1. Single nephron pressures, flows and resistances were studied in 17–18 week old genetically hypertensive and normotensive Kyoto rats resembling one another in renal weight, total filtration rate, morphology and number of glomeruli in the kidney.

2. Glomerular capillary pressures were similar in the two groups; however, the hypertensive rats had lower pressures at the end of the efferent arteriole than did the normotensive animals. Nephron filtration rates were similar in both groups. At the same time, the hypertensive rats had higher nephron filtration fractions, and their glomerular blood flow was lower than that of the normotensive rats. These changes in the hypertensive animals were due to their higher afferent and efferent arteriolar resistances. Despite their high blood pressure, the glomerular ultrafiltration forces in hypertensive rats were almost identical with those in normotensive rats.

3. We conclude that the regulation of glomerular capillary pressure and filtration rate in young Kyoto hypertensive rats is due to a balance of afferent—efferent arteriolar tonus. The normal glomerular capillary pressures and low efferent arteriolar pressures of these animals are most likely the functional basis for the lack of structural changes at these sites. These results suggest that the Kyoto animal's renal response to hypertension resembles that of most essential hypertensive patients, both in function and morphology.

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