Georgios Kounenis

Application and validation of a LC/fluorescence method for the determination of amoxicillin in sheep serum and tissue cage fluid.

A LC method with fluorescence detection after pre-column mercury dichloride derivation was developed and validated for the quantitative determination of amoxicillin in sheep blood serum and tissue cage fluid at levels down to 100 and 200ng/mL, respectively. Spiked blood serum and tissue cage fluid samples were deproteinized, derivatized with mercury dichloride and extracted prior to reversed phase LC analysis with fluorescence spectrophotometric detection at an excitation wavelength of 355nm and an emission wavelength of 435nm. Separation was carried out on a C(18) column with a mobile phase consisting of phosphate buffer, octanesulphonate sodium (OCT), and acetronitrile. A regression model using 1/concentration weighting was found the most appropriate for quantification. The intra-day precision for serum was 1.65-8.74% and for tissue cage fluid was 2.48-6.27%. The inter-day precision for serum was 0.39-3.57% and for tissue cage fluid was 0.44-2.54%. The overall precision over 3 days for blood serum using of 108 replicates was 1.70-9.44% and for tissue cage fluid using of 54 replicates was 2.51-6.76%. Studies of amoxicillin stability in blood serum and tissue cage fluid indicated that amoxicillin was stable after 4 weeks storage at -85 degrees C. The method was successfully applied for the determination of amoxicillin in blood serum and tissue cage fluid samples collected from rams after intravenous administration.

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.

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.

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.

Enhancing and inhibitory effects of H2-receptor antagonists on the GABA and the GABAA-agonist muscimol responses of the isolated guinea pig ileum: a pharmacodynamic interaction

This study investigates the effect of histamine H(2)-receptor antagonists on the GABA-responses of the intestine. GABA and the GABA(A)-agonist muscimol were applied to isolated ileal guinea pig preparations in the absence, and presence of two H(2)-receptor antagonists, famotidine and cimetidine. Both GABA and muscimol produced a concentration-dependent contractile effect on the guinea pig ileum. Famotidine and cimetidine modified this contractile effect, either by enhancing or by inhibiting it. The differing results depended not only on the antagonist concentration, but also on the concentration of GABA or muscimol. When tested at the concentration of 10(-5)M, famotidine enhanced the contractile response of the ileum to either GABA or muscimol, while cimetidine did not modify it. At the concentration of 3 x 10(-4)M, both H(2)-receptor antagonists tested inhibited the contractile effect of either GABA or muscimol. However, the famotidine-induced inhibition was more potent than the one produced by cimetidine. In conclusion, the interaction of H(2)-receptor antagonists with GABA receptors is not limited to the central nervous system, but it also extends to the peripheral nervous system. The receptor interaction mainly involves GABA(A)-receptors and depends on both the specific H(2)-antagonist and the concentration used.

Peripheral distribution of amoxicillin in sheep and influence of local inflammation.

The pharmacokinetics of amoxicillin (AMX) in blood serum (SBS) and tissue cage fluid (TCF) was studied in sheep. Four tissue cages, prepared from silicone rubber tubing, were subcutaneously inserted in the neck area (two on each side) of the experimental animals and AMX was administered both intravenously (IV) and intramuscularly (IM) at the dose rate of 15mg/kg bodyweight. The impact of local inflammation on AMX distribution in TCF was studied after intra-cavity injection of a lambda carrageenan solution in one of the two tissue cages used after each administration. In contrast to the three-compartment AMX disposition after IV injection, two-compartment, absorption-limited pharmacokinetics was observed after IM administration. Non-inflamed and inflamed TCF data revealed, in all cases, the attainment of low, but prolonged concentrations and absence of an inflammation-induced effect on AMX penetration into and elimination from TCF.

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.