Urinary cortisol metabolites are reduced in MDR1 mutant dogs in a pilot targeted GC-MS urinary steroid hormone metabolome analysis.

P-glycoprotein (P-gp) is the gene product of the multidrug resistance gene (MDR1, syn. ABCB1) that normally restricts the transfer of cortisol across the blood-brain barrier. In the absence of P-gp, cortisol access to the hypothalamus is increased and, by feedback inhibition, this finally leads to lower endogenous plasma cortisol levels in dogs with homozygous nt230(del4) MDR1 mutation (MDR1-/- mutant dogs). While a previous study only focused on plasma cortisol levels, the present study used urinary steroid hormone metabolites to analyze cortisol metabolism in MDR1-/- mutant dogs. Morning void urine was collected from 23 MDR1-/- mutant and 16 MDR1+/+ normal dogs and was subjected to targeted GC-MS steroid hormone metabolome analysis. Seven cortisol metabolites, cortisol itself, and 13 other steroid metabolites were detected. In general, all cortisol metabolites were lower in the urine of the MDR1-/- mutant dogs, with allo-tetrahydro-cortisol and β-cortol reaching the level of significance. In addition, 11-keto-pregnanetriol levels were significantly lower in the urine of the MDR1-/- mutant dogs, indicating that also the 17alpha-OH-progesterone-derived metabolism was altered. In conclusion, the present study provides the first steroid hormone metabolome analysis in the urine of MDR1-/- mutant dogs. Significant differences in the steroid metabolome of MDR1-/- mutant dogs point to a significant role of P-gp for cortisol metabolism and excretion and so indirectly also for hypothalamic-pituitary-adrenal axis regulation in dogs.