The regulation of oxidative drug metabolism by growth hormone in the dwarf goat: differences from and similarities to the mechanisms in rats.

The effects of bovine GH (BST), administered in different dose patterns, on in-vivo oxidative drug metabolism, were studied in female dwarf goats. Animals received recombinantly derived methionyl BST at a dose of 500 micrograms/kg body weight per 24 h for 6 days. It was administered to one group of goats as one s.c. injection per day, another group received a similar 24-h dose divided into three s.c. injections given at 8-h intervals, and the third group received 50 micrograms BST/kg body weight every 2.5 h by a pulsative i.v. infusion. Oxidative metabolic capacity was assessed by determining plasma sulphadimidine (SDD) elimination and urinary metabolite excretion. SDD shows a marked sex-dependent plasma elimination in dwarf goats, with male goats having a lower plasma clearance than female goats. When BST was given by daily injection, no clear effects on SDD plasma clearance or urinary metabolite excretion were observed. However, when the total dose was divided into three injections given at 8-h intervals, the plasma SDD elimination rate decreased. This was associated with a decrease in urinary excretion of the two main hydroxy SDD metabolites. When BST was given by discontinuous i.v. infusion, simulating the male endogenous plasma GH pattern, a marked decrease in SDD plasma clearance was observed. In addition, the excretion of the two urinary hydroxy metabolites was considerably reduced. These results suggest that GH can affect drug oxidation in dwarf goats via mechanisms similar to those suggested for rats. However, in the dwarf goat, the sex differences in drug metabolism are opposite to those in rats.

Cytochrome P450 induction and metabolism of alkoxyresorufins, ethylmorphine and testosterone in cultured hepatocytes from goats, sheep and cattle.

Very little is known of cytochrome P450 (P450) patterns and enzyme characteristics in food-producing animal species. Oxidative metabolism of alkoxyresorufins, ethylmorphine (EtM) and testosterone (TST) was used to monitor the effects of the P450 inducers phenobarbital (PB), beta-naphthoflavone (BNF), dexamethasone (DEX) and triacetyloleandomycin (TAO) in primary cultured hepatocytes from goats, sheep and cattle. BNF effectively and specifically induced ethoxyresorufin deethylase (> 20-fold), indicating the presence of an inducible P450 1A form, and down-regulated EtM demethylation and most selected TST hydroxylations. In non-induced hepatocyte cultures, TST was metabolized to 6 beta-, 2 beta-, 12 beta-, and 11 alpha-hydroxy-TST (OHT). PB and, to a lesser extent, DEX non-specifically induced all OHT formations, and EtM demethylation. TAO almost completely inhibited OHT formation and EtM demethylation. These results indicate the involvement of principally one P450 form, or a restricted number of related P450 forms, presumably belonging to the P450 3A subfamily. In western blot analysis, cross reactivity was found with rat anti-P450 3A1 and anti-sheep P450 3A. A more specific PB effect was observed for 16 alpha-OHT, which may be formed though a ruminant P450 2B form. None of the inducers influenced pentoxyresorufin depentylase (PROD) or EtM O-deethylation. Metabolite patterns and inducibility of selected activities in ruminant hepatocytes are in accordance with previous findings in goats in vivo. Cytochrome P450 characteristics in ruminants appear to differ from those in rats whereas similarities to the situation in humans appear to exist.

Hepatic cytochrome P450 induction in goats. Effects of model inducers on the metabolism of alkoxyresorufins, testosterone and ethylmorphine, and on apoprotein and mRNA levels.

Male and female African dwarf goats were treated orally with phenobarbital (PB) or triacetyloleandomycin (TAO), or subcutaneously with beta-naphthoflavone (BNF). Hepatic microsomal cytochrome P450 content was increased by PB and TAO, but not by BNF. PB effects on P450 activities were non-selective: ethoxyresorufin deethylase (EROD) and pentoxyresorufin depentylase (PROD), hydroxylation of testosterone (TST) and demethylation of ethylmorphine (ETM) were all induced by a factor of 2-3. A similar non-selective induction was observed with TAO, except for EROD and PROD (no effects). After PB and TAO treatment, increased levels of a protein cross-reactive with anti-sheep P450 3A and 2B were found. Thus, in dwarf goats, both PB and TAO appeared to be P450 3A inducers. Selective PB effects related to a P450 2B form on PROD are lacking but 16 alpha-hydroxylation of TST was induced markedly. At the mRNA level, PB induced an mRNA that showed good sequence homology with a human P450 3A4 cDNA probe, rather than with a rat 3A1 probe. BNF selectively induced EROD, whereas TST hydroxylation and ETM dealkylation were inhibited. With BNF-treated animals, increased concentrations of a protein cross-reactive with anti-rat P450 1A1/1A2 and of an mRNA that showed homology with a human 1A1 cDNA probe, but not with a mouse 1A1/1A2 probe, were observed.

Differential effect of pentoxifylline on lipopolysaccharide-induced downregulation of cytochrome P450.

It is now established that inflammatory stimuli such as lipopolysaccharides (LPS) and polyinosinic acid:polycytidylic (polyIC) suppress hepatic expression of cytochrome P450 (P450) genes in rat liver. Previous studies have suggested that LPS- or polyIC-induced downregulation of P450 was due to endogenously released inflammatory cytokines such as tumor necrosis factor-alpha (TNF-alpha), interleukin-1, interleukin-6, and interferons (IFNs). To improve our understanding of the role of inflammatory cytokines in mediating P450 depression, we investigated the possibility of preventing P450 downregulation with pentoxifylline. Pentoxifylline has been shown to inhibit LPS-induced TNF-alpha production by suppression of TNF-alpha gene expression. The present study shows that in uninduced male rats pentoxifylline selectively prevents the downregulation of microsomal P4501A2 and P4502B caused by LPS. No protective effect of pentoxifylline on the downregulation of P4502E1 and P4503A1/2 was observed. PolyIC-induced downregulation of P4501A2, P4502B, P4502E1, and P4503A1/2 was not affected by pentoxifylline. These results suggest that the LPS-induced downregulation of P4501A2 and P4502B is mediated to a large extent by TNF-alpha. Other cytokines might be involved in the suppression of P4502E1 and P4503A1/2. The fact that polyIC-induced downregulation is not protected by pentoxifylline is further evidence that this agent acts via a selective induction of IFNs.

Differential effects of pentoxifylline on the hepatic inflammatory response in porcine liver cell cultures. Increase in inducible nitric oxide synthase expression.

Pentoxifylline (PTX) has been shown to exert hepatoprotective effects in various liver injury models. However, little information is available about the effect of PTX on the hepatic acute phase response. In the present study, the effect of PTX on a lipopolysaccharide (LPS)-induced acute phase response in primary porcine liver cell cultures was examined. During 72 hr of incubation with or without LPS, the ability of PTX to influence the secretion of tumour necrosis factor-alpha (TNF-alpha), interleukin-6 (IL-6), acute phase proteins, and nitric oxide (NO) was assessed. PTX completely inhibited LPS-induced TNF-alpha production and attenuated IL-6 only after 48 hr of incubation. In contrast, PTX potentiated NO production and the expression of inducible nitric oxide synthase (iNOS) in hepatocytes after stimulation with LPS. The increased expression of iNOS and concurrent production of NO was also observed when liver cell cultures were incubated with dibutyryl cyclic adenosine monophosphate. No effect of PTX on acute phase protein secretion was observed during 72 hr of incubation. The present results show that PTX differentially affects the endotoxin-induced inflammatory response in primary porcine liver cell cultures by suppressing TNF-alpha and IL-6 while potentiating NO production.

The antipyretic effect of flurbiprofen.

Flurbiprofen, like its predecessor ibuprofen, possesses antipyretic properties in the endotoxin-fevered rabbit. Comparison of flurbiprofen and ibuprofen at varying dosages, reveals that flurbiprofen is at least 15 times more potent in this species. In goats, flurbiprofen is more potent than in rabbits. Flurbiprofen inhibited the pyrogenic effect of intravenously administered leucocytic pyrogen, but it did not alter the release of leucocytic pyrogen from peritoneal exudate cells in vitro. Moreover, flurbiprofen did not inhibit endotoxin-induced release of endogenous pyrogen in vivo. Incubation of leucocytic pyrogen with flurbiprofen did not alter its pyrogenic poteny. These results suggest that the antipyretic activity of flurbiprofen is due neither to interference with endogenous pyrogen synthesis and release, nor to inactivation of circulating endogenous pyrogen.

Effect of intracerebroventricular injection of PGE2 and 5HT on body temperature, heart rate and rumen motility of conscious goats.

Changes in rumen motility and heart rate following injection of 5-HT and PGE2 into the third cerebral ventricle were investigated in conscious goats. The doses used were known to produce predictable changes in thermoregulation in goats. The changes in body temperature, ear temperature (PGE2, 5-HT) and shivering (PGE2) were as reported by others. The i.c.v. injection of PGE2 and 5-HT inhibited rumen motility and slightly decreased the heart rate, probably due to a central action.

Stereoselectivity at the beta2-adrenoceptor on macrophages is a major determinant of the anti-inflammatory effects of beta2-agonists.

Previous research has shown that beta-adrenoceptor (beta-AR) agonists have potent anti-inflammatory capabilities, e.g. represented by suppression of release of the proinflammatory cytokines. Aim of this research was to determine whether the effects of beta-agonists on LPS-induced TNFalpha and IL-10 release are influenced by their different stereochemistry. In addition, the role of the beta-AR subtypes was studied. The effect of two stereoisomers of the selective beta2-AR agonist TA2005 [(R,R)- and (S,S)-] on the LPS-induced TNFalpha and IL-10 release by U937 macrophages was compared. The (R,R)-stereoisomer was 277 times more potent in inhibiting the TNFalpha release than the (S,S)-form. The (R,R)-stereoisomer also appeared to be more potent in increasing the IL-10 release. In radioligand binding studies the affinity of (R,R)-TA2005 for the beta-adrenoceptor was 755 times higher than the (S,S)-TA2005 stereoisomer. In addition, the elevation of intracellular cAMP in U937 cells appeared to be stereoselective: (R,R)-TA2005 was more potent in elevating intracellular cAMP. The effect of both stereoisomers on the LPS-induced TNFalpha release could almost completely be antagonized by preincubation with the selective beta2-AR-antagonist ICI-118551. Further evidence that the effect of the beta-agonists is mediated via the beta2-adrenoceptor subtype exclusively was acquired by incubation of U937 cells with selective beta1- and beta3-agonists. None of these receptor subtype agonists showed significant suppressive effect on TNFalpha release. This study provides additional proof that the anti-inflammatory effects of beta2-agonists are mediated via the beta2-adrenoceptor and indicates that these effects are highly dependent on the stereoselectivity of the ligand.

Influence of fever and flurbiprofen on trypanosome growth.

The administration of flurbiprofen, a potent non-steroidal anti-inflammatory drug (NSAID), to goats infected with trypanosomes resulted in high elevated parasitaemia and suppression of fever. In contrast to goats, rats infected with trypanosomes do not show febrile reactions. Therefore, the role of body temperature was investigated with yeast-induced fever in Trypanosoma evansi and T. brucei infected rats. These investigations did not support the hypothesis that a high body temperature causes a drop in parasitaemia. In goats infected with trypanosomes, it is also unlikely that fever has an inhibitory influence on the parasitaemia. In these animals, rises in parasitaemia could be provoked by doses of flurbiprofen as low as 1/20 of the normal doses and these doses did not or only partly suppressed fever. No effect on parasite growth could be obtained when flurbiprofen was added in concentrations up to 32 micrograms/ml directly to T. brucei cultures. Moreover, no growth promoting factor(s) could be identified in vitro in serum from flurbiprofen-treated goats.

Changes in food intake and forestomach motility of dwarf goats by recombinant bovine cytokines (IL-1 beta, IL-2) and IFN-gamma.

The present study was carried out to investigate the effects of rboIL-1 beta, rboIL-2, and rboIFN-gamma on food intake and forestomach motility in conscious dwarf goats. The intravenous injection of rboIL-1 beta (1 micrograms kg-1) resulted in tachycardia and an immediate fever that reached peak values 45 and 180 min after injection. At 9 to 13 min after rboIL-1 beta administration, both frequency and amplitude of rumen contractions rapidly diminished, being minimal at 30 min; during the fever, all goats refused to eat. Compared with the fever induced by rboIL-1 beta, that caused by rboIFN-gamma (2 micrograms kg-1 IV) was delayed in onset. Although the biphasic fever after rboIFN-gamma was more pronounced than after rboIL-1 beta, the changes in forestomach motility, food intake, and heart rate were less than after rboIL-1 beta. No changes in rectal temperature, heart rate, forestomach motility, and food intake were observed after rboIL-2 (1 micrograms kg-1 IV) injection. These results strongly indicate that the effects of cytokines on body temperature can be dissociated from their effects on food intake. Furthermore, these data suggest a possible relationship between forestomach motility and food intake.

In vitro susceptibility to antimicrobial drugs of 62 Salmonella strains isolated from horses in The Netherlands.

The in vitro activity of 17 antimicrobial drugs against strains of Salmonella typhimurium (n = 52), Salmonella thompson (n = 2), Salmonella heidelberg (n = 3), Salmonella hadar (n = 2), Salmonella enteritidis (n = 1), Salmonella infantis (n = 1) and Salmonella derby (n = 1) was tested using the agar dilution method. The strains were isolated from horses admitted to the Large Animal Clinics of Utrecht University. The majority of strains were susceptible to gentamicin, amikacin, kanamycin, enrofloxacin, ciprofloxacin, flumequine, colistine, furazolidone and ceftiofur. However, all strains of Salmonella typhimurium phage type 200 (n = 14), were multiresistant i.e. were resistant to ampicillin amoxycillin, amoxycillin in combination with clavulanic acid, chloramphenicol, nitrofurantoin, trimethoprim, aditoprim and baquiloprim. Two of these strains were also resistant to gentamicin. Based on the susceptibility data found in the present study in combination with pharmacokinetic data available in the literature a rationale for antimicrobial therapy in equine salmonellosis is given. As first choice, gentamicin at a dosage of 3 mg/kg combined with ampicillin at a dosage of 20 mg/kg given with a 8-12 hour dosing interval by intravenous route is advised. As an alternative, the intravenous administration of trimethoprim/sulfonamide combinations given twice daily at a combined dose of 30 mg/kg is suggested.

Primary bovine hepatocytes in the study of cytokine induced acute-phase protein secretion in vitro.

To investigate the utility of primary cultures of bovine hepatocytes for compartmentalized acute phase protein studies the secretion of serum amyloid-A (SAA) and haptoglobin (Hp) was measured after stimulation by pro-inflammatory cytokines (recombinant human IL-6 (rhIL-6) and recombinant human tumour necrosis factor-alpha (rhTNF-alpha)). During the incubation period of the experiment, the SAA and Hp secretion into the culture medium increased (P < 0.05). SAA concentrations showed an additional increase following treatment with each of the cytokines (P < 0.01). Hp concentrations remained unchanged, whereas incubation with a combination of both resulted in a significant increase of the medium concentration of both SAA (P < 0.01) and Hp (P < 0.05). From these findings it is concluded that primary bovine hepatocytes can be used for in vitro studies on acute-phase protein secretion.

The influence of nitrite, nitrate and nitric oxide on ammonia use and urea production in primary cultures of sheep hepatocytes.

Inorganic nitrite introduced into the living organism is rapidly converted into nitrate and nitric oxide (NO). It is known that nitrite decreases ammonia use and urea formation in vitro and it seems that nitrite also influences these processes in vivo. However, the mechanism underlying this effect is not known. Therefore, we decided to compare the influence of sodium nitrite (NaNO(2)), sodium nitrate (NaNO(3)) and NO on ammonia use and ureagenesis in monolayer cultures of sheep hepatocytes during 18 hr of cultivation. It was found that 0.5 and 2.5 mmNaNO(2) significantly reduced ammonia use in a dose-dependent manner for the first 6 hr of incubation; at higher concentrations (2.5 mm), it also decreased urea formation. Also, the presence of nitrite did not affect the lactate dehydrogenase (LDH) activity in the medium which indicates that the nitrite effect did not result from its cytotoxic action. NaNO(3) (0.5 and 2.5 mm) did not induce any changes in ammonia use and urea synthesis in hepatocytes. With sodium nitroprusside (SNP) (0.001, 0.01, 0.1, 0.5 and 1.0 mm) a decrease of ammonia use and urea production was observed corresponding to the nitrite effect, but contrary to nitrite exposure these changes in metabolism were persistent during the whole cultivation period. On the other hand, potassium cyanide (KCN) (0.1 and 0.5 mm) did not influence either urea formation or ammonia use. Thus, it can be concluded that in isolated hepatocytes nitrate and/or NO are not the mediators of nitrite effects on nitrogen metabolism. Furthermore, there is no evidence that nitrite effects are mediated by impaired mitochondrial respiration and energy production.

Characterization of cytochrome P450 isoenzymes in primary cultures of pig hepatocytes.

Despite the fact that pigs are increasingly used in pharmacological and toxicological studies, knowledge on the enzymes which metabolize xenobiotics, in particular cytochrome P450 (CYP) enzymes, in pigs is still very limited. Primary cultures of pig hepatocytes were used to characterize CYP enzymes. The characterization was performed at the level of enzymatic activities, apoprotein and mRNA analyses. Enzyme inducers investigated were beta-naphthoflavone (BNF), phenobarbital (PB), dexamethasone (DEX) and rifampicin (RIF). After 48hr of BNF treatment, CYP1A protein and mRNA levels were increased, and ethoxyresorufin O-deethylation and caffeine 3-demethylation were strongly induced. PB and RIF increased the levels of CYP3A apoprotein and mRNA, whereas BNF down-regulated CYP3A and related activities. PB and RIF treatment resulted in increased ethylmorphine N-demethylation and testosterone hydroxylation, which appears to be the result of CYP3A induction. Hybridization of pig RNA with a human CYP2C9 cDNA probe showed a PB and RIF inducible CYP, which was down-regulated by BNF. Similar inducing effects were observed for tolbutamide, a marker substrate for CYP2C. DEX was not a potent inducer, although some induction of CYP3A mRNA was observed. The present results indicate the absence of CYP2B and probably CYP2D enzymes and activities in pig liver. Despite some dissimilarities, the results indicate that pigs, apart from their very human-like physiology, might represent a more appropriate model species for oxidative drug metabolism in humans than rats.

Determination of sulfadimethoxine, sulfamethoxazole, trimethoprim and their main metabolites in porcine plasma by column switching HPLC.

A HPLC method for the determination of sulfadimethoxine, sulfamethoxazole, trimethoprim and their main metabolites in porcine plasma is reported. The metabolites under investigation were the N4-acetyl sulfonamides and 3'- and 4'-demethyl trimethoprim. In order to obtain a sensitivity of 25-50 ng ml-1, the application of column switching HPLC was investigated. An on-line preconcentration of the drugs and metabolites was preceded by an off-line sample pre-treatment. Parent compounds and metabolites were separated by reversed-phase HPLC followed by UV-detection. The mean recoveries for 4'-demethyl trimethoprim were greater than 80% while the mean recoveries for the other compounds were greater than 90%. Application of the method for analysis of plasma samples obtained from pharmacokinetic studies is described.

Clinical, haematological and blood biochemical changes in goats after experimental infection with tick-borne fever.

Tick-borne fever in goats caused by Ehrlichia (Cytoecetes) phagocytophila was characterised by high fever, dullness, anorexia, tachycardia and a slight to moderate inhibition of rumen motility. The animals developed a gradual decline in the total number of circulating white blood cells. There was a decrease in lymphocytes over a short period, followed by an increase. The number of neutrophils was higher on the 3rd day, causing considerable change in the lymphocyte:neutrophil ratios. The number of eosinophils increased slightly. Serum alkaline phosphatase (ALP) decreased during the febrile episodes, and a marked decline was observed in both plasma zinc and iron concentrations. Furthermore, there was a small but progressive decrease of haemoglobin and haematocrit values. Circulating endogenous pyrogen/leucocyte endogenous mediator could not be detected in plasma from febrile goats. Tick-borne fever was passively transmitted to kids with plasma obtained from these febrile animals.

Fever and changes in plasma zinc and iron concentrations in the goat. The effects of interferon inducers and recombinant IFN-alpha 2a.

Inflammation or invasion by pathogenic micro-organisms induces systemic changes, collectively known as the acute phase response. Among the varied alterations that together produce this response are fever, hypoferraemia and hypozincaemia. It is likely that these responses are mediated, in part, by production and release of cytokines such as interleukin-1 (Il-1), interferons (IFN-alpha) and tumour necrosis factor (TNF). The present report describes a comparative study in dwarf goats on recombinant human IFN-alpha 2a (0.5 x 10(5) IU per kg intravenously (i.v.) and 0.5 x 10(6) IU per kg intramuscularly (i.m.], Poly I:Poly C (an interferon inducer; 30 micrograms per kg i.v.), Newcastle disease virus La Sota strain (an interferon inducer; 0.5 ml per kg i.v.) and Escherichia coli endotoxin (an Il-1 and TNF inducer; 0.1 microgram per kg i.v.). The i.v. injection of recombinant IFN-alpha 2a caused characteristic monophasic febrile reactions, but no significant changes in plasma zinc and iron concentrations. The temperature responses were not due to endotoxin contamination because polymyxin B, which blocks endotoxin, had no inhibitory effect on the pyrogenicity of IFN-alpha 2a. In contrast, the IFN-alpha 2a-induced fever was completely prevented by flurbiprofen pretreatment (1 mg per kg i.v.). In contrast to the i.v. administration, i.m. injection of IFN-alpha 2a caused fever, hypoferraemia and hypozincaemia. Similar results were obtained after E. coli endotoxin, NCD La Sota strain and Poly I:Poly C injection. However, the shapes of the temperature curves and the changes in trace metal concentrations were markedly different. These data support the theory that fever and the changes in plasma zinc and iron concentrations are regulated by different mechanisms.

Fever and changes in plasma zinc and iron concentrations in the goat: the role of leukocytic pyrogen.

In goats with trypanosomiasis (T. vivax or T. congolense) no marked fall in plasma zinc concentration was seen despite high temperature peaks, whereas plasma concentrations of iron tended to undergo some decline. In goats infected with Ehrlichia phagocytophila, there was a marked decline in plasma zinc and iron to low values on the 3rd and 4th day, respectively. Circulating endogenous pyrogen (EP) or leukocytic endogenous mediator (LEM) could not be detected in plasma from febrile goats with tick-borne fever. The intravenous injection of leukocytic pyrogen (LP) in kids caused characteristic monophasic febrile reactions, whereas no significant changes in plasma trace metals were found. So, previous evidence purporting to show that LP is similar to or may be identical with LEM is demonstrably inconclusive. Intravenous injection of E. coli lipopolysaccharide (LPS) or staphylococcal enterotoxin B (SEB) induced fever and lowering of plasma zinc and iron concentrations. The decrease in those trace metal values was more persistent in goats given SEB than in those given E. coli LPS. After intramammary infusion of SEB or E. coli LPS, fever and significant decreases in plasma zinc and iron concentrations were observed but no clear relationship was found between the temperature responses and the alterations in plasma trace metal concentrations. Furthermore, the decrease in plasma iron concentration developed more rapidly in goats given SEB than in those given E. coli LPS, whereas the decrease in plasma zinc concentrations in the former was more delayed. These data support the theory that the concentrations of zinc and iron in plasma are regulated by different mechanisms, whereas febrile reactions are mediated by another type of endogenous protein.

Selective changes in oxidative xenobiotic metabolism in vivo and in vitro after parenteral administration of recombinant bovine somatotrophin to rats.

Effects of recombinant bovine somatotrophin (rBST) on in vivo and in vitro oxidative drug metabolism were studied in male rats. rBST was given subcutaneously at a dose of 250 or 500 micrograms 100 g-1 bodyweight 24 h-1 in different dosage patterns. Sulphadimidine (SDD) plasma clearance, urinary excretion of 6-hydroxy-SDD and the in vitro microsomal SDD-hydroxylations were only inhibited when rBST was given in three injections per 24 hours. The hepatic microsomal ethylmorphine N-demethylation rate and the testosterone hydroxylation rate at the 6 beta position were significantly reduced after one rBST injection per 24 hours. Microsomal testosterone hydroxylation rates at the 16 alpha and 2 alpha-positions were reduced depending on the frequency of rBST administration. It is concluded that the inhibition of in vivo and in vitro drug oxidation in rats by rBST is associated with selective changes in activity of cytochrome P450 enzymes in the liver.

Oral bioavailability and pharmacokinetics of baquiloprim in dwarf goats.

The pharmacokinetics of baquiloprim at a dose of 8 mg kg-1 bodyweight were determined after its intravenous and intra-ruminal administration to seven healthy female dwarf goats. After intravenous injection, the plasma elimination curve showed a rapid distribution phase (mean [SD] t1/2 alpha 0.89 [0.4] hours). The mean volume of distribution at steady-state (Vdss) was 14.1 (2.7) litres kg-1 bodyweight. The mean elimination half-life (t1/2 beta) was 14.0 (2.3) hours. After intra-ruminal administration its maximum concentration in plasma (Cmax) was 0.09 (0.01 microgram ml-1 and this maximum was not reached until approximately 35 hours after administration. The systemic oral bioavailability, calculated up to 48 hours after dosing, was 33.7 (7.1) per cent. Owing to a prolonged absorption phase, the data from only four of the goats fitted reasonably to a compartmental model.