Functional characterization of the microcirculation in tumors.

This review describes some aspects of tumor vessels and the influence of vasoactive agents on tumor blood flow, particularly the characteristic microcirculation of tumors with regard to its selective increase in blood flow. Elevation of blood pressure by infusion of angiotensin II produced a severalfold increase in tumor blood flow. The increase was selective and specific to the tumor vessels as long as the mean arterial blood pressure was kept under 150 mm Hg. Pressure elevation by angiotensin II also selectively increased tumor oxygen tension and influx of lymph flow from the primary transplanted lesion to the lymph node metastatic lesion. Newly devised techniques for analyzing microhemodynamics of tumor vessels showed that the velocity of tumor blood flow, the vascular area in tumor tissue, and the hydrostatic pressure difference between the tumor vessel and extravascular tissue were markedly enhanced. Thus, the extravasation of material into tumor tissues can be increased by the enhancement of blood flow. This demonstration allowed the development of a new approach to cancer chemotherapy, in which the delivery to tumor tissue of systemically administered anticancer drugs can be selectively enhanced.