What can the use of unmetabolizable lipophilic compounds tell about the importance of drug metabolism?

Fat feeding potentiates the diabetogenic effect of dexamethasone in Wistar rats

Background

The role of cortisol and its increased action/availability is implicated in the pathogenesis of insulin resistance associated with obesity and metabolic syndrome but the mechanism of increased action/availability is not known. Availability of several other lipophilic hormones, drugs and pollutants are also reported to be increased in obesity. Increased lipids in the circulation are reported to alter the fluidity and permeability of membranes. Hyperlipidemia is also reported to alter the pharmacokinetics and pharmacodynamics of lipophilic molecules and also membrane fluidity and permeability. In this context we assumed that the hyperlipidemia associated with human obesity might play a role in the altered action/availability of cortisol and this in turn might have initiated the metabolic complications. To evaluate our assumption we have administered dexamethasone [low [50 μg/kg/day] or high [250 μg/kg/day] dose] to high-fat [coconut oil & vanaspati] fed rats and the results were compared with rats administered with either dexamethasone or high-fat.

Results and Discussion

Within two weeks, the rats co-administered with high-fat and dexamethasone developed severe hyperglycemia, hyperlipidemia and insulin resistance compared to rats treated either of them alone. High-fat fed rats treated with higher dose of dexamethasone were presented with severe hyperglycemia, insulin resistance and also severe glycosuria. The hyperlipidemia caused by high-fat feeding might have altered the transport and distribution of dexamethasone, probably by altering the physical state of membranes and transport proteins.

Conclusion

From the results obtained, it can be speculated that the altered lipid and cortisol metabolism could affect one another, forming a vicious cycle.

Effects of 2,3,7,8-tetrachlorodibenzo-p-dioxin or 2,2',4,4',5,5'-hexachlorobiphenyl on vitamin K-dependent blood coagulation in female germfree WAG/Rij-rats

Newborn infants are susceptible to bleeding disorders caused by a vitamin K deficiency, so called 'haemorrhagic disease of the newborn' (HDN). These bleedings often occur in infants after medication of the mother with antiepileptics, such as phenobarbital or phenytoin. It has been suggested that an increase in the late type of HDN in exclusively breast-fed infants might be related to the presence of cytochrome P450-inducing polychlorinated biphenyls (PCBs), dibenzo-p-dioxins (PCDDs) and dibenzofurans (PCDFs) in human milk. In order to study this possible mechanistic relationship 5-week-old, germfree, female WAG/Rij-rats were exposed to a single oral dose of either 1 microgram 2,3,7,8-tetrachlorodibenzo-p-dioxin/kg body weight (TCDD) or 30 mg 2,2',4,4',5,5'-hexachlorobiphenyl/kg body weight (HxCB), representing cytochrome P-450 1A (3-methylcholanthrene type) and 2B (phenobarbital type) inducers. During the experiment blood coagulation time from each rat was measured. Also, hepatic 7-ethoxy-(EROD) and 7-pentoxyresorufin O-dealkylating (PROD) activities and total cytochrome P450 content were measured. Blood coagulation time (Thrombotest) in the HxCB-treated rats was significantly prolonged and positively correlated to PROD activity and total P450 content. No clear effect of TCDD on coagulation time could be observed under these experimental conditions. These results suggest involvement of P450 2B isoenzymes in vitamin K metabolism.

The use of 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) as an unmetabolizable lipophilic model compound

2,4,5,2',4'5'-Hexachlorobiphenyl (6-CB)--a polychlorinated biphenyl (PCB) congener resistant to metabolism in most species--has become a major residue in the biosphere including human adipose tissue. Its use as a model of unmetabolizable lipophilic compounds and as a tool in toxicokinetics in the last two decades is reviewed. This extremely water-insoluble compound is transported in plasma by albumin and lipoproteins. Binding to these plasma proteins appears to be important for uptake and release processes in different tissues. The redistribution kinetics of 6-CB as well as its pronounced adipose tissue storage and a very slow excretion with the faeces has been established in long-term animal studies. Excretion is strongly influenced by an increasing or diminishing adipose storage compartment size. Other minor pathways of elimination, e.g., via hair, become also important in the absence of metabolism and renal excretion. 6-CB has revealed the possibility of an almost quantitative transfer of the maternal body burden to the offspring via milk. The use of 6-CB in studies with tissue preparations in vitro is providing insight into transport mechanisms of uptake and release.