Effects of propranolol and epinephrine on density of capillaries in rat heart.

The influence of propranolol and epinephrine on the time course of capillary plasma filling and the maximal obtainable capillary density was studied in the hearts of anesthetized rats. The blood plasma was labeled in vivo with macromolecule-coupled fluorochromes (lissamine-rhodamine B 200 and fluorescein isothiocyanate) for different periods of time and the density of marked capillaries determined in frozen sections of the heart. In the propranolol-treated (1.0 mg/kg) rats complete filling was observed after 40 s (controls: 10 s). When epinephrine (5.0 micrograms X kg-1 X min-1) was infused, plasma filling was completed within 5 s. Maximal capillary densities were higher in the drug-treated groups than in the control group (controls 3,530 capillaries/mm2; propranolol 3,890 capillaries/mm2; epinephrine 3,930 capillaries/mm2, subepicardium). This increase was induced by a reduction in mean diameter of muscular fibers due to a higher distension of the left ventricular wall in the drug-treated rats. It is concluded that changes in plasma transit times and in intercapillary distances due to fiber lengthening were induced by high doses of propranolol or epinephrine.