Calculation of a mean functional diameter of capillaries of isolated rabbit hearts and changes of this diameter during hypoxia. Pathophysiological and pharmacological studies.

Isolated rabbit hearts were perfused aerobically (45 min) and hypoxically (105 min), using a modified Langendorff technique. The mean functional diameter of capillaries (MFDC) was calculated from the perfusion rate per minute and the inflow resistance by the model of Hagen-Poiseuille. The MFDC expresses the mean lumen of all capillaries of the heart, regardless of the different behaviour of the capillaries in the different regions of the myocardium. The MFDC decreased very rapidly after the onset of a hypoxia from 3.5 +/- 0.3 micron to about 70% of the initial diameter within 5-10 min (p less than 0.01) and then in a slower range within the following 70 min to about 44% of the initial diameter. The release of lactate dehydrogenase (LDH) from the hypoxic myocardium was detectable after 45 min of hypoxia and rose drastically after 60 min of hypoxia in our model. The decrease of the MFDC as well as the release of LDH from the hypoxic myocardium can be diminished by application of O-(beta-hydroxyethyl)-rutoside or 10 mM mannitol to the hypoxic perfusion medium. Both substances have antioxidant activities. It is discussed that the injury of the microvasculature is an early process during hypoxia, which can potentiate the hypoxic changes of the myocardial cells by additional diminution of the supply with oxygen and substrates. The protecting activities of substances with antioxidant actions to hypoxic myocardium were supported.