title: Long-term stability and circadian variation in circulating levels of surfactant protein D.
abstract: 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.
authors.affiliation: Medical Biotechnology Center, Institute of Medical Biology, University of Southern Denmark, Winsløwparken 253, 5000 Odense C, Denmark.
authors.forename: Silje Vermedal
authors.initials: SV
authors.lastname: Hoegh
authors.forename: Grith Lykke
authors.initials: GL
authors.lastname: Sorensen
authors.forename: Ida
authors.initials: I
authors.lastname: Tornoe
authors.forename: Tine
authors.initials: T
authors.lastname: Lottenburger
authors.forename: Henriette
authors.initials: H
authors.lastname: Ytting
authors.forename: Hans Jørgen
authors.initials: HJ
authors.lastname: Nielsen
authors.forename: Peter
authors.initials: P
authors.lastname: Junker
authors.forename: Uffe
authors.initials: U
authors.lastname: Holmskov
mesh_terms: 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*
chemical_list: D037663:Pulmonary Surfactant-Associated Protein D