AUTORADIOGRAPHIC LOCALIZATION OF ACTIVE RENIN IN THE JUXTAGLOMERULAR APPARATUS OF THE DOG KIDNEY - EFFECTS OF SODIUM-INTAKE

1. The effects of dietary sodium intake on active renin binding in the juxtaglomerular apparatus (JGA) of superficial and juxtamedullary cor tex of the dog kidney were examined by quantitative in vitro autoradio graphy using a radiolabelled renin inhibitor [I-125]-H77, which has hi gh affinity for dog renin. 2. Changes in sodium intake resulted in mar ked alterations of active renin binding in the radiolabelled JGA. In c omparison with the control kidney (190.8 +/- 7.7 Bq/mm(3)), a higher d ensity of binding occurred in the labelled JGA of sodium-depleted kidn ey (277.7 +/- 6.2 Bq/mm(3)), while a lower density of binding was foun d in the labelled JGA of sodium-loaded kidney (99.3 +/- 7.4 Bq/mm(3)). 3. Active renin binding in the labelled JGA was significantly higher in superficial JGA than in their juxtamedullary counterparts, irrespec tive of sodium intake. 4. Pre-incubation with trypsin (0.5 mg/mL), a p rocedure known to activate prorenin, markedly increased active renin b inding in the labelled JGA of control (+ similar to 35%; P < 0.01) and sodium-loaded kidneys (+ similar to 75%; P < 0.01), but had little ef fect on binding in the labelled JGA of the sodium-depleted kidney (+/- similar to 5-10%; NS). The proportions of active renin as a percentag e of total renin were 60, 75 and 95% in the labelled JGA of sodium-loa ded, control, and sodium-depleted kidneys, respectively. 5. Emulsion m icroscopic autoradiography revealed that the binding was exclusively l ocalized in the JGA, including the afferent and efferent arterioles, m acula densa and extraglomerular mesangium, Labelling extended to the i nterlobular arteries in sodium depleted kidney. 6. These results indic ate that autoradiography combined with the in vitro binding of radiola belled renin inhibitors may provide a useful tool to measure active an d prorenin renin and thereby study the physiological regulation of ren in in the kidney.