Hemodynamics of the recently opened glomeruli. Micropuncture study in 22- to 26-day-old puppies

In 22- to 26-day-old beagle puppies, just after superficial nephrons have begun to function, the single nephron glomerular filtration rate is extremely low (6.7 nl.min-1) approximating a tenth of the adult value (57 nl.min-1) even though blood pressure is as high as 90 mm Hg. Glomerular vascular resistance is almost six times the value found in the adult dog (1.8 vs. 0.32 mm Hg.ml-1.min), this increase being mainly due to a rise in afferent (80%) rather than in efferent (20%) resistance. The glomerulus, having only recently started to function, is characterized by filtration pressure equilibrium, a phenomenon atypical of superficial glomeruli in the adult dog. These findings favor the concept that a low glomerular blood flow rate might be the main cause of the very low SNGFR in the new glomeruli, although some contribution of an altered ultrafiltration coefficient cannot be excluded.