Hemodynamic and hormonal responses to hemorrhage are similar in conscious rabbits before pregnancy and at midgestation

Near the end of gestation, pregnant animals are less able to maintain arterial blood pressure (BP) during hemorrhage (H) and exhibit reduced baroreflex gain, suggesting that the reduced gain is responsible for the inability to tolerate H. Reflex gain is unaltered at midgestation. Therefore, the hypothesis that the hemodynamic and hormonal responses to H are normal in rabbits at midgestation was tested. Rabbits (n=5) were bled at 2% of their blood volume (BV) per min until BP fell, before pregnancy (non-P) and again at 2 wks gestation (2wk-P; term is 31 days). BV was larger (p<0.05) in 2wk-P (201±15 ml) than in non-P (177±7 ml) rabbits, but basal BP was similar (69±2 mmHg, non-P; 70±2 mmHg, 2wk-P). In both groups, BP was maintained at basal levels during H until abruptly falling after the loss of a similar % BV (38.6±2.0%, non-P; 39.3±1.8%, 2wk-P). H produced similar increases (p<0.05) in heart rate (157±9 to 241±13 bpm, non-P; 162±12 to 250±7 bpm, 2wk-P). Plasma angiotensin II increased (p<0.05) from 10±1 to 21±6 pg/ml (non-P) and from 8±2 to 24±6 pg/ml (2wk-P), corticosterone increased (p<0.01) from 4±1 to 39±8 ng/ml (non-P) and from 6±3 to 35±11 ng/ml (2wk-P), vasopressin increased (p<0.05) from 0.9±0.4 to 37.7±3.8 pg/ml (non-P) and from 2.1±0.7 to 32.5±10.7 pg/ml (2wk-P), and these increases were not different between groups. In conclusion, the emergence of the inability to maintain BP during H coincides with the appearance of reduced baroreflex gain near the end of pregnancy, suggesting that attenuated baroreflex function mediates the inadequate hemodynamic response.