Effects of Recovery from Anesthesia and Surgery on Renal Sodium Handling in Conscious Rats

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Abstract
The accumulating evidence that the delivery of fluid from the proximal tubules to the loop of Henle (Vprox) can be measured in conscious rats by the lithium clearance (CLi)technique has renewed interest in developing a method by which also the glomerular filtration rate can be measured in conscious rats in a steady-state condition without influence from anesthesia and surgery. In the present study, Wistar rats of both sexes were put into a restraining cage, catheters were implanted in the jugular vein and the bladder, and renal parameters were determined under various conditions: different types of surgery, absence or presence of infusion with saline or glucose, normal or reversed diurnal rhythm, and examination at various times after surgery. In acutely operated and restrained rats given saline infusion, the proximal tubular fluid output (CLi) as well as the urinary excretion of sodium (UNaV) increased markedly during the first hours after anesthesia and surgery. After 5 h, both variables were significantly higher than in unoperated, unrestrained rats (CLi 364 ± 40 vs. 151 ± 38 µl/min/100g; UNaV 1,243 ± 433 vs. 219 ± 88 nmol/min/lOOg; means ± SD). Reversal of the diurnal rhythm did not change this pattern. Rats infused with 150 ml glucose instead of saline showed similar increases in CLi and UNaV, although the absolute levels were lower than in saline-infused rats. Rats given no infusion at all had subnormal values of CLi and UNaV. Rats operated 1–3 days before experiments and infused with saline showed enhanced although more stable values of CLi and UNaV. It is concluded that preceding anesthesia and surgery cause a lowering, and restraining and infusion an enhancement of Vprox and UNaV. As a consequence, the acutely catheterized, restrained rat model may not be suited for studies on physiological changes in tubular sodium handling.