Regulation of capillary perfusion by small arterioles is spatially organized

To explore a mechanism for spatial recruitment of capillaries, this study determined whether the arterioles controlling capillary perfusion, which typically arise as sequential branches along a transverse arteriole, 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 separate 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 changes when applied locally to individual vessels. However, 10^-9 mol/L NE added to the tissue superfusate, or 5% added superfusate oxygen (also locally subthreshold), each induced significant diameter changes (both constrictions and dilations), in different branches, that were presumably due to summation of individually subthreshold events that changed the prevailing conditions at the point of observation. These significant diameter changes were related to the maximal diameter or to initial tone of the branches, but these changes occurred in different ways for NE versus oxygen. With NE, the branch arterioles that constricted (versus dilated) were significantly larger (maximal diameter, 22.3±2.6 versus 15.9±2.1 μm) and had higher tone (fractional constriction, 0.53±0.05 versus 0.63±0.05); with oxygen, those that constricted were the same size as those that dilated (maximal diameter, 28.6±1.1 versus 30.5±2.7 μm), but constrictors had lower tone (fractional constriction, 0.49±0.04 versus 0.39±0.06). Both vessel diameter and tone were themselves significantly dependent on the sequential branch position. In addition, the spatial position of the branch along the transverse arteriole modified the extent to which diameter or tone influenced the responses; with NE, the arterioles that were located distally along the transverse arteriole dilated (axial distance versus response, y=0.0001x-0.05; F test on slope, P=0.01), yet with oxygen, the 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 change is different when prevailing conditions (eg, local pressure/flow) are altered by adrenergic versus metabolic means.