Reduced venoconstrictor reserve in spontaneously hypertensive rats subjected to hemorrhagic stress

Anesthetized spontaneously hypertensive rats (SHR) and normotensive WKY controls were subjected to either a constant pressure (CPH) (constant mean arterial pressure of 35 mmHg) or an incremental volume (IVH) hemorrhage protocol (successive 1-ml blood withdrawals separated by 20 minute compensatory periods). Diameters and pressures were measured in small (400-600 micron) intestinal mesenteric veins before and during hemorrhage. Before hemorrhage, vasodilator suffusion produced a significant venodilation in SHR but not in WKY. During hemorrhage, an active compensatory venoconstriction occurred in WKY but not in SHR. SHR were also less tolerant of hemorrhage than WKY. During CPH, venous pressures in WKY were significantly higher than those in SHR. However, venous pressures in SHR and WKY were not significantly different during CPH with vasodilator suffusion. This study indicates that a decreased reserve capacity for compensatory venoconstriction may contribute to the reduced ability of SHR to tolerate hypotensive stress.