Density of plasma-perfused capillaries in the rat heart during carbocromene-induced vasodilation

Urethane-anesthetized thoracotomized rats were used to ascertain whether the density of plasma-perfused capillaries increases in the heart during pharmacologically induced vasodilation. Carbocromene, in a dose of 3.0 mg/(kg x min) i.v., infused for 5 min, raised coronary blood flow from 6.2 +/- 0.6 to 15.6 +/- 0.1 ml/(min x g); heart rate and blood pressure were only slightly changed. Capillary density was determined by timed infusions of a plasma label (FITC or RB 200 coupled with gamma-globulin), infused for different periods of time in the same animal. No significant difference could be observed in the number of capillaries marked for 10 min in the carbocromene-treated rats as compared to the controls (carbocromene-treated rats: 3,630 +/- 90 cap/mm2 subepicardium, 3,360 +/- 70 cap/mm2 subendocardium; controls: 3,750 +/- 140 cap/mm2 subepicardium, 3,210 +/- 90 cap/mm2 subendocardium). In those rats treated with the vasodilator the filling of the microcirculatory system was nearly complete within a labelling period of 1 sec (3,500 +/- 170 cap/mm2 subepicardium, 3,070 +/- 110 cap/mm2 subendocardium), whereas significantly lower values were found when the dye was infused for 1 sec in the controls (2,560 +/- 460 cap/mm2 subepicardium, 1,960 +/- 400 cap/mm2 subendocardium). The results indicate that the filling of the cardiac microcirculatory system is accelerated by a pharmacologically induced vasodilation, the maximal density of plasma-perfused capillaries is not raised, however.