Effects of induced hyperthermia on pharmacokinetics of ropivacaine in rats

Effects of Low-Dose Escherichia coli Lipopolysaccharide-Induced Endotoxemia on Morphine Pharmacokinetics in an Animal Model

OBJECTIVE

Systemic inflammation may change the bioavailability and pharmacokinetics of opioids. However, there are insufficient data on morphine pharmacokinetics in mild inflammatory conditions. This study aimed to determine the pharmacokinetics of morphine during low-dose endotoxemia in rabbits.

DESIGN

In two experiments (separated by a 14-day washout period), 10 rabbits received intravenous morphine at a dose of 3 mg/kg. In the second set of experiments, morphine infusion was preceded by low-dose endotoxemia induced with lipopolysaccharide (Escherichia coli 0111: B4) at a dose of 5 µg/kg. The kinetics of systemic morphine concentrations and chosen physiological parameters were measured at specific time intervals up to 6 hours after morphine administration.

RESULTS

In endotoxemia, decreased elimination half-life (P = 0.017), mean residence time (P = 0.022), and volume of distribution (P = 0.037) as well as an increased elimination rate constant (P = 0.013) and total body clearance (P = 0.023) were noted. The inverse linear correlation between morphine clearance versus the percentage (%) change in body temperature and pulse rate observed under control conditions was abolished under endotoxemia.

CONCLUSIONS

Low-dose endotoxemia is correlated with significant alterations in morphine pharmacokinetics in rabbits, leading to the faster elimination of the drug.

CLINICAL IMPLICATIONS

These findings may have important implications in patients with low-grade inflammation and imply the need to modify morphine dosing regimens to ensure optimal analgesia. The issue warrants further experimental and clinical investigation.

Effects of Transfer from Breeding to Research Facility on the Welfare of Rats

Simple Summary

This study measured effects of transfer on body temperature, stress hormone levels, body weight, behavior and water and food intake in rats. Environmental temperature strongly affected body temperature of rats and needs to be controlled. Male rats need to habituate for at least one week, females for two weeks after transfer.

Abstract

Transfer from the breeding facility to a research facility is a stressful event for laboratory animals. Heat stress has been reported to constitute one of the major concerns during transport of animals. This study measured ambient and body temperature, corticosterone and glucose levels, body weight, behavior and water and food intake before, during and after transfer in Wistar rats. Decreased body weight, water and food intake were observed on the day of transfer in rats. Environmental temperature strongly affected body temperature of rats and needs to be controlled. Male rats need to habituate for at least one week, females for two weeks after transfer.

Nanoanesthesia: a novel, intravenous approach to ankle block in the rat by magnet-directed concentration of ropivacaine-associated nanoparticles

BACKGROUND

As an alternative to current methods of local nerve block, we studied the feasibility of producing ankle block in the rat with IV injection of magnetic nanoparticles (MNPs) associated with ropivacaine and application of a magnet at the ankle.

METHODS

The anesthetic effect of magnet-directed ropivacaine-associated MNPs (MNP/Ropiv) was tested in the rat using paw withdrawal latencies from thermal stimuli applied to the hindpaw. The MNP/Ropiv complexes consisted of 0.7% w/v ropivacaine and 0.8% w/v MNPs containing 12% w/w magnetite (F₃O₄). The effect of IV injection of MNP/Ropiv with 15, 30, and 60-minute magnet application to the right ankle was compared with the effect without magnet application on the left hindpaw, to conventional ankle block with 0.1% or 0.2% ropivacaine, and to IV injection of MNPs alone with 30-minute magnet application to the right ankle. In addition, the pharmacokinetics of the MNP/Ropiv complexes were determined.

RESULTS

IV injection of MNP/Ropiv with magnet application at the ankle significantly increased paw withdrawal latencies from thermal stimuli compared with pretreatment baselines in the same paw (P < 0.0001) and compared with the contralateral paw without magnet application (P < 0.0001). IV injection of MNPs alone had no significant effect on paw withdrawal latency. Absolute ropivacaine concentrations in ankle tissue, and ankle tissue-to-plasma concentration ratios were higher in the MNP/Ropiv group with 30-minute magnet application compared with MNP/Ropiv group without magnet application (mean ± SEM, 150 ± 10 ng/g vs 105 ± 15 ng/g, respectively, and 6.1 ± 0.8 vs 4.2 ± 0.7, respectively).

CONCLUSIONS

The current study establishes proof of principle that it is possible to produce ankle block in the rat by IV injection of MNP/Ropiv complexes and magnet application at the ankle. The results indicate that further study of this approach is warranted.

Midazolam in rabbits terminates dysrhythmias caused by intracerebroventricular ropivacaine

The current study was designed to investigate the mechanisms by which ropivacaine may act within the central nervous system (CNS) to produce cardiotoxicity. Eighty New Zealand rabbits were divided into four groups randomly. In Group 1, 20 rabbits received intracerebroventricular (icv) saline, and then received icv ropivacaine 30 min later. In Group 2, 20 rabbits received icv ropivacaine. Whenever dysrhythmias continued for more than 5 min, 0.1 ml saline was administered into the left cerebral ventricle. Ten minutes later, 0.1 ml midazolam was given into the left lateral ventricle. In Group 3, 20 rabbits received icv ropivacaine, and once the dysrhythmias developed, the inspired isoflurane concentration was increased from 0.75% to 1.50%. In Group 4, 20 animals received an intravenous (iv) phenylephrine infusion until dysrhythmias occurred. In Group 1, the rabbits did not develop dysrhythmias in response to icv saline, whereas dysrhythmias did develop in these animals after icv ropivacaine. In Group 2, icv saline had no effect on the dysrhythmias; however, icv midazolam terminated cardiac dysrhythmias. In Group 3, an increase in the concentration of the inspired isoflurane had no effect on dysrhythmias. In Group 4, icv midazolam had no effect on dysrhythmias in response to iv phenylephrine. Ropivacaine administered directly into the CNS is capable of producing cardiac dysrhythmias; midazolam terminated dysrhythmias presumably by potentiation of γ-aminobutyric acid (GABA) receptor activity. Our results suggest that ropivacaine produces some of its cardiotoxicity not only by the direct cardiotoxicity of the drug, but also by the CNS effects of ropivacaine.