Effect of arginine vasopressin on regional adrenal blood flow and plasma cortisol concentration in fetal sheep

Abstract

Objective: To determine whether arginine vasopressin (AVP) at plasma concentrations measured during moderate hypoxemia affects adrenal blood flow. Study design: Regional blood flows were measured in 5 unanesthetized normoxemic fetuses (124–128 days' gestation) during a 24-h intravenous infusion of aVP in isotonic saline solution. Another 5 fetuses received an infusion of vehicle. Blood flows were determined before the infusion and at 2 h and 24 h from its onset using radionuclide-labeled microspheres. Results: At 2 h and 24 h of AVP infusion, fetal plasma concentrations of IR-AVP had risen from 4.7 ± 0.9 pg/ml to 9.8 ± 1.1 pg/l and 9.4 ± 0.7 pg/ml, respectively. Thus we achieved plasma concentrations of IR-AVP comparable to those previously reported during moderate hypoxemia. There was no significant effect of treatment on fetal plasma concentrations of immunoreactive adrenocorticotropic hormone (ACTH) or cortisol. AVP infusion significantly decreased fetal heart rate and raised cotyledonary blood flow from 198 ± 18 ml/min per 100 g to 235 ± 17 ml/min and 218 ± 10 ml/min per 100 g at 2 h and 24 h, respectively, from the start of the AVP infusion. Basal values for adrenal medullary and cortical blood flows were similar in the AVP and saline groups, and did not change significantly during the infusions. Conclusion: These findings suggest that the rise in adrenal blood flow seen after hypoxemia is not due to a direct action of systemic AVP, but is attributable to other influences, likely including changes in circulating ACTH.