Selective vasodilators redistribute intestinal blood flow and depress oxygen uptake

In this paper we report the effects of adenosine and isoproterenol on intestinal oxygen consumption. Using the recently developed technique of laser-Doppler velocimetry (LDV), we also studied the effects of these two agents on regional blood flow in the canine intestinal mucosa and muscularis. In isolated gut loops perfused at constant blood flow, both vasodilators depressed O2 uptake, but isoproterenol increased rubidium extraction while adenosine depressed rubidium extraction. Under constant pressure perfusion both dilators depressed O2 uptake but increased total flow. Isoproterenol increased the capillary filtration coefficient (Kf), but adenosine depressed Kf. LDV flowmetry showed adenosine increased muscularis flow and depressed mucosal flow. Both LDV flowmetry and radioactive microspheres showed isoproterenol increased mucosal but depressed muscularis flow. Our data indicate that the deleterious effects of isoproterenol and adenosine in depressing intestinal oxygen uptake can be explained by the ability of these agents to selectively vasodilate one of the regional circulations within the gut wall and by a "vascular steal" mechanism to reduce blood flow in the other vascular bed not as sensitive to the agent in question. The reduction in the regional blood flow thus depresses the O2 consumption of the gut as a whole. By comparing our regional laser-Doppler blood flow data with a simple analog model, we show that the seemingly paradoxical effect of vasodilators in depressing a regional blood flow while increasing total perfusion is explicable by a "vascular steal" mechanism.