Regional blood flow in the calf and plasma endothelin during prolonged orthostasis in humans

This study tested the hypothesis that circulating plasma levels of the vaso constrictor endothelin increase during prolonged orthostasis in association with changes in both calf regional blood flow, systemic arterial blood pre ssure and heart rate, and that the (133)Xenon wash-out method can be used f or prolonged registration of skeletal muscle blood flow. The simultaneous e ffects of prolonged orthostasis on plasma endothelin, calf skeletal muscle and subcutaneous blood flow, heart rate and arterial blood pressure were in vestigated in 13 healthy subjects before, during and after 30 min of 50 deg rees head-up tilt. Blood flow rate was measured by the local (133)Xenon was h-out method in the supine position and during 50 degrees head-up tilt. The method was evaluated for skeletal muscle for prolonged observation in the supine position in five subjects and compared with blood flow rates measure d with plethysmography. Plasma endothelin was unchanged during head-up tilt , despite a maximal reduction in skeletal muscle blood flow rate of 47% (p < 0.001) and subcutaneous blood flow rate of 80% (p < 0.01) and a maximal i ncrease in heart rate of 15% (p < 0.001) and diastolic (12%, p < 0.01) bloo d pressure. The skeletal muscle wash-out curves for (133)Xenon were monoexp onential from 30 min after injection and at least during the next hour and correlated with total limb blood flow rate assessed simultaneously with ple thysmography (r = 0.76, p < 0.0001). Circulating endothelin does not appear to be in the first line of regulation of calf microcirculation, systemic a rterial blood pressure or heart rate during prolonged orthostasis. The (133 )Xenon wash-out method can be used for prolonged measurement of skeletal mu scle blood flow.