Sites of autoregulatory escape of blood flow in the mesenteric vascular bed.

1. Stimulation of the sympathetic nerves to the intestinal vascular bed results in an initial decrease in blood flow followed by a recovery towards the control level. This recovery was termed autoregulatory escape by Folkow and his co-workers and they suggested it was associated with a redistribution of blood flow within the intestinal wall. This theory has been examined in cats anaesthetized with pentobarbitone sodium. 2. The sympathetic nerves to the intestinal vascular bed were stimulated for 4 min periods at a submaximal frequency (4 Hz). The blood flows to individual parts of the superior mesenteric arterial bed (whole intestine, mucosal and submucosal layer, muscle layer of intestine, mesentery and lymph nodes, appendix and colon) were measured using radioactive microspheres before, at the peak of the vasoconstriction (30 sec), after autoregulatory escape had occurred (3-5 min) and during the hyperaemia after cessation of nerve stimulation. 3. All parts of the mesenteric vascular bed showed a significant initial vasoconstriction followed by a recovery in the flow to a level not significantly different from the pre-stimulation control flow. All parts showed a significant hyperaemia after cessation of stimulation. The distribution of the superior mesenteric flow at the peak of the vasoconstriction, after autoregulatory escape had occurred and during the hyperaemia after cessation of nerve stimulation was not significantly different from that during the control period. 4. It is concluded that all parts of the mesenteric vascular bed show autoregulatory escape and that this phenomenon is not associated with a redistribution of blood flow within the intestinal wall. Autoregulatory escape must involve relaxation of the same vessels which were originally constricted and various theories on the mechanism of the escape are discussed.