Endocrine Abstracts

Introduction: Recent studies demonstrated that circulating concentration of many metabolites fluctuates with circadian rhythmicity. However, detailed information on amino acid and biogenic amine daily variations is still lacking. Dried blood spot (DBS) sampling appears to be a convenient tool for studies requiring multiple daily sampling.

Aim: To verify the validity of DBS as a suitable device, we compared serum and DBS levels and changes along the day of a large panel of amino acids and biogenic amines.

Methods: Six healthy normal weight (BMI=18.5-25.0 kg/m²) subjects (3/3 men/women) aged 26-50y were enrolled. All had a standard Mediterranean normocaloric diet during the observation and the 6 preceding days. Six paired serum, from venous, and DBS, from capillary, blood sampling were collected 30 min before and two hours after breakfast (07:30), lunch (13:30) and dinner (19:30). Twenty amino acids and 21 biogenic amines were measured by liquid chromatography-tandem mass spectrometry.

Results: All amino acids were measurable in both serum and DBS. Of 21 biogenic amines, 14 could be measured in both serum and DBS. One was only detected in serum. Concentrations of amino acids and biogenic amines were 16.4-796.2 and 0.049–96.2 µM in serum and 18.9–550.9 and 0.17–157.6 µM in DBS, respectively. Among amino acids, 15 were higher and 3 were lower in serum vs DBS (P: <0.001–0.018). Among biogenic amines, 6 were higher and 5 were lower in serum vs DBS (P: <0.001–0.010). Direct correlations between serum and DBS levels were observed for 14 amino acids (P:<0.0001–0.022) and 11 biogenic amines (P: <0.0001–0.047). Strongest correlations were observed for amino acids in leucine (r=0.794), isoleucine (r=0.760), valine (r=0.740) and proline (r=0.739), and for biogenic amines in acetylornithine (r=0.941), creatinine (r=0.655), putrescine (r=0.590) and serotonin (r=0.582). Parallel significant daily variations were observed both in serum and in DBS for isoleucine, leucine, methionine, phenylalanine, proline, tryptophan, tyrosine, valine and methionine sulfoxide (P: 0.001–0.068). The positive quadratic trend indicated higher levels in the morning and evening, and lower levels at midday. Additional significant reduction along the time points was observed in DBS for creatinine (P<0.001) and kynurenine (P0.007).

Conclusions: Information conveyed by DBS parallels those by serum for most of the analytes. Furthermore, DBS provided additional evidences on creatinine and kynurenine daily variations. DBS represents a valid opportunity to study amino acid and biogenic amine circadian variations in physiologic and clinical conditions related to the HPA axis.