Autoregulatory responses of superficial nephrons and their association with sodium excretion during arterial pressure alterations in the dog.

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Abstract
Micropuncture experiments were conducted to assess autoregulatory behavior of superficial nephrons and to evaluate the possible contribution of alterations in tubular and peritubular capillary pressures to the mechanism responsible for arterial pressure effects on urine flow and Na excretion. In response to decreases in renal arterial pressure (RAP), autoregulation of renal blood flow (RBF) and glomerular filtration rate (GFR) was highly efficient to blood pressures as low as 80 mm Hg. Proximal tubule pressure (PTP), peritubular capillary pressure (PCP), distal tubule pressure and single nephron GFR (SNGFR) remained within 5% of control values with reductions in RAP to 90 mm Hg. Further decreases in RAP to the lower autoregulatory range caused significant decreases in PTP and PCP; slight decreases in SNGFR were not significant. Within the autoregulatory range, GFR and filtered Na load were not altered, and urine flow and Na excretion responses were due to changes in net fractional Na and water reabsorption. No quantitative relationships could be established between magnitude of changes in urine flow or Na excretion and PTP, PCP or proximal tubule fractional reabsorption. Superficial nephrons apparently autoregulate in association with the total nephron population except for a proportionately greater reduction when RAP is reduced to the lower autoregulatory range. Urinary responses to reduced arterial pressure occur even in the absence of quantitatively associated alterations in proximal tubule function or PCP.