Endocrine Abstracts

Background: Acylcarnitines and sphingomyelins are key players of lipid metabolism at cytosolic, vesicular and cell membrane level. Their metabolism was found deranged in diseases of the hypothalamus-pituitary-adrenal axis. Little information is available about levels of these compounds in the bloodstream and their circadian fluctuations. Dried blood spots (DBS) allow multiple autonomous and painless collections per day. However, poor biological validation data are available about lipid measurement in DBS.

Aim: To evaluate acylcarnitine and sphingomyelin levels and daily variations in paired serum and DBS collections.

Methods: Three women and three men aged 26-50 y were enrolled. All were healthy, normal weight and followed a Mediterranean standardized diet for one week, ending with the collection day. Serum and DBS were obtained 30 min before and 2 h after breakfast (7.30), lunch (13.30) and dinner (19.30). Forty acylcarnitines and 15 sphingomyelins were quantified by flow injection - tandem mass spectrometry.

Results: Twenty-four acylcarnitines were undetectable, 6 were only detectable in serum, 2 only in DBS and 8 in both; mean levels ranging 0.104-40.5 µM in serum and 0.160-21.8 µM in DBS. Four acylcarnitines were higher (P<0.001), whereas carnitine was lower (P=0.003) in DBS vs serum. Direct correlations between DBS and serum levels were observed for 7 analytes (P: <0.0001-0.019), with strongest correlations displayed by carnitine (R=0.797), butyrylcarnitine (R=0.714) and propionylcarnitine (R=0.686). Serum propionylcarnitine, valerylcarnitine, octanoylcarnitine, dodecanoylcarnitine and tetradecenoylcarnitine showed significant daily variations (P: 0.002-0.038), with positive quadratic or negative linear trend. Butyrylcarnitine, acetylcarnitine and hexadecanoylcarnitine showed significant daily variations in DBS (P: 0.009-0.037), with negative linear trend. All sphingomyelins were detectable, ranging 0.197-119.9 µM in serum and 0.281-136.5 µM in DBS. Five and 6 molecules were higher and lower in DBS vs serum, respectively (P: <0.001-0.026). Nine sphingomyelins displayed direct correlations between DBS and serum levels (P: <0.0001-0.028), with strongest correlations displayed by sphingomyelins(SM)-OH-C22:2 (R=0.790), SM-C16:1 (R=0.760) and SM-C20:2 (R=0.690). No analyte fluctuations were found in serum, whereas significant daily changes were observed in DBS for 11 sphingomyelins (P: <0.001-0.036), mostly with negative linear trend. Strongest variations were observed for SM-OH-C16:1, SM-C20:2, SM-C24:0, SM-OH-C24:1, SM-C26:0 and SM-C26:1.

Conclusion: Despite significant correlations between serum and DBS levels, acylcarnitines and sphingomyelins showed different patterns of circadian fluctuations in the two matrices. This may depend on the different distribution of individual molecules in soluble and cellular bloodstream fractions. Further studies are necessary to establish the relevance of serum and DBS measurements for the characterization of circadian rhythm imbalances.