Vasoactive agents and splanchnic oxygen uptake.

Many vasoactive agents are known to alter oxygen uptake by splanchnic organs. Data from the literature indicate that, in general, vasodilators increase, whereas vasoconstrictors decrease oxygen uptake. We compare and contrast the effects of vasoactive agents on oxygen uptake observed in vivo, under constant-flow and free-flow conditions, to those observed in vitro. The discrepancies between the in vivo and in vitro data are discussed relative to the effects of vasoactive agents on blood flow, intraorgan blood flow distribution, the countercurrent exchange of oxygen, capillary exchange capacity, and oxidative metabolism. Changes in blood flow, oxidative metabolism, and capillary density appear to be the major mechanisms by which vasoactive agents alter splanchnic oxygen uptake in vivo. Experimental designs are proposed that may help minimize inconsistencies in the data in future studies.