Defense reaction alters the response to blood loss in the conscious rabbit

The interaction of sensory stressors with the cardiovascular response to bl ood loss has not been studied. The cardiovascular response to a stressor (i .e., the defense reaction) includes increased skeletal muscle blood flow an d perhaps a reduction in arterial baroreflex function. Arterial pressure ma intenance during blood loss requires baroreflex-mediated skeletal muscle va soconstriction. Therefore, we hypothesized that the defense reaction would limit arterial pressure maintenance during blood loss. Male, New Zealand Wh ite rabbits were chronically prepared with arterial and venous catheters an d Doppler flow probes. We removed venous blood in conscious rabbits until m ean arterial pressure decreased to <40 mmHg. We repeated the experiment wit h (air) and without (sham) simultaneous exposure to an air jet stressor. Ai r resulted in a defense reaction (e.g., mean arterial pressure = 94 +/- 1 a nd 67 +/- 1 mmHg for air and sham, respectively). Contrary to our hypothesi s, air increased the blood loss necessary to produce hypotension (19.3 +/- 0.2 vs. 16.9 +/- 0.2 ml/kg for sham). Air did not reduce skeletal muscle va soconstriction during normotensive hemorrhage. However, air did enhance ren al vasoconstriction (97 +/- 3 and 59 +/- 3% of baseline for sham and air, r espectively) during the normotensive phase. Thus the defense reaction did n ot limit but rather extended defense of arterial pressure during hemorrhage .