Absence of estimated glomerular pressure autoregulation during interrupted distal delivery

Micropuncture studies in dogs have suggested that a distal tubule-to-afferent arteriole feedback system may participate in the autoregulation mechanism at the single-nephron level. To evaluate the effect of interrupted distal delivery on glomerular capillary pressure (GP) and its autoregulation, the proximal tubule was blocked with oil and maximal stop-flow pressure was measured with a micropressure servo-null system. The GP was estimated from the sum of stop-flow pressure and the plasma colloid osmotic pressure (membrane oncometer). In 18 dogs given a mild mannitol load, average +/- SD control arterial pressure was 118 +/- 16 mmHg, proximal tubule pressure was 24 +/- 5 mmHg, and estimated GP averaged 70 +/- 10 mmHg. There was a highly significant relationship between estimated GP and arterial blood pressure. Similar results were obtained in hydropenic dogs. In response to decreases in renal arterial pressure in individual dogs, stop-flow pressure and estimated GP failed to exhibit autoregulation although autoregulation of renal blood flow, GFR, and proximal tubule pressure was observed over an arterial pressure range of 150-95 mmHg. These results indicate that interruption of normal distal delivery by proximal tubule blockage interferes with the ability of the nephron to autoregulate glomerular pressure. They provide further evidence in support of the concept that a distal tubular feedback mechanism participates, at least in part, in the autoregulatory control of glomerular pressure.