Cardiac and peripheral vascular contributions to hypotension in spinal cats

On transection of the cervical spinal cord, substantial decreases in systemic arterial pressure and in discharge of many sympathetic nerves suggest the absence of sympathetic support to the cardiovascular system. However, discharge of mesenteric and splenic nerves is well maintained in spinal cats (R. L. Meckler and L. C. Weaver. J. Physiol. Lond. 396: 139-153, 1988; R. D. Stein and L. C. Weaver. J. Physiol. Lond. 396: 155-172, 1988). We proposed that the low arterial pressure in spinal animals was caused predominantly by decreased cardiac output and vasodilation in muscle and some visceral vascular beds but that sustained mesenteric and splenic discharge was causing significant splanchnic vasoconstriction and partial support of arterial pressure. Therefore, changes in cardiac output, total peripheral resistance, and resistance of constant-flow-perfused mesenteric visceral and hindlimb skeletal muscle vascular beds caused by interruption of cervical spinal pathways were assessed. Blockade of cervical pathways decreased arterial pressure as much by decreasing cardiac output as by decreasing total peripheral resistance. Resistances of the muscle and mesenteric vascular beds decreased equally. In conclusion, hypotension in spinal cats is caused by decreased cardiac output and by vasodilation, which is as prominent in mesenteric as it is in muscle vascular beds. The maintained mesenteric sympathetic discharge in spinal cats appears unable to produce significant support of vascular arterial resistance.