Effects of surgery on plasma volume and salt and water excretion in rats

Surgical preparation of rats for micropuncture resulted in a marked decrease in sodium excretion (UNaV) compared to awake animals. Associated with surgery, hematocrit (Hct) rose. Studies were performed to determine whether the rise in Hct resulted from reduced plasma volume (PV) or increased red cell volume (RCV) and to explore the relation of such alterations to the fall in UNaV. PV and RCV were determined in the calm awake rat using 125I-albumin and 51Cr-labeled red blood cells. Micropuncture surgery was performed and RCV, PV, and Hct again measured. After anesthesia and femoral artery catheterization, Hct was not different from Hct in awake animals (42.9 +/- 2.8%). The Hct increased following surgery to 48.2 +/- 2.8% (P less than 0.001), accompanied by a large fall in PV (-18.9 +/- 2.3%, P less than 0.001) with no change in RCV. Plasma volume repletion to awake values restored UNaV toward levels appropriate for dietary intake in animals on a high salt diet. Althouth plasma repletion slightly increased UNaV above awake values in low salt diet rats, they continued to avidly retain salt with respect to total salt load.