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

Abstract

The accumulating evidence that the delivery of fluid from the proximal tubules to the loop of Henle (V_prox) can be measured in conscious rats by the lithium clearance (C_Li)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 (C_Li) as well as the urinary excretion of sodium (U_NaV) increased markedly during the first hours after anesthesia and surgery. After 5 h, both variables were significantly higher than in unoperated, unrestrained rats (C_Li 364 ± 40 vs. 151 ± 38 µl/min/100g; U_NaV 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 C_Li and U_NaV, although the absolute levels were lower than in saline-infused rats. Rats given no infusion at all had subnormal values of C_Li and U_NaV. Rats operated 1–3 days before experiments and infused with saline showed enhanced although more stable values of C_Li and U_NaV. It is concluded that preceding anesthesia and surgery cause a lowering, and restraining and infusion an enhancement of V_prox and U_NaV. As a consequence, the acutely catheterized, restrained rat model may not be suited for studies on physiological changes in tubular sodium handling.

The effect of chronic sodium depletion on renal function in conscious rats

“…Even previous studies in conscious rats have provided conflicting results: Shalmi et al (1990) reported a reduced GFR in rats maintained on a low-sodium diet, whereas the same laboratory later found no such effect (Shalmi & Thomsen, 1993). It may be significant that the earlier study used restrained animals which had been surgically instrumented under short-term anaesthesia only hours previously; there is accumulating evidence that such preparations do not yield results which reflect renal function in unstressed rats even in sodium-replete states (Walker, Buscemi-Bergin & Gellai, 1983; Thomsen, Zanoni & Christensen, 1988;Petersen, Shalmi, Lam & Christensen, 1991). It is notable that in the later study of Shalmi & Thomsen (1993), which did not find an effect of sodium restriction on GFR, surgery was performed some 5-6 days previously.…”