Morphometric analysis of the actions of angiotensin II on renal arterioles and glomeruli

To study the effects of ansiotensin II on afferent and efferent arteriole diameters and on intraglomerular dimensions, angiotensin II (20 ng · kg^-1 · min^-1) or saline vehicle was infused intravenously for 20 min into anesthetized rabbits pretreated with enalapril. Both kidneys were perfusion fixed (glutaraldehyde), and vascular casts were made of the right kidneys using methacrylate. Morphometric analysis of the left kidneys using transmission electron microscopy revealed no significant effects of angiotensin II within the glomerulus, including the degree of mesangial contraction. The diameters of the afferent and efferent arteriole casts from the right kidneys were measured at 20, 50, and 75 μm from the glomerulus by scanning electron microscopy. In the outer cortex the mean diameters of the afferent and efferent arterioles were 14.1 ± 0.8 and 9.7 ± 0.5 μm, respectively, in the angiotensin II-infused rabbits, significantly less than in the control (vehicle) rabbits, 17.0 ± 0.7 μm (P <0.001) and 10.7 ± 0.4 μm (P <0.005), respectively. Calculation of the relative changes in vascular resistance, however, indicated that the effects of angiotensin II on efferent arteriole resistance (average difference 2.4 ± 1.2 units/μm) were significantly greater per unit length than the effects on afferent arteriole resistance (average difference 0.9 ± 0.3 units/μm). Thus infused angiotensin II caused greater reduction in afferent arteriolar diameter than in efferent, but the calculated increase in vascular resistance per micron was greater in efferent vessels due to their smaller resting diameter.