Endocrine Abstracts

Background: Hormones oscillate in circadian and ultradian rhythms. Consequently, single time point measurements are very difficult to interpret. To address this, we developed an automated system of 24-hour ambulatory microdialysis. This allows measurement of free hormone concentrations in subcutaneous interstitial fluid collected while participants continue normal daily activities. To validate the technique for adrenal steroid hormones, we simultaneously sampled interstitial fluid and blood plasma over 24 hours in a cohort of healthy women and men.

Methods: Participants (age 18–68, no regular medications, no active medical conditions, BMI 16-29) were recruited for hormone profile analysis. A 20 kDa linear microdialysis sampling catheter was inserted in abdominal subcutaneous tissue. Catheters were perfused at 1 microl/min using a portable CMA107 microdialysis pump attached to our novel fraction collector (U-RHYTHM) worn in an elasticated waist band. Microdialysate samples within the fraction collector were separated by air bubble every 20 minutes. Simultaneous blood samples were obtained from peripheral venous blood using an automated blood sampling system (HABS). A standard routine with respect to food, light exposure and sleep was maintained throughout the 24-hour sampling period. Multiplex analysis of steroid concentrations in both blood and microdialysate was achieved using triple quadrupole mass spectrometry. Plasma adrenocorticotropic hormone was measured using the IMMULITE 2000 Immunoassay System.

Results: We will present data from the first 5 recruited participants. 72 consecutive samples of blood and microdialysate were analysed for each participant. Analyte profiles detected in both compartments include cortisol (F), cortisone (E), tetrahydrocortisol (THF), tetrahydrocortisone (THE), corticosterone (CCS), 18-OH-cortisol (18-OHC), 18-OH-corticosterone (18-OHCCS), and aldosterone. All 24-hour profiles demonstrated circadian and/or ultradian rhythms.

Conclusions: U-RHYTHM interstitial microdialysis reliably and accurately detects dynamic fluctuations in steroid physiology. Measurements in interstitial fluid are highly correlated with plasma. Therefore we propose that U-RHYTHM ambulatory microdialysis is a more powerful, credible and accurate alternative to traditional single-time point assessments of adrenal function. U-RHYTHM therefore has broad potential as a clinical tool in the diagnosis and monitoring of adrenal disorders, including Cushing’s syndrome and adrenal insufficiency.