Influence of systemically applied angiotensin II on the microcirculation of glomerular capillaries in the rat

The effect of intravenous infusion of angiotensin II on microvascular parameters of the renal microcirculation of rats was studied. With the aid of fluorescence microscopy and a high sensitivity video system we observed the passage of fluorescence-labeled erythrocytes through single glomerular capillaries on the surface of the rat kidney. From videotaped recordings, we measured the velocity and the flux of erythrocytes using a modified dual-slit technique with support of a microprocessor system. Angiotensin II was administered intravenously at a rate of either 0.2 or 0.4 microgram/min/kg of body wt. Angiotensin II decreased renal blood flow in a dose-dependent fashion (a 32% decrease with 0.2 microgram/min/kg and a 42% decrease with 0.4 microgram/min/kg). The higher rate of angiotensin II infusion had a variable effect on red cell velocity in glomerular capillaries with an overall effect to decrease velocity by 18%. Red cell flux in capillaries was similarly decreased by 25% with angiotensin II infusion. Three successive infusions of angiotensin II did not significantly diminish the effect of the peptide on red cell velocity or flux. Volume flow through the glomerular capillaries (calculated from erythrocyte velocity and vessel diameter) decreased during angiotensin II infusion (0.4 microgram/min/kg) from 3.2 to 2.4 nl/min despite no change in capillary diameter or hematocrit (ratio of erythrocyte flux to volume flow). These data indicate that alterations of the ultrafiltration coefficient (Kf) are not induced by uniform capillary vasoconstriction mechanisms, as others have suggested.