Interglomerular heterogeneity of filtration fraction among superficial nephrons: fact or artifact

The variance (S2inter-G) in protein concentration (CE) of plasma samples collected by micropuncture from randomly chosen surface efferent arterioles was compared to the variance (S2intra-G) of samples collected and re-collected from the same efferent arterioles in 23 Munich-Wistar rats. During efferent arteriolar blood collection, an oil droplet was maintained with gentle suction downstream to the collection pipette to avoid disturbance of natural glomerular blood outflow. S2inter-G averaged 0.067 +/- 0.014 (SE), a value comparable to mean S2intra-G (0.075 +/- 0.016, P greater than 0.50), indicating that the degree of interglomerular heterogeneity in the value of filtration fraction is too small to detect with this micropuncture technique. When ten additional samples were collected using uncontrolled suction pressure, S2inter-G averaged 1.15 +/- 1.30, a value markedly greater than that obtained with controlled pressure. In six nephrons which possessed both glomeruli and efferent arterioles on the surface of the kidney, glomerular capillary pressure (PGC) was monitored during efferent arteriolar blood collection. Applying widely varied suction pressure, PGC changed as much as 42 mmHg, suggesting that improper suction pressure can alter glomerular pressures and flows, hence filtration fraction and CE. Thus, unlike renal proximal tubules, some of the rat renal efferent arterioles do not function as Starling resistors, which may, in turn, account for the wide variability in CE values reported recently by some investigators.