Asymmetric dimethylarginine : assay methodology and serum levels in non-pregnant and pregnant women

Asymmetric dimethylarginine (ADMA) is a naturally occurring methylated amino acid which is released into the circulation during protein metabolism. Elevated ADMA levels reduce the formation of nitric oxide (NO) by inhibiting nitric oxide synthase and are associated with endothelial dysfunction. The female sex hormone, estrogen, is known to have an effect on ADMA concentration by regulating the expression of ADMA degrading enzyme dimethylarginine dimethylaminohydrolase (DDAH). Earlier it has been observed that the ADMA concentration is elevated in women with complicated pregnancies, such as pre-eclampsia. In the present series of studies, the effects of hormonal cyclic changes on the levels of ADMA and its related compounds were investigated in healthy non-pregnant women with and without hormonal contraceptive use. Another aim was to examine if there was an association between ADMA levels and flow-mediated dilatation and factors that are known to be involved during normal pregnancy, such as hypercholesterolemia and changes in the immune system. In order to measure the concentrations of ADMA, symmetric dimethylarginine (SDMA), Lhomoarginine and L-arginine in human serum and plasma, the HPLC method developed by Teerlink et al. (2002) was optimized and it proved to have good recovery, excellent linearity, low detection limits and good separation efficiency of the analytes. ADMA and its related compound concentrations were measured from two study populations; The Cardiovascular Risk in Young Finns Study and The Complicated Pregnancy Study. Brachial artery flow-mediated dilatation (FMD) was measured by ultrasound. It was found that the ADMA concentration was dependent on hormonal status of women without oral contraceptive (OC) use. The use of oral contraceptives decreased ADMA and SDMA concentrations as compared with non-OC users but there was no variation in the ADMA concentration during menstrual cycle phases in women with OC-use. Furthermore, it was found that ADMA, SDMA and L-arginine concentrations declined and L-homoarginine concentrations were elevated in normal pregnancy in comparison with the levels in non-pregnant females. During normal pregnancy, serum levels of ADMA and SDMA were not associated with hypercholesterolemia or circulating cytokines, IL-6 and TNF-α, or C-reactive protein. FMD was enhanced in normal pregnancy but it was not associated with ADMA levels. In summary, the present studies provide new information about ADMA and related compounds regarding pregnancy related phenomenon: endothelial function, immune system upregulation and increased lipid concentration during normal pregnancy. In this thesis, it was found that the ADMA concentration varied during menstrual cycle. Thus if women of fertile age are used in studies, the menstrual cycle phase and not only their possible use of hormonal contraception should be taken into account. Furthermore, also the type of contraception is important. This applies also to CRP measurements where the use of OC has been shown to increase the circulating CRP concentration. On the contrary, endothelial function measured as FMD seemed to be unaffected by the menstrual cycle phase. These results may be useful when ADMA concentrations from normal pregnancy are compared with the results from complicated pregnancies. National Library of Medicine Classification: WQ 244, QU 60 Medical Subject Headings: Humans; Female; Pregnancy; Arginine/blood; C-Reactive Protein; Cytokines; Homoarginine/blood; Interleukin-6/blood; Endothelium, Vascular; Menstrual Cycle; Nitric Oxide/blood; Tumor Necrosis Factor-alpha; Vasodilation