Influence of perfusate osmolality on stop-flow pressure feedback responses in the dog

Abstract
Single nephron microperfusion experiments were conducted to evaluate the influence of changes in perfusate osmolality on feedback-mediated responses in stop-flow pressure. Both nonelectrolyte- and NaCI-containing solutions with varying osmolalities were perfused toward the distal nephron from a late proximal segment at perfusion rates (PR) ranging from 16-68 nl/min. A wax block was placed in an intermediate segment and stop-flow pressure (SFP) was measured from an early proximal tubule segment. Perfusion with hypertonic NaCI solutions (1000 mosmol/kg) resulted in decreases in stop-flow pressure such that maximum responses occurred at lower PR than in tubules perfused with isotonic electrolyte solutions, while hypotonic NaCl solutions (100 mosmol/kg) failed to elicit feedback responses even at maximum perfusion rates. During perfusion with a 300 mosmol/kg mannitol solution SFP was significantly lower than in nonperfused tubules (35 .+-. 2 vs. 51 .+-. 1 mmHg) and was not altered greatly with increases in PR. During perfusion with hypotonic mannitol solutions (150 and 75 mosmol/kg), urea solutions and glucose solutions (300 mosmol/kg) decreases in SFP of about 15-25 mmHg occurred. Perfusion with more dilute mannitol solutions failed to elicit decreases in SFP. The addition of furosemide (10-3 M) to the mannitol solutions led to complete inhibition of the SFP responses to increases in PR. In the dog significant feedback-mediated decreases in SFP can occur with nonelectrolyte, as well as electrolyte perfusion solutions.

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