Studies on Propofol in Sheep and Rats

Correia, Dacia Alzira de Augusto (1994) Studies on Propofol in Sheep and Rats. MSc(R) thesis, University of Glasgow.

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Abstract

The studies described in this thesis were carried out in two phases. Studies were performed in rat and sheep hepatic microsome preparations and in rat lung microsome preparations in vitro. A pilot study to choose an optimum incubation time for studies using rat hepatic microsomes indicated that propofol, 28muM, 56muM, 84muM and 140 muM, was rapidly degraded, depending on the initial propofol concentration. For all subsequent studies, an incubation time of 15 min was chosen. There were no significant differences in the rate of degradation of propofol by hepatic microsomes at any propofol concentration between rats and sheep. The mean maximal velocity (Vmax; muMoles/min.mg of protein) and the mean Michaelis constant values (Km; muM) were 0.43 +/- 0.04 & 48.9 +/- 2.2 for propofol in rat preparations and 0.43 +/- 0.06 & 42.4 +/-7.1 for sheep tissues. However, the rate of degradation of propofol was significantly different between Sprague Dawley and Wistar rats (n=3 ; P < 0 .05). Microsome preparations obtained from lung tissue showed a high capacity of degradation of propofol (5.6, 11, 28 & 56 muM). However, the amount of cytochrome P450 present in lung microsomes could not be determined. The Vmax and Km values for lung tissue were 0.93 +/- 0.28 Umoles/min.mg of protein and 13.7 +/- 4.8 muM , respectively. 1-aminobenzotriazole (12 mM), inhibited the degradation of propofol by rat hepatic microsomes by 75.4% - 29.5%, depending on the concentration of propofol assayed (28 - 140 muM), thus indicating that the degradation of propofol in this system is highly cytochrome P450-dependent. Ketamine, 5 ?g/ml, and alfentanil, 200 ng/ml, incubated with propofol (28muM, 56muM, 84muM & 140 muM) did not alter the rate of degradation of propofol in hepatic microsomes obtained from 6 Sprague Dawley rats. Mean (+/- sem) Vmax values (muMoles/min.mg protein) for propofol alone (P) and in the presence of ketamine (K) and alfentanil (A) were 0.52 +/- 0.08 (P), 0.56 +/- 0.06 (K); 0.43 +/- 0.04 (P), 0.39 +/- 0.03 (A). Mean (+/- sem) Km values (muM) for propofol alone (P) and in the presence of ketamine (K) and alfentanil (A) were 46.5 +/- 2.5 (P), 53.2 +/- 3.2 (K); 48.9 +/- 2.2 (P), 48.7 +/- 4.4 (A). However, aminopyrine, 5mM, impaired the degradation of propofol at all concentrations. The mean (+/- sem) Vmax and Km values for propofol alone (P) and for propofol in the presence of aminopyrine (Am) were 0.42 +/- 0.04 (P), 0.43 +/- 0.06 (Am) muMoles/min.mg protein and 43.6 +/- 5.0, 66.2 +/- 7.6 (Am) muM. The significant increase in Km value for propofol in the presence of aminopyrine (p < 0.05) indicates that the inhibition is competitive. Propofol (5.6muM, 56muM, 280muM, & 561muM) also inhibited dose dependently the formation of formaldehyde from the demethylation of aminopyrine. Propofol, 56muM, impaired the demethylation of aminopyrine by 34.9% (mean), which suggests that propofol has the potential in vivo to inhibit the degradation of drugs that are metabolised in this way. Propofol, alone (group 1 (G1), n=5) or in combination with ketamine (group 2 (G2), n=5) was used to anaesthetise 10 Scottish Blackface sheep undergoing superficial body surface surgery. All sheep were premedicated with acepromazine, 0.05 mg/kg and papaveretum, 0.4 mg/kg, i.m., 30 min before induction of anaesthesia with either propofol, 4 mg/kg (Gl) or propofol, 3 mg/kg, and ketamine, 1 mg/kg, injected i.v. over 60 sec. Immediately after induction of anaesthesia, a variable infusion rate of either propofol alone (G1), 0.3 - 0.5 mg/kg/min or propofol 0.2 - 0.3 mg/kg/min and ketamine, 0.1 - 0.2 mg/kg/min (G2) was started, adjustments being made according to the clinical and cardiovascular and respiratory status of each animal. Blood samples for propofol analysis, carried out by high performance liquid chromatography, were obtained during the infusion period and for 24 h after the infusion was stopped. The mean infusion periods were 64.8 +/- 3.1 min and 60 +/- 0 min for G1 and G2 respectively. Induction and maintenance of anaesthesia was satisfactory for each group, although endotracheal intubation was not possible in one sheep in G1 after the induction dose of propofol. The inclusion of ketamine in the anaesthetic regime had a propofol-sparing effect. The total dose of propofol used was 801 +/-39 mg (G1) and 468 +/- 32 mg (G2), while the total dose of ketamine used was 267 +/-15 mg. There were no significant differences in pulse rates or systolic arterial blood pressures recorded during anaesthesia between the 2 groups, however, the respiratory rates were consistently higher in G2 than in G1, although there was no evidence of respiratory depression in sheep in Gl. No attempt was made to measure the 'depth' of anaesthesia in either group. Recovery from anaesthesia was rapid for animals in both groups.

Item Type: Thesis (MSc(R))
Qualification Level: Masters
Additional Information: Adviser: Andrea Nolan
Keywords: Veterinary science
Date of Award: 1994
Depositing User: Enlighten Team
Unique ID: glathesis:1994-76333
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 19 Nov 2019 15:35
Last Modified: 19 Nov 2019 15:35
URI: http://theses.gla.ac.uk/id/eprint/76333

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