Arteriolar bifurcation angles vary with position and when flow is changed

Flow distribution at microvascular bifurcations is influenced by the geometry of the bifurcation region, an area which also is altered pathologically. Bifurcation geometry was measured by in vivo microscopy of the cremaster muscle of anesthetized (Nembutal, 70 mg/kg) golden hamsters (N = 40), at rest and during maximal dilation (10(-4) M adenosine). The sequential branches had progressively smaller angles of bifurcation at rest: (first position) 118 +/- 5 degrees; (second) 89 +/- 6 degrees; (third) 78 +/- 5 degrees; (last) 58 +/- 4 degrees. Between flow conditions, the angle at any bifurcation changed by up to +/- 50 degrees, and the angle change was related to position. For the first position, the angles that decreased vs those that increased were significantly different at rest (130 +/- 6 degrees vs 109 +/- 7 degrees), but not during maximal dilation (119 +/- 6 degrees vs 118 +/- 7 degrees). Conversely, at the last bifurcation, the resting angles were not different (58 +/- 5 degrees vs 56 +/- 7 degrees), but became significantly different during maximal dilation (48 +/- 6 degrees vs 68 +/- 6 degrees). The axial distance to the first branch ranged between 57 and 857 microns; the angle at the first position was significantly smaller for those first branches that arose further distally along the feed. Further, angles that decreased (vs those that increased) were from significantly longer transverse arterioles (total length: 1820 +/- 77 microns vs 1560 +/- 61 microns). Resting tone was related to the angle as the smaller angles at the first position, but not at the last position, were more constricted in diameter. Tone was differently related to angle change as the bifurcations that decreased (vs those that increased) in angle were significantly more constricted at the last position (branch diameter rest/maximal: 0.57 +/- 0.05 vs 0.81 +/- 0.08) but not at the first position (0.66 +/- 0.09 vs 0.64 +/- 0.05). Thus, we show that the angle of bifurcation varies systematically for sequential branches arising along a single transverse arteriole and that the angles change with flow. This systematic organization for the geometric shape of sequential bifurcation regions may participate in the regulation of flow distribution within this group of arterioles.