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

Trends in veterinary clinical and fundamental pharmacology: past and future in The Netherlands

Veterinary pharmacology has undergone a gradual development in the Netherlands during this century. Starting from a historical perspective the paper aims to provide an overview of future trends and important issues in the area of veterinary pharmacology and toxicology. It is pointed out that this discipline comprises several subdisciplines as the comparative aspect of both, pharmacology and toxicology, is inherent to veterinary medicine which has to address a broad variety of animal species. Thus, the comparison of drug effects, side effects, and drug disposition as well as the comparison of the species-specific susceptibility to xenobiotics are obvious challenges in this discipline. Several areas in clinical pharmacology are highlighted to indicate future research needs. Finally, the principles of Good Veterinary Practice are presented as the 'golden standard' in veterinary clinical pharmacology.

Effect of ingesta and of tablets of different strengths on the systemic availability of digoxin in normal dogs

The plasma concentrations of digoxin were measured in eight normal dogs given digoxin on four occasions, using three different feeding regimens and tablets of two strengths. Although ingesta tended to slow the absorption of digoxin, the systemic availability of the drug, based on measurements of Cmax, tmax and AUC did not differ when digoxin tablets were given with canned food, with dry food, or without food. However, some of the pharmacokinetic characteristics and smaller individual variations with the dry food regimen would be considered advantageous for maintenance therapy. Tablets containing 62.5 micrograms or 250 micrograms of digoxin had a similar relative bioavailability. The peak plasma digoxin concentrations were higher in female dogs, and the trends in other data also suggested that the systemic availability of digoxin was better in female dogs.

Gender differences in drug metabolism regulated by growth hormone

Gender differences in drug metabolism in rats have been known for more than 60 years when it was first reported that the much shorter duration of drug action in the male was due to the effects of testicular androgens. More recent studies have demonstrated that this sexual dimorphism in rat drug metabolism results from the differential expression of a possible dozen, or so sex-dependent hepatic forms of cytochrome P450. Moreover, it is the sexually dimorphic plasma profiles of growth hormone, and not androgens, that directly regulate the expression of these individual forms of hepatic cytochrome P450. Male rats secrete growth hormone in an "on-off" episodic rhythm in which interpulse periods contain no detectable levels of the hormone. Growth hormone secretion in the female rat is also pulsatile, but can be characterized as "continuous" since hormone levels are always present in the circulation. It would appear that the duration of the interpulse period is at least one "signal" in the growth hormone profile regulating hepatic expression of the sex-dependent forms of cytochrome P450, and thus establishing the gender differences in drug metabolism. The exaggerated gender differences in rat drug metabolism (i.e. 300-500%) have made it the standard, and understandably an ideal model in which to investigate the mechanisms regulating these dimorphisms. However, it is also possible that these studies have limited value when extrapolated to other species, such as humans, in which the magnitude of the sexual differences are much smaller, and the dimorphism may be reversed (F > M). In this regard, the mouse model, in which the sexual differences (F > M) in drug metabolizing enzyme activities vary by only 40-100%, are also regulated by sex-dependent plasma growth hormone profiles, and may be more representative of the vast majority of outbred species in which only subtle gender differences occur in drug metabolism.

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.