The effects of dietary sodium intake on active renin binding in the juxtaglomerular apparatus (JGA) of superficial and juxtamedullary cortex of the dog kidney were examined by quantitative in vitro autoradiography using a radiolabelled renin inhibitor [125I]-H77, which has high affinity for dog renin. 2. Changes in sodium intake resulted in marked alterations of active renin binding in the radiolabelled JGA. In comparison with the control kidney (190.8 +/- 7.7 Bq/mm3), a higher density of binding occurred in the labelled JGA of sodium-depleted kidney (277.7 +/- 6.2 Bq/mm3), while a lower density of binding was found in the labelled JGA of sodium-loaded kidney (99.3 +/- 7.4 Bq/mm3). 3. Active renin binding in the labelled JGA was significantly higher in superficial JGA than in their juxtamedullary counterparts, irrespective of sodium intake. 4. Pre-incubation with trypsin (0.5 mg/mL), a procedure known to activate prorenin, markedly increased active renin binding in the labelled JGA of control (+ approximately 35%; P < 0.01) and sodium-loaded kidneys (+ approximately 75%; P < 0.01), but had little effect on binding in the labelled JGA of the sodium-depleted kidney (+/- approximately 5-10%; NS). The proportions of active renin as a percentage of total renin were 60, 75 and 95% in the labelled JGA of sodium-loaded, control, and sodium-depleted kidneys, respectively. 5. Emulsion microscopic autoradiography revealed that the binding was exclusively localized in the JGA, including the afferent and efferent arterioles, macula densa and extraglomerular mesangium. Labelling extended to the interlobular arteries in sodium depleted kidney. 6. These results indicate that autoradiography combined with the in vitro binding of radiolabelled renin inhibitors may provide a useful tool to measure active and prorenin renin and thereby study the physiological regulation of renin in the kidney.
