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Circulating platelets contain high concentrations of TGF-β1 in their α-granules and release it on platelet adhesion/activation. We hypothesized that uncontrolled in vitro release of platelet TGF-β1 may confound measurement of plasma TGF-β1 in mice and that in vivo release and activation may contribute to cardiac pathology in response to constriction of the transverse aorta, which produces both high shear and cardiac pressure overload. Plasma TGF-β1 levels in blood collected from C57Bl/6 mice by the standard retro-bulbar technique were much higher than those obtained when prostaglandin E₁ was added to inhibit release or when blood was collected percutaneously from the left ventricle under ultrasound guidance. Even with optimal blood drawing, plasma TGF-β1 was lower in mice rendered profoundly thrombocytopenic or mice with selectively low levels of platelet TGF-β1 because of …