In vitro perfusion of juxtamedullary nephrons in rats.

Anatomical studies of rat kidneys revealed the presence of a unique population of juxtamedullary glomeruli (JMG) located at the inside cortical surface apposed to the pelvic lining and arcuate veins. These "superficial" JMG were exposed by longitudinally bisecting the kidney, reflecting the papilla, removing the pelvic mucosa, and transecting the venous walls. A microperfusion system was developed to allow blood perfusion of these nephrons via arcuate arteries. At a perfusion pressure of 100 mmHg, with most of the preglomerular pressure drop being localized to the terminal afferent arteriolar segment. In subsequent studies, blood hematocrit was reduced to approximately 30% with physiological solutions devoid of or containing albumin, and rats were treated with a converting enzyme inhibitor. In these conditions, single nephron glomerular filtration rate averaged 34 +/- 4 nl/min (low plasma colloid osmotic pressure, PCOP) and 23.3 nl/min (maintained PCOP). Proximal tubule reabsorption ranged from 17 to 29%. In conclusion, the integrity of nephron function is maintained in this model, which may provide further insights into the dynamics of filtration and reabsorption processes of juxtamedullary nephrons.