Effect of an experimental malaria infection on the metabolism of phenacetin in the rat isolated perfused liver.

1. The effect of infection with the rodent malaria parasite Plasmodium berghei on the metabolism of phenacetin has been investigated in a rat isolated perfused liver preparation. 2. A bolus dose of phenacetin (10 mg) was introduced into the perfusate reservoir of both control (n = 4) and malaria-infected (n = 4) liver preparations, and samples of bile and perfusate were collected (0-4 h) for hplc analysis of phenacetin, paracetamol and its phase II metabolites. 3. Whereas malaria had no effect on the hepatic clearance of phenacetin (control: 0.64 +/- 0.15 versus malaria: 0.66 +/- 0.14 ml min-1), there was a significant reduction in the hepatic clearance of generated paracetamol (control: 1.22 +/- 0.15 versus malaria: 0.41 +/- 0.08 ml min-1) and the total recovery in bile and perfusate of paracetamol glucuronide (control: 1.18 +/- 0.44 versus malaria: 0.29 +/- 0.20 mg). There was no significant change during malaria infection in the total recovery of either phenacetin (control: 1.30 +/- 0.73 versus malaria: 0.79 +/- 0.36 mg) or paracetamol sulphate (control: 0.81 +/- 0.25 versus malaria: 0.74 +/- 0.16 mg),

Possible isozyme-specific effects of experimental malaria infection with Plasmodium berghei on cytochrome P450 activity in rat liver microsomes.

We have investigated the effect of experimental malaria infection on rat cytochrome P450-mediated drug metabolism using ethoxyresorufin and metoprolol as probe compounds. Malaria infection caused a significant reduction in total intrinsic clearance of ethoxyresorufin in both low and high parasitaemia malaria compared with control (control 18.7 +/- 7.2; low parasitaemia 10.5 +/- 4.1; high parasitaemia 4.3 +/- 1.4 mL min-1). However, clearance of metoprolol was unchanged in malaria infection compared with control (control 2.7 +/- 1.2; malaria 4.0 +/- 1.7 mL min-1). The change in clearance of ethoxyresorufin was the result of a decrease in Vmax, with no apparent change in Km. There was no change in either Vmax or Km of metoprolol. These results indicate a possible isozyme-selective effect of experimental malaria.

The disruption of brain microtubules in vitro by the phospholipase inhibitor p-bromophenacyl bromide.

The influence of p-bromophenacyl bromide (pBPAB) and structural analogues on the assembly and Ca2+ sensitivity of porcine brain microtubules (MTs) was studied by spectrophotometric measurements in vitro. MT assembly was inhibited by 36 microM pBPAB but not by the structural analogues p-chlorophenacyl chloride or acetophenone. In the presence of pBPAB, but not structural analogues, the addition of 10 mM Ca2+ induced aggregation of polymerized MT protein, whereas a decrease in turbidity (due to MT disassembly) was observed in controls. The effects of pBPAB on both MT assembly and Ca2+ sensitivity were blocked by glutathione, but not by N-acetyl L-cysteine, N-acetyl L-lysine nor L-tyrosine, indicating that a highly reduced sulphydryl group(s) may be involved. Western blotting analyses of drug-treated MTs revealed a form of tubulin with altered electrophoretic characteristics, probably caused by a covalent interaction with pBPAB. MT preparations polymerized in the presence of the drug contained fewer MTs than control samples, the predominant structures being identified as amorphous aggregates of MT proteins. The fact that pBPAB affects MT integrity at an effective anti-inflammatory dose in vitro may reflect the involvement of MT disruption in some of the pharmacological effects of this drug. pBPAB is not therefore a suitable tool for studying the specific involvement of phospholipase A2 in cellular events.

Metabolism of caffeine and theophylline in rats with malaria and endotoxin-induced fever.

1. The effects of malaria infection due to Plasmodium berghei and Escherichia coli endotoxin-induced fever on the metabolism of orally-administered caffeine (CA: 10 mg/kg) to its primary metabolites (theobromine (TB), paraxanthine (PX) and theophylline (TH)) were studied in 5-week-old male Wistar rats (n = 5 for each treatment). In separate experiments, the effects of malaria and endotoxin-induced fever on the clearance of i.v.-administered theophylline (TH; 15 mg/kg) were studied in another group of rats. 2. The ratios of CA to the three primary metabolites (TB/CA, PX/CA, PH/CA) determined in a single plasma sample obtained 3 h after CA administration were significantly reduced (p < 0.05) both by malaria and fever compared with control (saline) treatment. The clearance of TH determined from the concentration of TH in a single plasma sample obtained 6 h after TH administration was significantly reduced (p < 0.05) by fever but not malaria (4.0 +/- 0.7 ml/min/kg in controls; 4.2 +/- 0.5 in malaria; 2.4 +/- 0.4 in fever). 3. These results suggest that malaria and fever have different effects on CA and TH metabolism in vivo, probably as a result of different effects on the hepatic isozymes involved.

Effect of malaria infection and endotoxin-induced fever on phenacetin O-deethylation by rat liver microsomes.

We have investigated the effect of malaria infection with the rodent parasite Plasmodium berghei and fever induced by Escherichia coli endotoxin on the metabolism of phenacetin to paracetamol by rat liver microsomes from young (4 weeks old) male Wistar rats (N = 5 in control and fever groups; N = 10 in malaria-infected group). Following determination of % parasitaemia, the malaria-infected group was divided into a low parasitaemia subgroup (N = 5; mean % parasitaemia = 9.87 +/- 2.6) and a high parasitaemia subgroup (N = 5; mean % parasitaemia = 36.6 +/- 8.1). The control group received normal saline. Total microsomal protein was not significantly affected by fever or malaria infection while cytochrome P450 levels were reduced by approximately 50% in the high parasitaemia subgroup, 20% in the low parasitaemia subgroup and 20% in the endotoxin-treated group. Phenacetin-O-deethylation kinetics were biphasic in both control and malaria-infected rats, but monophasic in endotoxin-treated rats. Total apparent intrinsic clearance (CL(int),total; calculated as Vmax/Km; Vmax is maximum velocity, Km is Michaelis constant) of phenacetin was reduced approximately 6-fold in low parasitaemia, 30-fold in high parasitaemia and 35-fold in fever. There was a poor correlation between CL(int),total and % parasitaemia (r = -0.6). However, log CL(int),total correlated inversely with % parasitaemia (r = -0.9), suggesting that Cl(int),total decreased exponentially with an increase in % parasitaemia. Phenacetin O-deethylation is a marker for cytochrome P4501A2 activity and the results of the present study suggest that both malaria infection and fever might specifically reduce P4501A2 activity in the rat.

Effect of malaria infection and endotoxin-induced fever on the metabolism of antipyrine and metronidazole in the rat.

Antipyrine and metronidazole were administered as a cocktail to young (4 weeks old) male Wistar rats (N = 12 for each treatment) to investigate the effect of malaria infection due to the rodent parasite Plasmodium berghei and Escherichia coli endotoxin-induced fever on the metabolism of the two compounds in vivo. Control rats received normal saline. Antipyrine and metronidazole clearances were estimated from a single saliva sample while the formation clearances of their metabolites (in malaria-infected and control rats) were estimated from the product of clearance of parent drug and the fraction of the administered dose excreted as metabolites in urine in 24 hr. Rats treated with endotoxin produced no urine during this period. Malaria infection had no effect on clearance of antipyrine or on formation clearance of any of its metabolites. However, the clearance of metronidazole was reduced by approximately 20% compared with controls as a result of decreased formation of hydroxymetronidazole. Fever decreased clearance of both antipyrine and metronidazole by approximately 36% and 23%, respectively. These results demonstrate that both malaria infection and fever can influence P450-dependent drug metabolism and the effects seen appear to be isozyme-selective.