For the juxtamedullary nephrons only the loop of Henle is easily accessible for micropuncture. We have shown that the tubuloglomerular feedback (TGF) mechanism is highly activated in these nephrons. Techniques have been developed for measurements on these nephrons without disrupting the normal flow to the macula densa. The tubular flow rate was measured by a real-time videodensitometric method in juxtamedullary nephrons in Lewis x DA rats. For the determination of tubular fluid composition a small fraction (less than 10%) of the fluid was sampled. Measurements were also performed according to the conventional technique, where all the tubular fluid was sampled. Despite a more than twofold increase in tubular flow rate when the flow to the macula densa region was interrupted, sodium concentration was only reduced by 25%, whereas potassium concentration was reduced by 40%. The results demonstrate that interruption of the urine flow to the macula densa results in a 52% increase in the tubular load of Na in the loop of Henle of juxtamedullary nephrons, and a corresponding increase of 24% for K. Thus, TGF exerts a strong influence on these nephrons which is why previous data on water and electrolyte loads, based on stop-flow measurements, greatly overestimate these loads.
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