Venulo-arteriolar communication and propagated response. A possible mechanism for local control of blood flow.

The effect of microinjection of norepinephrine (10(-5) M) into precapillary microvessels of the rat mesentery was studied using intravital microscopy. Upon application, in 29 out of 40 cases (73%) flow ceased at the site of drug application, although in most cases the precapillary microvessels themselves did not show a diameter change due to a lack of smooth muscle cells as confirmed by transmission electron microscopy. In 17 out of the 29 cases with flow cessation (59%), an intimate contact between the venule draining the site of application and the supplying arteriole was found. Initial constriction was seen at the site where the venule crossed the arteriole. Constriction propagated both up- and downstream along the arteriole, and also across arteriolo-arteriolar arcades. Arteriolar constriction could be abolished by intentionally occluding the venule draining the norepinephrine solution. It is proposed that venuloarteriolar contacts and propagated vasomotor response may contribute to local blood flow regulation by providing a feedback loop between tissue capillaries and resistance arterioles. In three complete mesenteric microvessel networks, the arterioles (n = 34) supplying 273 out of 401 capillaries (68%) were in close proximity to venules draining these same capillaries. Each of these arterioles served, on average, 43 capillaries, showing a bimodal distribution with peaks at 4 to 16 and at 64 to 256 capillaries. On average, 62% of all capillaries drained by a given venule crossing an arteriole originated from this very arteriole, indicating a reasonably effective feedback.