Hormonal and hemodynamic responses to vena caval obstruction in fetal sheep

To test the hypothesis that ACTH and vasopressin responses are quantifiable as functions of induced changes in central venous or arterial pressures, we produced various degrees of vena caval obstruction in fetal sheep (118--134 days gestation). In seven experiments, vena caval obstruction increased heart rate 18 +/- 7 beats/min and carotid arterial oxygen saturation 8.4 +/- 2.1%, but did not alter any measured vascular pressure or hormones. More severe vena caval obstruction (n = 10) decreased mean arterial pressure 13 +/- 2 mmHg, central venous pressure 1.3 +/- 0.3 mmHg, and heart rate 47 +/- 12 beats/min, and increased fetal plasma ACTH 1,047 +/- 448 pg/ml, cortisol 4.4 +/- 2.2 ng/ml, and vasopressin 47.9 +/- 24.2 pg/ml, but did not alter 11-deoxycortisol. The stimulus increased plasma cortisol (radioimmunoassay after chromatography) 100% and "corticosteroids" (radiotransinassay without chromatography) 20%, demonstrating the nonlinear relationship between these two variables. End-inflation plasma ACTH and vasopressin concentrations were significantly related to the induced decreases in mean arterial and central venous pressures, suggesting that the hormonal responses to vena caval obstruction were mediated by cardiovascular mechanoreceptors. Plasma vasopressin concentrations were linearly related to plasma ACTH concentrations (4 = 0.94; P less than 0.001), suggesting parallel release of the two hormones.