Cardiovascular and hormonal response to hemorrhage in conscious rats

The effect of arginine vasopressin (AVP) pressor blockade on the response to graded hemorrhage was investigated on conscious, unstressed, freely moving rats. The parameters studied were mean arterial pressure (MAP), heart rate (HR), blood velocity in the ascending aorta (ABV), and plasma concentrations of AVP (pAVP), renin (PRC), corticosterone (pCS), and catecholamines. After the first hemorrhage (0.75% of body wt over 5 min), MAP remained unchanged while ABV declined and HR increased. The two subsequent hemorrhages brought about significant reduction in MAP, HR, and ABV. pAVP, pCS, and PRC increased gradually during the experiment, while plasma catecholamine levels remained unchanged except for epinephrine, which increased after the third hemorrhage. After pretreatment with the AVP-pressor antagonist, [d(CH2)5Tyr(Me)]AVP, the hemorrhage-induced cardiovascular changes were practically identical to those seen in control animals. Results with AVP blockade performed after the third hemorrhage were also negative. A pressor role of AVP after hypotensive hemorrhage could only be revealed in the presence of converting-enzyme inhibition and alpha-adrenergic blockade. In addition, [d(CH2)5Tyr(Me)]AVP did not modify the effect of hemorrhage on pCS and catecholamines and caused only a slight enhancement of the increase in PRC. It is concluded that conscious, nonstressed rats, if all compensatory mechanisms are allowed full expression, exhibit a normal cardiovascular compensatory response to hemorrhage in the absence of functional AVP pressor receptors.