Quantification of arteriolar density and embolization by microspheres in rat myocardium

We developed a technique for in vitro determination of arteriolar densities. Hearts, obtained from anesthetized rats and perfused by the Langendorff method, were fully dilated with adenosine and were arrested with an elevated potassium concentration. After a stabilization period, the hearts were perfused with a buffered fixative for some minutes to obtain a nonvital vasculature. After fixation, perfusion was switched back to control for some minutes. The hearts were then perfused with a medium containing a low concentration of microspheres: either pollen grains [Urtica dioica (15.4 microns), Betula (23.5 microns), or Phleum pratense (36.5 microns)] or polystyrene microspheres (15 microns). The perfusion was switched back to the standard medium after flow had been reduced to between 83 and 57% of control flow. Microscopic observations of microsections revealed that the percentages of arterioles embolized by one microsphere were 64% for the 15.4-microns, 74% for the 15-microns, 78% for the 23.5-microns, and 72% for the 36.5-microns microspheres. The percentages of arterioles embolized with two microspheres were 20, 15, 15, and 16%, respectively. The arteriolar densities were calculated from the total fractional reduction in coronary flow, the amount of microspheres injected, the wet heart weight, and the degree of occupancy, which corrects for the multiple embolization of the arterioles. The arteriolar densities in the rat hearts were 162.4 +/- 54.9 mg-1 (N = 6) for polystyrene microspheres of 15 microns, 161.5 +/- 81.1 mg-1 (N = 15) for 15.4-microns microspheres, 31.6 +/- 7.8 mg-1 (N = 9) for 23.5-microns microspheres, and 13.0 +/- 2.4 mg-1 (N = 8) for 36.5-microns microspheres.