Renal function in normal and potassium-depleted rats before and after preparation for micropuncture experimentation

Renal function was examined in unrestrained conscious rats maintained on either a control diet or a low-potassium diet, then re-examined in the same animals after thiobutabarbital (Inactin) anaesthesia and preparation for micropuncture studies. In conscious rats, inulin clearance (C_In) was not significantly different in the two groups (control 1012±43, low-K 904±58 l/min per 100g body wt; mean±SE), but lithium clearance (C _Li; used as an estimate of end-proximal fluid delivery) and fractional lithium excretion (FE_Li) were substantially reduced in the low-K group (C _Li: 246±11 vs 126±8 l/min per 100g body wt, P<0.001; FE_Li: 0.245±0.009 vs 0.143±0.008, P<0.001). Following anaesthesia and preparation for micropuncture, there were significant reductions in urine flow rate and sodium excretion in the control group, but not in the low-K rats. Potassium excretion increased in both groups, but values in the potassium-depleted animals remained extremely low. In neither group of rats was preparation for micropuncture associated with significant changes in C _In, C _Li or FE_Li. Thus, differences in tubular function between control and potassium-depleted rats were still apparent. The results suggest that preparation for micropuncture disturbs the function of the distal nephron, but that rates of glomerular filtration and proximal tubular reabsorption remain similar to values in conscious animals.