The effect of anaesthesia and standard clearance procedures on renal function in the rat

Renal function was assessed in unrestrained conscious rats during either their active period (i.e. the hours of darkness) or their inactive period. On the following day, measurements were repeated after Inactin anaesthesia and preparation for clearance studies. In rats anaesthetized during their active period, preparation for clearance studies had no effect on inulin clearance (used as a measure of glomerular filtration rate), lithium clearance (used as an estimate of end-proximal fluid delivery) or fractional lithium excretion. In rats anaesthetized during their inactive period, the same procedures resulted in increases in all three variables, to reach values indistinguishable from those in animals studied during their active period. In both groups of rats there were increases in the fractional reabsorption of sodium and water in the distal nephron and in the urinary excretion of potassium. It is concluded that in anaesthetized rats prepared for clearance studies, rates of glomerular filtration and proximal tubular reabsorption (as indicated by lithium clearance) are similar to those in conscious animals during their active period.