Videomicroscopic method for direct determination of blood flow to the papilla of the kidney.

We adapted the technique of videomicroscopy for direct determination of blood flow in individual capillaries of the papilla of the kidney, the ascending vasa recta (AVR) and descending vasa recta (DVR). The papilla was exposed in anesthetized rats and positioned under a video-camera-microscope and viewed under epiillumination. The intravenous infusion of fluorescein-isothiocyanate (FITC)-labeled gamma globulin was combined with fluorescence microscopy to enhance the contrast among plasma, red blood cells and capillary walls. On the television monitor, the walls were clearly outlined, enabling the measurement of capillary diameter. The velocity of red cells (Vrbc) in individual vasa recta was measured using the dual slit technique. From the videotape recorded microscopic image of a vas rectum, two photometric signals were obtained by integrating the light intensity from two electronic "windows" positioned closely together over the same capillary. Red cell velocity was calculated by dividing the distance between the two windows by the time delay between signals. The delay was determined using analog correlation tracking or digital cross correlation techniques. Single vasa recta blood flow was calculated from capillary diameter, Vrbc, and F (Fahraeus factor), which converts Vrbc to average whole blood velocity, Vblood. In quartz capillaries the same size as vasa recta, the ratio F = Vrbc/Vblood = 1.42 +/- 0.06. Total papillary blood inflow and outflow was calculated by multiplying the total number of DVR or AVR times the mean single capillary blood flow for DVR or AVR, respectively.