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Introduction: Microvesicles are elevated in the circulation during cardiac stress; a response that is blunted in those with cardiovascular disease. Microvesicles play both activating and regulatory roles in the pathogenesis of hypertension, and their release is believed to be a protective mechanism during cardiac stress. Hypothesis: We hypothesised that microvesicle release during exercise-induced cardiac stress differs in young adults with and without hypertension. Methods: Microvesicle release was measured in 106 participants (79 normotensive, 27 hypertensive). Blood samples were obtained during maximal exercise testing at three time-points: before, immediately post and following 20 minutes recovery. The level of microvesicle procoagulant activity was determined in all participants. Microvesicles were phenotyped by flow cytometry in a subset of participants. Results: Exercise was associated with a significant elevation in the total procoagulant activity of microvesicles (p<0.001) and this increase was significantly higher in the normotensive group than the hypertensives (median(IQR) +0.7(0.0-2.1) vs -0.1(-0.4-0.4), p=0.001). Circulating procoagulant, platelet, endothelial, leucocyte, granulocyte and monocyte-derived microvesicles were shown to be elevated during exercise (p<0.05). The elevation in endothelial and leucocyte microvesicle levels returned to baseline within 20 minutes recovery. The significant elevation in procoagulant, platelet, granulocyte and monocyte-derived microvesicles was only seen in the normotensive participants, not in those with hypertension. Furthermore, amongst the normotensive participants, those with a blunted release of platelet microvesicles had higher peak diastolic blood pressures (86.8±8.4 vs 73.5±14.5, p=0.028) during exercise and increased global arterial stiffness measured at rest by the cardio-ankle vascular index (CAVI: 6.1±0.6 vs 5.4±0.5, p=0.022). Conclusions: In young adult hypertension there is a blunted release of microvesicles during exercise-induced cardiac stress. Normotensive young adults with a blunted platelet microvesicle release demonstrate early markers of future hypertension risk with greater peripheral resistance and arterial stiffness. [ABSTRACT FROM AUTHOR]