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Hepatic drug metabolism studies in streptozotocin and spontaneously diabetic rate : the possible influence of [³H]-estradiol binding proteins

University of British Columbia

We have examined the effect of recent onset diabetes on several aspects of hepatic microsomal metabolism in both chemically-induced and spontaneously BB (Bio Breeding) diabetic male and female Wistar rats. Experiments were performed either 4 days post-streptozotocin injection or 4 days after withdrawal of insulin (BB rats). Differential alterations of the diabetic state on hepatic microsomal enzyme activities were observed. Female diabetic rats exhibited no change in benzo[a]pyrene hydroxylase activity, a decrease in testosterone A⁴ hydrogenase, and an increase in aniline hydroxylase. On the other hand, male diabetic rats demonstrated a decrease in hepatic benzo[a]pyrene hydroxylase activity, no change in testosterone A⁴ hydrogenase, and an increase in aniline hydroxylase. Insulin treatment reversed these effects. Benzo[a]pyrene hydroxylase kinetic studies did not reveal marked differences between control and diabetic rats. There were no marked differences between the chemically-induced and genetic models of diabetes with respect to the metabolism studies. Serum testosterone levels were significantly lower than control in BB diabetic males, whereas no change was apparent in female diabetics. Serum insulin determinations suggested that the BB diabetic animals we examined were not severely diabetic although they did exhibit hyperglycemia. Electrophoresis of hepatic microsomal proteins indicated that spontaneous diabetes of short duration altered the protein distribution in the cytochrome R450 region. Two [³H]-estradiol binding sites were detected in rat liver cytosol by Scatchard analysis with a ligand concentration range of 0.05 to 200 nM. The high affinity site, which was specific for estrogens, exhibited a K[sub=d] of ~10⁻¹⁰M and a capacity of ~100 fmol/mg protein in the 50% ammonium sulfate fraction. Unexpectedly, the data suggested that the capacity of this site was greater in males than in females. The moderate affinity binding site exhibited a k[sub=d] of ~10⁻⁷M and a capacity of M0 pmol/mg protein in the whole cytosol fraction. Binding at this site was markedly pH dependent. Both estradiol and dihydrotestosterone competed for binding to this site. A sex difference existed for moderate affinity binding because it was present only in males. We obtained unexpected results in binding studies conducted on a relatively small number of BB diabetic rats. In diabetic males, the capacity of the high affinity site was reduced to 50% of control, whereas the reduction in moderate affinity binding was not nearly so marked. Additional studies using a larger sample size and more sophisticated data analysis are required to verify these results. We concluded that alterations in sex dependent drug metabolism evident in streptozotocin-induced diabetic rats were also seen in the spontaneously diabetic rat model, and were accompanied by changes in the relative disposition of electrophoretically separable microsomal proteins. Changes in circulating androgen levels were also found in BB diabetic males, along with changes in the capacities of certain hepatic steroid binding sites. It is not yet possible to establish mechanistic relationships between these [³H]-estradiol binding sites and modulation of hepatic drug metabolism.

Text

2010-04-15

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http://hdl.handle.net/2429/23664

Pharmacology

University of British Columbia

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