Vascular adrenergic interactions during hemorrhagic shock

The objective of this paper is to review the sequence of vascular events that follows severe hemorrhage. The initial cardiovascular imbalance is a fall in the volume/vascular capacity relationship that leads to reductions in cardiac output and mean arterial pressure (MAP). Peripheral sensors detect the fall in MAP and changes in blood chemistry that cause withdrawal of the normal inhibitory tone from the cardiovascular control centers in the central nervous system. The resulting increased sympathetic activity initiates a series of events that include stimulation of peripheral adrenergic nerves and the adrenal medulla. The magnitude of the compensatory vasoconstriction that follows is the net result of the interaction of the epinephrine (E) from the adrenal medulla and norepinephrine (NE) from the peripheral nerves on the peripheral vascular adrenoreceptors as well as other nonadrenergic mechanisms not discussed here (i.e., angiotensin endogenous opiates). By using pharmacological blocking agents, these adrenoreceptors have been subclassified as: innervated postsynaptic alpha 1; presynaptic alpha 2 (Ps alpha 2); and extrasynaptic alpha 2 (Es alpha 2) adrenoreceptors. The action of E and NE on the alpha 1 and Es alpha 2 receptors initiates the compensatory vasoconstriction, whereas action of these catecholamines on the Ps alpha 2 located on the presynaptic membrane inhibits further release of NE from peripheral nerve terminals, thereby reducing the effect of the innervated alpha 1 receptors. This autoinhibition together with a similar action by prostaglandin E on NE release is thought to be, at least in part, responsible for the vascular decompensation known to occur in the skeletal muscle after hemorrhage. Thus, one of the factors determining survival after hemorrhage may be related to the relative dominance of alpha 1 and Es alpha 2 receptors during the initial compensatory response.