Are ( 13 C)cortisol and ( 3 H)cortisol metabolized identically to natural cortisol in adrenalectomized piglets?

Abstract Adrenalectomized piglets were intravenously administered a mixture of ( 13 C 4 )cortisol and ( 3 H)cortisol and natural cortisol to determine if the two tracers are metabolized identically to natural cortisol. Urine was collected after 0.5, 1.0, 1.5 and 2.0 days and the isotope dilution was measured in the four major urinary cortisol metabolites, namely tetrahydrocortisone (THE), tetrahydrocortisol THF), α‐and β‐cortolone in the cumulative urines. In contrast to other studies, because of the sensitivity of the method used to measure the 13 C 4 enrichment, non‐cumulative urine collections were also analysed. Quantification of the 13 C 4 isotope enrichement was carried out by gas chromatography/mass spectrometry with selected ion monitoring. The specific activities of the metabolites from the cumulative urine collections were determined by high‐performance liquid chromatography and scintillation counting. Small secondary isotope effects seemed to occur during the metabolism of ( 13 C 4 )cortisol, as a decrease in isotope enrichment in all four metabolites was measured. These effects were easily observed with α‐and β‐cortolone isolated from the cumulative urine collections; the enrichment decreased by 19% and 14%, respectively. The lowering in isotope dilution in THE observed in the 2.0 day cumulative urine collection in piglets 1 and 2 were 4% and 3%, respectively. A lowering in isotope dilution in THF in the 2.0 day cumulative urine collection could be observed in piglet 2, namely 7%, but no change in isotope dilution could be seen in piglet 1. These secondary isotope effects could only be observed in the 2 days cumulative urine, and not in the cumulative urines collected over shorter times. The non‐cumulative urines collected at half‐day periods showed a significant decrease in isotope dilution in THE and THF isolated from the urine collected after 1 day. No statistically significant isotope effects were observed with the metabolism of ( 3 H)cortisol, except at 0.5 day when the specific activity in the cortolones was lower and that in THF was higher. However, at 0.5 day with THE and 1.0 day with THF and the cortolones the specific activities remained approximately 6% higher than that administered in the cortisol. Secondary isotope effects with tritiated cortisol may have occurred but because of the relatively large imprecision of the measurement (SD = 3–4% with THE and THF and the cortolones (SD ≈ 8) compared to the measurements of the 13 C 4 enrichment (SD ≈ 2%) these effects could not statistically be proven. The 13 C enrichment: specific activity ratios of THE and THF isolated from the cumulative urines further illustrated that the metabolites were more preferentially enriched in 3 H than in 13 C. The apparent biological secondary isotope effects observed in the cumlative 2 day urine collections are smaller for ( 13 C 4 )cortisol than for ( 3 H 2 )cortisol. If these isotope effects also occur in man, then the 13 C 4 ‐labelled tracer would be better than the 3 H tracer for measuring the urinary cortisol production rate.