REGULATION OF CAPILLARY PERFUSION BY SMALL ARTERIOLES IS SPATIALLY ORGANIZED

To explore a mechanism for spatial recruitment of capillaries, this st udy determined whether the arterioles controlling capillary perfusion, which typically arise as sequential branches along a transverse arter iole, could respond differently from each other in situ in a spatially ordered way. Diameter changes were measured for these arterioles at a known location in the intact microvasculature in the cremaster muscle of anesthetized Golden hamsters (N=67); each arteriole controls separ ate capillary groups. These arterioles all had the same concentration dependence to locally (by micropipette) applied norepinephrine (NE, 10 (-9) to 10(-3) mol/L), and 10(-9) mol/L NE did not induce diameter cha nges when applied locally to individual vessels. However, 10(-9) mol/L NE added to the tissue superfusate, or 5% added superfusate oxygen (a lso locally subthreshold), each induced significant diameter changes ( both constrictions and dilations), in different branches, that were pr esumably due to summation of individually subthreshold events that cha nged the prevailing conditions at the point of observation. These sign ificant diameter changes were related to the maximal diameter or to in itial tone of the branches, but these changes occurred in different wa ys for NE versus oxygen. With NE, the branch arterioles that constrict ed (versus dilated) were significantly larger (maximal diameter, 22.3/-2.6 versus 15.9+/-2.1 mum) and had higher tone (fractional constrict ion, 0.53+/-0.05 versus 0.63+/-0.05); with oxygen, those that constric ted were the same size as those that dilated (maximal diameter, 28.6+/ -1.1 versus 30.5+/-2.7 mum), but constrictors had lower tone (fraction al constriction, 0.49+/-0.04 versus 0.39+/-0.06). Both vessel diameter and tone were themselves significantly dependent on the sequential br anch position. In addition, the spatial position of the branch along t he transverse arteriole modified the extent to which diameter or tone influenced the responses; with NE, the arterioles that were located di stally along the transverse arteriole dilated (axial distance versus r esponse, y=0.0001x-0.05; F test on slope, P=0.01), yet with oxygen, th e distal arterioles constricted (y=-0.0001x-0.02; P=0.03). Thus, this study shows that the small arterioles that control capillary perfusion are capable of responding differently from each other in a spatially organized way and, further, that the spatial pattern of diameter chang e is different when prevailing conditions (eg, local pressure/flow) ar e altered b adrenergic versus metabolic means.