2~10-~-3.2~10~’ mol 1-l). In a second series of experiments, the preparations were exposed to cumulatively increasing concentrations of acetylcholine ( 10-7-10~4 mol ll’), then to acetylcholine 3 min after pretreatment with o-tubocurarine (4~10~~ mol 1-l) and finally to acetylcholine in the presence of D-tubocurarine 3 min after pretreatment with nizatidine or ranitidine (at 3.2~10~~ and 3.2~10~” mol ll’). The addition of each cumulatively increasing concentration of acetylcholine or carbachol was made after the contractile response to the previous concentration had reached a maximum.
The responses obtained were expressed as a percentage of the maximum height attained in the control curve (acetylcholine or carbachol alone). Statistical evaluation of the data was performed using Student’s t-test for paired data. The data were expressed as mean+ssM and P values <0.05 were considered to be significant.
RESULTS RPsp0lrsi~,rncss nixtiditle
IO ucetylcholinc
twfore
and after yretr.eatment
with
or ranitidine
The contractile responses induced by acetylcholine (3.2~10~’ - IO-’ mol 1-l) were augmented by the pretreatment with nizatidine or ranitidine at a concentration of 3.2~10~’ mol 1 ‘, while at a concentration of lo-’ mol 1-l they were not significantly modified. On the other hand, the contractile responses induced by acetylcholine were inhibited by pretreatment with nizatidine or ranitidine at a concentration of 3.2~10~’ mol 1-l. This inhibition produced a shift to the right of the concentration-response curves for acetylcholine with a marked reduction of the maximum response (Fig. 1).
1
la)
100
I
100
Concentration
Fig. 1. Cumulative concenlration-response curves for acetylcholine-induced contractures of the toad rectus abdominis muscle in the absence (0) and in the presence of nizatidine (a) or mnitidine (b) at concentrations of 3.2~10~’ (0), IO-’ (A) and 3.2x10-j mol I-’ (0). The ordinatcs are expressed as a percentage of the mean maximum response induced by acetylcholine (control). Each point represents the meankseM obtained from 24 preparations for acetylcholine alone and eight preparations for acetylcholine in the presence of each concentration of ranitidine or nizatidine. *Indicates the significant values of acetylcholine augmentation or inhibition caused by ranitidine or nizatidine (P
Concentration Fig. 2. Cumulative concentration-response curves for carbachol-induced contractures of the toad rectus abdominis muscle in the absence (0) and in the presence of nizatidine (a) or ranitidine (b) at the concentrations of 3.2~10~ (0), IO-’ (A) and 3.2~10~’ mol I-’ (0). The ordinates are expressed as a percentage of the mean maximum response induced by carbachol (control). Each point represents the meanfsEM obtained from 24 preparations for carbachol alone and eight preparations for carbachol in the presence of each concentration of nizatidine or rdnitidine. Carbachol inhibited values from 10e6 to 3.2~10~ mol I-’ were significant (PcO.05).
to car-hachol before und after pretr-eutment with nixtidine or runitidine The contractile responses induced by carbachol (3.2~10~~-3.2~10-” mol IK’) were inhibited by pretreatment with nizatidine or ranitidine at concentrations of 3.2x10-‘-3.2~10~’ mol l-‘, in a concentration-dependent manner. This inhibition produced a shift to the right of the concentration-response curves for carbachol with a marked reduction of the maximum response (Fig. 2). Responsilvness
Responsiveness
to ucetylcholine
before and after- pretreatment
wtubocrrrar-ine und to ucetylcholine in the presence pretreatment with nizutidine or runitidine
w?th
of wtuhocurur-ins
uftel
The contractile responses induced by acetylcholine (3.2~10~~-10~’ mol I-‘) were inhibited by the pretreatment with D-tubocurarine at a concentration of 4x lo-’ mol 1-l. This o-tubocurarine activity on the acetylcholine-induced responses was inhibited by pretreatment with nizatidine or ranitidine at a concentration of 3.2x10 ’ mol 1-l. On the other hand, nizatidine and ranitidine at a concentration of 3.2~10~’ mol 1-l augmented the o-tubocurarine activity. This augmentation produced a shift of the curves to the right with a marked decrease of the maximum response (Fig. 3).
Pharmoc~o/o,qical
Researc~h. Vol. 29. No. 2. 1994
1001
lo-’
10 5
tbl
100
TN+.
lo-”
159
t
10-4 Concentration
Cm011~ ’I
Fig. 3. Cumulative concentration-response curves for acetylcholine-induced contractures of the toad rectus abdominis muscle in the absence (0) and in the presence of o-tubocurarine at a concentration of 4~10~~ mol I-’ (0); to acetylcholine in the presence of o-tubocurarine at the concentration of 4~10-~ mol IF’plus nizatidine (a) or ranitidine (b) at the concentrations of 3.2x lOA (A) and 3.2~10~~ mol If’ (0). The ordinates are expressed as a percentage of the maximum response induced by acetylcholine (control). Each point represents the mean+saM obtained from IX preparations for acetylcholine alone; 18 preparations for acetylcholine in the presence of o-tubocurarine and nine preparations for acetylcholine in the presence of o-tubocurarine plus each concentration of nizatidine or ranitidine. *Indicates the significant values of n-tubocurarine inhibition or augmentation caused by nizatidine or ranitidine (P
DISCUSSION In the present study it was shown that both nizatidine and ranitidine, at concentrations of 3.2~10~’ mol I-‘, potentiated the action of acetylcholine on the toad rectus abdominis muscle, but at lo-’ mol 1-l they did not modify it. On the other hand, at an even higher concentration (3.2~10~’ mol ll’) nizatidine and ranitidine inhibited the action of acetylcholine and produced a shift to the right of the concentration-response curves for acetylcholine, with a marked reduction of the maximum response. In a previous investigation concerning the toad rectus abdominis muscle under the same experimental conditions, it was also noticed that ranitidine and cimetidine at high concentrations inhibited the contractile responses caused by acetylcholine, while at relatively lower ones they augmented them [4]. The potentiation caused by nizatidine and ranitidine, as observed in this study, can be attributed to their anticholinesterase activity, since in previous investigations it was shown that both the above Hz-receptor antagonists possess an anticholinesterase activity [5, 6, lo]. On the other hand, the inhibition which nizatidine and ranitidine caused on the acetylcholine activity could suggest that the respective Hz-receptor antagonists possess a neuromuscular blocking activity as well. This neuromuscular blocking activity is probably mediated by ion-channel
Phormuc~ologic~ul Rusrurc~h. Vd. 29, No. 2. I994
160
block, since the shift to the right of the acetylcholine concentration-response curve accompanied by the reduction of the maximum response are characteristic of this type of block [4]. In the present study it was also found that nizatidine and ranitidine, at all concentrations used, inhibited the responses of the toad rectus abdominis muscle to carbachol.
This
inhibition
was characterized
by a concentration-dependent
shift
of the test curves
to the right with a marked reduction of the maximum response, probably because the respective HZ-receptor antagonists induce neuromuscular blockade on the carbachol activity by producing ion-channel block. It is of special interest to note that the dual effect of nizatidine and ranitidine on the tissue responses to acetylcholine was not obtained when carbachol was used as an agonist. This can be explained by considering that while acetylcholine is rapidly hydrolysed by cholinesterase enzymes, carbachol is totally resistant to hydrolysis by them [ 111. These results provide further evidence that nizatidine and ranitidine may cause neuromuscular blockade. Furthermore, according to our results it was shown that D-tubocurarine competitively inhibited the responses of the toad rectus abdominis muscle to acetylcholine and that this blockade was potentiated by nizatidine and ranitidine at the higher concentration, with a marked reduction of the maximum response, while it was reversed at the lower concentration used. This comes in accordance with the observations of other investigators who showed that cimetidine, at high doses, potentiated the neuromuscular blockade produced by D-tubocurarine in anaesthetized cat [8] and by atracurium in anaesthetized rat [9]. This neuromuscular blocking activity, which is revealed at high concentrations of the above HZ-receptor antagonists, may interfere with or mask some of their anticholinesterase activity observed at the lower concentrations used. The conclusion which may be drawn is that the Hz-receptor antagonists, nizatidine and ranitidine, possess diverse cholinergic activities including an anticholinesterase activity at relatively low concentrations and a neuromuscular blocking activity as the concentration increases. Thus, the D-tubocurarine neuromuscular blocking activity is potentiated by nizatidine and ranitidine at high concentrations, while it is reversed at relatively lower concentrations.
REFERENCES 1. Helman CA, Tim LO. Pharmacology and clinical efficacy of ranitidine, a new H?receptor antagonist. Pharmacotherapy 1983; 3: 185-92. 2. Lim TM, Evans DC, Warrick MW, Pioch RP. Actions of nizatidine, a selective histamine HZ-receptor antagonist, on gastric acid secretion in dogs, rats and frogs. J Pharmacol Exp Ther 1986; 239: 406-l 0. 3. Albert CC, Baker JD, Cooke
JE. A rational approach to anaesthetic premeditation. Drug 1989; 37: 219-28. 4. Cheach LS, Lee HS, Gwee MCE. Anticholinesterase activity of and possible ion-channel block by cimetidine, ranitidine and oxmetidine in the toad rectus abdominis muscle. C/in Exp Pharmacol Physiol 198.5; 12: 353-7. 5. Hansen WE, Bert1 S. Inhibition of cholinesterase by ranitidine. Lam-et 1983; i: 235. 6. Kounenis G, Koutsoviti-Papadopoulou M, Elezoglou V. The inhibition of acetylcholinesterase by ranitidine. J Pharmacobio-Dyn 1986; 9: 941-5.
Pl~ar~mac~olo~iccrl
Resewch.
Vol. 29. No.
2. I994
161
7. Hansen WE, Bcrtl S. The inhibition of acetylcholinesterase and pseudocholinesterase by cimetidine. Arrneim Forsch (Drug Res) 1983; 33: 161-2. 8. Aw LLF, Cheah LS, Gwee MCE. Cimetidine and tubocurarine interaction in the anaesthetized cat. In Proc XVI Singapore-Malaysia Congress of Medicine, Academy of Medicine Singapore, 1982; 4024. 9. Mishra Y, Ramzan I. Enhancement by cimetidine of neuromuscular paralysis induced by atracurium in rats. Arch Znt Pharmac~odyn 1992; 318: 97-106. IO. Kounenis G, Voutsas D, Koutsoviti-Papadopoulou M, Elezoglou V. Inhibition of acetylcholinesterase by the HZ-receptor antagonist, nizatidine. .I Pharmacohio-Dyn 1988; 11: 767-71. I I. Taylor P. Cholinergic agonists. In: Goodman Gilman A, Rail TW, Nies AS, Taylor P. eds, Thr pharmacological basis of therapeutics. 9th edn Chapter 6. New York: McMillan. 1990: 122-30.
155
EFFECT OF NIZATIDINE AND RANITIDINE ON THE D-TUBOCURARINE NEUROMUSCULAR BLOCKADE IN THE TOAD RECTUS ABDOMINIS MUSCLE G. KOUNENIS, M. KOUTSOVITI-PAPADOPOULOU Department of Pharmacology, Aristotelian University,
and V. ELEZOGLOU
Faculty of Veterinary Medicine, 540 06 Thessaloniki, Greece
Received in final form 24 September 1993
SUMMARY The influence of varying concentrations of the Hz-receptor antagonists nizatidine and ranitidine on the acetylcholine- and carbachol-induced contractures on the toad rectus abdominis muscle, as well as the possible interaction between the above Hz-receptor antagonists and D-tubocurarine were studied. Nizatidine and ranitidine at a concentration of 3.2~10~~ mol 1-l augmented, and at 3.2~10~~ mol 1-l inhibited, the acetylcholine-induced contractures on the toad rectus abdominis muscle, while at concentrations from 3.2~10~~ to 3.2~10~~ molll’ they inhibited the carbachol-induced contractures, in a concentration-dependent manner. In addition, nizatidine and ranitidine at a concentration of 3.2~10-~ mol ll’ reversed the D-tubocurarine blocking activity on the acetylcholine-induced contractures, but at a concentration of 3.2~10~~ mol 1-l they augmented it. These findings provide evidence that the above Hz-receptor antagonists produce either cholinesterase inhibition or neuromuscular blockade, depending on their concentration. Thus, the D-tubocurarine neuromuscular blocking at high activity is potentiated concentrations of nizatidine and ranitidine. while it is reversed at lower ones. KEY WORDS: nizatidine, abdominis muscle.
ranitidine,
acetylcholine,
carbachol,
o-tubocurarine,
toad
rectus
INTRODUCTION The Hz-receptor antagonists nizatidine and ranitidine have a potency in reducing gastric acid production and have been prescribed for the treatment of peptic ulcer disease [ 1, 21, as well as before induction of anaesthesia to minimize aspiration pneumonitis [3]. On the other hand, it was shown that ranitidine and another Hzreceptor antagonist cimetidine at high concentrations inhibit the acetylcholineinduced contractures on the toad rectus abdominis muscle, while at relatively lower concentrations they augment them [4]. The above inhibition has been attributed to their neuromuscular blocking activity [4], and the augmentation to their anticholinesterase activity [5-71. It has also been reported that cimetidine 104346 18/94/020 155-07/$08.00/0
0 1994 The Italian Pharmacological Society
156
Pharmacologral
Rrseurc~h, Vol. 29, No. 2. 1994
potentiates the blocking activity caused by o-tubocurarine in the anaesthetized cat [8] and by atracurium in the anaesthetized rat [9]. On the basis of the above findings it was decided to investigate: the influence of varying concentrations of the Hz-receptor antagonists nizatidine and ranitidine on the acetylcholineand carbachol-induced contractures of the toad rectus abdominis muscle, and the possible interaction between the above H,-receptor antagonists and the nondepolarizing blocking agent n-tubocurarine.
MATERIALS
AND METHODS
Preparation of the toad rectus ahdominis muscle The toad (Bufo melanostictus) was decapitated after stunning and was pithed using a pithing needle. Two rectus abdominis muscles were obtained from each toad. The preparations were mounted in 20 ml organ baths (Hugo Sachs Electronik KG, Germany) containing Ringer solution of the following composition (in mmol I-‘): NaCl, 111; KCl, 1.9; CaCl*, 1.2; NaHC03, 2.4; NaH2P04, 0.064; dextrose 1.1. The solution in the organ baths was bubbled with 95% 02--5% CO1 gas and maintained at a temperature of 20°C. A resting tension of 500 mg was applied to the preparations which then were allowed to stabilize for a period of 30 min before any compound addition. During this period the preparations were washed with fresh Ringer solution every 10 min. The isotonic responses of the preparations were recorded on a physiograph recorder (desk model, type DMP-4A, Narco Co., USA) via isotonic myograph transducers (Narco Co., USA). Drugs The following compounds were used: nizatidine (Eli Lilly, USA), ranitidine hydrochloride (Glaxo, UK), acetylcholine chloride (E. Merck, Germany), carbachol chloride (Sigma Chemical Co., USA) and o-tubocurarine (Sigma Chemical Co., USA). The solutions of the above compounds were freshly prepared, before each experiment, using Ringer solution as a solvent. Concentration-response curves After the 30 min stabilization period, the preparations were exposed to cumulatively increasing concentrations of acetylcholine ( 10~7-10~4 mol ll’) or carbachol (3.2x10-‘-3.2~10-~ mol I-‘) to obtain full concentration-response curves and then to acetylcholine or carbachol 3 min after pretreatment with nizatidine or ranitidine (3.2~10-~-3.2~10~’ mol 1-l). In a second series of experiments, the preparations were exposed to cumulatively increasing concentrations of acetylcholine ( 10-7-10~4 mol ll’), then to acetylcholine 3 min after pretreatment with o-tubocurarine (4~10~~ mol 1-l) and finally to acetylcholine in the presence of D-tubocurarine 3 min after pretreatment with nizatidine or ranitidine (at 3.2~10~~ and 3.2~10~” mol ll’). The addition of each cumulatively increasing concentration of acetylcholine or carbachol was made after the contractile response to the previous concentration had reached a maximum.
The responses obtained were expressed as a percentage of the maximum height attained in the control curve (acetylcholine or carbachol alone). Statistical evaluation of the data was performed using Student’s t-test for paired data. The data were expressed as mean+ssM and P values <0.05 were considered to be significant.
RESULTS RPsp0lrsi~,rncss nixtiditle
IO ucetylcholinc
twfore
and after yretr.eatment
with
or ranitidine
The contractile responses induced by acetylcholine (3.2~10~’ - IO-’ mol 1-l) were augmented by the pretreatment with nizatidine or ranitidine at a concentration of 3.2~10~’ mol 1 ‘, while at a concentration of lo-’ mol 1-l they were not significantly modified. On the other hand, the contractile responses induced by acetylcholine were inhibited by pretreatment with nizatidine or ranitidine at a concentration of 3.2~10~’ mol 1-l. This inhibition produced a shift to the right of the concentration-response curves for acetylcholine with a marked reduction of the maximum response (Fig. 1).
1
la)
100
I
100
Concentration
Fig. 1. Cumulative concenlration-response curves for acetylcholine-induced contractures of the toad rectus abdominis muscle in the absence (0) and in the presence of nizatidine (a) or mnitidine (b) at concentrations of 3.2~10~’ (0), IO-’ (A) and 3.2x10-j mol I-’ (0). The ordinatcs are expressed as a percentage of the mean maximum response induced by acetylcholine (control). Each point represents the meankseM obtained from 24 preparations for acetylcholine alone and eight preparations for acetylcholine in the presence of each concentration of ranitidine or nizatidine. *Indicates the significant values of acetylcholine augmentation or inhibition caused by ranitidine or nizatidine (P
Concentration Fig. 2. Cumulative concentration-response curves for carbachol-induced contractures of the toad rectus abdominis muscle in the absence (0) and in the presence of nizatidine (a) or ranitidine (b) at the concentrations of 3.2~10~ (0), IO-’ (A) and 3.2~10~’ mol I-’ (0). The ordinates are expressed as a percentage of the mean maximum response induced by carbachol (control). Each point represents the meanfsEM obtained from 24 preparations for carbachol alone and eight preparations for carbachol in the presence of each concentration of nizatidine or rdnitidine. Carbachol inhibited values from 10e6 to 3.2~10~ mol I-’ were significant (PcO.05).
to car-hachol before und after pretr-eutment with nixtidine or runitidine The contractile responses induced by carbachol (3.2~10~~-3.2~10-” mol IK’) were inhibited by pretreatment with nizatidine or ranitidine at concentrations of 3.2x10-‘-3.2~10~’ mol l-‘, in a concentration-dependent manner. This inhibition produced a shift to the right of the concentration-response curves for carbachol with a marked reduction of the maximum response (Fig. 2). Responsilvness
Responsiveness
to ucetylcholine
before and after- pretreatment
wtubocrrrar-ine und to ucetylcholine in the presence pretreatment with nizutidine or runitidine
w?th
of wtuhocurur-ins
uftel
The contractile responses induced by acetylcholine (3.2~10~~-10~’ mol I-‘) were inhibited by the pretreatment with D-tubocurarine at a concentration of 4x lo-’ mol 1-l. This o-tubocurarine activity on the acetylcholine-induced responses was inhibited by pretreatment with nizatidine or ranitidine at a concentration of 3.2x10 ’ mol 1-l. On the other hand, nizatidine and ranitidine at a concentration of 3.2~10~’ mol 1-l augmented the o-tubocurarine activity. This augmentation produced a shift of the curves to the right with a marked decrease of the maximum response (Fig. 3).
Pharmoc~o/o,qical
Researc~h. Vol. 29. No. 2. 1994
1001
lo-’
10 5
tbl
100
TN+.
lo-”
159
t
10-4 Concentration
Cm011~ ’I
Fig. 3. Cumulative concentration-response curves for acetylcholine-induced contractures of the toad rectus abdominis muscle in the absence (0) and in the presence of o-tubocurarine at a concentration of 4~10~~ mol I-’ (0); to acetylcholine in the presence of o-tubocurarine at the concentration of 4~10-~ mol IF’plus nizatidine (a) or ranitidine (b) at the concentrations of 3.2x lOA (A) and 3.2~10~~ mol If’ (0). The ordinates are expressed as a percentage of the maximum response induced by acetylcholine (control). Each point represents the mean+saM obtained from IX preparations for acetylcholine alone; 18 preparations for acetylcholine in the presence of o-tubocurarine and nine preparations for acetylcholine in the presence of o-tubocurarine plus each concentration of nizatidine or ranitidine. *Indicates the significant values of n-tubocurarine inhibition or augmentation caused by nizatidine or ranitidine (P
DISCUSSION In the present study it was shown that both nizatidine and ranitidine, at concentrations of 3.2~10~’ mol I-‘, potentiated the action of acetylcholine on the toad rectus abdominis muscle, but at lo-’ mol 1-l they did not modify it. On the other hand, at an even higher concentration (3.2~10~’ mol ll’) nizatidine and ranitidine inhibited the action of acetylcholine and produced a shift to the right of the concentration-response curves for acetylcholine, with a marked reduction of the maximum response. In a previous investigation concerning the toad rectus abdominis muscle under the same experimental conditions, it was also noticed that ranitidine and cimetidine at high concentrations inhibited the contractile responses caused by acetylcholine, while at relatively lower ones they augmented them [4]. The potentiation caused by nizatidine and ranitidine, as observed in this study, can be attributed to their anticholinesterase activity, since in previous investigations it was shown that both the above Hz-receptor antagonists possess an anticholinesterase activity [5, 6, lo]. On the other hand, the inhibition which nizatidine and ranitidine caused on the acetylcholine activity could suggest that the respective Hz-receptor antagonists possess a neuromuscular blocking activity as well. This neuromuscular blocking activity is probably mediated by ion-channel
Phormuc~ologic~ul Rusrurc~h. Vd. 29, No. 2. I994
160
block, since the shift to the right of the acetylcholine concentration-response curve accompanied by the reduction of the maximum response are characteristic of this type of block [4]. In the present study it was also found that nizatidine and ranitidine, at all concentrations used, inhibited the responses of the toad rectus abdominis muscle to carbachol.
This
inhibition
was characterized
by a concentration-dependent
shift
of the test curves
to the right with a marked reduction of the maximum response, probably because the respective HZ-receptor antagonists induce neuromuscular blockade on the carbachol activity by producing ion-channel block. It is of special interest to note that the dual effect of nizatidine and ranitidine on the tissue responses to acetylcholine was not obtained when carbachol was used as an agonist. This can be explained by considering that while acetylcholine is rapidly hydrolysed by cholinesterase enzymes, carbachol is totally resistant to hydrolysis by them [ 111. These results provide further evidence that nizatidine and ranitidine may cause neuromuscular blockade. Furthermore, according to our results it was shown that D-tubocurarine competitively inhibited the responses of the toad rectus abdominis muscle to acetylcholine and that this blockade was potentiated by nizatidine and ranitidine at the higher concentration, with a marked reduction of the maximum response, while it was reversed at the lower concentration used. This comes in accordance with the observations of other investigators who showed that cimetidine, at high doses, potentiated the neuromuscular blockade produced by D-tubocurarine in anaesthetized cat [8] and by atracurium in anaesthetized rat [9]. This neuromuscular blocking activity, which is revealed at high concentrations of the above HZ-receptor antagonists, may interfere with or mask some of their anticholinesterase activity observed at the lower concentrations used. The conclusion which may be drawn is that the Hz-receptor antagonists, nizatidine and ranitidine, possess diverse cholinergic activities including an anticholinesterase activity at relatively low concentrations and a neuromuscular blocking activity as the concentration increases. Thus, the D-tubocurarine neuromuscular blocking activity is potentiated by nizatidine and ranitidine at high concentrations, while it is reversed at relatively lower concentrations.
REFERENCES 1. Helman CA, Tim LO. Pharmacology and clinical efficacy of ranitidine, a new H?receptor antagonist. Pharmacotherapy 1983; 3: 185-92. 2. Lim TM, Evans DC, Warrick MW, Pioch RP. Actions of nizatidine, a selective histamine HZ-receptor antagonist, on gastric acid secretion in dogs, rats and frogs. J Pharmacol Exp Ther 1986; 239: 406-l 0. 3. Albert CC, Baker JD, Cooke
JE. A rational approach to anaesthetic premeditation. Drug 1989; 37: 219-28. 4. Cheach LS, Lee HS, Gwee MCE. Anticholinesterase activity of and possible ion-channel block by cimetidine, ranitidine and oxmetidine in the toad rectus abdominis muscle. C/in Exp Pharmacol Physiol 198.5; 12: 353-7. 5. Hansen WE, Bert1 S. Inhibition of cholinesterase by ranitidine. Lam-et 1983; i: 235. 6. Kounenis G, Koutsoviti-Papadopoulou M, Elezoglou V. The inhibition of acetylcholinesterase by ranitidine. J Pharmacobio-Dyn 1986; 9: 941-5.
Pl~ar~mac~olo~iccrl
Resewch.
Vol. 29. No.
2. I994
161
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