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KMID : 0378019910340030039
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New Medical Journal
1991 Volume.34 No. 3 p.39 ~ p.52
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Changes of Plasma Catecholamine Levels and Cardiac Output during Simulated Weightlessness
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Abstract
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Head-down tilt(HDT) to -6° has been commonly used as the experimental model of weightlessness in both man and animals. The purpose of this study was to observe the transient changes of cardiovascular hemodynamics including cardiac output(Q)and peripheral skin blood flow(SBF), and to observe the changes of sympathoadrenal activity response to -6° HDT within a relatively early period.
Present study was done on 12 healthy young adult males. The postural changes were performed from sitting to supine control position(SCP). Minus 6° HDT which followed SCP lasted for 30 minutes. Then subjects were returned to supine recovery position (SRP) and remained for 10 minutes.
Q was determined by the computerized cardiopulmonary exercise system, ´and by equilibrium method of CO2 rebreathing by indirect Fick´s principle. Blood pressure(BP) and heart rate(HR) were also measured. Total peripheral vascular resistance(R) was calculated by dividing respective mean BP values by Q. The concentration of plasma catecholamines (CA) was measured by a modified radioenzymatic method of Peuler and Johnson. In addition, photoelectric pulse amplitude(PEPA) and skin temperature(Ts) to evaluate the attitudes of SBF in the ear lobe, finger and toe were recorded simultaneously.
The results obtained are as follows
BP, HR and R were decreased(p<0.01) and Q was increased (p<0.01) in the postural change from the sitting to SCP. In the head-down posture after SCP, Q was increased, and HR, R and CA were decreased insignificantly. SBF of the ear lobe and the toe were increased(P<0.05), but the finger SBF was decreased. In SRP, Q was decreased (pFrom the above results, it may be suggested that depressor reflex causing the decrease of the peripheral vascular tone and the concentration of plasma catecholamines by the consistent central fluid shift during -6° HDT are triggered by cardiopulmonary baroreceptors. In HDT, depressor actions of the cardiopulmonary baroreceptors and the increase of the transmural pressure might give rise to reflex vasodilation in the ear lobe. On the contrary, the increase of SBF in the toe may be due to vasodilation induced by local myogenic autoregulation by the decrease of the transmural pressure by the elevated foot position in addition to the baroreceptor reflex. It seems likely that the vessels of the finger are predominantly mobilized in temperature regulation and responsible to non-specific emotional stimuli including blood sampling etc., than those of the ear and the toe.
Present study provides evidence that the increase of the R and the decrease of the peripheral SBF in SRP were mediated by the pressor reflex of the cardiopulmonary baroreceptors mainly act through the vasomotor tone in order to maintain a constant blood flow and blood pressure in the central portion of the body.
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