INTRODUCTION I N CONTRAST TO LOW- LAND SPECIES such as the sheep the llama fetus has a striking increase in total peripheral resistance, during hypox- emia and there is no significant augmentation of cerebral blood flow. We speculate that chronic exposure to hypoxia in the llama has resulted in genetically fixed mechanisms that allow the llama to tolerate chronic hypoxemia at altitude. We have shown previously that this peripheral vasoconstriction is not due to HIGH ALTITUDE MEDICINE & BIOLOGY Volume 1, Number 3, 2000 Mary Ann Liebert, Inc. Cardiovascular Responses to Arginine Vasopressin Blockade During Acute Hypoxemia in the Llama Fetus EMILIO A. HERRERA, 1 RAQUEL A. RIQUELME, 3 EMILIA M. SANHUEZA, 1 CRISTIAN GAJARDO, 1 JULIAN T. PARER, 4 and ANÍBAL J. LLANOS 1,2 ABSTRACT Herrera, Emilio A, Raquel A. Riquelme, Emilia M. Sanhueza, Cristian Gajardo, Julian T. Parer, and Aníbal J. Llanos. Cardiovascular responses to arginine vasopressin blockade during acute hypoxemia in the llama fetus. High Alt Med Biol 1:175–184, 2000.—The fetal llama has a marked increase in the peripheral vascular resistance and no augmentation of brain blood flow during hypoxemia. In spite of the substantial plasma arginine-vasopressin (AVP) increase during hy- poxemia, up to 8 times greater than in fetal sheep, there are no changes of carotid and femoral blood flows during hypoxemia with a V 1 receptor blockade, as is seen in the fetal sheep. The aim of this study was to assess the role of AVP function in mediating the combined ventricular output and organ blood flow in the hypoxemic llama fetus. Six fetal llamas at 0.65 of gestation were instrumented under general anesthesia, and cardiorespiratory responses and blood flows determined under normoxemic and hypoxemic conditions. The AVP effect was determined us- ing a V 1 antagonist during normoxemic and hypoxemic conditions. Organ blood flows were measured with the radioactive microsphere technique. No significant differences in organ blood flow or in their vascular resistances were seen between the control and treated fetuses during hypoxemia. We conclude that V 1 blockade did not have any important role in the cardiovascu- lar response to acute hypoxemia in the llama fetus, in contrast with lowland fetuses. AVP may be playing a role in other regions, possibly in kidney or lung, during hypoxemia. Key Words: organ blood flow; hypoxemia; V 1 blockade; hypoxia; organ oxygen delivery 1 Laboratorio de Fisiología y Fisiopatología del Desarrollo, Programa de Patología, Instituto de Ciencias Biomédi- cas (ICBM), Facultad de Medicina, Universidad de Chile, Santiago, Chile. 2 Centro Internacional de Estudios Andinos (INCAS), Universidad de Chile, Santiago, Chile. 3 Departamento de Bioquímica y Biología Molecular, Facultad de Ciencias Químicas y Farmacéuticas, Universidad de Chile, Santiago, Chile. 4 Department of Obstetrics, Gynecology and Reproductive Sciences and Cardiovascular Research Institute, Univer- sity of California, San Francisco, CA. 175