Head-down tilt (HDT) has been commonly used as the simulation of a microgravity to evaluate the hemodynamic and the hormonal responses to the central blood shift observed during the spaceflight. But there were some differences in the systemic responses according to species or angle of tilt, and there are few reports in the hemodynamic response and the regulation on polmonary vascular bed during the early stage of HDT. This study was attempted to clarify the regulatory mechanism on the systemic and pulmonary vascular responses through the low pressure receptor, and the hormonal changes to the -6 degrees HDT, and to investigate the effect of anticholinergic agent, glycopyrrolate, on the hemodynamic and the hormonal responses. In 28 anesthetized dogs(8~14 kg), 15 dogs(TILT group) were tilted from the supine position to the -6 degrees head-down position(HDP ) for 30 minutes, then back to the supine recovery position for the next 30 minutes, and the same tilt procedure was taken at 20 minutes after the intra venous administration of 0.2g glycopyrrolate. The rest 13 dogs were kept in the supine position without tilt as the control group(CONT group ) for the same time course of the TILT group. Cardiac output(Q), heart rate(HR), stroke volume(SV), index of contractility(IC) and thoracic fluid volume(TFV) were deterkuned by the impedance cardiograph. Systemic arterial pressure (SAP) and pulmonary arterial pressure(PAP) were measured at the brachial and pulmonary arteries respectively, and central venous pressure(CVP) also at the right atrium. And systemic and pulmonary vascular resistances (SVR and PVR) were calculated. The concentration of plusma ANP and ADH in the arterial blood were measured by the radioimmunoassay in 8 dogs in each group at 5 minutes before, and 25 and 55 minutes after begining of the tilt. During the HDP, Q, SV and TFV increased to 9%,10.6% and 12.6ml respectively in the TILT group, where as they decreased by 10%, 13.8% and 7.5ml respectively in the CONT group. SAP, PAP and HR did not change significantly in the TILT group, but SAP and HR increased in the CONT group. At the same time, SVR and PVR reduced significantly by 4.5% and 8.9% respectively at 15 minutes of HDP in the TILT group, where as they elevated gradually to 21.8% and 13.4% respectively at the first 30 runutes In the CONT group. There were no significant differences in the changes of HR, PAP and CVP between the TILT and the CONT group. And the plasma ANP and ADH did not change significantly in the both groups. After the administration of glycopyrrolate, SAP and SVR Increased to 3.7mmHg and 5.1% respectively, and PAP, PVR, Q, HR, and SV did not change significantly during the HDP in the TILT group. There was no significant difference in the change of each hemodynamic parameter between the TILT and the CONT group, except in TFV, which showed the same change as that before the administration of glycopyrrolate in the both groups. The plsma ANP showed nonslgnificant increased trend at the HDP in the TILT group but did not change In the CONT group, and the plasma ADH showed nonsignificant decreased trend in the both groups. In summary, the cardiopulmonary baroreceptors induced the vasodilations of both systemic and pulmonary vessels by the expansion of thoracic fluid volume at the head-down tilt, and the anticholinergic agent(glycopyrrolate) tended to attenuate the hemodynamic responses to the tilt. The concentration of the plasma ANP and ADH did not show significant changes in this study. It is suggested that the regulatory mechanism inducing the systeruc and pulmonary vats odilation during the early phase of the head-down tilt is mediated by the cholinergic vasodilator fiber in the dog.
Animals ; Arterial Pressure ; Atrial Natriuretic Factor ; Dogs ; Electric Impedance ; Glycopyrrolate ; Head-Down Tilt* ; Heart ; Heart Atria ; Hemodynamics* ; Plasma ; Pressoreceptors ; Pulmonary Artery ; Radioimmunoassay ; Space Flight ; Stroke ; Supine Position ; Thoracic Surgery, Video-Assisted ; Weightlessness

The aim of this study was to determine the adaptation of atrial natriuretic peptide(ANP) to simulated weightlessness. The level of immunoreactive plasma ANP level and the expression of right atrial ANP mRNA were assessed in male Sparague-Dawley rats during 2 weeks of head-down suspension(HDS) The rats were placed in a -45 degrees anti-orthostatic position. The plasma ANP level was investigated by radioimmunoassay and ANP mRNA was expressed by Northern blot analysis. The changes in daily water intake, body weight and arterial hematocrit did not show the statistical significances during HDS. The plasma ANP level slightly increased after 1 day of HDS, the response was transient and then decreased below the control levels. The expression of the ANP mRNA increased after 1 day, the peak value occurred at 3 days of HDS and thereafter ANP mRNA showed the similar patterns compared to the changes of plasma level. On the basis of these findings, we conclude that the initial adaptation of ANP to increased control blood volume in rats occurs between 3 and 7 days of HDS, and the ANP acts an important role during regulatory process to central hypervolemia at both synthetic and secretory levels within the early stage of HDS.
Animals ; Atrial Natriuretic Factor ; Blood Volume ; Blotting, Northern ; Body Weight ; Drinking ; Hematocrit ; Humans ; Male ; Plasma ; Radioimmunoassay ; Rats* ; RNA, Messenger* ; Weightlessness

Cells and tissues of human and animal are protected against free radicals by several complex mechanisms including the action of antioxidant enzymes, such as superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) and catalase. There had been a few reports that simulated or actual weightlessness induced the decrease in activity of antioxidant enzymes and the increase in lipid peroxidation, The purpose of this study was to observe the time-course variation of antioxidant enzymes activities in rats during 14 days of head-down suspension(HDS) at -45 degrees as a model of simulated weightlessness. During HDS, the hepatic activity of SOD significantly decreased (p<0.05) at 3 day of HDS and then maintained a lower value compared to control horizontal position, GSH-Px activity also significantly decreased (p<0.05) at 3 and 7 day of HDS, thereafter showed a slight increasing trend to control horizontal value. The activity of hepatic catalase increased during HDS and the value at the end of HDS showed significant increase (p<0.05). From these results, there is a possibility that weightlessness induce the increase of oxygen free radicals according to the decrease of some antioxidant enzyme activities. The main scavenger to oxygen free radicals is operated via catalase system in rats during HDS. Therefore, we suggest that it is necessary to administrate the antioxidants for protection of the body against oxygen free radicals during first 1 week after exposure of weightlessness, at least.
Animals ; Antioxidants ; Catalase ; Free Radicals ; Glutathione Peroxidase ; Humans ; Lipid Peroxidation ; Oxygen ; Rats* ; Superoxide Dismutase ; Weightlessness*