Tubular pressure gradients and filtration dynamics during urinary stop flow in the rat

Summary

Proximal tubular pressure during 0.9 and 5% NaCl diuresis in rats remained above ureteral pressure for periods of occlusion of the ureter for as long as an hour, indicating continuous reabsorption-replacement. With 5% mannitol diuresis, the pressures came to apparent equilibrium in 20–25 min, and with 10–20% mannitol, in ca. 13 min. Inulin injected after the initiation of stop flow with intense mannitol diuresis (15 min blockade) was localized in the nephron beyond the site of the sodium “low”, taken to be location of the early distal convoluted tubule, thus verifying continued reabsorption-replacement during the period of ureteral occlusion. When inulin was injected after 30 min of stop flow with 5% NaCl diuresis, even greater concentrations of inulin were obtained in the nephron beyond the site of the sodium low, when the ureteral blockade was released at 45 min. In this series, U/P inulin averaged 72 (in the stop flow urine), compared to 13 in the mannitol group. C _IN during stop flow in the mannitol series averaged 0.16 ml/min/100 g B.W. (both kidneys), and with 5% NaCl, 0.434 ml/min/100 g B.W. The average values per g of kidney weight were 0.175 and 0.494 ml/min, respectively. These values approximate 25 and 60% of expected free flow values. A further complication of the stop flow method in the rat is that cortical blood flow declines after 10–15 min of blockade during mannitol diuresis, apparently the result of increased intrarenal pressure impairing flow in susceptible segments of the vasculature.