Effects of barium-induced cardiac contraction on large- and small-vessel intramyocardial blood volume

We have measured the effects of barium-induced cardiac contraction on the intramyocardial blood volume content of all vessels and have independently measured the blood volume content of vessels with a diameter greater than 100 μm in rat myocardium. Measurements of total intramyocardial blood volume were made by using [¹²⁵I]albumin as a plasma marker and technetium-99m as a red blood cell marker. In one group of rats (n = 8), diastolic arrest was induced by an intravenous injection of KCl; in a second group (n = 8), systolic arrest was induced by an intravenous injection of BaCl₂. In both groups, the hearts were frozen in situ immediately after heart arrest while aortic pressure was decaying from its former physiological level. The left ventricular free wall was sectioned transmurally in a cryomicrotome, and the blood volume within each tissue sample was calculated from its radioactivity. The volume of blood in vessels larger than 100 μm was independently estimated from the exposed cross-sectional area of these vessels in photographs of the frozen tissue taken during tissue sectioning in the cryomicrotome. Total intramyocardial blood volume was found to decrease by about 42% from 8.6 ± 1.3 ml/100 g (mean ± SEM) in the KCl group to 5.0 ± 0.7 ml/100 g in the BaCl₂ group (p < 0.01). Total volume was greater in the epicardial than in the endocardial layers of both groups (p < 0.05). The volume of blood contained in vessels larger than 100 μm decreased from 1.2 ± 0.3 ml/100 g in the KCl group to 0.3 ± 0.1 ml/100 g in the BaCl₂ group (p < 0.05). The volume of blood contained in vessels smaller than 100 μm, calculated by subtracting large-vessel volume from total volume, decreased from 7.4 ml/100 g in the KCl group to 4.6 ml/100 g in the BaCl₂ group. These results show that barium-induced cardiac contraction strongly affects the volume of blood in the smaller coronary blood vessels. Although differences exist between barium contraction and a normal systolic contraction, the results suggest that during the normal cardiac cycle the small vessels play an important role in the determination of phasic coronary blood flow.