Renal extraction and clearance of p‐aminohippurate during saline and dextrose infusion in the rat.

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Abstract
The reliability of the use of p-aminohippurate (PAH) as a clearance marker for the determination of renal plasma flow during saline or dextrose infusion was investigated in conscious and anesthetized rats. In the conscious rat (wt. 350-400 g) infused for 4 h with 0.85% saline followed by 3 h of 2.5% dextrose, PAH clearance averaged 11.50 .+-. 1.45 ml/min during saline infusion and 7.83 .+-. 0.82 ml/min during dextrose infusion. The difference was significant (P < 0.02). Inulin clearance, however, was unchanged. With dextrose as the initial infusate, PAH clerance averaged 5.86 .+-. 0.65 ml/min and increased (P < 0.01) to 8.74 .+-. 0.71 ml/min following a change of infusate to saline, although inulin clearance was again unchanged. In separate groups of rats anesthetized with sodium pentobarbitone, a comparison was made between renal blood flow values calculated from PAH clearance and H, and those determined simultaneously using an electromagnetic flow probe placed on the left renal artery. During 0.85% saline infusion, the calculated value was 88.1 .+-. 8.4% of that measured using the flow probe. During 2.5% dextrose infusion the value determined from PAH clearance was only 47.3 .+-. 4.2% of that obtained using the flow probe, and this is significantly (P < 0.01) lower than the ratio found during saline infusion. In another study, renal venous blood samples were obtained from sodium-pentobarbitone-anesthetized rats during 0.85% saline or 2.5% dextrose infusions, for calculation of the renal PAH extraction ratio. During saline infusion, PAH extraction averaged 68.3 .+-. 2.5%, whereas during dextrose infusion PAH extraction was lower (P < 0.001) and averaged only 48.8 .+-. 3.2%. PAH clearance grossly underestimates renal plasma flow during 2.5% dextrose infusion in the rat.