Effects of vasoconstrictors on intestinal vascular resistance and oxygen extraction

To delineate the mechanism through which vasoactive compounds alter intestinal oxygen consumption and to determine the pharmacological nature of the receptors involved, the authors quantitated the effects of vasoconstrictors on arteriovenous oxygen difference and on vascular resistance in isolated constant flow perfused canine small bowel. Norepinephrine (NE) and sympathetic stimulation (SS) increased vascular resistance and depressed O₂ extraction. These effects were not altered by β-blockade, but were abolished by α-blockade. Since capillary filtration coefficients at constant pressure perfusion and ⁸⁶Rb extraction at constant flow are reported to diminish during NE and SS, it follows that these agents reduce O₂ extraction by an α-adrenergic closure of precapillary sphincters. Vasopressin had similar effects which were not affected by adrenergic blocking agents. Epinephrine (Epi) in high doses or after propranolol produced the same effects as NE and SS. By contrast, Epi in low doses increased O₂ and ⁸⁶Rb extraction. This response to low doses of Epi was not affected by phentolamine, but was reversed by propranolol. The authors conclude that Epi in high doses or after propranolol depresses intestinal O₂ extraction by the same mechanism as NE and SS, but the mechanism through which Epi increases intestinal O₂ extraction is unclear.