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Notes: The metabolism of deutrated cortisol (9,12,12,-2H)cortisol, (2H3-F) was compared to that of radioactive cortisol (3H2-F) and natural cortisol, when these three compounds were administered simultaneously to an adrenalectomized piglet. The relative isotope dilution of tritium was determined from the specific activities of the main urinary neutral cortisol metabolites, tetrahydrocortisone (THE) and tetrahydrocortisol (THF), normalized to that of the cortisol mixture administered. To obtain a comparison of the isotope dilution of deuterium in the metabolites THE and THF to that in the cortisol mixture, the three steroids were converted to the common oxidation product 11-oxo-aetiocholanolone, and deivatized to the methoxime-tert-butyl-dimethylsilyl ether. The relative 2H-isotope dilution then was measured by gas chromatography/mass spectrometry. It was found that the specific activity of THE in the cumulative urine collections was similar to that of the cortisol mixture administered; the two-day value was, however, less. The specific activity of THF was slightly but significantly smaller than 1 (∼0.9) at all times. The relative 2H-isotope dilution in THE was slightly but significantly larger than one (∼1.1) at all times, whereas that in the THF was larger than 1.0 at 9 and 32 h or equal to 1.0 at 20 and 47 h of urine collection. When comparing the metabolism of the two tracer cortisol species the quotient of the 3H- and the 2H-isotope dilutions in THE and THF was smaller than 1.0. It can be concluded that (2H3)cortisol may be used for the determination of the cortisol production rate.

Notes: An isotope dilution mass spectrometric method to determine the urinary cortisol production rate (CPR) in babies and children is described. The method uses stable isotopically labelled (1,2,3,4-13C)cortisol. The tracer is intravenously administered to the patient and urine is collected for the following three days. Following extraction, enzymic hydrolysis, purification and isolation by high-performance liquid chromatography (HPLC) the urinary cortisol metabolites tetrahydrocortisone, tetrahydrocortisol, α- and β-cortolone are separately oxidized to the common product, 11-oxo-aetiocholanolone. The methyl oxime tert-butyldimethylsilyl ether derivative (MO TBDMS) was analysed by gas chromatography mass spectrometry. Quantification of the isotope enrichment was carried out by selected ion monitoring of the base fragment ion at m/z 344[M-103]+ for unlabelled, and at m/z 348 for labelled 11-oxo-aetiocholanolone. HPLC isolation of the metabolites together with the oxidation step allowed very small isotope enrichments, sometimes down to 0.1% (1:1000), to be reliably measured against a linear calibration graph containing 0 to 1% (13C4) enrichments. The standards for the calibration graph were synthesized from mixtures of labelled (13C4) cortisol and natural cortisol, and the calibration graph was prepared each time samples were measured. The long term instrumental precision of the isotope dilution analyses was 0.91% for a derivatized sample containing a (13C4) enrichment of 0.5% (measured on six different days over seven months). The coefficient of variation of the complete procedure for the four cortisol metabolites was between 1.17 and 2.14%. The clinical applicability of the method is demonstrated by presenting the results of a CPR determination in a patient.

Notes: Adrenalectomized piglets were intravenously administered a mixture of (13C4)cortisol and (3H)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 13C4 enrichment, non-cumulative urine collections were also analysed. Quantification of the 13C4 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 (13C4)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 (3H)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 13C4 enrichment (SD ≈ 2%) these effects could not statistically be proven. The 13C enrichment: specific activity ratios of THE and THF isolated from the cumulative urines further illustrated that the metabolites were more preferentially enriched in 3H than in 13C. The apparent biological secondary isotope effects observed in the cumlative 2 day urine collections are smaller for (13C4)cortisol than for (3H2)cortisol. If these isotope effects also occur in man, then the 13C4-labelled tracer would be better than the 3H tracer for measuring the urinary cortisol production rate.