Background:Cardiovascular diseases are united in pathology by atherosclerosis; signal transduction is essential in this process for the expression of cell adhesion molecules early in the disease, capturing, tethering and transmigrating monocytes into the sub-endothelial space. Mitogen and stress activated kinases (MSK) 1/ 2 are critical for the homeostatic control of cytokine expression through interleukin-10 and dual specificity protein-1 (DUSP-1). Macrophages from MSK 1/ 2 knock-out (KO) animals display reduced IL-10, DUSP-1 expression and bare a hyperinflammatory phenotype. Calcitrol, the active form of vitamin D, exerts its anti-inflammatory effects through up regulation of IL-10 and DUSP-1 in vitro, with an ability to curb cytokine expression (tumour necrosis factor-α and IL-6); however the effector pathway remains poorly understood with limited in vivo studies. Aim:To elucidate the relationship between Calcitrol and MSK 1/ 2 in endothelial dysfunction. Method:Skin microcirculation was assessed on the flanks of anesthetised (1.5-2% isoflurane with oxygen) 16 week old C57/Black MSK 1/ 2 KO (n=24) mice. Following baseline measurements animals were fed a proatherogenic diet (2% cholesterol) and subdivided (n=12) to receive vehicle (carboxymethyl cellulose sodium 0.5%) or 200ng Calcitrol twice a week for the study duration (16 weeks). Laser Doppler Imaging (LDI) in combination with iontophoresis of phenylepherine (PE), acetylcholine (ACh), sodium nitroprusside (SNP) and localised skin heating (440C) were used to measure vascular function. Results are expressed as group mean, skin perfusion in arbitrary units (AU)±standard error of mean (SEM). Blood pressure was assessed non-invasively using a tail cuff system (mmHg±SEM); cardiac hypertrophy was calculated by: cardiac mass (mg)/body weight (g).T-test analysis was used for statistical significance (p <0.05). Results:There were no significant differences at baseline for vascular responses. Vehicle (259±6AU) treated animals displayed reduced ACh responses at 16 weeks when compared to Calcitrol (292±5AU) treated mice (p = 0.0006). There were no significant differences for endothelium-independent responses (SNP) or cardiac mass between the two groups. Calcitrol supplemented animals displayed lower systolic (119±3 mmHg VS. vehicle 129±3 mmHg, p = 0.04) and diastolic (84±3 mmHg VS. vehicle 99±2 mmHg, p = 0.01) blood pressures. Conclusion:ACh induced vasodilation was significantly reduced in MSK 1/ 2 KO vehicle mice, but was preserved in Calcitrol treated animals. Maximal dilatory capacity was not significantly different between groups, suggesting modulation at the level of the endothelium. For the first time we provide in vivo longitudinal data showing the protective effects of vitamin D on endothelial dysfunction, possibly through modulation of inflammation.