Giussani D A, Riquelme R A, Moraga F A, McGarrigle H H, Gaete C R, Sanhueza E M, Hanson M A, Llanos A J
Departamento de Medicina Experimental, Facultad de Medicina, Universidad de Chile, Casilla, Chile.
Am J Physiol. 1996 Jul;271(1 Pt 2):R73-83. doi: 10.1152/ajpregu.1996.271.1.R73.
We tested the hypothesis that the llama fetus has a blunted cardiovascular chemoreflex response to hypoxemia by investigating the effects of acute hypoxemia on perfusion pressure, heart rate, and the distribution of the combined ventricular output in 10 chronically instrumented fetal llamas at 0.6-0.7 gestation. Four llama fetuses had the carotid sinus nerves sectioned. In the intact fetuses, there was a marked bradycardia, an increase in perfusion pressure, and a pronounced peripheral vasoconstriction during hypoxemia. These cardiovascular responses during hypoxemia in intact fetuses were accompanied by a pronounced increase in plasma vasopressin, but not in plasma angiotensin II concentrations. Carotid denervation prevented the bradycardia at the onset of hypoxemia, but it did not affect the intense vasoconstriction during hypoxemia. Plasma vasopressin and angiotensin II levels were not measured in carotid-denervated fetuses. Our results do not support the hypothesis that the carotid chemoreflex during hypoxemia is blunted in the llama fetus. However, they emphasize that other mechanisms, such as increased vasopressin concentrations, operate to produce an intense vasoconstriction in hypoxemia. This intense vasoconstriction in the llama fetus during hypoxemia may reflect the influence of chronic exposure to the hypoxia of high altitude on the magnitude and gain of fetal cardiovascular responses to a superimposed acute episode of hypoxemia.
我们通过研究急性低氧血症对10只处于妊娠0.6 - 0.7期、已进行长期仪器植入的胎儿羊驼的灌注压、心率及联合心室输出量分布的影响，来检验羊驼胎儿对低氧血症的心血管化学反射反应减弱这一假设。四只羊驼胎儿的颈动脉窦神经被切断。在完整的胎儿中，低氧血症期间出现明显的心动过缓、灌注压升高以及显著的外周血管收缩。完整胎儿在低氧血症期间的这些心血管反应伴随着血浆血管加压素的显著升高，但血浆血管紧张素II浓度未升高。颈动脉去神经支配可防止低氧血症开始时的心动过缓，但不影响低氧血症期间强烈的血管收缩。未对颈动脉去神经支配的胎儿测量血浆血管加压素和血管紧张素II水平。我们的结果不支持低氧血症期间羊驼胎儿颈动脉化学反射减弱这一假设。然而，这些结果强调，其他机制，如血管加压素浓度升高，在低氧血症时发挥作用以产生强烈的血管收缩。羊驼胎儿在低氧血症期间的这种强烈血管收缩可能反映了长期暴露于高海拔低氧环境对胎儿心血管对叠加急性低氧血症发作反应的幅度和增益的影响。
