Capillary density in rat myocardium during timed plasma staining.

In urethan-anesthetized thoracotomized rats the density of myocardial capillaries that had been stained during timed infusions of a plasma label was determined. gamma-Globulin-coupled fluorochromes [lissamine-rhodamine B 200 (RB 200) and fluoresceinisothiocyanate (FITC)] were applied to the vascular system for different intervals of time. The circulation in the heart was freeze stopped, and the dye-containing capillaries were counted in histological cross sections of the organ. When the dyes had been infused into the left atrium for 1, 2, 5, or 10 s the number of stained vessels continuously increased (1,960 +/- 170, 2,780 +- 160, 3,310 +/- 130, and 3,340 +/- 140 cap/mm2 in the subepicardium and 1,990 +/- 210, 2,490 +/- 190, 3,090 +/- 160, and 3,210 +/- 160 cap/mm2 in the subendocardium, respectively). However, further prolongation of dye exposure (2, 10, or 30 min) did not increase significantly the number of marked capillaries. The induction of a general hypoxia (30-s stop of ventilation) after dye application also did not induce higher capillary counts than were found in the normoxic animals during long-time plasma labeling. Changes in red blood cell-containing capillaries were not covered by these observations. The distribution of stained capillaries in the evaluated areas was more inhomogeneous after incomplete filling with RB 200 for 1 s than after complete staining with FITC for 4 min. The results favor the view that in the rat heart at rest the blood plasma passes the microcirculation in an inhomogeneous manner; in all capillaries the slowest transit times do not exceed 5 s, however.