Pharmacodynamics of amoxicillin against Mannheimia haemolytica and Pasteurella multocida and pharmacokinetic/pharmacodynamic (PK/PD) correlation in sheep.

Silicone-made tissue cages were implanted in sheep. Blood serum (SBS) and tissue cage fluid (TCF) samples were collected after amoxicillin intravenous and intramuscular administrations, at the dose of 15 mg/kg. Amoxicillin pharmacodynamics were studied in an artificial culture medium, SBS and TCF with use of a Mannheimia haemolytica and a Pasteurella multocida strain. A concentration-independent antimicrobial activity of amoxicillin was confirmed for levels higher than 0.79-1.75×MIC. This result favored the use of the percentage of the 24 h dosing interval during which drug levels remain above MIC as the appropriate pharmacokinetic/pharmacodynamic index. The subsequent correlation revealed that intravenous administration could be considered effective against "deep" infections caused by bacteria with MICs<1 µg/mL or "shallow" infections caused by bacteria with MICs<0.1 µg/mL. Intramuscular administration could be safely considered effective against both "deep" and "shallow" infections when the MICs of the targeted pathogens are lower than 1 µg/mL.

Influence of the injection site on the pharmacokinetics of amoxicillin after intramuscular administration of a conventional and a long-acting formulation in sheep.

The pharmacokinetics of amoxicillin (AMX) were investigated in sheep following intravenous (i.v.) and intramuscular (i.m) injection, comparing two different drug formulations, a conventional and a long-acting AMX-trihydrate suspension. For the i.m. application two different injections sites, the neck area and the hind limb were used to identify possible differences in the kinetic parameters related to the site of injection. A three-compartment open model could best describe AMX disposition after i.v. administration. Data analysis after i.m. administration of the conventional suspension at both injection sites revealed the occurrence of a flip-flop phenomenon, clearly indicating that absorption of AMX is the rate-limiting step of its overall disposition. A moderate effect of the injection site was observed with a tendency for the neck area to be advantageous, mainly in terms of rate rather than extent of absorption. Injection of the long-acting formulation led to a focal depot formation, thus yielding lower but remarkably prolonged serum AMX levels reflected in the respective terminal half-lives. The concentration-time profile of AMX after administration of the long-acting formulation was less affected by the injection site, but the low serum levels justify its use only in cases in which a high susceptibility of the involved bacterial population is confirmed.

Cisapride reverses the anticholinergic effect of disopyramide on the isolated guinea-pig urinary bladder.

The present investigation aims to examine whether the prokinetic agent cisapride is able to reverse disopyramide's anticholinergic effect on the isolated guinea-pig urinary bladder. Acetylcholine, at concentrations ranging from 10(-7) to 10(-3) M, produced a stimulatory effect on the urinary bladder (pEC(50) value=5.1). Disopyramide competitively antagonized the contractile effect of acetylcholine with an ID(50)=4.4 x 10(-6) M. Although cisapride by itself had either no intrinsic contractile action or a modest effect on the urinary bladder, at concentrations ranging from 3 x 10(-7) to 10(-6) M, it significantly reversed the above inhibitory effect of disopyramide, and produced a parallel leftward shift of the concentration-response curve for acetylcholine in the presence of disopyramide. The pEC(50) values for acetylcholine in the presence of 3 x 10(-6) M and 10(-5) M disopyramide were 4.7 and 4.2, respectively, while in the presence of 10(-5) M disopyramide, after pretreatment with 5 x 10(-7) M cisapride, the pEC(50) value for acetylcholine was 4.6. It is concluded that cisapride is effective in reversing the anticholinergic activity of disopyramide on the isolated guinea-pig urinary bladder, probably by facilitating cholinergic neurotransmission.

New simple liquid chromatographic method for the determination of trimethoprim, sulfadiazine and N4-acetylsulfadiazine in plasma of broilers.

A new method for simultaneous quantification of trimethoprim, sulfadiazine and N4-acetylsulfadiazine in plasma of broilers at levels down to 13-16 ng/ml has been developed. Samples were deproteinized with acetonitrile, defatted with hexane, and extracted with dichloromethane. Chromatographic analysis was carried out on a C18 column in the presence of tetrabutylammonium hydrogen sulfate, a competing base, while detection was performed at 240 nm for trimethoprim, and 270 nm for both sulfadiazine and N4-acetylsulfadiazine. Accuracy and precision data showed recoveries and relative standard deviation values better than 87.3% and 3.1%, respectively, for all three analytes. The good analytical characteristics of the method could allow limits of detection in the low ng/ml range to be realised. The method was successfully applied to determine drug concentrations in plasma samples from broilers administered a combination of sulfadiazine and trimethoprim.

Biphenylacetic acid enhances the antagonistic action of fluoroquinolones on the GABA(A)-mediated responses of the isolated guinea-pig ileum.

This paper examines the effect of biphenylacetic acid on the antagonistic action of norfloxacin and enoxacin on the GABA(A)-mediated responses of the isolated guinea-pig ileum. GABA produced transient contractions followed by relaxation. The contractile effect of exogenously applied GABA was concentration-dependent with EC(50)= 9.8 x 10(-6) M. This contractile effect was not significantly modified by biphenylacetic acid, and the EC(50) value for GABA in the presence of 10(-5) M biphenylacetic acid was 1.15 x 10(-5) M. The GABA contractile effect was inhibited, dose-dependently, by either norfloxacin or enoxacin, but only at concentrations higher than 10(-5) M. The response of the ileum to GABA (at EC(50)) was reduced to 35 and 36% by pretreatment with 10(-5) M norfloxacin or enoxacin, respectively. However, in the presence of 10(-5) M biphenylacetic acid, the response of the ileum to GABA was reduced to 2.2% by pretreatment with 10(-5) M enoxacin, while it was completely abolished by pretreatment with 10(-5) M norfloxacin and the IC(50) values were 5.5 x 10(-7) and 1.5 x 10(-6) M for norfloxacin and enoxacin, respectively. These data show that biphenylacetic acid whilst having no effect at the GABA(A)-mediated contractile response of the guinea-pig ileum, enhances the antagonistic effect of both enoxacin and norfloxacin. This suggests that combined administration of fluoroquinolones and biphenylacetic acid synergistically inhibits GABA(A)-receptors at the intestinal level.

Synergistic and antagonistic pharmacodynamic interaction between ranitidine and cisapride: a study on the isolated rabbit intestine.

The present study examines the pharmacodynamic interaction between the H(2)-receptor antagonist ranitidine and the prokinetic agent cisapride on the isolated rabbit intestine. Ranitidine produced a concentration-dependent contractile effect on the duodenal, ileal and ascending colon preparations, with EC(50)values of 1.35 x 10(-4)M for the duodenum, 1.2 x 10(-4)M for the ileum and 1.15 x 10(-4)M for the ascending colon. The effect of cisapride on the ranitidine contractile effect was dependent on the cisapride concentration used. Thus, cisapride, at concentrations from 10(-10)up to 5 x 10(-7)for the duodenum and the ascending colon and up to 10(-6)M for the ileum, potentiated the contractile responses of the preparations to ranitidine. However, at higher concentrations cisapride produced a non-competitive inhibition of the intestinal contractile responses to ranitidine with IC(50)values of 4.2 x 10(-5)M for the duodenum, 1.65 x 10(-5)M for the ileum and 3.2 x 10(-6)M for the ascending colon. These data show that cisapride may modify the contractile responses of the isolated rabbit intestine to ranitidine, having a potentiating effect up to a certain concentration and an antagonistic one at higher concentrations. In conclusion, co-administration of the above drugs may lead to enhanced or reduced intestinal motility.

Propofol's biphasic effect on GABA(A)-receptor-mediated response of the isolated guinea pig ileum.

This study examines the effect of the anaesthetic agent propofol on the guinea pig ileum and whether this effect derives from an interaction of the drug with GABA receptors. Propofol produced a biphasic effect consisting of a dose-dependent contractile effect (EC(50)=2.2x10(-5)m) followed by an 'after relaxation'. This propofol effect was similar to the one produced by GABA and was bicuculline-sensitive (ID(50)=3.2x10(-7)m). Propofol, at concentrations of 10(-7)and 10(-6)m, potentiated the ileum contractile responses to GABA, but only at the lower dose range of applied GABA, while at a concentration of 10(-5)m, it inhibited the contractile effect over the entire dose range of applied GABA. In addition, while the contractile response of the ileum to exogenously applied acetylcholine was not influenced by propofol at concentrations of up to 7x10(-6)m, it was antagonised by higher concentrations of propofol. In conclusion, the above data permit us to suggest that propofol's contractile effect on the guinea pig ileum is mediated by an interaction of the drug with GABA(A)-receptors.

Cisapride counteracts the anticholinergic effect of disopyramide on the guinea pig isolated small but not large intestine.

This study examines the influence of the prokinetic agent cisapride on the disopyramide's effect on the isolated duodenum and ileum, as well as on the ascending colon of the guinea pig. Acetylcholine produced the well known stimulatory effect on the intestinal parts with EC50 values 9.6 x 10(-8), 7 x 10(-9) and 1.1 x 10(-6) M for the duodenum, ileum and ascending colon, respectively. Disopyramide inhibited, in a concentration-dependent manner, the contractile responses of the intestinal parts to acetylcholine. Cisapride, at concentrations ranging from 5 x 10(-9) M to 10(-6) M, significantly reversed the above inhibitory effect of disopyramide on the guinea pig duodenum and ileum and produced a parallel leftward shift of the concentration-response curve for acetylcholine in the presence of disopyramide. However, cisapride was not found able to reverse the inhibitory effect of disopyramide on the ascending colon. In conclusion, cisapride may counteract the inhibitory effect of disopyramide on the guinea pig duodenum and ileum, probably by facilitating the cholinergic neurotransmission, while it is not able to reverse the disopyramide's inhibitory effect on the guinea pig ascending colon.

Effect of combined fenthion and cimetidine use in rats on lethality, blood cholinesterase activities, and serum cholinesterase isoenzymes.

H2-receptor antagonists inhibit cholinesterase (ChE) activity. We examined perturbations in ChE isoenzyme patterns and ChE activities of rats from the combined effects of fenthion (FEN) and cimetidine (CIM). Sixty-four female Sprague-Dawley rats were divided into 8 groups. Four rat groups were given FEN or gum arabic solution and each group divided into 2 small groups according to the CIM or gum arabic administration. FEN was administered po at 12.3 mg/kg (1/20 LD50) or 24.5 mg/kg (1/10 LD50) for 14 days or 49 mg/kg (1/5 LD50) every 4 days. CIM was given po at 1,500 mg/kg from days 7 to 13. Samples were collected 3 h after CIM administration on days 8 and 13. CIM did not influence ChE isoenzyme patterns or ChE activity. FEN inhibited both the ChE isoenzyme patterns and ChE activities without producing clinical signs. Although 1 rat in the 12.5 mg FEN/kg + CIM group died on day 10, all rats in other FEN (24.5 mg/kg or 49 mg/kg) + CIM groups died on days 8-10. Differences in suppression of ChE isoenzyme patterns were detectable between the FEN-dosed and FEN + CIM-dosed groups. There were no differences in ChE activities between the FEN-dosed and FEN + CIM-dosed groups. The i.p. administration of 500 mg CIM/kg (LD50) did not suppress ChE activities.

Effect of cisapride on the isolated guinea pig gall bladder and common bile duct.

1. In this study, the effect of cisapride on the isolated guinea pig gall bladder (GB) and common bile duct (CBD) motility was investigated. 2. Cisapride, up to a certain concentration, produced an increase in tone of the GB (EC50 1.35 x 10(-8) M) and the CBD (EC50 2.75 x 10(-9) M), whereas, at concentrations higher than 3 x 10(-5) M for the GB and 5 x 10(-6) M for the CBD, it produced a transient increase in tone followed by a sustained decrease in tone or in the contraction amplitude or in both. 3. The cisapride-induced increase in tone was antagonized by atropine on both the GB and the CBD. 4. Cisapride up to 2 x 10(-5) M for the GB and 3 x 10(-6) M for the CBD did not modify, whereas, at concentrations higher than 2.8 x 10(-5) M for the GB and 4 x 10(-6) M for the CBD, it antagonized, noncompetitively, the concentration-response curve to exogenously applied acetylcholine. The IC50 values for cisapride on the EC50 of acetylcholine on the GB and the CBD were 9.4 x 10(-5) M and 8.2 x 10(-6) M, respectively. 5. In conclusion, on the guinea pig GB and CBD, low concentrations of cisapride produce a stimulating effect, probably of cholinergic origin, whereas higher concentrations produce a transient stimulating effect, probably of cholinergic origin, followed by a sustained relaxing effect, which may involve both cholinergic and noncholinergic pathways.

Inhibitory effect of flumequine, enoxacin and norfloxacin on the GABA-induced contractile effect on the guinea pig ileum.

1. In the present study the effect of flumequine, enoxacin and norfloxacin on the GABA-elicited contractions (EC50 1.10 x 10(-5) M) on the isolated guinea pig ileum was investigated, in a comparative manner. 2. All three fluoroquinolones dose-dependently antagonised the GABA-induced contractions in a non-competitive manner, but no statistical difference in the degree of the antagonism they produced was noted. Besides, they did not influence the ileal cholinergic contractions induced by exogenous acetylcholine. 3. These results suggest that the above fluoroquinolones tested dose-dependently inhibit the contractile effect of GABA on the guinea pig ileum and this inhibition seems to be associated with an antagonistic action of the fluoroquinolones at GABAA-receptors present in the guinea pig ileum.

Ketamine may modify intestinal motility by acting at GABAA-receptor complex; an in vitro study on the guinea pig intestine.

In the present study the effect of ketamine, a dissociative anaesthetic, on the GABA- and on the specific GABAA-agonist muscimol-induced responses of the isolated guinea pig ileum was investigated. GABA as well as muscimol produce a concentration-dependent contractile effect on the duodenum, jejunum and ileum. The sensitivity of the intestinal parts to both the above substances increases from the duodenum to the ileum. Ketamine produces a non-competitive inhibition of the GABA- and muscimol-induced contractions of the ileum, while it does not influence the ileal cholinergic contractions induced by exogenous acetylcholine. These results suggest that ketamine may modify intestinal motility through its antagonistic action at the GABAA-receptor complex.

The effect of xylazine on the isolated sheep trachea.

The effect of xylazine on the isolated sheep trachea and its possible interactions with the alpha 2-adrenergic antagonist, atipamezole, and the anticholinergic agent, atropine, was studied. The mechanical responses of the tracheal preparations were recorded after exposing each one to cumulatively increasing concentrations of xylazine alone or in the presence of atipamezole or atropine. Xylazine exerted a concentration-dependent contractile effect, with a threshold concentration of 10(-7) M while the maximum activity was produced at a concentration of 10(-5) M (EC50 = 2.3 x 10(-7). This xylazine-induced contractile effect was inhibited by atipamezole, but not significantly modified by atropine. Thus, it is concluded that alpha 2-adrenoceptors exist in the sheep trachea and it is suggested that alpha 2-adrenoceptor agonists may act on airways in sheep directly through stimulation of peripheral alpha 2-adrenergic receptors and indirectly via central alpha 2-adrenergic receptor activation of parasympathetic tone.

Ketamine protects acetylcholinesterase against inhibition by propoxur and phoxim.

In the present study the effect of ketamine on the contractions caused by propoxur and phoxim on the isolated guinea pig ileum was investigated. Ketamine was found able to inhibit in a concentration-dependent manner the contractile responses of the ileum to propoxur and phoxim, while it did not significantly modify the contractions induced by acetylcholine. Propoxur and phoxim augmented the contractile responses induced by acetylcholine in the presence of acetylcholinesterase. This augmentation was prevented by ketamine, in a concentration-dependent manner. These findings suggest that ketamine inhibits the contractile effect of propoxur and phoxim on the guinea pig ileum and this inhibition seems to be associated with the protection of acetylcholinesterase against the action of these two compounds.

Ketamine inhibits the anticholinesterase activity of ranitidine and physostigmine.

1. The present study was performed to investigate the effect of ketamine on the contractile responses induced by the H2-receptor antagonist ranitidine and the anticholinesterase agent physostigmine on the isolated guinea pig ileum. 2. The contractile responses induced by ranitidine and physostigmine on the guinea pig ileum were significantly prevented, in a concentration-dependent manner, by ketamine, while the ones induced by acetylcholine were not modified. The contractile responses induced by acetylcholine were inhibited by exogenous acetylcholinesterase. Ranitidine and physostigmine inhibited the above reduction, and this effect was significantly prevented by ketamine in a concentration-dependent manner. 3. These findings show that ketamine inhibits the contractile effect of ranitidine and physostigmine on the guinea pig ileum. This inhibition caused by ketamine seems to be associated with the protection of acetylcholinesterase against the inhibition by ranitidine and physostigmine.

Effect of nizatidine and ranitidine on the D-tubocurarine neuromuscular blockade in the toad rectus abdominis muscle.

The influence of varying concentrations of the H2-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 H2-receptor antagonists and D-tubocurarine were studied. Nizatidine and ranitidine at a concentration of 3.2 x 10(-4) mol l-1 augmented, and at 3.2 x 10(-3) mol l-1 inhibited, the acetylcholine-induced contractures on the toad rectus abdominis muscle, while at concentrations from 3.2 x 10(-4) to 3.2 x 10(-3) mol l-1 they inhibited the carbachol-induced contractures, in a concentration-dependent manner. In addition, nizatidine and ranitidine at a concentration of 3.2 x 10(-4) mol l-1 reversed the D-tubocurarine blocking activity on the acetylcholine-induced contractures, but at a concentration of 3.2 x 10(-3) mol l-1 they augmented it. These findings provide evidence that the above H2-receptor antagonists produce either cholinesterase inhibition or neuromuscular blockade, depending on their concentration. Thus, the D-tubocurarine neuromuscular blocking activity is potentiated at high concentrations of nizatidine and ranitidine, while it is reversed at lower ones.

Effect of erythromycin on different parts of the rabbit intestine: comparison with motilin.

1. The effect of erythromycin on the rabbit intestinal smooth muscle was investigated and was compared to that of motilin. 2. Whole isolated segments from the duodenum, jejunum, ileum and ascending colon, as well as strips from the circular and longitudinal smooth muscle of the ascending colon were used. 3. Erythromycin was found to possess a concentration-dependent contractile effect on the above intestinal parts but in different degree of intensity. 4. The order of the sensitivity of the intestinal parts to erythromycin was duodenum = jejunum > ascending colon > ileum. 5. The circular smooth muscle of the ascending colon was found to be more sensitive than the longitudinal smooth muscle. 6. A similar contractile effect, but at lower concentrations, and the same regional specificity were observed with motilin.

Comparative study of erythromycin, troleandomycin and tylosin on the rabbit intestine.

The macrolide antimicrobial agents, erythromycin, troleandomycin and tylosin were tested for their effect on isolated whole segments of the rabbit duodenum, jejunum, ileum and ascending colon, as well as on strips of the circular and longitudinal smooth muscle of the ascending colon. The 14-membered macrolides erythromycin and troleandomycin were found to possess a concentration-dependent contractile effect on the intestinal smooth muscle. The order of potency was: erythromycin > troleandomycin. The 16-membered macrolide tylosin was found to have a much weaker potency than erythromycin and troleandomycin. In addition, the circular smooth muscle of the ascending colon was found to be more sensitive to the compounds tested than the longitudinal smooth muscle.

Effect of cimetidine on the histamine and dimaprit induced responses on the guinea pig ileum during development.

1. In ileal segments obtained from 1, 15, 30, 45 and 60-day-old guinea pigs, histamine produced a dose-dependent significant contractile response from a threshold concentration of 3.2 x 10(-8) M. 2. The mean contractile responses (at 10(-5) M) of the ileum from the 1, 15, 30 and 45-day-old guinea pigs compared to that from the 60-day-old ones were 73.79, 92.30, 94.07 and 94.41% respectively. 3. Dimaprit (at 10(-5) M) had no effect on the ileum of the 1-day-old guinea pigs, while it produced a significant contractile response on the ileum of the guinea pigs at all other ages. 4. The mean contractile responses of the ileum from the 15, 30 and 45-day-old guinea pigs compared to that from the 60-day-old ones were 74.44, 66.66 and 76.94% respectively. 5. Cimetidine (at 10(-5), 3.2 x 10(-5) and 10(-4) M) significantly potentiated the histamine-induced contraction on the ileum of the 1-day (from 10(-7) to 3.2 x 10(-6) M) and the 15-day-old guinea pigs (at 10(-8) M). 6. On the contrary, cimetidine (at 3.2 x 10(-5) M) significantly inhibited the contractile response of the ileum to dimaprit. 7. In conclusion, the guinea pig ileum appears increasingly sensitive to histamine from the newborn to the 60-day-old ones, but its sensitivity to dimaprit, not detectable in the newborn, fluctuates during development. 8. Cimetidine produces a potentiating effect, which declines with age, on the on the histamine-induced contraction, while it inhibits the dimaprit contractile response on the ileum.

Comparative study of the H2-receptor antagonists cimetidine, ranitidine, famotidine and nizatidine on the rabbit stomach fundus and sigmoid colon.

The H2-receptor antagonists, cimetidine, ranitidine, famotidine and nizatidine, were tested for their effect on the isolated smooth muscle strips from the rabbit stomach fundus and sigmoid colon. These H2-receptor antagonists were found to possess a concentration-dependent contractile effect on the above smooth muscle preparations and the order of potency was: ranitidine = nizatidine > famotidine > cimetidine. In addition, the smooth muscle preparations from the sigmoid colon were significantly more sensitive to the above compounds than the smooth muscle preparations from the stomach fundus.