Alpha-adrenergic receptor antagonism prevents intestinal vasoconstriction but not hypoperfusion following...

Resuscitation (RES) after hemorrhage (HEM) results in persistent arteriolar constriction and hypoperfusion of the small intestine (SI) despite restoration of mean arterial pressure (MAP) and cardiac output (CO) to normal values. We postulated that increased adrenergic activity contributes to this vasoconstriction and impairment of flow. A loop of SI from decerebrate rats was exteriorized and suffused with Krebs' solution (37 degrees C, pH 7.4). In initial experiments, the effectiveness of alpha-adrenergic receptor antagonism by phentolamine (PHEN) was assessed. Subsequent groups received either topical PHEN (10(-6) M, n = 6) or saline (n = 6) in the suffusion and were then bled to 50% baseline (BL) MAP for 60 min and resuscitated to BL with shed blood/lactated Ringer's. Intravital microscopy and optical Doppler velocimetry were used to measure large (A1) and small, premucosal (A3) arteriolar diameters and RBC velocity; microvascular blood flow was calculated. MAP and transpulmonary CO were measured. During HEM, control animals developed A1 constriction and hypoperfusion with A3 arteriolar dilation. PHEN treatment prevented A1 constriction and enhanced A3 dilation but did not improve flow. Immediately after RES in controls, microvascular diameters and A1 flow returned to BL; however, over the 2-hr post-RES period there was progressive A1 and A3 vasoconstriction and hypoperfusion despite maintenance of BL MAP and CO. After RES in PHEN-treated animals, A1 flow returned to BL, but progressive hypoperfusion was only partially prevented. alpha-Adrenergic-mediated vasoconstriction contributes to intestinal hypoperfusion after HEM, but other mechanisms are also involved in microvascular responses during RES.(ABSTRACT TRUNCATED AT 250 WORDS)