Endothelin, microcirculation and hemorheology

Although a large amount of data concerning microcirculation and cardiovascu lar disease exists, little is known about microcirculation and hypertension . This is largely due to the difficulty in selectively examining capillarie s and metarterioles, independently from small arteries or large vessels. The physiological role of capillaries and metarterioles, the two elements t hat make up the microcycle, is peculiar and closely related to metabolic ex change. During the hypertensive state, several factors can alter these mechanisms. These include elevated plasma viscosity, abnormal membrane properties of re d blood cells, and an increase in fibrinogen, LTL and hematocrit levels. The question of whether an abnormal release of endothelium derived vasoacti ve factors from capillaries, or an abnormal production of chemical factors by blood cells running through the vasculature area is present in hypertens ives is fascinating, but unfortunately neither experimental nor clinical da ta has yet been able to answer it. Recently, evidence of the formation of endothelin by red blood cells from e ndogenous precursors was given, suggesting that red blood cells may modulat e the vascular tone both directly, through the release of ATP or endothelin -1, and indirectly, when hemolysis occurs and hemoglobin is released. The p athological significance of these findings has not been dearly demonstrated in hypertension thus far, although it is reasonable to hypothesise that th ere are clinical implications for the pathogenesis and the progression of v ascular damage during the hypertensive state.