Abstract
Summary
In a previous study, the distribution of nephron filtration rates was not affected by variations in dietary sodium, but anesthesia may have blunted a physiological response. Therefore, unanesthetized littermate rats placed on high (7 mEq/day) or low (0.4 mEq/day) sodium intake were studied. Daily urinary sodium excretion was measured in metabolic cages. One hour after recovery from ether anesthesia and surgery, the inulin clearance and urinary sodium excretion were measured. 14C-ferrocyanide was then infused during 12 sec and the left kidney pedicle tied off. Arterial pressure did not change. A rapidly fatal dose of pentobarbital preceded removal of the kidney. Urinary sodium excretion rate was reduced relative to mean daily cage measurements, to the same extent in conscious as in anesthetized rats on high sodium intake. Separate night and day urine collections revealed a marked circadian rhythm of sodium excretion rate in all rats, with lower rates during the day sufficient to account for those observed during the experiment. The superficial/juxtamedullary nephron filtration ratio was 0.90 ± 0.04 (SE) in six unanesthetized rats on low sodium and 0.84 ± 0.06 on high sodium (p > 0.10). Paired analysis of five littermate pairs also showed no difference (-0.069 ± 0.050) in spite of a significant difference in urinary sodium excretion (2.20 ± 0.38 μEq/min, p < 0.005). This study supports the view that dietary sodium does not affect the intrarenal distribution of nephron filtration rates.
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