Long-term stability and circadian variation in circulating levels of surfactant protein D.
Surfactant protein D (SP-D) is an oligomeric calcium-dependent lectin with important roles in innate host defence against infectious microorganisms. Several studies have shown that patients with inflammatory lung disease have elevated levels of circulating SP-D, and serum SP-D has been suggested to be used as a biomarker for disease e.g. in COPD. We aimed to investigate the variation of circulating SP-D in healthy individuals in and between days for 6 months. In addition, we studied the SP-D response to a standardized physical exercise programme. SP-D was measured in serum using a 5-layered ELISA technique. We found that circulating SP-D remained constant over a 6-month period. However, during the course of one day SP-D varied significantly. Median SP-D peaked at 10 a.m. at 1009 ng/ml (95% CI: 803-1497), subsequently decreasing to nadir at 10 p.m. at 867 ng/ml (95% CI: 650-1148)(P<0.00005). Median pre-exercise level of SP-D was 746 ng/ml (95% CI: 384-2035), and immediately after cessation of physical activity the median SP-D level was 767 ng/ml (95% CI: 367-1885) (P=0.248). Our findings underscore the importance of standardized blood sampling conditions in future studies on the potential role of SP-D as a biomarker. Importantly, stable measures of systemic SP-D over a prolonged period support that SP-D is suitable for biomarker studies.
Hoegh
Silje Vermedal
SV
Medical Biotechnology Center, Institute of Medical Biology, University of Southern Denmark, Winsløwparken 253, 5000 Odense C, Denmark.
Sorensen
Grith Lykke
GL
Tornoe
Ida
I
Lottenburger
Tine
T
Ytting
Henriette
H
Nielsen
Hans Jørgen
HJ
Junker
Peter
P
Holmskov
Uffe
U
D000328:Adult; D000368:Aged; D002940:Circadian Rhythm; D015444:Exercise; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D055550:Protein Stability; D037663:Pulmonary Surfactant-Associated Protein D / Q000097:blood* / Q000378:metabolism*
D037663:Pulmonary Surfactant-Associated Protein D